Influence of Soil Moisture on Boundary Layer Cloud Development

Transcription

1 86 JOURNAL OF HYDROMETEOROLOGY VOLUME 5 Influence of Soil Moiture on Boundary Layer Cloud Development M. B. EK National Center for Environmental Prediction, Environmental Modeling Center, Suitland, Maryland A. A. M. HOLTSLAG Meteorology and Air Quality Section, Wageningen Univerity, Wageningen, Netherland (Manucript received 22 February 2002, in final form 10 May 2003) ABSTRACT The daytime interaction of the land urface with the atmopheric boundary layer (ABL) i tudied uing a coupled one-dimenional (column) land urface ABL model. Thi i an extenion of earlier work that focued on modeling the ABL for 31 May 1978 at Cabauw, Netherland; previouly, it wa found that coupled land atmophere tet uing a imple land urface cheme did not accurately repreent urface fluxe and coupled ABL development. Here, finding from that earlier tudy on ABL parameterization are utilized, and include a more ophiticated land urface cheme. Thi land urface cheme allow the land atmophere ytem to repond interactively with the ABL. Reult indicate that in coupled land atmophere model run, realitic daytime urface fluxe and atmopheric profile are produced, even in the preence of ABL cloud (hallow cumulu). Subequently, the role of oil moiture in the development of ABL cloud i explored in term of a new relative humidity tendency equation at the ABL top where a number of procee and interaction are involved. Among other iue, it i hown that decreaing oil moiture may actually lead to an increae in ABL cloud in ome cae. 1. Introduction The interaction of the land urface with the atmopheric boundary layer (ABL) include many procee and important feedback mechanim with additional interaction in the cae of cloud (Fig. 1; e.g., ee Wetzel et al. 1996; Pielke et al. 1998; Bett 2000; Freedman et al. 2001). It i the purpoe of thi tudy to explore the feedback mechanim, in particular, the role of oil moiture on ABL cloud development. The cae tudy by Holtlag et al. (1995, hereafter HMR95) examined ABL model run driven by oberved urface fluxe, and reproduced the oberved boundary layer tructure for a cae tudy at Cabauw, Netherland (ee, alo, Stull and Driedonk 1987). But in their coupled land urface ABL model run HMR95 found that they could not reproduce oberved fluxe and boundary layer tructure uing a imple land urface cheme (uing contant urface conductance). In thi tudy we ue the ame cae tudy day a HMR95, but we alo model land urface ABL interaction uing an ABL cheme coupled with a more ophiticated land urface cheme. To thi end, for the tudy here we ue the Coupled Correponding author addre: M. B. Ek, National Center for Environmental Prediction, Environmental Modeling Center, 5200 Auth Road, Room 207, Suitland, MD michael.ek@noaa.gov Atmopheric Boundary Layer Plant-Soil (CAPS) model that conit of coupled land urface and ABL cheme, and wa developed to repreent interaction of the land urface with the ABL. Originally the CAPS model wa intended for incluion in large-cale numerical weather prediction model where computational efficiency i important, yet the equation ued are comprehenive enough to approximate the phyical procee thought to be mot important (e.g., Holtlag et al. 1990; Pan 1990; Holtlag and Boville 1993; F. Chen et al. 1997; Bett et al. 1997). The land urface cheme in the CAPS model ha been ued in a tand-alone mode for a number of enitivity experiment in different geophyical condition (e.g., Kim and Ek 1995; Chen et al. 1996) and, for the ame purpoe, a part of the Project for Intercomparion of Land-Surface Parameterization Scheme (PILPS; e.g., T. H. Chen et al. 1997; Wood et al. 1998; Chang et al. 1999); the tudy by Chang et al. (1999) include a comprehenive decription of the current phyic in the CAPS model land urface cheme. In addition, a number of tudie have pecifically examined land atmophere interaction uing the CAPS model in a coupled land urface ABL column mode (e.g., Ek and Mahrt 1994; Ek and Cuenca 1994; Xinmei and Lyon 1995; Cuenca et al. 1996; Holtlag and Ek 1996). In thi tudy we firt decribe the dataet at Cabauw (ection 2); then give an overview of the CAPS model 2004 American Meteorological Society

2 FEBRUARY 2004 EK AND HOLTSLAG 87 FIG. 1. Important interaction between the urface and atmopheric boundary layer for condition of daytime urface heating. Solid arrow indicate the direction of feedback that are normally poitive (leading to an increae of the recipient variable). Dahed arrow indicate negative feedback. Two conecutive negative feedback make a poitive one. Note the many poitive and negative feedback loop that may lead to increaed or decreaed relative humidity and cloud cover (adapted from Ek and Mahrt 1994, their Fig. 1). (ection 3); followed by land urface only, ABL-only, and coupled land urface ABL model run (ection 4); then examine the influence of oil moiture on ABL cloud (hallow, fair-weather cumulu) development (ection 5); and finally cloe with a ummary and concluion (ection 6). 2. Cabauw ite and dataet In thi tudy we ue obervation made on 31 May 1978 at or near the Cabauw ite in central Netherland that provide the neceary information for model initialization, forcing, and verification. The region urrounding the Cabauw ite i rather flat for a ditance of at leat 20 km, with many field and cattered canal, village, orchard, and line of tree. One of the main branche of the Rhine, the River Lek, flow about 1 km outh of the Cabauw ite, approximately 45 km eat of the North Sea. The Cabauw ite itelf i located in an open field nearly completely covered by hort gra that extend for everal hundred meter in all direction, with a erie of hallow, narrow ditche that provide drainage for the ite. Under the od layer (3 cm) the oil conit of heavy clay down to about 0.6 m, with a nearly aturated peat layer below. Soil moiture meaurement uing a neutron probe were taken covering the tudy day at three ample ite in the micrometeorology tower plot adjacent to the Cabauw tower; meaurement were made at 10-cm interval down to 50 cm, and at 1 m (Weel 1983). The cae tudy day wa the beginning of a dry down period, though oil moiture value were till ufficient o that tranpiration wa not overly limited. There had not been any precipitation for 1 week, and thi wa to lat three more week into later June before the next ubtantial precipitation event. The 213-m tower at the Cabauw ite include atmopheric obervation of wind and wind tre, temperature, and pecific humidity at multiple level, a well a enible and latent heat fluxe determined from profile and Bowen ratio method. Incoming olar and longwave radiation, low-level urface and oil temperature, and low-level pecific humidity meaurement were made at the micrometeorological ite adjacent to the Cabauw tower (within 200 m). The downward longwave radiation i upect, however, being anomalouly low. An etimate of downward longwave radiation i made via a reidual by taking the difference between the oberved net radiation, and the um of the net olar radiation and outgoing terretrial (longwave) radiation (computed from the infrared radiometer, auming an emiivity of 1). Soil heat fluxe were meaured by tranducer buried at depth of 5 and 10 cm; urface oil heat flux wa inferred from extrapolation of thee meaurement (Beljaar and Boveld 1997). See Monna et al. (1987) and Weel (1984) for further information on Cabauw obervation, and van Ulden and Wieringa (1996) for an extenive review of Cabauw boundary layer reearch. Four radioonde were launched from the Cabauw ite during the morning of the tudy day providing temperature and moiture profile above the tower level. Additionally the dataet i upplemented with information from radioonde launched at De Bilt (about 25 km to the northeat) everal time during the day, providing additional meaurement of wind, temperature, and moiture. Becaue of the proximity and imilarity in urface condition, the De Bilt obervation are thought to be repreentative of the Cabauw ite, epecially above the urface layer (ee HMR95). For our cae tudy day of interet, the ynoptic weather pattern over wetern Europe wa dominated by urface high preure with generally fair weather and light wind from the eat, with a frontal ytem to the wet of the Britih Ile. 3. CAPS model The CAPS model conit of a land urface cheme with multiple oil layer (Mahrt and Pan 1984), and a imple plant canopy (Pan and Mahrt 1987) that i modified to include the effect of vegetation uing a big leaf approach for canopy conductance, following Noilhan and Planton (1989), and i decribed in Holtlag and Ek (1996). Thi more empirically baed approach for canopy conductance follow the original work by Jarvi (1976) and Stewart (1988) where canopy conductance i modeled a a function of atmopheric forcing and oil moiture availability. Under thi approach, for thi tudy we adopt the canopy conductance formulation more pecific to Cabauw following Beljaar

3 88 JOURNAL OF HYDROMETEOROLOGY VOLUME 5 FIG. 2. Initial oil moiture profile for 31 May 1978 at Cabauw, Netherland: oberved (), initial model oil moiture reference profile interpolated to model oil-level midpoint (, and olid line), and oberved oil temperature at 2 cm() and initial model oil temperature reference profile (, and heavy olid line). and Boveld (1997) who examined the influence of vegetation evaporative control on urface moiture fluxe at Cabauw. The oil heat flux formulation implicitly account for vegetation-reduced thermal conductivity (and, thu, oil heat flux), allowing more available energy for enible and latent heat fluxe; thi formulation follow the one ued in the Tiled European Centre for Medium-Range Weather Forecat (ECMWF) Scheme for Surface Exchange over Land (TESSEL) model (Viterbo and Baljaar 1995; van den Hurk et al. 2000). Thi i in principle imilar to the formulation decribed in Peter-Lidard et al. (1997) that ue a more explicit dependence on vegetation denity (via a leaf area index). We et the depth of the firt oil layer in our model the ame a in the TESSEL model (7 cm; Fig. 2) in order to ue the ame Cabauw-calibrated coefficient in the oil heat flux formulation. Following Beljaar and Boveld (1997), our ubequent oil layer match the bottom of the higher root denity zone (18-cm depth), a zone of lower root denity down to the bottom of the root zone (60 cm), with a ubroot zone below (1.5-m total depth), and an implicit oil column bottom at 3.0 m. A nonlinear root ditribution with exceively high root denity near the urface may lead to improper rapid drying of the higher root denity oil layer in the root zone (Zeng et al. 1998). Thi can yield le accurate prediction of latent heat flux, and ubequently the urface energy budget. A uch, in our tudy here we aume a near-uniform root ditribution becaue bulk method (uniform root ditribution) are more conitent with the current level of undertanding and, thu, are preferred over rootweighted method (Deborough 1997). Thi may mitigate the problem of treating the root zone a tatic when in fact it may be rather dynamic in term of the ability of vegetation to extract water from where it i available in the root zone, depite the root denity ditribution. The ABL cheme ue the original combined local (K theory) and nonlocal (boundary layer cale mixing) development by Troen and Mahrt (1986) with an update to nonlocal mixing of heat and moiture following Holtlag and Boville (1993), and quite imilar to the ABL cheme ued in the HMR95 tudy. The boundary layer height formulation ha been modified to account for boundary layer with relatively high wind peed and upper boundary layer tratification, and include the effect of turbulence due to urface friction under nearneutral and table condition (for further detail, ee Vogelezang and Holtlag 1996). A imple fractional boundary layer cloud-cover formulation (Ek and Mahrt 1991) i included in the ABL cheme, which i baed on a Gauian ditribution of total-water (vapor plu liquid) relative humidity near the boundary layer top, where cloud cover i defined a the area under the Gauian curve above aturation. The relative humidity ditribution include turbulent and meocale variation, where the turbulent variation i formulated in term of dry-air entrainment at the boundary layer top, while the meocale (ubgrid) variation i pecified a a function of horizontal grid ize (aumed to be on the order of 100 km, correponding to a meocale relative humidity variation of 5% acro the domain of central Netherland). When patial fluctuation of relative humidity are large, boundary layer cloud firt form at a lower patially averaged relative humidity. Thi formulation wa developed uing Hydrological Atmopheric Pilot Experiment (HAPEX) Modéliation du Bilan Hydrique (MOBILHY) data (continental fairweather cumulu), but ha alo hown quite favorable performance in the tudy of Mocko and Cotton (1995) uing data from the Boundary Layer Experiment (BLX) (alo continental fair-weather cumulu) a well a from the Firt International Satellite Cloud Climatology Project (ISCCP) Regional Experiment (FIRE) (marine tratocumulu). With the CAPS model in a fully interactive mode, the land urface cheme i coupled with the ABL cheme, and ABL cloud predicted by the cloud-cover formulation are allowed to alter the radiation budget at the urface (via a imple urface radiation cheme included a an option in the CAPS model), thereby affecting urface procee (e.g., fluxe). Incoming clear-ky olar and downward longwave radiation reaching the urface are calculated following the method of Collier and Lockwood (1974) (imilar to Holtlag and van Ulden 1983), and Satterlund (1979), repectively, and include olar attenuation by ABL cloud via a tranmiion function following Liou (1976) for climatological cu-

4 FEBRUARY 2004 EK AND HOLTSLAG 89 mulu cloud, a cloud enhancement to the downward longwave radiation, following Paltridge and Platt (1976), and a Cabauw-pecific hortwave albedo formulation following Duynkerke (1992). The lowet atmopheric model level are at 20, 40, 80, 120, and 200 m (matching the Cabauw tower obervation height), with 100-m vertical reolution from 200 m to a height of 2 km, then 200-m reolution above to the top of the model domain (10 km). The time tep ued in the model run i 180, which i felt to be appropriate for the model (vertical) reolution. 4. Model evaluation We firt examine model run of the land urface forced by atmopheric condition, followed by model run of the ABL forced by oberved urface heat and moiture fluxe. Thee tand-alone or offline land urface only and ABL-only (uncoupled) tet allow u to iolate the procee reponible for land urface fluxe (land urface cheme without ABL interaction) and ABL development (ABL cheme without land urface interaction) eparately before coupling the land urface and ABL cheme. In a coupled mode, more complicated interaction and feedback are poible, including the formation and preence of ABL cloud. FIG. 3. Atmopheric forcing data for 31 May 1978 at Cabauw, Netherland: (a) 20-m temperature, pecific humidity, and wind peed (large ymbol from Cabauw and DeBilt radioonde data), and (b) incoming and reflected olar and downward longwave radiation. (a) Time erie of air temperature and pecific humidity for ABL-only, and coupled land urface ABL model run, and (b) time erie of incoming olar, reflected olar, and downward longwave radiation for coupled land urface ABL model run uing modeled radiation (with ABL cloud interaction). a. Land urface modeling reult (atmopheric forcing) We ue the ame cae tudy day a HMR95, and firt examine a bae tate model run to how land urface behavior in repone to oberved atmopheric forcing (Fig. 3) before making coupled land urface ABL model run. We initialize our land urface model uing oil moiture obervation interpolated to the midpoint of the model oil level (Weel 1983; Fig. 2). Soil temperature i initialized at the firt model oil layer (3.5 cm) uing 2 cm obervation; thi difference i not expected to be ignificant at thi time of day. Soil temperature obervation are not available below 2 cm, o to initialize oil temperature at ubequent model oil level we make approximation from the average of the previou week, month, and 3-month 2-m air temperature, repectively, for the lowet three model level, with the annual 2-m air temperature ued a the implicit bottom temperature. In our land urface model run driven by oberved atmopheric condition, the latent heat flux i lightly underpredicted (overpredicted) in the morning (afternoon), while enible heat flux i generally well predicted, though lightly high around midday (Fig. 4a). Thi may be due to the light underprediction in the canopy conductance in the morning (reult not hown). Net radiation and oil heat flux are generally well predicted (Fig. 4b), though the oil heat flux i lightly overpredicted (underpredicted) in the morning (early afternoon), with oil temperature till comparing favorably (reult not hown). Thee reult are not completely urpriing becaue the parameter ued in the variou urface formulation have been calibrated for Cabauw data, although not pecifically for our cae tudy day here (31 May 1978). b. Atmopheric boundary layer modeling reult (urface forcing) We alo examine a bae tate model run to how ABL behavior in repone to oberved urface fluxe before making coupled land urface ABL model run. Overall, our reult are imilar to thoe of HMR95. Following

5 90 JOURNAL OF HYDROMETEOROLOGY VOLUME 5 FIG. 4. Time erie of (a) latent and enible heat flux, and (b) net radiation and oil heat flux (obervation indicated by ymbol), and model run (line) for land urface only and coupled ABL land urface model run uing precribed or modeled radiation (31 May 1978 at Cabauw, Netherland). HMR95, large-cale (horizontal) advection wa unknown but thought to be weak, conidering the ynoptic ituation, o thi ame condition i applied to the ABL model run here. A in HMR95, we initialize the ABL cheme with temperature and pecific humidity profile (Fig. 5a), and drive the ABL by precribing oberved fluxe on 31 May 1978 (ee Fig. 4a). Becaue a column model cannot adequately repreent meocale momentum dynamic, we precribe the wind profile at each time tep by interpolating radioonde wind data (above 200 m) taken at 0600, 1200, and 1800 UTC; below 200 m we ue UTC interpolated FIG. 5. (a) Initial atmopheric profile of potential temperature (), and pecific humidity (q) and aturation pecific humidity (q at ), and (b) wind peed profile from De Bilt radioonde and Cabauw tower wind ued for precribing wind in model run (31 May 1978). 30-min Cabauw tower wind obervation that are conitent with radioonde wind (Fig. 5b). So we depart lightly from HMR95 where wind wa modeled (though they precribed geotrophic wind); thi may alo have contributed to the le accurately modeled urface fluxe in HMR95. By avoiding modeling the wind we can focu more effectively on the ABL mixing of heat and moiture, and on the interaction with the land urface in the cae of coupled land urface ABL model run (decribed in the next ection). Thi method of precribing wind wa alo uccefully employed by Holtlag and Ek (1996) to deal with a complicated wind ituation and allowed them to focu on boundary layer

6 FEBRUARY 2004 EK AND HOLTSLAG 91 FIG. 6. Maximum afternoon boundary layer depth (dahed line), and 1600 UTC (light gray) and maximum boundary layer (dark gray) cloud cover v precribed vertical motion in ABL model run forced by oberved urface fluxe (31 May 1978 at Cabauw, Netherland). FIG. 7. Time erie of (a) ABL height and (b) cloud cover for ABLonly, and coupled land urface ABL model run (31 May 1978 at Cabauw, Netherland). heat and moiture mixing, and interaction with the urface. Additionally, we turn on the ABL cloud in order to ae the performance of the ABL cloud formulation, though there i no interaction with the urface becaue urface fluxe are precribed; that i, in thi phae of teting, ABL cloud do not modify the urface radiation budget and ubequent ABL development. In full coupled land urface ABL teting (the ubject of the next ection), we include the effect of a cloud-modified urface radiation budget. (However, note that in our cloud-cover formulation o far, there i no enhanced turbulent mixing due to ABL cloud, that i, by hallow cumulu.) Large-cale vertical motion wa not known in the HMR95 tudy where it wa et to zero; however, the precribed vertical motion affect boundary layer growth, which influence ABL cloud development. So we firt make a erie of enitivity tet of the ABL model forced by oberved urface fluxe where vertical motion i varied between 1.0 cm 1 (at 2 km, then linearly decreaing to zero at the urface). Modeled ABL cloud are firt predicted in the midafternoon, and generally increae with increaing vertical motion (increaing boundary layer depth), with modeled ABL cloud cover varying between near zero and complete coverage (Fig. 6). With increaing precribed vertical motion, ABL cloud form and increae in their fractional coverage becaue with an increaingly deeper ABL, cooling at the ABL top i ufficient for the relative humidity to reach a threhold value (influenced by the ABL-top relative humidity variation) for cloud to form. For our cae tudy day here, modeled cloud cover remain mall until poitive value of vertical motion are precribed, after which cloud cover increae greatly (while boundary layer depth increae lightly). Reult ugget that a nominally mall value of vertical motion (0.5 cm 1 ) give a fractional ABL cloud cover that i qualitatively conitent with the ynoptic weather ituation decribed earlier, where ABL cloud (fair-weather cumulu) firt formed in the early to midafternoon with 3/8 5/8 coverage by late afternoon acro the region around Cabauw in central Netherland. Note that when uing a precribed large-cale vertical motion of zero (a in HMR95), modeled cloud coverage i quite mall, though profile and time erie of temperature and pecific humidity are not overly enitive (a in HMR95) (reult not hown). In our ABL model run driven by oberved urface fluxe, modeled ABL growth i lightly too vigorou in the morning hour; ABL depth i better repreented in the afternoon, and during the late afternoon ABL tranition to a hallow table boundary layer (Fig. 7). ABL growth i alo lightly more vigorou in the late morning than that found in HMR95, with noon time value a few hundred meter deeper, though well within the range of uncertainty in the oberved ABL depth at De Bilt (ued a an etimate for Cabauw). Midafternoon value of modeled ABL depth were about m deeper than found by HMR95, though lack of obervation during

7 92 JOURNAL OF HYDROMETEOROLOGY VOLUME 5 thi period make thi comparion inconcluive. Our precription of wind profile throughout the model run period veru modeled wind in HMR95 may have lead to difference in diagnoed ABL height. Alo recall that in our tudy here we have updated the ABL height formulation (decribed in ection 3), though ue of the original formulation for ABL height yield little difference in the development of the ABL, owing to the daytime convective nature of the ABL (reult not hown). The modeled evolution of both temperature and moiture i conitent with the ABL development, and, a in HMR95, the modeled 20-m (potential) temperature wa found to be lightly warmer than oberved in the morning hour, and about 1C cooler than oberved during the afternoon hour (Fig. 3a). Specific humidity i comparable to obervation, though with a lightly maller midmorning peak that i often oberved prior to late-morning rapid ABL growth (Fig. 3a). Alo note the 1200 UTC profile of potential temperature and pecific humidity (Fig. 8), which how the modeled potential temperature profile cooler than oberved by about 1.0 K (1.5 K) compared to Cabauw tower (De Bilt radioonde) obervation, and pecific humidity about 0.5 gkg 1 greater (le) than the Cabauw tower (De Bilt radioonde) obervation. A in HMR95, the difference in the profile may be attributed to modification in the air ma not repreented by the forcing, that i, urface fluxe, and initial temperature and humidity and pecified wind profile. We refer the reader to HMR95 for further detail of their enitivity experiment on ABL repone to, for example, variou choice of advection, initial temperature and pecific humidity profile, and other model enitivity tet. c. Coupled modeling reult (urface atmophere interaction) In coupled run, the model i initialized the ame a in previou land urface only (ection 4a) and ABLonly (ection 4b) model run, but now the land urface i allowed to operate interactively with the ABL, but with the oberved radiation at the urface precribed for our firt tet. Note that if the reult from coupled model run improve compared to reult from model run with the land urface cheme operating alone (atmophere and radiation forced), or with the ABL cheme operating alone (urface forced), then compenating interaction could be reponible for any noted improvement. On the other hand, a coupled model tudy may reveal that the model fluxe are more repreentative on the ABL cale (e.g., on the order of ten of kilometer) compared to thoe oberved (at the Cabauw tower ite). Marguli and Entekhabi (2001) explored land atmophere interaction uing an adjoint framework, and point out the importance of uing a coupled (e.g., land urface ABL) model in examining the enitivity of urface parameter (veru typical uncoupled teting). In our tudy here, generally we hope that in coupling the land urface and FIG. 8. ABL profile at 1200 UTC for (a) potential temperature and (b) pecific humidity for ABL-only, and coupled land urface ABL model run (31 May 1978 at Cabauw, Netherland). ABL cheme that the reult will not diverge ignificantly. Thi appear to be the cae in the coupled model run for urface heat fluxe (Fig. 4), and ABL development and cloud formation (Fig. 3a, 7, and 8). (Note that in the cae here, predicted ABL cloud are paive in that they do not affect the radiation reaching the urface.) A an additional tet, we utilize the imple urface radiation cheme (decribed in ection 3) to predict the radiation budget at the urface. Thi remove an additional degree of freedom in coupled model run o that it i more fully interactive (except for the pecified evolution of the wind profile). The ABL cloud cover predicted by the cloud-cover formulation attenuate incoming olar radiation and lightly enhance downward longwave radiation that reache the urface, which af-

8 FEBRUARY 2004 EK AND HOLTSLAG 93 fect land urface procee (canopy conductance, urface fluxe, etc), ubequent boundary layer development, cloud cover, and o on. In thi cae, the incoming and reflected olar (S and S, repectively) and downward longwave (L ) radiation are predicted fairly well (Fig. 3b), which aee the performance of our imple urface radiation cheme and the interaction with ABL cloud. The modeled urface heat fluxe (Fig. 4) are imilar to thoe uing oberved radiation, with imilar reult for ABL development and cloud cover (Fig. 3a, 7, and 8). 5. Impact of oil moiture on ABL cloud development a. Coupled model reult To fully explore the interaction of the land urface with the ABL and the effect on boundary layer cloud development, we make a erie of model run (a reference et) where we change the oil moiture from quite dry to quite wet. Initial condition and forcing are the ame a in our previou coupled model run (except here we pecify a uniform oil moiture profile to better illutrate difference between model run), and vary oil moiture from below the wilting point ( wilt ) to near aturation ( at ). We note that for the variou model run, a we decreae the initial oil moiture from intermediate oil moiture value (cloe to obervation, 0.43) to below the wilting point, ABL cloud cover decreae to zero (Fig. 9a), a omewhat intuitive reult. However, a we increae the initial oil moiture from intermediate oil moiture value to near aturation, ABL cloud cover decreae lightly, a omewhat counterintuitive reult. Certainly there are a number of procee that account for thi behavior, that i, interaction between the land urface, atmopheric boundary layer (including ABL cloud), free atmophere, and initial ABL condition (Fig. 1). Before attempting an explanation of thi repone, we alo examine the role of atmopheric tability ( ) above the ABL in land urface interaction with the evolving boundary layer becaue ha a trong influence on boundary layer growth. We make two additional et of model run a above, except now we precribe one et with increaed atmopheric tability above the oberved afternoon boundary layer top (compared with the reference et of model run above), and another et with decreaed atmopheric tability (ee Fig. 5a). We then examine the reulting afternoon ABL depth and fractional cloud cover, and the midday urface energy budget a it change with changing precribed initial oil moiture (Fig. 9b). The et of model run with tronger atmopheric tability ha hallower ABL depth than the reference et and le cloud cover for drier oil, with increaing cloud cover for model run with increaed oil moiture. However, in great contrat, the et of model run with FIG. 9. Impact of variation in volumetric oil moiture for different et of model run (reference, and increaed and decreaed above- ABL atmopheric tability) on (a) ABL depth and cloud cover, and (b) component of the urface energy budget (Rn: net radiation; LE: latent heat flux; H: enible heat flux; and G: oil heat flux). weaker atmopheric tability above the ABL ha deeper ABL depth (a one would expect) and yet much greater cloud cover for drier oil, with decreaing cloud cover for increaing oil moiture. Thi i in general agreement with the finding by Wetzel et al. (1996). In the next ection, we will attempt to explain thi reult in term of a tendency equation for relative humidity at the ABL top. b. Analytical reult The role of oil moiture in ABL cloud development involve a complex interaction of urface and atmopheric procee. Ek and Mahrt (1994) examined the daytime evolution of ABL-top relative humidity that i expected to control ABL cloud development. They howed that the relative humidity tendency at the ABL top involve a number of competing mechanim, with relative humidity directly increaing due to urface evaporation and due to ABL growth (ABL-top temperature decreae), and relative humidity directly decreaing due to urface enible heat flux and due to entrainment of warm and dry air into the ABL from above. The indirect role of urface evaporation i to reduce

9 94 JOURNAL OF HYDROMETEOROLOGY VOLUME 5 urface heating, thereby competing with ABL growth that i reduced due to reduced urface heating, although thi diminihe ABL-top warm- and dry-air entrainment. In a imilar type of tudy, De Bruin (1983) examined the effect of different land urface and ABL procee on the Prietley Taylor parameter (ued in relating urface available energy to urface evaporation). To further undertand the role of oil moiture and other factor on ABL cloud development, we extend the work of Ek and Mahrt (1994) and examine a ueful new equation for relative humidity tendency at the ABL top (ee appendix for development), where RH Rn G [ef ne(1 e f)], (1) t Lhq where R n G i available energy at the urface (R n i net radiation and G i oil heat flux), i air denity, L i latent heat, h i ABL depth, and q i aturation pecific humidity jut below the ABL top. In (1), e f i the urface evaporative fraction (of urface energy available for evaporation) defined a LE LE ef, (2) R G H LE n where LE (L wq ) and H (c p w ) are the urface latent and enible heat fluxe, repectively. Furthermore, ne(1 e f ) reflect the direct effect of nonevaporative procee on relative humidity tendency, where ne i given by [ ] L q c2 ne (1 C ) RH c 1, (3) c h p where c p i pecific heat, C i the (negative of the) ratio of urface to ABL-top enible heat flux, q i the pecific humidity drop above the ABL (negative), i the potential temperature lape rate above the ABL, and c 1, c 2 are function of urface preure, temperature and preure at the ABL top, and contant (ee appendix). Here, ne conit of three term [each multiplied by (L / c p )(1 C )]: ABL-top dry-air entrainment [q/(h ), a negative contribution to ABL-top relative humidity tendency], boundary layer growth (RHc 2 /, a poitive contribution), and boundary layer heating through urface warming and ABL-top warm-air entrainment (RHc 1,a negative contribution). From (1) we ee that the relative humidity tendency i proportional to available energy and inverely proportional to ABL depth and temperature (via aturation pecific humidity), while the ign of the relative humidity tendency i determined by the ign of e f ne(1 e f ). Examining (1), it i apparent that the direct role of e f i to increae the ABL-top relative humidity, while the indirect role of urface evaporation (via reduced urface heating, and diminihed ABL growth and entrainment) i found in the expreion ne(1 e f ). Figure FIG. 10. ABL-top relative humidity tendency equation, e f ne(1 e f ) (normalized by the available energy term), a a function of evaporative fraction (e f ) v nonevaporative procee (ne) with Cabauw (Netherland, 31 May 1978) value and time indicated (dot). See text for detail. 10 how how e f ne(1 e f ) depend on e f veru ne, where e f ne(1 e f ) i the relative humidity tendency, RH/t, normalized by the available energy term, (R n G)/( hq ). When the above-abl atmopheric tability i rather trong (larger ), or if the tability i rather weak and the above-abl air i rather dry (larger q), then ne 1 o that RH/t increae a e f increae, confirming intuition. (For the range 0 ne 1, RH/t 0 and increae with increaing e f, while for ne 0, RH/t 0 only when e f exceed ome threhold value that increae for increaingly negative value of ne.) Here, oil moiture act to increae ABL-top relative humidity and, thu, increae the probability of ABL cloud development given a ufficient initial ABL relative humidity. On the other hand, with weaker above-abl tability (maller ), boundary layer growth i le retricted over drier oil than over moiter oil compared to the cae with tronger tability. So with above-abl air that i not too dry, then ne 1 o that RH/t increae a e f decreae, which i omewhat counterintuitive. Here, oil moiture act to limit the increae of ABL-top relative humidity and, thu, decreae the probability of ABL cloud development. Note that the larget value of RH/t are achieved for ne 1, uggeting that the greatet potential for ABL cloud development i not over moit oil, but rather over dry oil with weak tability and above-abl air that i not too dry. From (1) (3), note that with drier air above the ABL (increaingly negative q), the value of ne decreae,

10 FEBRUARY 2004 EK AND HOLTSLAG 95 and that a the oil moiture increae, generally e f increae (depending on the precie relationhip between oil moiture and urface evaporation). But a change in tability above the ABL ( ) affect both dry-air entrainment and boundary layer growth, two oppoing procee in the ABL-top relative humidity tendency equation. So, only if the above-abl pecific humidity drop i greater (le negative) than ome threhold q RHhc 2 (at the ABL top) will ne increae with decreaing tability, which correpond to ne (L /c p ) (1 C )RHc 1 (to the left of the heavy vertical dahed line in Fig. 11). Note that thi threhold value of q decreae (become more negative) for increaing RH, h, and c 2 (decreaing T); thi i the cae at Cabauw from morning to midday. Finally, a q 0, ne 0 for c 2 /c 1 g/c p 1C (100 m) 1 (dry adiabatic lape rate). Before we proceed, we note that the outcome of (1) (3) (a preented in Fig. 10 and 11) agree well with the output of the coupled model (confirmed by more than a thouand run), a long a h/l 5, which i required for the aumption of mixed-layer condition (ee Holtlag and Nieuwtadt 1986). c. Dicuion We can examine the variou ABL-top relative humidity tendency term in (1) (3) for Cabauw data during period of poitive urface fluxe and when h/l 5 (Table 1, Fig. 10). From midmorning until midday, the dry-air entrainment term decreae in magnitude (become le negative) with time becaue of increaing ABL depth and a omewhat teady value of dry air above the ABL (depite decreaing atmopheric tability jut above the growing ABL), while the ABL growth term increae greatly a the atmopheric tability decreae. During thi ame time period the ABL warming term diminihe only modetly, and the evaporative fraction increae only lightly. Here, the effect of oil moiture i to increae the ABL-top relative humidity (ne 1), except during the midday rapid ABL growth when the effect of oil moiture only modetly increae ABL-top relative humidity (ne 1). We note that the ABL-top relative humidity increaed ufficiently for ABL cloud (both modeled and oberved) to form by mid- to late afternoon (ee dicuion in ection 4b). We now focu on the rapid ABL growth period (e.g., 1115 UTC at Cabauw), during or after which ABL cloud are generally initiated, and examine the effect of changing evaporative fraction and atmopheric tability on the relative humidity tendency. Uing the initial oil moiture value near that oberved at Cabauw, note that a with the Cabuaw obervation, ne 1 for the reference et model run a well (Table 2). For a drier oil in thi cae, normalized relative humidity tendency decreae lightly, with the ABL cloud cover alo decreaing. In a deeper growing boundary layer due to larger urface enible heat flux, a larger h yield a FIG. 11. Value of ne a a function of above-abl atmopheric tability ( ) v above-abl pecific humidity drop (q) for (a) morning condition at Cabauw (Netherland, 31 May 1978; RH 0.80, h 400 m, and T 12.0C at 0850 UTC, with Cabauw value baed on obervation, upper left, black dot) and (b) midday condition (RH 0.90, h 1875 m, and T 5.0C at 1115 UTC, with Cabauw value middle left, black dot). maller actual relative humidity tendency [ee (1) (3)], and le cloud cover (Fig. 9a). Here, tronger warmand dry-air entrainment negate the effect of ABL-top cooling on the increae of ABL-top relative humidity. For a moiter oil, normalized relative humidity tendency increae lightly, although with a hallower ABL depth the actual relative humidity tendency i le with ubequently le cloud cover. In thi cae the greatet relative humidity tendency (and, thu, cloud cover) occur for intermediate oil moiture. Thi i in agreement

11 96 JOURNAL OF HYDROMETEOROLOGY VOLUME 5 TABLE 1. Quantitie ued to evaluated the relative humidity tendency term from obervation via Eq. (1) (3) for 31 May 1978 at Cabauw, Netherland, correponding to time at the Cabauw tower in the morning hallower boundary layer (0645 and 0715 UTC), and radioonde launche at Cabauw (0748 and 0850 UTC) and at De Bilt (1115 UTC). See text for explanation. Time (UTC) R n G (W m 2 ) (kg m 3 ) h (m) q at (g kg 1 ) e f () q (g kg 1 ) Time (UTC) (K km 1 ) RH () T (C) R d/c (p/p ) p() Time (UTC) Dry-air entrainment () ABL growth () ABL warming () ne () e f () ne (1 e f )() h/l () e f ne (1 e f )() drh/dt (h 1 ) with our aement of the role of oil moiture on ABL cloud development baed on the development in the previou ection. For the two et of model run where the atmopheric tability above the boundary layer i changed, note that q i greater (le negative) than the threhold value, o that an increae (decreae) in atmopheric tability ( ) hould yield a decreae (increae) in ne (ee previou ection, Fig. 11). So for the et of model run with increaed (tronger) atmopheric tability, ABL depth i hallower (a one would expect), and becaue ne 1 there i a decreae in ABL-top relative humidity tendency and, thu, le cloud cover for drier oil (Table 2, Fig. 9a), with increaing cloud cover for increaing oil moiture (ne 0). In contrat, for the et of model run with decreaed (weaker) atmopheric tability, ABL depth i deeper (a one would expect), and yet becaue ne 1 there i an increae in ABL-top relative humidity tendency and, thu, more cloud cover for drier oil, with decreaing cloud cover for increaing oil moiture (ne k 1). Note that the larget value of RH/ t and, thu, ABL cloud cover are achieved for a mall evaporative fraction (lower oil moiture) with weak TABLE 2. Midday normalized relative humidity tendency, e f ne(1 e f )[(RH/t) Lhq /(R n G)]; for different urface evaporative fraction and atmopheric tability condition. Stability above ABL [ (K km 1 )] Increaed (7.0) Reference (4.0) Decreaed (1.0) Nonevaporative term (ne) Dry (0.20) Evaporative fraction (e f ) Oberved (0.79) Moit (0.95) tability (ne k 1), a wa uggeted in the relative humidity tendency development in the previou ection. Thee finding are qualitatively conitent with the Ek and Mahrt (1994) HAPEX MOBILHY data (ummer 1986, outhwet France) who found that a fair-weather day with trong atmopheric tability above the ABL and a large oberved evaporative fraction (via higher oil moiture) gave a imilar midday relative humidity at the ABL top a a fair-weather cae 9 day later with weaker atmopheric tability and decreaed oil moiture. 6. Summary In thi coupled model tudy we have examined land atmophere interaction uing model run with obervational verification. Reult indicate that in coupled land urface atmopheric boundary layer (ABL) model run, realitic daytime urface fluxe and atmopheric profile, including ABL cloud, are produced uing the CAPS model. Both land urface and ABL model run yielded encouraging reult operating eparately, and when coupled together interactively, even in the preence of model-predicted ABL cloud. Thi ugget that in thi coupled land atmophere ytem, procee are well-repreented by the CAPS model. The role of oil moiture on ABL cloud development wa explored in term of a new ABL-top relative humidity tendency equation, where a number of land urface and atmopheric procee interact. It wa hown with good agreement between model run, an analytical development, and analyi of Cabauw data, that the effect of oil moiture i to increae the ABL-top relative humidity tendency and, thu, the potential for ABL

12 FEBRUARY 2004 EK AND HOLTSLAG 97 cloud formation only if the tability above the boundary layer i not too weak (and given ufficient initial ABL relative humidity, and above-abl air that i not too dry). On the other hand, for weak tability above the boundary layer, drier oil yield a greater ABL-top relative humidity tendency and, thu, cloud cover. There i great interet in the tudy of land atmophere interaction and a large number of dataet from many field program repreenting divere geophyical location with which to tudy thee interaction. The new relative humidity tendency equation preented here may provide a ueful quantitative framework for future land urface ABL interaction tudie in the formation of ABL cloud. Acknowledgment. Thi reearch wa upported by the NOAA Climate and Global Change Program under award number NA36GP0369, the Air Force Office of Scientific Reearch under contract F , and the Royal Netherland Meteorological Intitute (KNMI), which provided the Cabauw dataet. KNMI ponored Michael Ek a a viiting cientit while on abbatical leave from Oregon State Univerity, prior to joining NCEP/EMC under the Univerity Corporation for Atmopheric Reearch Viiting Scientit Program. We alo wih to thank Fred Boveld and Bart van den Hurk at KNMI, Anton Beljaar at ECMWF, Larry Mahrt and Richard Cuenca at Oregon State Univerity, Ken Mitchell at NCEP/EMC, and a hot of other cientit at Wageningen Univerity, KNMI, and elewhere for their helpful comment, patience, and upport during the progre of thi work. In addition, we wih to acknowledge the ueful comment from reviewer Chrita Peter-Lidard at NASA/GSFC and two anonymou referee, and AMS Journal of Hydrometeorology editor Dara Entekhabi. APPENDIX Relative Humidity Tendency at the ABL Top Relative humidity at the atmopheric boundary layer (ABL) top i thought to control the development of ABL cloud. In order to undertand the relevant procee we initially follow the development in Ek and Mahrt (1994) (with a modification by Chang and Ek 1996), and analyze the relative humidity (RH) tendency at the ABL top, which may be written a RH q 1 q RH q 1 q RH e t t q q t q t q t q t p 1 q 1 p 1 de T RH, (A1) q t p t e dt t where q i the pecific humidity, q i aturation pecific humidity (e /p), i the ratio of dry air to water vapor ga contant, e i aturation vapor preure, p i air preure, de /dt i the lope of the aturation vapor preure curve, and T i temperature. FIG. A1. Schematic of idealized well-mixed boundary layer ued in analyzing the relative humidity tendency at the ABL top, where h i boundary layer depth, q and q are the above-abl pecific humidity drop and lape rate, repectively, and and are the above-abl potential temperature jump and lape rate (tability), repectively. With well-mixed condition for and q (typical for a dry convective boundary layer where h/l 5), the relative humidity reache a maximum near the boundary layer top, which will be the reference level in the following development. The relative humidity tendency combine the eparate influence of change in moiture and change in temperature, the firt and econd term on the right-hand ide of (A1), repectively, where thee tendencie are influenced by different boundary layer and land urface procee. Thi development i continued to explicitly account for thee different procee. Temperature tendency in a well-mixed boundary layer can be expreed a [ ] R d/cp T p, (A2) t t p which can be eventually written a R d /cp T p Rd T p, (A3) t p t c p t where i potential temperature, p i urface preure, c p i pecific heat of air, and the equation of tate and the definition of potential temperature have been ued. Uing the hydrotatic approximation and neglecting the local change of preure at a fixed height, the preure tendency can be written a p p h h pg h g, (A4) t z t t RdT t where h i the boundary layer depth, z i height, i air denity, and g i gravity. Subtituting (A4) into (A3) give p

13 98 JOURNAL OF HYDROMETEOROLOGY VOLUME 5 R d/cp T p g h. (A5) t p t c t The Clauiu Clapeyron equation can be written a 1 de L, (A6) e dt RT 2 where L i latent heat. Subtituting (A5) and (A6) into (A1) give where RH 1 q RH h c2 c 1, (A7) t q t q t t R d/cp L q p c1 2 ; R T p L q cp q g c2. (A8) 2 R T R T c p d p For our well-mixed ABL aumption we can ue equation for the boundary layer moiture and thermodynamic budget from Tenneke (1973) (in the advectionfree cae): q (wq wq h) ; t h (w w h), (A9) t h where wq and w are the moiture and heat fluxe, repectively, and the ubcript and h refer to the urface and the level jut below the boundary layer top, repectively. Subtituting (A9) into (A7) give RH (wq wq h) t hq [ ] RH h w (1 C ) c2 c 1, (A10) q t h where C w h/ w, the (negative of the) ratio of urface to ABL-top enible heat flux. Equation (A10) i quite imilar to the relative humidity tendency equation from Ek and Mahrt [1994, their Eq. (9)]. Next, we aume a imple bulk well-mixed ABL (Fig. A1) o that the ABL depth tendency may be approximated a (Tenneke 1973; Bett 1973): h w (1 C ), (A11) t h where i the vertical gradient of potential temperature above the ABL. ABL-top dry-air entrainment i h wqh q, (A12) t where q i the change in pecific humidity acro the ABL top [which i normally negative, and the mean large-cale vertical motion i zero, analogou to Tenneke (1973, hi Eq. (1), and other) (Fig. A1)]. Subtituting (A11) and (A12) into (A10) eventually yield (1), the relative humidity tendency at the ABL top. REFERENCES Beljaar, A. C. M., and F. C. Boveld, 1997: Cabauw data for the validation of land urface parameterization cheme. J. Climate, 10, Bett, A. K., 1973: Non-precipitating cumulu convection and it parameterization. Quart. J. Roy. Meteor. Soc., 99, , 2000: Idealized model for equilibrium boundary layer over land. J. Hydrometeor., 1, , F. Chen, K. E. Mitchell, and Z. I. Janjić, 1997: Aement of the land urface and boundary layer model in two operational verion of the NCEP Eta Model uing FIFE data. Mon. Wea. Rev., 125, Chang, S., and M. Ek, 1996: Note on Daytime evolution of relative humidity at the boundary layer top. Mon. Wea. Rev., 124, , D. Hahn, C.-H. Yang, D. Norquit, and M. Ek, 1999: Validation tudy of the CAPS model land urface cheme uing the 1987 Cabauw/PILPS dataet. J. Appl. Meteor., 38, Chen, F., and Coauthor, 1996: Modeling of land-urface evaporation by four cheme and comparion with FIFE obervation. J. Geophy. Re., 101, , Z. Janjić, and K. Mitchell, 1997: Impact of atmopheric urfacelayer parameterization in the new land-urface cheme of the NCEP meocale Eta model. Bound.-Layer Meteor., 85, Chen, T. H., and Coauthor, 1997: Cabauw experimental reult from the Project for Intercomparion of Land-urface Parameterization Scheme. J. Climate, 10, Collier, L. R., and J. G. Lockwood, 1974: The etimation of olar radiation under cloudle kie with atmopheric dut. Quart. J. Roy. Meteor. Soc., 100, Cuenca, R. H., M. Ek, and L. Mahrt, 1996: Impact of oil water property parameterization on atmopheric boundary-layer imulation. J. Geophy. Re., 101, De Bruin, H. A. R., 1983: A model for the Prietley Taylor parameter. J. Climate Appl. Meteor., 22, Deborough, C. E., 1997: The impact of root-weighting on the repone of tranpiration to moiture tre in land urface cheme. Mon. Wea. Rev., 125, Duynkerke, P. G., 1992: The roughne length for heat and other vegetation parameter for a urface of hort gra. J. Appl. Meteor., 31, Ek, M., and L. Mahrt, 1991: A formulation for boundary-layer cloud cover. Ann. Geophy., 9, , and R. H. Cuenca, 1994: Variation in oil parameter: Implication for modeling urface fluxe and atmopheric boundarylayer development. Bound.-Layer Meteor., 70, , and L. Mahrt, 1994: Daytime evolution of relative humidity at the boundary layer top. Mon. Wea. Rev., 122, Freedman, J. M., D. R. Fitzjarrald, K. E. Moore, and R. K. Sakai, 2001: Boundary layer cloud and vegetation atmophere feedback. J. Climate, 14, Holtlag, A. A. M., and A. P. van Ulden, 1983: A imple cheme for daytime etimate of the urface fluxe from routine weather data. J. Climate Appl. Meteor., 22, , and F. T. M. Nieuwtadt, 1986: Scaling the atmopheric boundary layer. Bound.-Layer Meteor., 36, , and B. Boville, 1993: Local veru nonlocal boundary-layer diffuion in a global climate model. J. Climate, 6, , and M. Ek, 1996: Simulation of urface fluxe and boundary layer development over the pine foret in HAPEX MOBILHY. J. Appl. Meteor., 35,

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