J4.12 REGIONAL HYDROLOGICAL CYCLE AND WEATHER AND CLIMATE IN THE CONTIGUOUS UNITED STATES

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1 J4.12 REGIONAL HYDROLOGICAL CYCLE AND WEATHER AND CLIMATE IN THE CONTIGUOUS UNITED STATES 1. INTRODUCTION i Hu 1 nd Song Feng Climte nd Bio-Atmospheric Sciences rogrm School of Nturl Resource Sciences University of Nebrsk-Lincoln Lincoln, Nebrsk Hydrologicl cycle in the Erth-tmosphere system is not only mnifesttion of the wether nd climte but lso driving force for chnges of the climte, prticulrly when the cycle is disturbed by nthropogenic chnges of lnd surfce conditions. Understnding the hydrologicl cycle nd its vribility nd reltion to wether nd climte on vrious time scles will ssist us to understnd both regionl nd globl climte nd how nthropogenic effects my lter the wter cycle nd the current climte. In this study, we ddress the reltionship between properties of regionl hydrologicl cycle nd wether nd climte nd emphsize the equilibrium of the hydrologicl cycle nd its corresponding sttisticl fetures of precipittion in the regionl climte. Hydrologicl cycles for different regions in the contiguous United Sttes were first nlyzed using NCE-NCAR Renlysis dt of Then, the reltionship of the hydrologicl cycle intensity nd other components in the cycle re exmined with regionl precipittion nd temperture. The dt nd methods used in this study re discussed in the next section followed in section 3 by presenttions nd discussions of the mjor results. Some issues relted to chnge of the hydrologicl cycle nd wether nd climte re highlighted in section DATA AND METHODS The components in regionl hydrologicl cycle, e.g., surfce evportion, locl wter recycling, tmospheric moisture fluxes nd convergence, nd precipittion, were obtined nd clculted from severl dt sources. Specificlly, the moisture fluxes nd convergence were computed using 6-hourly NCE-NCAR Renlysis Model dt on sigm levels nd then trnsformed to the physicl domin. The surfce evportion ws obtined from 2.5º 2.5º NCE-NCAR Renlysis Model. Although the evportion dt my contin bis in some isolted spots nd cuse spurious locl lnd moisture sources, effects of such bises on clculted hydrologicl cycle could be minimized using regionl verge nd for temporl scles longer thn month (Trenberth nd Guillemot 1998; Dirmeyer nd Brubker 1999; Bosilovich nd Schubert 2001). Accordingly, we clculted the monthly nd nnul hydrologicl cycle in lrge-scle regions (L>1000km) in the contiguous U.S. The precipittion dt used in exmining the hydrologicl cycle were lnd precipittion dt on 2.5º 2.5º grid (Chen et l. 2002). In ddition, dily precipittion dt from 1062 sttions in the U.S. (Esterling et l. 1999) lso were used to derive precipittion properties tht were used in evlutions of reltionship between precipittion climte nd locl hydrologicl cycle. To clculte the hydrologicl cycle, we used the method described in Brubker et l. (1993), which is bsed on the conceptul frmework of Budyko (1974). The model hs two mjor ssumptions: 1) vritions in evportion nd precipittion re liner cross the study region, nd 2) the tmosphere efficiently mixes the evported wter ( m ) with dvected moisture ( ) nd the rtio of vs. m in the totl precipittion is constnt. These ssumptions hve been exmined nd shown to cuse bout 10% error in regionl wter budget for the conditions in the centrl U.S. nd using long-term time verge vlues in the clcultions (Brubker et l. 1993; Burde nd Zngvil 2001). Bsed on these ssumptions, the totl moisture content in the tmospheric column of region is composed of n dvection portion, W, nd portion from locl evportion, W m, thus, W= W + W m. Consequently, the totl precipittion,, is mde from two sources: dvection,, nd locl evportion, m, i.e., = + m. The two components in ply different roles in regionl hydrologicl cycle nd chrcterize regionl climte. In prticulr, in 1 Corresponding uthor ddress: Dr. i Hu, 237 L.W. Chse Hll, University of Nebrsk-Lincoln, Lincoln, NE E-mil: HU2@unl.edu. Internet:

2 the locl wter recycle, wter recycling rtio hs been used to define the contribution of locl evportion to the totl precipittion (Elfith nd Brs 1996), nd is recognized s mjor chrcter of region s climte (Brubker et l. 1993; Trenberth 1999). A lrger recycling rtio describes more significnt role of locl evportion in the regionl precipittion nd climte. A smller recycling rtio, on the other hnd, indictes weker role of evportion in rinfll development but dominnt role of moisture flux in the region s rinfll nd climte. If we define the inwrd flux of moisture to region s the integrl of the tmospheric moisture + inflow cross ll the boundries of the region, i.e., F = wγ Vγ dγ, where, w γ nd V γ = ( uγ, vλ ) re the moisture content nd the velocity perpendiculr to the boundry surfce, respectively, nd Γ in is the boundries of the domin with inwrd moisture flux, then the totl moisture input to the region is ( E ) A = F + +, where A is the re of the region nd the second term on the rhs includes the net 2 moisture input from the surfce (Brubker et l. 1993; Burde nd Zngvil 2001). In, the moisture dvected into the region is clculted from: F Γin A 2 = +. For time scles of month nd longer, the tmosphere cn be ssumed to be well mixed, mening tht t given region the wter molecules of dvective origin nd those of locl evportive origin re well mixed nd hve equl probbilities to be precipitted, i.e., from r m = = =. So, the locl wter recycling rtio, r, cn be clculted in terms of nd =. Becuse the wter cycle hs different equilibri in the different climte (Elfith nd Brs 1996), we divided the continentl U.S. into 11 regions with similr climte conditions nd clculted the hydrologicl cycle nd its reltionship with precipittion nd temperture climte in those regions. 3. MAJOR RESULTS In discussing the results, we focus on the reltionship of the regionl hydrologicl cycle nd precipittion events of different chrcteristics, e.g., intensity nd durtion, nd elborte the differences in hydrologicl cycles corresponding to different precipittion climte. In Fig. 1, we show the coefficients of correltion of locl wter recycling rtio vs. precipittion in different ctegories of intensity nd durtion. We should remind tht these correltions were clculted using 52-yer dt from A generl feture in Fig. 1 is the negtive correltion between the recycling rtio nd precipittion in regions long the west cost, in southestern nd estern U.S., nd in the Gret Lkes re. ositive but smll correltions re found in the west centrl nd mountinous regions in the west. Consistently, the reltionship between the recycling rtio nd the drought event hs reversed sign in the regions. With regrd to the definition of the recycling rtio, negtive correltion between recycling rtio nd precipittion in prticulr ctegory, for exmple, the lrgest or most intense precipittion events in yer, would indicte lrge in the totl precipittion,, so tht - is lwys much smller thn, which then, bsed on the definition of the recycling rtio, domintes the chnge in the rtio. This cretes n inverse reltionship nd results in negtive correltion between r nd. Thus, in Fig. 1 the regions with lrge negtive correltions (<-0.3) indicte dvection of moisture into the regions s n essentil process ffecting the regions precipittion nd wter cycle. On the other hnd, the regions with positive correltion hve smller contributions from to nd, thus, weker dependence of their precipittion nd wter cycle on dvected moisture. In these regions, surfce evportion plys n importnt role in regionl precipittion nd wter cycle.

3 This difference in dependence of the precipittion on dvected moisture nd locl evportion mong the regions is further described in the result of Fig. 2 showing the reltionship between surfce evportion nd regionl precipittion. The correltion coefficients re lrge in the regions of the centrl U.S. where the dvection effect is wek, nd smll in the western nd estern U.S. where the dvected moisture plyed mjor role in locl precipittion nd locl wter cycle. This role of the moisture dvection in precipittion is explicitly shown in Fig. 3; the high correltions in the regions long the west costs, southestern U.S., nd the Gret Lkes re depict significnt effect of moisture flux on precipittion development nd low correltions in the centrl nd prticulrly north-centrl U.S. show rther wek influence of moisture dvection on precipittion nd wter cycle in those regions. Such wek effect in the centrl U.S. could be result of lcking mechnisms trnsporting moisture to the centrl portions of the continent. Similr results lso hve been obtined from nlyses for the northern summer. 4. DISCUSSION These mutully consistent results differentite hydrologicl or wter cycle in different climtes cross the contiguous U.S. nd ssocited chrcteristics of regionl precipittion. These chrcteristics revel not only the processes unique to the regionl precipittion development but lso the different sources tht could cuse the precipittion vritions in vrious regions. For exmple, in the centrl U.S., regionl lnd surfce chnge could hve substntil effect on precipittion, wter cycle, nd climte becuse of strong dependence of the wter cycle on the surfce evportion. However, similr mgnitude of lnd surfce chnge my hve much weker effect on locl precipittion nd climte in the costl regions becuse of the wek dependence of locl wter cycle on the surfce evportion in those regions. Therefore, lnd surfce chnge effects on nd vegettion feedbcks to regionl climte could be more significnt in the in-lnd regions, wheres flow nomlies nd ssocited chnges in circultion dynmics should be mjor concerns of wter cycle, precipittion, nd climte vritions in regions in the west nd est costl regions in the contiguous U.S. Acknowledgements: This reserch hs been supported by grnt from NOAA GCI nd by USDA Coopertive Reserch roject NEB REFERENCES BosilovichM.G. nd S.D. Schubert, 2001: recipittion recycling over the centrl United Sttes dignosed from the GEOS-1 dt ssimiltion system. J. Hydrometeorology, 2, Brubker, K.L., A. Entekhbi, nd.s. Egleson, 1993: Estimtion of continentl precipittion recycling. J. Climte, 6, Burde G.I. nd A. Zngvil, 2001: The estimtion of regionl precipittion recycling prt one: review of recycling models. J. Climte, 14, Budyko, M.I., 1974: Climte nd Life. Acdemic ress, 508 pp. Chen M.,. Xie, J.E. Jnowik nd.a. Arkin, 2002: Globl lnd precipittion: 50-yr monthly nlysis bsed on guge observtions. J. Hydrometeorology, 3, Dirmeyer,.A. nd K.L. Brubker, 1999: Contrsting evportive moisture sources during the drought of 1988 nd the flood of J. Geophys. Res., 104, Esterling, D.R., T.R. Krl, J.H. Lwrimore, nd S.A. Del Greco, 1999: United Sttes historicl climtology network dily temperture, precipittion, nd snow dt ( ). CDIAC Communictions. Elfith, E.A.B. nd R.L. Brs, 1996: recipittion recycling. Rev. Geophys., 34, Trenberth K.E. nd C.J. Guillemot, 1996: hysicl processes involved in the 1988 drought nd 1993 floods in North Americ. J. Climte, 9, Trenberth K.E. nd C.J. Guillemot, 1998: Evlution of the tmospheric moisture nd hydrologicl cycle in the NCE/NCAR renlysis. Cli. Dynmics, 14, Trenberth, K.E., 1999: Atmospheric recycling: Role of dvection nd locl evportion. J. Climte, 12,

4 Figure 1: Correltion coefficients of locl wter recycling rtio vs. precipittion nd drought. Figure 2: Correltion coefficients of locl surfce evportion vs. precipittion nd drought.

5 Figure 3: Correltion coefficients of moisture flux vs. precipittion nd drought.