Soil-layer configuration requirement for large-eddy atmosphere and land surface coupled modeling

Transcription

1 ATMOSPHERIC SCIENCE LETTERS Atmos. Sci. Let. 14: (2013) Published online in Wiley Online Library (wileyonlinelibrary.com) DOI: /asl2.426 Soil-layer configuration requirement for large-eddy atmosphere and land surface coupled modeling Shaofeng Liu 1,2, * and Yaping Shao 1 1 Institute for Geophysics and Meteorology, University of Cologne, Cologne, Germany 2 Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, China *Correspondence to: S. Liu, Institute for Geophysics and Meteorology, University of Cologne, Cologne, Germany. sliu03@uni-koeln.de Received: 25 July 2012 Revised: 3 December 2012 Accepted: 19 January 2013 Abstract To investigate the soil-layer configuration requirement for numerical studying large-eddy scale atmosphere and land surface interactions, we conduct two experiments with an atmosphere and land surface coupled large-eddy model, one with the usual soil-layer configuration and the other with a finer one. It is found that the former configuration prevents the soil to respond to atmospheric large eddies and the soil acts solely as a fixed external condition to the atmospheric turbulence, and is thus inadequate for large-eddy atmosphere and land surface coupled modeling. It is shown that soil-layer configuration has a profound impact on the simulated atmospheric boundary layer dynamics. Copyright 2013 Royal Meteorological Society Keywords: atmosphere land interaction; large-eddy simulation; soil-layer configuration 1. Introduction Land surface mass, energy and momentum fluxes are important quantities to determine in weather and climate models. These fluxes can be strongly affected by land surface heterogeneity (Shao et al., 2001; Heinemann and Kerschgens, 2005). To better understand heterogeneous atmosphere and land surface interactions, three-dimensional atmospheric and land surface data are needed. Such data are difficult to obtain from field or laboratory measurements, but can be synthetically generated using large-eddy atmosphere and land surface coupled models (LES-ALMs). Large-eddy simulation (LES) models have been under development since the 1960s (Smagorinsky, 1963; Deardorff, 1970; Moeng, 1984), and are now widely used for atmospheric turbulent flow simulations (Sullivan et al., 1998; Beare et al., 2004; Kleissl et al., 2006; Kumar et al., 2006). Earlier LES models were not coupled with land surface schemes, and the simulations were mostly done with prespecified land surface forcing (Hechtel et al., 1990; Avissar and Schmidt, 1998; Albertson et al., 2001; Letzel and Raasch, 2003; Huang et al. 2008). More recently, LES-ALMs have been developed and applied to the simulation of boundary layer flows over synthetic (Patton et al., 2005; Courault et al., 2007) and natural (Huang and Margulis, 2010; Brunsell et al., 2011) heterogeneous land surfaces. However, the land surface schemes used in these studies are almost the same as those designed for weather and climate modeling, in which the soil layers are configured to represent the atmosphere and land surface exchanges on diurnal and longer time scales. Table I summarizes the soil-layer configurations used in some of the recent LES-ALM studies, with the typical thickness of the top soil layer being about 0.1 m. Given that such thick soil layers are used, one must ask whether the atmosphere and land surface interactions on large-eddy time scale were realistically represented, and more generally whether the impact of land surface heterogeneity on the atmospheric boundary layer can be correctly represented. For example, Brunsell et al. (2011) concluded that land surface heterogeneity has little impact on the dynamics of the atmospheric boundary layer. The validity of this statement requires rigorous testing, because it may be a model artifact arising from the use of a thick top-layer soil which prohibits the atmosphere and land surface from interaction on large-eddy time scales. Our own experience shows that soil-layer configuration is of fundamental importance to the simulation of large-eddy atmosphere land surface interactions, especially over heterogeneous areas, which deserves careful consideration. In this paper, we examine the soil-layer configuration requirement for LES-ALM. The soil-layer configurations summarized in Table I are tested. They are found to be inadequate in that they do not permit the response of the land surface to the effects of atmospheric large eddies. As a consequence of the thick soil-layers used, the land surface only exerts a quasi-stationary force on the atmosphere, rather than interacts with it. Sensitivity experiments are carried out using a LES-ALM (Shao et al., 2013) for different soil-layer configurations, and the model results are compared with field measurements. Guidelines for soil-layer configuration for LES-ALM are proposed. Copyright 2013 Royal Meteorological Society

2 Soil-layer configuration requirement in LES 113 Table I. A review on the soil-layer configuration used in recent LES-ALM studies. Reference LES-LSM coupled? LSM model Soil layers Study description Patton et al. (2005) Yes Noah LSM (Chen and Dudhia, 2001) 4 layers: 0.1, 0.3, 0.6, 1 m Influence of idealized surface heterogeneity Courault et al. (2007) Yes ISBA (Noilhan and Mahfouf 1996), some kind of 2 layer: 0.1 and 1m Impact of synthetic surface heterogeneity on the boundary layer force-restore LSM Huang and Margulis (2010) Yes A force-restore LSM (Noilhan and Planton 1989) Same as above Land and atmosphere feedbacks for natural heterogeneous surface Huang et al. (2011) Same as above On particular land use: urban Brunsell et al. (2011) Same as above On the impacts of the scales of surface heterogeneity on boundary layer dynamics Table II. Settings of the model simulations. Characteristic Exp 1 Exp 2 Simulation Domain Spatial resolution A km 3 domain in western Germany, covered with nine land use types, i.e. bare soil, settlement, bog, water, harvested rapeseed and grain, beet, pasture and forest. Detailed distribution refers to Shao et al Horizontal: x = y = 60 m; vertical: logarithmically stretched with z varying between 2 m near the surface to 24 m for z 80 m Grids Time step 0.2 s Same as Exp 1 Simulation period UTC, 5 August 2009 Lateral BCs Periodic Upper BCs Constant pressure with zero vertical velocity Soil layers Default 4 layers, 0.1, 0.3, 0.6, 1 m 4 layers, 0.01, 0.03, 0.06, 0.1 m 2. Model and numerical experiments The LES-ALM model used in this study is as described in Shao et al. (2013). It is based on the WRF (Weather Research and Forecast) large-eddy flow model and the land surface scheme is based on the Noah land surface model (LSM) (Chen and Dudhia, 2001). Several developments are made to the land surface scheme to ensure its adequacy for large-eddy simulation of atmosphere and land surface processes. Here, we only give a brief description of the techniques for computing surface sensible- and latent-heat fluxes and the related soil processes. Surface sensible heat flux, H, and latent heat flux, LE, can be expressed as (T a T s ) H = ρc p (1) r h LE = ρlβ (q a q s (T s )) r q (2) where ρ, T a and q a are air density, air temperature and specific humidity at the lowest model level; c p and L are the air specific heat capacity and latent heat of vaporization; T s the surface skin temperature and q s (T s ) the saturation specific humidity at T s.theβ parameter is assumed to be a linear function of the soil moisture in the top-layer soil. The calculations of the resistances r h and r q are as described in Shao et al. (2013). Soil temperature obeys the heat diffusion equation T s t = z ( ν G T s z ) + s T (3) where T s is soil temperature, ν G the soil thermal diffusivity and s T a temperature source. The soil moisture θ obeys the Richards equation θ = ( ) (ψ θ + z) K W + s θ (4) t z z Here, ψ θ is the hydraulic suction head, K W the hydraulic conductivity and s θ a moisture source. In LSMs used for weather predictions, it is appropriate to select relatively thick soil layers. For example, in Noah LSM, the default thicknesses of the four soil layers from top to bottom are 0.1, 0.3, 0.6 and 1.0 m, respectively, as used in Patton et al. (2005). In a force-restore scheme, the soil layers are typically set to 0.1 m for the top-layer soil and 1 m for the bulk soil layer, as is the case in Courault et al. (2007), Huang and Margulis (2010), Huang et al. (2011) and Brunsell et al. (2011). However, for large-eddy modeling, much thinner layers must be selected to allow the land surface to respond to the effects of large eddies. Suppose the typical time scale of the atmospheric system is t A. Then, because the soil thermal diffusivity, ν G,isofthe order of magnitude of 10 6 to 10 7 m 2 s 1, the corresponding thickness of the soil layer, s, must satisfy s v G t A (5) For t A = 1day, s is about 0.2 m; for t A = 10 min, s is about 0.01 m. To allow for soil response to the large eddy fluctuations, we set the thickness of the first soil layer as 0.01 m. To examine the sensitivity of the LES-ALM simulations on soil-layer configuration,

3 114 S. Liu and Y. Shao Figure 1. (a) Comparison of the simulated soil temperature for Exp 1 (dashed line) and Exp 2 (solid line) with observation (dots); (b) as (a), but for soil moisture; (c) 60-min average of top-layer soil temperature (thick dashed line for Exp 1 and thick solid line for Exp 2, left axis) and deviation (thin dashed line for Exp 1 and thin solid line for Exp 2, right axis); (d) as (c), but for top-layer soil moisture. Figure 2. Comparison of the simulated and observed surface H and LE for a harvested wheat surface. (a) 30-min averages over a period of 12 h; (b) instantaneous values for a 1-h time interval. Solid lines are for Exp 2, dashed for Exp 1 and full dots for observations. experiments with two different soil-layer configurations, as shown in Table II, are carried out for a heterogeneous land surface in Germany. The simulations are done for 5 August 2009, a day with weak south easterly winds of 3 4 m s 1 in the entire boundary layer and no clouds except for some thin cirrus in the afternoon. The simulation domain is a flat area of roughly km 2 about 100 m above mean sea level and the landscape dominated by field crops of sugar beet and two grain species (winter wheat and winter barley). For details of the land use distribution and related surface characteristics refer to Shao et al. (2013). 3. Results 3.1. Response of soil The two simulations use the same initial conditions for the atmosphere and soil temperature/moisture as in Shao et al. (2013). In Exp 1, in comparison with the measurements, the (top layer 5 cm) soil temperature increase was too slow in the morning (Figure 1(a)), as well as the soil moisture decreases (Figure 1(c)). In Exp 2, the 5-cm soil temperature and moisture vary more rapidly with time (Figure 1(a) and (c)). For the

4 Soil-layer configuration requirement in LES cm soil temperature, Exp 2 reaches its peak earlier in better accordance with the observations. On the other hand, in Exp 2, the 5-cm soil moisture decreases more quickly leading to a drier top-layer soil (Figure 1(d)). On turbulent time scales, the top-layer soil temperature in Exp 2 has much stronger variations (Figure 1(b)), although such strong variations are absent in the toplayer soil moisture (Figure 1(d)) Response of fluxes As Equation ([2]) shows, the top-layer soil moisture is a key parameter in determining the partition of net radiation into sensible and latent heat fluxes. Lower top-layer soil moisture means less evaporation. Consequently, Exp 1 and Exp 2 have different flux partitions. For a harvested wheat land surface, both Exp 1 and Exp 2 show a reasonable agreement between the simulated and observed H andlefluxesondiurnal time scale (Figure 2(a)). However, compared to Exp1, Exp 2 obtained a higher H but lower LE for most of the time. Figure 2 also compares the simulated instantaneous H and LE fluxes. Exp 2 shows much stronger variations in H (Figure 2(b)), consistent with the instantaneous soil temperature variations seen in Figure 1(b). For example, Exp 2 has high H values just before 1310 UTC, in correspondence with the low top-layer soil temperature values; the low H values between 1330 and 1340 UTC correspond well with the high top-layer soil temperature values. The simulation thus clearly shows that there is a significant coupling between atmosphere and land surface processes on the large-eddy scale. On average, LE simulated in Exp 1 is almost twice that simulated in Exp 2, which is apparently related to their difference in the top-layer soil moisture. The difference in the variations of LE is not striking, as the difference in the top-layer soil moisture on large-eddy time scales (Figure 1(d)). This is because, for the case we studied, the top-layer soil is rather dry and the evaporation is limited by soil moisture supply. Figure 3 shows the comparison of the profiles of the time-and-domain averaged H and LE between the two simulations. Both H profiles are typical for a convective boundary layer, i.e. H linearly decreases with height until the inversion layer (Figure 3(a)). Both LE profiles show that LE increases with height to the inversion layer (Figure 3(b)). These latter results are consistent with those of Deardorff (1974). However, the near-surface H from Exp 2 is about 50 W m 2 ( 20%) higher than that from Exp 1, while LE is lower by the same margin. The quantitative differences between Exp 1 and Exp 2 in the H and LE profiles persist through almost the entire boundary layer. For the present case studied, the thin soillayer configuration resulted in a more convective atmospheric boundary layer. Figure 3. Profiles of H (a) and LE (b) averaged over the simulation domain and the time period of UTC. Solid lines are Exp 2 and dashed Exp 1. Figure 4. Comparison of the energy spectra of the 2-m air temperature over a bare soil surface. Solid curve is for Exp 2 and red dashed Exp Response of large eddies We have shown in Sections on Response of Soil and Response of Fluxes that the thin soil-layer configuration allows the land surface to respond to the atmospheric large eddies. Then, conversely, how do large eddies respond to the fluctuations of land surface quantities? Figure 4 shows the energy spectrum of 2-m air temperature fluctuations over a bare soil surface. In the range of energy-containing eddies, distinct differences between Exp 1 and Exp 2 are seen in the frequency range between and s 1. Exp 2 shows weaker temperature variations on 10 min time scales. Spectral differences appear for the frequency smaller than s 1. On 20 min and longer time scales, Exp 2 has stronger variation. This is due to the larger H in Exp 2, which makes the atmospheric boundary thermally more unstable, as a consequence of the drier top-layer soil. The LES-ALM model simulation suggests the rapid land atmosphere exchanges due to the thinner soil-layer configuration weakens the atmospheric large eddies. To better understand this phenomenon, we reexamine Figures 1(b) and 2(b). In Exp 2, when a cool large eddy approaches (a little before 1310 UTC), it fosters the surface H (Figure 2(b)) and thereby cools the top-layer soil (Figure 1(b)). After the passage of cool eddy, the atmosphere should resume its relatively warm state, but the cooled land surface

5 116 S. Liu and Y. Shao Figure 5. (a) Spectra of sensible heat fluxes of SP1 in logarithm for the fine soil-layer configuration; (b) spectral differences between SP2 and SP1 in linear scale; (c) as (b), but between SP3 and SP1; (d), (e) and (f) are the same as (a), (b) and (c), but for the thick soil-layer configuration; (g) PDF distribution of heterogeneity length scale. continues to cool the atmosphere above and prohibits the recovery. On the other hand, when a warm large eddy approaches (between 1330 and 1340 UTC), it suppresses the surface H (Figure 2(b)) and warms the top-layer soil (Figure 1(b)). After the passage of the warm eddy, the warmed soil continues to warm the air above and again, prohibits the recovery. There is thus a negative feedback which weakens the large eddies. Such a feedback mechanism between land surface and large eddies can only be seen by using a fine soil-layer configuration Discussion As discussed in Section Response of Large Eddies, in large-eddy modeling a distinct feedback mechanism can be found between land surface and large eddies only if the fine soil-layer configuration is used. It is perceivable, that very different, if not entirely opposite, conclusions with respective to the atmospheric boundary layer dynamics may be obtained if the soil-layer configuration is not carefully chosen. To better understand this, several LES-ALM simulations are carried out in addition to those listed in Table II. In the simulation domain, three artificial land use patterns are generated by keeping the total vegetation fraction unchanged: in SP1, vegetation is randomly distributed and heterogeneity occurs on very small length scales; in SP2, the dominant heterogeneity scales is 6 20 times model grid spacing (dx); in SP3, the dominant heterogeneity scale is times dx (Figure 5(g)). Thus, in SP1, SP2 and SP3, land surface heterogeneity occurs on different scales. Combining these three land surface patterns with the two soil-layer configurations, six experiments are done. The spectral differences of the simulated sensible heat fluxes between SP2 and SP1 and between SP3 and SP1 clearly show the impact of surface heterogeneities. The scales at which the difference peaks of sensible heat fluxes occur reasonably well correspond to the typical scales of land surface heterogeneity, as seen in Figure 5. For SP1, the differences of the spectral levels between the two kinds of soil-layer configuration are insignificant (Figure 5(a) and (c)). By comparing Figure 5(b) and (c) (fine soillayer configuration) with Figure 5(e) (g) (thick soillayer configuration), however, it is found that the thin soil-layer configuration allows the impact of surface heterogeneity to propagate to much higher levels in the atmospheric boundary layer. It is conclusive from our simulation, that in modeling atmosphere and land surface interaction on large-eddy scales, the soil-layer configuration must be sufficiently fine. Otherwise, the model results can be misleading. 4. Conclusions The sensitivity of large-eddy simulation of the atmospheric boundary layer to soil-layer configuration is investigated by using a LES-ALM. Two simulations are conducted, one with the conventional soil-layer configuration and the other with a finer soil-layer configuration. The results show that the latter configuration allows the land surface to respond to, and to influence, the atmospheric large eddies.

6 Soil-layer configuration requirement in LES 117 The conventional soil-layer configuration designed for weather and climate models, but widely used for LES-ALM, does not allow the feedbacks between the atmosphere and land surface on large-eddy time scales. Thus, in LES-ALM, if the conventional soillayer configurations are used, the land surface mainly acts as a quasi-stationary external forcing to the atmospheric turbulence, even though a surface skin layer is applied in the LSM. This is because the skin layer has no capacity for heat and moisture storage, and the characteristic of the land-surface response to turbulent fluctuations is determined by the thickness of the soil layers. Using the fine soil-layer configurations, we have demonstrated that a negative feedback between the atmosphere and land surface exists on the large-eddy scale, which cannot be seen from the conventional soil-layer configurations. Our study shows that the requirement on soil-layer configuration depends on the nature of the problems under investigation. If the goal is to study the structure of the boundary layer and the land surface s forcing on it on diurnal or longer time scales, or only the impacts of prescribed surface heterogeneity on the boundary layer, as for the first four papers listed in Table I, the conventional soil-layer configuration might be adequate. But if the goal is to study atmosphere and land surface interactions on the large-eddy scales, then soil-layer configuration must be sufficiently fine. Using additional experiments, we have demonstrated that the fine-soil layer configuration is also important for studying the impact of land surface heterogeneity. The soil-layer configuration used in this study can be used as a reference for LES-ALM. Acknowledgements This work is jointly supported by the DFG Transregional Cooperative Research Centre 32 Patterns in Soil-Vegetation- Atmosphere-Systems: Monitoring, Modelling and Data Assimilation and the 863 project (Grant 2010AA012304). References Albertson JD, Kustas WP, Scanlon TM Large-eddy simulation over heterogeneous terrain with remotely sensed land surface conditions. Water Resources Research 37: Avissar R, Schmidt T An evaluation of the scale at which ground-surface heat flux patchiness affects the convective boundary layer using large-eddy simulation model. Journal of the Atmospheric Sciences 55: Beare RJ, Macvean MK, Holtslag AAM, Cuxart J, Esau I, Golaz J-C, Jimenez MA, Khairoutdinov M, Kosovic B, Lewellen D, Lund TS, Lundquist JK, McCabe A, Moene AF, Noh Y, Raasch S, Sullivan PP An intercomparison of large-eddy simulations of the stable boundary layer. Boundary-Layer Meteorology 118: Brunsell NA, Mechem DB, Anderson MC Surface heterogeneity impacts on boundary layer dynamics via energy balance partitioning. Atmospheric Chemistry and Physics 11: Chen F, Dudhia J Coupling an advanced land surface-hydrology model with the Penn State-NCAR MM5 Modeling System. Part I: model implementation and sensitivity. Monthly Weather Review 129: Courault D, Drobinski P, Brunet Y, Lacarrère P, Talbot C Impact of surface heterogeneity on a buoyancy-driven convective boundary layer in light winds. Boundary-Layer Meteorology 124: Deardorff JW A numerical study of three-dimensional turbulent channel flow at large Reynolds numbers. Journal of Fluid Mechanics 41: Deardorff JW On the entrainment rate of a stratocumulus-topped mixed layer. Quarterly Journal of the Royal Meteorological Society 102: Hechtel LM, Moeng CH, Stull RB The effects of nonhomogeneous surface fluxes on the convective boundary layer: a case study using large-eddy simulation. Journal of the Atmospheric Sciences 47: Heinemann G, Kerschgens M Comparison of methods for area-averaging surface energy fluxes over heterogeneous land surfaces using high-resolution non-hydrostatic simulations. International Journal of Climatology 25: , DOI: /joc Huang HY, Stevens B, Margulis SA Application of dynamic subgrid-scale models for large-eddy simulation of the daytime convective boundary layer over heterogeneous surfaces. Boundary- Layer Meteorology 126: Huang HY, Margulis SA Evaluation of a fully coupled largeeddy simulation-land surface model and its diagnosis of landatmosphere feedbacks. Water Resources Research 46:W06512,DOI: /2009WR Huang HY, Margulis SA, Chu CR, Tsai HC Investigation of the impacts of vegetation distribution and evaporative cooling on synthetic urban daytime climate using a coupled LES-LSM model. Hydrological Processes 25: Kleissl J, Kumar V, Meneveau C, Parlange MB Numerical study of dynamic Smagorinsky models in large-eddy simulation of the atmospheric boundary layer: validation in stable and unstable conditions. Water Resources Research 42: W06D10, DOI: /2005WR Kumar V, Kleissl J, Meneveau C, Parlange MB Large-eddy simulation of a diurnal cycle of the atmospheric boundary layer: atmospheric stability and scaling issues. Water Resources Research, DOI: /2005WR Letzel MO, Raasch S Large eddy simulation of thermally induced oscillation in the convective boundary layer. Journal of the Atmospheric Sciences 60: Moeng CH A large-eddy simulation model for the study of planetary boundary-layer turbulence. Journal of the Atmospheric Sciences 41: Noilhan J, Mahfouf JF The ISBA land surface parameterization scheme. Global and Planetary Change 13: Noilhan J, Planton S A simple parameterization of land surface processes for meteorological models. Monthly Weather Review 117: Patton EG, Sullivan PP, Moeng CH The influence of idealized heterogeneity on wet and dry planetary boundary layers coupled to the land surface. Journal of the Atmospheric Sciences 62: Shao Y, Sogalla M, Kerschgens M, Brücher W Effects of landsurface heterogeneity upon surface fluxes and turbulent conditions. Meteorology and Atmospheric Physics 78: Shao Y, Liu S, Schween J, Crewell S Large-Eddy Atmosphere Land Surface Modeling over Heterogeneous Surfaces: Model Development and Comparison with Measurements. Boundary-Layer Meteorology, in press. Smagorinsky J General circulation experiments with the primitive equations, part I: the basic experiment. Monthly Weather Review 91: Sullivan PP, Moeng CH, Stevens B, Lenschow DH, Mayor SD Structure of the entrainment zone capping the convective atmospheric boundary layer. Journal of the Atmospheric Sciences 55: