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Downloaded 10/02/21 08:25 AM UTC 15 NOVEMBER 2006 A R O R A A N D B O E R 5875 The Temporal Variability of Soil Moisture and Surface Hydrological Quantities in a Climate Model VIVEK K. ARORA AND GEORGE J. BOER Canadian Centre for Climate Modelling and Analysis, Meteorological Service of Canada, University of Victoria, Victoria, British Columbia, Canada (Manuscript received 4 October 2005, in final form 8 February 2006) ABSTRACT The variance budget of land surface hydrological quantities is analyzed in the second Atmospheric Model Intercomparison Project (AMIP2) simulation made with the Canadian Centre for Climate Modelling and Analysis (CCCma) third-generation general circulation model (AGCM3). The land surface parameteriza- tion in this model is the comparatively sophisticated Canadian Land Surface Scheme (CLASS). Second- order statistics, namely variances and covariances, are evaluated, and simulated variances are compared with observationally based estimates. The soil moisture variance is related to second-order statistics of surface hydrological quantities. The persistence time scale of soil moisture anomalies is also evaluated. Model values of precipitation and evapotranspiration variability compare reasonably well with observa- tionally based and reanalysis estimates. Soil moisture variability is compared with that simulated by the Variable Infiltration Capacity-2 Layer (VIC-2L) hydrological model driven with observed meteorological data. An equation is developed linking the variances and covariances of precipitation, evapotranspiration, and runoff to soil moisture variance via a transfer function. The transfer function is connected to soil moisture persistence in terms of lagged autocorrelation. Soil moisture persistence time scales are shorter in the Tropics and longer at high latitudes as is consistent with the relationship between soil moisture persis- tence and the latitudinal structure of potential evaporation found in earlier studies. In the Tropics, although the persistence of soil moisture anomalies is short and values of the transfer function small, high values of soil moisture variance are obtained because of high precipitation variability. At high latitudes, by contrast, high soil moisture variability is obtained despite modest precipitation variability since the persistence time scale of soil moisture anomalies is long. Model evapotranspiration estimates show little variability and soil moisture variability is dominated by precipitation and runoff, which account for about 90% of the soil moisture variance over land surface areas. 1. Introduction phasizes both the varying properties of the soil and the direct role of vegetation in determining the surface en- Land surface models (LSMs) provide the connection ergy and water balance, particularly by taking into ac- between the atmosphere and the land surface in general count the physiological properties of vegetation [leaf circulation models (GCMs). LSMs range in complexity area index (LAI) and stomatal resistance]. The SVAT from the simple “bucket” model with fixed water- approach may be enhanced to include the coupling be- holding capacity (Manabe 1969), to models of interme- tween photosynthesis and stomatal conductance (Sell- diate complexity, which implicitly represent some ef- ers et al. 1996b; Dickinson et al. 1998) and the dynamic fects of soil properties and vegetation (e.g., Noilhan treatment of vegetation (e.g., Foley et al. 1996; Arora and Planton 1989), to more sophisticated soil–vegeta- 2002). tion–atmosphere–transfer (SVAT) schemes that in- The role of LSMs is to mediate the fluxes of energy, clude explicit treatment of vegetation (e.g., Sellers et al. moisture, and momentum that connect the atmosphere 1986; Dickinson et al. 1986). The SVAT approach em- to the underlying land surface. Soil moisture is the dom- inant quantity affecting these surface fluxes. Shukla and Mintz (1982) highlight the role played by soil moisture Corresponding author address: Vivek Arora, Canadian Centre for Climate Modelling and Analysis, Meteorological Service of in the extratropics and make an analogy with energy in Canada, University of Victoria, Victoria, BC V8W 2Y2, Canada. the ocean. The ocean stores some of the radiational E-mail: [email protected] energy it receives in summer and uses it to heat the Unauthenticated | Downloaded 10/02/21 08:25 AM UTC JCLI3926 5876 JOURNAL OF CLIMATE VOLUME 19 atmosphere in winter. The soil stores some of the pre- precipitation, evapotranspiration, and runoff and inves- cipitation it receives in winter and uses it to humidify tigate the connection between the variability of these the atmosphere in summer. The role of soil moisture in moisture budget components and that of soil moisture. influencing near-surface atmospheric variability (Del- Monthly values of moisture budget quantities from the worth and Manabe 1988, 1989; Manabe and Delworth second Atmospheric Model Intercomparison Project 1990), in affecting the persistence of atmospheric circu- (AMIP2) simulation made with the Canadian Centre lation (Namias 1952, 1959), and the effect of anomalies for Climate Modelling and Analysis (CCCma) third- of soil moisture and temperature on atmospheric circu- generation atmospheric general circulation model lation (Charney et al. 1977; Walker and Rowntree 1977; (AGCM3) are used. The SVAT scheme used in the Rowntree and Bolton 1983; Yeh et al. 1984; Dickinson CCCma AGCM3, the Canadian Land Surface Scheme and Henderson-Sellers 1988; Shukla et al. 1990; Koster (CLASS), explicitly models interactions between and Suarez 1995; Avissar and Liu 1996) have been in- canopy, snow, and ground (soil) moisture reservoirs. A vestigated by many researchers. Realistic spatial distri- brief description of AGCM3 and the land surface butions of soil moisture have been shown to improve scheme is given in section 2, which also describes the rainfall patterns, reduce errors in near-surface tempera- AMIP2 simulation. Comparisons of the variance of tures, and improve interannual variability in simulated simulated surface hydrological quantities with observa- climate (Fennessey and Shukla 1999; Dirmeyer 2000; tionally based variance estimates are presented in sec- Douville and Chauvin 2000). tion 3. The connection between the variability of soil While the role of soil moisture in affecting the near- moisture and other moisture budget components is in- surface atmosphere has been the focus of a number of vestigated in section 4, and the results are summarized studies, very few have focused on the nature and causes in section 5. of soil moisture variability itself. Delworth and Manabe (1988) describe the variability of soil moisture in a 2. Model and experimental design GCM (using a bucket LSM with a field capacity of 15 a. The CCCma third-generation AGCM cm) in terms of the variability of precipitation and the control of potential evaporation. In the light of stochas- The CCCma third-generation atmospheric GCM tic theory, they report that the forcing of the land sur- (Scinocca and McFarlane 2004) is the latest in the series face system by precipitation (a white noise input vari- of AGCMs described in Boer et al. (1984) and McFar- able) will yield soil moisture (the output variable) with lane et al. (1992). The results analyzed here are ob- a red spectrum, the redness of which is controlled by tained with a T47 L32 version of the model. Dynamic the dependence of evaporation on soil moisture and terms are calculated at triangular T47 spectral trunca- potential evaporation (the damping term). They con- tion, and the physical terms on a 96 ϫ 48 (3.75°) hori- clude that the increasing redness of soil moisture spec- zontal linear grid. The vertical domain extends to 1 hPa tra at high latitudes is a result of declining potential with the thicknesses of the model’s 32 layers increasing evaporation. Entekhabi and Rodriguez-Iturbe (1994) monotonically with height from approximately 100 m at derive an analytical expression for the space–time the surface to 3 km in the lower stratosphere. power spectrum of soil moisture that is related to the While many of the parameterized physical processes space–time power spectrum of the rainfall through a in the third-generation model are qualitatively similar hydrological gain function. They use a simple soil water to AGCM2, key new features include 1) a new param- balance model in which losses (combined evapotrans- eterization of cumulus convection (Zhang and McFar- piration, surface runoff, and drainage) are represented lane 1995), 2) an improved treatment of solar radiation as a linear function of soil moisture. The space–time that employs four bands in the visible and near-infrared power spectrum of rainfall is estimated by assuming region, 3) an “optimal” spectral representation of to- that rainbands arrive in space–time according to a ho- pography (Holzer 1996), 4) a revised representation of mogeneous Poisson process. As with Delworth and turbulent transfer coefficients at the surface (Abdella Manabe (1988), Entekhabi and Rodriguez-Iturbe and McFarlane 1996), 5) a hybrid moisture variable (1994) conclude that the hydrological gain function (Boer 1995), and 6) CLASS for treatment of the land (whose parameters depend on climate and surface hy- surface processes (Verseghy 1991; Verseghy et al. drological processes) acts as a low-pass filter that pref- 1993). erentially dampens high-frequency space and time fluc- b. The Canadian Land Surface Scheme tuations of rainfall to produce low-frequency and large- scale soil moisture variations. The CLASS land surface
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