Elfatih A. B. Eltahir

Simulation of regional-scale water and energy budgets: Representation of subgrid cloud and precipitation processes within RegCM

A new large-scale cloud and precipitation scheme, which accounts for the sub- grid-scale variability of clouds, is coupled to NCARs Regional Climate Model (RegCM). This scheme partitions each grid cell into a cloudy and noncloudy fraction related to the average grid cell relative humidity. Precipitation occurs, according to a specified autoconversion rate, when a cloud water threshold is exceeded. The specification of this threshold is based on empirical in-cloud observations of cloud liquid water amounts. Included in the scheme are simple formulations for raindrop accretion and evaporation. The results from RegCM using the new scheme, tested over North America, show significant improvements when compared to the old version. The outgoing longwave radiation, albedo, cloud water path, incident surface shortwave radiation, net surface radiation, and surface temperature fields display reasonable agreement with the observations from satellite and surface station data. Furthermore, the new model is able to better represent extreme precipitation events such as the Midwest flooding observed in the summer of 1993. Overall, RegCM with the new scheme provides for a more accurate representation of atmospheric and surface energy and water balances, including both the mean conditions and the variability at daily to interannual scales. The latter suggests that the new scheme improves the models sensitivity, which is critical for both climate change and process studies.

A soil moisture-rainfall feedback mechanism. 1. Theory and observations

This paper presents a hypothesis regarding the fundamental role of soil moisture conditions in land-atmosphere interactions. We propose that wet soil moisture conditions over any large region should be associated with relatively large boundary layer moist static energy, which favors the occurrence of more rainfall. Since soil moisture conditions themselves reflect past occurrence of rainfall, the proposed hypothesis implies a positive feedback mechanism between soil moisture and rainfall. This mechanism is based on considerations of the energy balance at the land-atmosphere boundary, in contrast to similar mechanisms that were proposed in the past and that were based on the concepts of water balance and precipitation recycling. The control of soil moisture on surface albedo and Bowen ratio is the fundamental basis of the proposed soil moisture-rainfall feedback mechanism. The water content in the upper soil layer affects these two important properties of the land surface such that both variables decrease with any increase in the water content of the top soil layer. The direct effect of soil moisture on surface albedo implies that wet soil moisture conditions enhance net solar radiation. The direct effect of soil moisture on Bowen ratio dictates that wet soil moisture conditions would tend to enhance net terrestrial radiation at the surface through cooling of surface temperature, reduction of upwards emissions of terrestrial radiation, and simultaneous increase in atmospheric water vapor content and downwards flux of terrestrial radiation. Thus, under wet soil moisture conditions, both components of net radiation are enhanced, resulting in a larger total flux of heat from the surface into the boundary layer. This total flux represents the sum of the corresponding sensible and latent heat fluxes. Simultaneously, cooling of surface temperature should be associated with a smaller sensible heat flux and a smaller depth of the boundary layer. Whenever these processes occur over a large enough area, the enhanced flux of heat from the surface into the smaller reservoir of boundary layer air should favor a relatively large magnitude of moist static energy per unit mass of the boundary layer air. The dynamics of localized convective storms as well as the dynamics of large-scale atmospheric circulations have been shown to be sensitive to the distribution of boundary layer moist static energy by several previous studies. These theoretical concepts are tested using field observations from Kansas and explored further in a companion paper (Zheng and Eltahir, this issue) using a simple numerical model.

Atmospheric Controls on Soil Moisture–Boundary Layer Interactions. Part I: Framework Development

This paper investigates the influence of soil moisture on the development and triggering of convection in different early-morning atmospheric conditions. A one-dimensional model of the atmospheric boundary layer (BL) is initialized with atmospheric sounding data from Illinois and with the soil moisture set to either extremely wet (saturated) or extremely dry (20% of saturation) conditions. Two measures are developed to assess the lowlevel temperature and humidity structure of the early-morning atmosphere. These two measures are used to distinguish between four types of soundings, based on the likely outcome of the model: 1) those soundings favoring deep convection over dry soils, 2) those favoring deep convection over wet soils, 3) those unlikely to convect over any land surface, and 4) those likely to convect over any land surface. Examples of the first two cases are presented in detail. The early-morning atmosphere is characterized in this work by the newly developed convective triggering potential (CTP) and a low-level humidity index, HIlow. The CTP measures the departure from a moist adiabatic temperature lapse rate in the region between 100 and 300 mb (about 1‐3 km) above the ground surface (AGS). This region is the critical interface between the near-surface region, which is almost always incorporated into the growing BL, and free atmospheric air, which is almost never incorporated into the BL. Together, these two measures form the CTP-HIlow framework for analyzing atmospheric controls on soil moisture‐boundary layer interactions. Results show that in Illinois deep convection is trigged in the model 22% of the time over wet soils and only 13% of the time over dry soils. Additional testing varying the radiative conditions in Illinois and also using the 1D model with soundings from four additional stations confirm that the CTP-HI low framework is valid for regions far removed from Illinois.

An analysis of the soil moisture-rainfall feedback, based on direct observations from Illinois

Many global and regional climate modeling studies have demonstrated the importance of the initial soil water condition in their simulations of regional rainfall distribution. However, none of these modeling studies has been tested against directly observed data. This study tests the hypothesis that soil saturation is positively correlated with subsequent precipitation by analyzing a 14-year soil moisture data set from the state of Illinois. The linear correlation between an initial soil saturation condition and subsequent rainfall is significant during the summer months, reaching a peak of r2 > 0.4 in mid-June. This result is consistent with the hypothesis that knowledge of late spring/early summer soil moisture conditions can aid in the prediction of drought or flood years, but it does not necessarily prove that feedback from anomalous soil moisture reservoirs is the cause of anomalous summer conditions. Further analyses indicate that from early June to mid-August, persistence in rainfall cannot fully account for the observed correlations, suggesting the likelihood of a physical feedback mechanism linking early summer soil saturation with subsequent precipitation. However, spatial and temporal data limitations restrict the potential for drawing strong new conclusions from the Illinois study.

Dynamics of Wet and Dry Years in West Africa

Abstract This paper proposes a theoretical framework for describing interannual climatic variability over West Africa. The dynamical theory of zonally symmetrical thermally direct circulations suggests that a meridional monsoon circulation must develop over any tropical region (off the equator) when the absolute vorticity near the tropopause reaches a threshold value of zero. However, for a moist atmosphere that satisfies a quasi-equilibrium balance between moist convection and the radiative forcing, the absolute vorticity at upper-tropospheric levels is a function of both latitude and the meridional distribution of boundary-layer entropy. Hence, the onset of a monsoon circulation depends in a nonlinear fashion on these two factors. The theory predicts that a flat distribution of entropy does not drive any circulation and that a relatively large gradient of entropy should drive a strong monsoon circulation. The location of the region of West Africa, relatively close to the equator, dictates that the dynam...

Atmospheric Controls on Soil Moisture–Boundary Layer Interactions. Part II: Feedbacks within the Continental United States

The CTP-HIlow framework for describing atmospheric controls on soil moisture‐boundary layer interactions is described in a companion paper, Part I. In this paper, the framework is applied to the continental United States to investigate how differing atmospheric regimes influence local feedbacks between the land surface and the atmosphere. The framework was developed with a one-dimensional boundary layer model and is based on two measures of atmospheric thermodynamic properties: the convective triggering potential (CTP), a measure of the temperature lapse rate between approximately 1 and 3 km above the ground surface, and a low-level humidity index, HIlow. These two measures are used to distinguish between three types of early-morning atmospheric conditions: those favoring moist convection over dry soils, those favoring moist convection over wet soils, and those that will allow or prevent deep convective activity, independent of the surface flux partitioning. Analyses of multiyear CTP-HIlow scatterplots from radiosonde stations across the contiguous 48 United States reveal that during the summer months (June, July, and August) positive feedbacks between soil moisture and moist convection are likely in much of the eastern half of the country. Over the western half of the country, atmospheric conditions and the likelihood of moist convection are largely determined by oceanic influences, and land surface conditions in the summer are unlikely to impact convective triggering. The only area showing a potential negative feedback is in the dryline and monsoon region of the arid Southwest. This potential arises because of the topography of this and surrounding regions. A relatively narrow band of stations lies in between the eastern and western portions of the country, in some years behaving like the stations to the west and in other years behaving like the stations to the east.

El Niño and the Natural Variability in the Flow of the Nile River

Natural variability in the annual flow of the Nile River has been the subject of great interest to the civilizations that have historically occupied the banks of that river. Here we report results from analysis on two extensive data sets describing sea surface temperature of the Pacific Ocean, and the flow of water in the Nile River. The analysis suggests that 25% of the natural variability in the annual flow of the Nile is associated with El Nino oscillations. A procedure is developed for using this observed correlation to improve the predictability of the Nile flood. A simple hypothesis is presented to explain physically the occurrence of the Hurst phenomenon in the Nile flow.

Pathways Relating Soil Moisture Conditions to Future Summer Rainfall within a Model of the Land–Atmosphere System

In this paper, the key pathways and mechanisms through which soil moisture conditions affect future rainfall over the U.S. Midwest are investigated using a regional climate model. A series of numerical experiments are performed to identify these pathways using the drought of 1988 and flood of 1993 as representative events. The results suggest that the soil moisture‐rainfall feedback is an important mechanism for hydrologic persistence during the late spring and summer over the midwestern United States. They indicate that the feedback between soil moisture and subsequent rainfall played a significant role in enhancing the persistence of the drought of 1988 and the flood of 1993. It is found that there is a pronounced asymmetry in the sensitivity of simulated rainfall to specified initial soil moisture. The asymmetry acts to favor a stronger soil moisture‐rainfall feedback during drought conditions as opposed to flood conditions. Detailed analyses of the simulations indicate that the impact of soil moisture on both the energy and water budgets is crucial in determining the strength of the soil moisture‐rainfall feedback. Anomalously high soil moisture tends to 1) increase the flux of high moist static energy air into the planetary boundary layer from the surface via an increase in net surface radiation, 2) reduce the planetary boundary layer height thus increasing the moist static energy per unit mass of air, and 3) reduce the amount of entrained air of low moist static energy from above the planetary boundary layer. Each of these effects are additive and combine to increase the moist static energy per unit mass of air in the planetary boundary layer. This increase results in an increase in the frequency and magnitude of convective rainfall events and a positive feedback between soil moisture and subsequent rainfall.

Representation of Water Table Dynamics in a Land Surface Scheme. Part I: Model Development

Most of the current land surface parameterization schemes lack any representation of regional groundwater aquifers. Such a simplified representation of subsurface hydrological processes would result in significant errors in the predicted land surface states and fluxes especially for the shallow water table areas in humid regions. This study attempts to address this deficiency. To incorporate the water table dynamics into a land surface scheme, a lumped unconfined aquifer model is developed to represent the regional unconfined aquifer as a nonlinear reservoir, in which the aquifer simultaneously receives the recharge from the overlying soils and discharges runoff into streams. The aquifer model is linked to the soil model in the land surface scheme [Land Surface Transfer Scheme (LSX)] through the soil drainage flux. The total thickness of the unsaturated zone varies in response to the water table fluctuations, thereby interactively coupling the aquifer model with the soil model. The coupled model (called LSXGW) has been tested in Illinois for an 11-yr period from 1984 to 1994. The results show reasonable agreements with the observations. However, there are still secondary biases in the LSXGW simulation partially resulting from not accounting for the spatial variability of water table depth. The issue of subgrid variability of water table depth will be addressed in a companion paper.

On the asymmetric response of aquifer water level to floods and droughts in Illinois

Here we analyze observed characteristics of the natural variability in the regional-scale hydrological cycle of Illinois, including the soil and atmospheric branches. This analysis is based on a consistent data set that describes several hydrological variables: the flux of atmospheric water vapor, incoming solar radiation, precipitation, soil moisture content, aquifer water level, and river flow. The climatology of the average regional hydrological cycle has been estimated. Variability in incoming solar radiation, not precipitation, is the main forcing of the seasonal variability in evaporation, soil moisture content, aquifer water level, and river flow. While precipitation plays a minor role in shaping the natural variability in the regional hydrological cycle at the seasonal timescale, variability in precipitation is the major factor in shaping the natural variability in the regional hydrological cycle at the interannual timescale. The anomalies in the different variables of the regional hydrological cycle have been computed and the persistence patterns of extreme floods and droughts have been compared. The 1988 drought left a signature in the aquifer water level that is significantly more persistent than the corresponding signature for the 1993 summer flood. The discharge from unconfined groundwater aquifers to streams (base flow) provides an efficient dissipation mechanism for the wet anomalies in aquifer water level. However, the nonlinear dependence of the groundwater discharge on aquifer water level (groundwater rating curve) may explain why droughts leave a significantly more persistent signature on groundwater hydrology, in comparison to the signature of floods. This nonlinearity has been attributed to the increasing degree by which the unconfined aquifers get connected to the channels network, as the aquifer water level rises leading to higher drainage density. The potential implications of these results regarding the impact on regional water resources due to any future climate change are discussed.