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Roni Avissar and Fei Chen

Abstract

Mesoscale circulations generated by landscape discontinuities (e.g., sea breezes) are likely to have a significant impact on the hydrologic cycle, the climate, and the weather. However, these processes are not represented in large-scale atmospheric models (e.g., general circulation models), which have an inappropriate grid-scale resolution. With the assumption that atmospheric variables can be separated into large scale, mesoscale, and turbulent scale, a set of prognostic equations applicable in large-scale atmospheric models for momentum, temperature, moisture, and any other gaseous or aerosol material, which includes both mesoscale and turbulent fluxes is developed. Prognostic equations are also developed for these mesoscale fluxes, which indicate a closure problem and, therefore, require a parameterization. For this purpose, the mean mesoscale kinetic energy (MKE) per unit of mass is used, defined as Ẽ = 0.5 〈u i ′2〉 where u i represents the three Cartesian components of a mesoscale circulation (the angle bracket symbol is the grid-scale, horizontal averaging operator in the large-scale model, and a tilde indicates a corresponding large-scale mean value). A prognostic equation is developed for Ẽ, and an analysis of the different terms of this equation indicates that the mesoscale vertical heat flux, the mesoscale pressure correlation, and the interaction between turbulence and mesoscale perturbations are the major terms that affect the time tendency of Ẽ. A state-of-the-art mesoscale atmospheric model is used to investigate the relationship between MKE, landscape discontinuities (as characterized by the spatial distribution of heat fluxes at the earth's surface), and mesoscale sensible and latent heat fluxes in the atmosphere. MKE is compared with turbulence kinetic energy to illustrate the importance of mesoscale processes as compared to turbulent processes. This analysis emphasizes the potential use of MKE to bridge between landscape discontinuities and mesoscale fluxes and, therefore, to parameterize mesoscale fluxes generated by such subgrid-scale landscape discontinuities in large-scale atmospheric models.

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Bin Li and Roni Avissar

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The impact of subgrid-scale variability of land characteristics on land-surface energy fluxes simulated in atmospheric models (e.g., GCMs) was investigated with Patchy Land-Atmosphere Interactive Dynamics (PLAID), a land-surface scheme developed by Avissar and Pielke that represents the land surface as a mosaic of patches. Eleven different distributions of the five predominant characteristics of land-surface schemes (i.e., stomatal conductance, soil-surface wetness, leaf area index, surface roughness, and albedo) were considered. A total of 5 580 900 steady-state simulations was produced to thoroughly analyze this impact under a broad range of atmospheric conditions. The authors found that the more skewed the distribution within the range of land-surface characteristics that is related nonlinearly to the energy fluxes, the larger the difference between the energy fluxes calculated with the distribution and the corresponding mean. Among the various distributions considered in the study, the lognormal distribution produced the largest such difference, and negatively skewed beta distributions resulted in negligible difference. In general, the latent beat flux was the most sensitive to spatial variability and the radiative flux emitted by the surface was the least sensitive. The results indicate that it is very important to consider the spatial variability of leaf area index, stomatal conductance, and, in bare land, soil-surface wetness. The spatial variability of surface roughness is mostly important under neutral and stable atmospheric conditions. It appears that the relationship between albedo and surface energy fluxes is almost linear, and therefore, using a mean value of this characteristic is appropriate. This analysis emphasizes the need to develop land-surface schemes able to account for spatial variability in atmospheric models, as well as the necessity to provide higher statistical moments when creating datasets of land-surface characteristics.

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Roni Avissar and David Werth
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Fei Chen and Roni Avissar

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Vertical heat fluxes associated with mesoscale circulations generated by land-surface wetness discontinuities are often stronger than turbulent fluxes, especially in the upper part of the atmospheric planetary boundary layer. As a result, they contribute significantly to the subgrid-scale fluxes in large-scale atmospheric models. Yet they are not considered in these models. To provide some insights into the possible parameterization of these fluxes in large-scale models, a state-of-the-art mesoscale numerical model was used to investigate the relationships between mesoscale heat fluxes and atmospheric and land-surface characteristics that play a key role in the generation of mesoscale circulations. The distribution of land-surface wetness, the wavenumber and the wavelength of the land-surface discontinuities, and the large-scale wind speed have a significant impact on the mesoscale heat fluxes. Empirical functions were derived to characterize the relationships between mesoscale heat fluxes and the spatial distribution of land-surface wetness. The strongest mesoscale heat fluxes were obtained for a wavelength of forcing corresponding approximately to the local Rossby deformation radius. The mesoscale heat fluxes are weakened by large-scale background winds but remain significant even with moderate winds.

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Yongqiang Liu and Roni Avissar

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Persistence in the land–atmosphere system simulated with the National Center for Atmosphere Research Community Climate Model Version 2 (CCM2) coupled with the Biosphere–Atmosphere Transfer Scheme (BATS) is examined. BATS simulates various vegetation and soil types and explicitly predicts soil temperature. Thus, it is well equipped to study persistence in different climatic regions, and to compare the relative importance of soil hydrological and thermal processes. An evaluation of a 10-yr simulation produced with CCM2–BATS indicates that this coupled model is able to reproduce the observed spatial patterns of soil moisture and soil temperature in China. Also, the magnitude of these two soil variables in the simulation are, in general, close to observations. The major exception is soil temperature during wintertime. Analysis of this simulation indicates a significant persistence in soil moisture and soil temperature. The timescales of the persistence are of the order of months to seasons. In comparison with soil temperature, soil moisture has a much stronger persistence, as indicated by larger autocorrelations and longer timescales. Persistence of the simulated soil moisture depends on latitude and regional climatology. This regional dependence is also found in the observations. This study emphasizes that persistence of soil moisture is determined mainly by actual evaporation, and its impact on atmospheric persistence is determined mainly by the nature of internal moisture exchanges in the land–atmosphere system.

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David Werth and Roni Avissar

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The annual cycle of evapotranspiration (ET) is an important component of the Amazon hydrological balance, which is of critical importance to the global water cycle. Understanding the changing water balance in this region is particularly important to estimate future global and regional hydroclimate change in response to projected deforestation of the rain forest in this region.

Several methods have been used to estimate the annual ET cycle in the Amazon basin. These different methods, which result in a spread of annual means, ranges, and phases of the ET cycle, are evaluated here. In an attempt to reconcile the differences between them, both the data and the assumptions upon which the methods are based are scrutinized. The differences seem to originate from the geographic site where radiation and ET are simulated and/or observed and, more significantly, from the way that vegetation controls ET in the different models being used.

While field campaigns conducted during the Large-Scale Biosphere Atmosphere (LBA) experiment in the Amazon have provided many new insights into the Amazon hydroclimate, additional observations of ET and precipitation in that region are needed to understand better the processes involved.

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Yongqiang Liu and Roni Avissar

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In a companion paper, Y. Liu and R. Avissar analyzed the features of persistence in the land–atmosphere system simulated with the National Center for Atmospheric Research Community Climate Model Version 2 coupled with the Biosphere–Atmosphere Transfer Scheme (CCM2–BATS). To interpret the results obtained in that study, a fourth-order land–atmosphere analytical model is developed and used to investigate the timescales of disturbances in the land–atmosphere system, and the major parameters and processes affecting them. This analytical model has four damping timescales, namely seasonal, monthly, weekly, and daily. It is found that the seasonal scale is caused by self-feedback of soil moisture, and its length increases significantly due to the interactions between soil moisture and the other system variables. A sensitivity analysis performed with the Fourier amplitude sensitivity test indicates that the seasonal timescale is mostly affected by the physical parameters related to hydrological processes (namely, evaporation, runoff, and soil moisture diffusion), while the thermal characteristics of the land–atmosphere system mostly affect the monthly timescale. Thus, the results of this analytical study indicate that the persistence obtained in the CCM2–BATS simulation is an inherent property of the land–atmosphere system. They also emphasize the importance of soil moisture disturbances on persistence in the climatic system.

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Roni Avissar and Hai Pan

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Lake Kinneret is a 168-km2 lake located in northern Israel. It provides about 50% of the drinking water consumed in this arid country. To manage correctly this vital water resource, it is essential to understand the various hydrometeorological processes that affect its water budget and, in particular, its evaporation. The complexity of the terrain in this region (varying from ≈2800 m to ≈−410 m within a short distance), combined with different types of soil and ground covers surrounding the lake, results in complicated microscale and mesoscale atmospheric motions, including sea, lake, and land breezes, as well as anabatic and katabatic winds. The Regional Atmospheric Modeling System (RAMS), a state-of-the-art nonhydrostatic model with two-way interactive multigrid nesting and four-dimensional data assimilation capabilities, was used, together with observations collected near the western and eastern shores of the lake, to study these processes. It was configured with two nested grids centered in the middle of the lake: 1) a coarse grid with 4 km × 4 km grid elements representing a 264 km × 240 km domain including Mount Hermon, the Dead Sea, the Golan Heights, and the Mediterranean coast; and 2) a fine grid with 1 km × 1 km grid elements covering a 42 km × 50 km domain. Two three-day periods in the summers of 1992 and 1993, during which hydrometeorological observations were available, were simulated. To account for synoptic conditions, the National Centers for Environmental Prediction–National Center for Atmospheric Research mandatory-level reanalyses produced every 6 h for these periods were assimilated by the model. The strength and timing of the various atmospheric motions that develop in that region and their interactions significantly affect the hydrometeorological processes of the lake, which are subject to important diurnal and spatial variations of wind intensity and direction, temperature, humidity, and fluxes. Since these processes have a strong feedback on the lake hydrodynamics and thermal structure, it is concluded that the development of a coupled lake–atmosphere model is needed to provide good estimates of lake evaporation when lake water surface temperatures are not available. Here, it is demonstrated that RAMS performs properly, given the particular complexity of the Lake Kinneret system and the uncertainty inherent in observations above turbulent water.

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Roni Avissar and David Werth

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Past studies have indicated that deforestation of the Amazon basin would result in an important rainfall decrease in that region but that this process had no significant impact on the global temperature or precipitation and had only local implications. Here it is shown that deforestation of tropical regions significantly affects precipitation at mid- and high latitudes through hydrometeorological teleconnections. In particular, it is found that the deforestation of Amazonia and Central Africa severely reduces rainfall in the lower U.S. Midwest during the spring and summer seasons and in the upper U.S. Midwest during the winter and spring, respectively, when water is crucial for agricultural productivity in these regions. Deforestation of Southeast Asia affects China and the Balkan Peninsula most significantly. On the other hand, the elimination of any of these tropical forests considerably enhances summer rainfall in the southern tip of the Arabian Peninsula. The combined effect of deforestation of these three tropical regions causes a significant decrease in winter precipitation in California and seems to generate a cumulative enhancement of precipitation during the summer in the southern tip of the Arabian Peninsula.

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Roni Avissar and Tatyana Schmidt

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The effects on the convective boundary layer (CBL) of surface heterogeneities produced by surface sensible heat flux waves with different means, amplitudes, and wavelengths were investigated here. The major objective of this study was to evaluate at which scale surface heterogeneity starts to significantly affect the heat fluxes in the CBL. The large-eddy simulation option of the Regional Atmospheric Modeling System developed at Colorado State University was used for that purpose. Avissar et al. evaluated this model against observations and demonstrated its reliability. It appears that the impact of amplitude and wavelength of a heat wave is nonlinearly dependent upon the mean heating rate. The circulations (or rolls) resulting from surface heterogeneity are strong when the amplitude and the wavelength of the heat wave are large, especially at low mean heating rate. In that case the profiles of horizontally averaged variables are quite strongly modified in the CBL. The potential temperature is not constant with elevation, and the sensible heat flux considerably departs from the linear variation with height obtained in a typical CBL that develops over a homogeneous domain. The mean turbulence kinetic energy profile depicts two maxima, one near the ground surface and one near the top of the CBL, corresponding to the strong horizontal flow that develops near the ground surface and the return flow at the top of the CBL. In a dry atmosphere, a weak background wind of 2.5 m s−1 is strong enough to considerably reduce the impact of ground-surface heterogeneity on the CBL. A moderate background wind of 5 m s−1 virtually eliminates all impacts that could potentially be produced in realistic landscapes. Water vapor does not significantly affect the CBL. However, the formation of rolls at preferential locations within the heterogeneous domain results in “pockets” of high moisture concentration, which have a strong potential for clouds formation. Such clouds may not form over homogeneous domains where moisture is more uniformly distributed, and the CBL is not as high as in heterogeneous domains. From this study, it can be concluded that as long as the “patchiness” of the landscape has a characteristic length scale smaller than about 5–10 km (even without background wind), the “mosaic of tiles” type of land surface scheme suggested by Avissar and Pielke can be applied to represent the land surface in atmospheric models. At larger scales, the impact of landscape heterogeneity may be significant, especially when the atmosphere is humid. Therefore, this study supports previous estimates, which were based on theoretical analyses.

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