Relevance of soil moisture for seasonal climate predictions: a preliminary study

In the framework of the Global Soil Wetness Project (GSWP), the ISBA land-surface scheme of the ARPEGE atmospheric general circulation model has been forced with meteorological observations and analyses in order to produce a two-year (1987–1988) soil moisture climatology at a 1^°×1^° horizontal resolution. This climatology is model dependent, but it is the climatology that the ARPEGE model would produce if its precipitation and radiative fluxes were perfectly simulated. In the present study, ensembles of seasonal simulations (March to September) have been performed for 1987 and 1988, in which the total soil water content simulated by ARPEGE is relaxed towards the GSWP climatology. The results indicate that the relaxation has a positive impact on both the model's climatology and the simulated interannual variability, thereby confirming the utility of the GSWP soil moisture data for prescribing initial or boundary conditions in comprehensive climate and numerical weather prediction models. They also demonstrate the relevance of soil moisture for achieving realistic simulations of the Northern Hemisphere summer climate. In order to get closer to the framework of seasonal predictions, additional experiments have been performed in which GSWP is only used for initialising soil moisture at the beginning of the summer season (the relaxation towards GSWP is removed on 1st June). The results show a limited improvement of the interannual variability, compared to the simulations initialised from the ARPEGE climatology. However, some regional patterns of the precipitation differences between 1987 and 1988 are better captured, suggesting that seasonal predictions can benefit from a better initialisation of soil moisture.     

Sensitivity of the hydrological cycle to increasing amounts of greenhouse gases and aerosols

The coupled atmosphere–ocean Climate Model of the Centre National de Recherches Météorologiques (CNRM) has been used to run a time-dependent climate change experiment to study the impact of increasing amounts of greenhouse gases and aerosols on the simulated water cycle. This simulation has been initialised with the oceanic temperature and salinity profiles and the atmospheric trace gas concentrations observed in the 1950s, and has been carried out for 150 years after a 20-year spin-up. The simulated climate change has been analysed as the difference between two 30-year time slices: 1970–2000 and 2070–2100 respectively. The model achieves a reasonable simulation of present-day climate and simulates a general increase in precipitation throughout the twenty first century. The main exceptions are the subtropics, where the enhanced Hadley circulation has a drying impact, and the mid-latitude continents, where the increased evaporation in spring and decreased moisture convergence in summer lead to a relative summer drying. Global and regional analyses suggest that the precipitation increase is generally limited by a decrease in the water vapour cycling rate and in the precipitation efficiency, which appear as key parameters of the simulated water cycle. In order to reduce the spread between climate scenarios, more efforts should be devoted to estimate these parameters from satellite observations and meteorological analyses, and their possible evolution over recent decades. In the present study, the impacts of global warming on the surface hydrology have been also investigated. The main findings are the amplification of the annual cycle of soil moisture in the mid-and-high latitudes, and the decrease in the Northern Hemisphere snow cover, at a rate that is consistent with recent satellite estimations and should increase during the twenty first century. The runoff simulated over the 1950–2100 period has been converted into river flow using a linear river routeing model. The trends simulated over recent decades are surprisingly consistent with the river flow measurements available from the Global Runoff Data Centre. These trends can differ from those estimated over the whole 150-year integration, thereby indicating that it is not safe to predict hydrological impacts just by extrapolating the trends found in the available observations. Our climate model seems likely to provide qualitative hydrological scenarios over large river basins, but it still shows serious biases in the simulation of present-day river flows. Regional hydrological projections remain a challenge for the global climate modelling community and downscaling techniques are still necessary for this purpose.     

Detectability of Summer Dryness Caused by Greenhouse Warming

This study investigates the temporal and spatial variation of soil moisture associated with global warming as simulated by long-term integrations of a coupled ocean-atmosphere model conducted earlier. Starting from year 1765, integrations of the coupled model for 300 years were performed for three scenarios: increasing greenhouse gases only, increasing sulfate-aerosol loading only and the combination of both radiative forcings. The integration with the combined radiative forcings reproduces approximately the observed increases of global mean surface air temperature during the 20th century. Analysis of this integration indicates that both summer dryness and winter wetness occur in middle-to-high latitudes of North America and southern Europe. These features were identified in earlier studies. However, in the southern part of North America where the percentage reduction of soil moisture during summer is quite large, soil moisture is decreased for nearly the entire annual cycle in response to greenhouse warming. A similar observation applies to other semi-arid regions in subtropical to middle latitudes such as central Asia and the area surrounding the Mediterranean Sea. On the other hand, annual mean runoff is greatly increased in high latitudes because of increased poleward transport of moisture in the warmer model atmosphere. An analysis of the central North American and southern European regions indicates that the time when the change of soil moisture exceeds one standard deviation about the control integration occurs considerably later than that of surface air temperature for a given experiment because the ratio of forced change to natural variability is much smaller for soil moisture compared with temperature. The corresponding lag time for runoff change is even greater than that of either precipitation or soil moisture for the same reason. Also according to the above criterion, the inclusion of the effect of sulfate aerosols in the greenhouse warming experiment delays the noticeable change of soil moisture by several decades. It appears that observed surface air temperature is a better indicator of greenhouse warming than hydrologic quantities such as precipitation, runoff and soil moisture. Therefore, we are unlikely to notice definitive CO₂-induced continental summer dryness until several decades into the 21st century.     

A global monthly climatology of soil moisture and water balance

Global monthly climatology of available soil moisture content is derived on a 4° by 5° grid from observed precipitation and air surface temperature by use of a simple water budget model. The governing equations and methods of calculation for deriving these fields, which follow the formulation of Thornthwaite, are first described and the importance of the various assumptions and simplifications of this approach are discussed. The derived global fields are then presented. A comparison of some of the derived fields with other calculations is also made in order to permit an evaluation of the results: For example, our indirect estimate of the river run-off is generally in good agreement with more direct estimates, except for high latitude regions where the freezing of the soil may play an important role.Yale Mintz died on 27 April 1991. This work was carried out jointly over a number of years preceding his death     

Uncertainties in the GSWP-2 precipitation forcing and their impacts on regional and global hydrological simulations

The Global Soil Wetness Project (GSWP) is an international initiative aimed at producing global data sets of soil wetness and energy and water fluxes by driving land surface models with state-of-the-art 1° by 1° atmospheric forcing and land surface parameters. It also provides a unique opportunity to develop and test land surface parameterizations at the global scale, using multi-year off-line simulations that are not affected by the systematic errors found in atmospheric models. Nevertheless, the accuracy and reliability of the 10?year GSWP-2 atmospheric forcing remain questionable. A first comparison using the high-resolution Rhône-AGGregation (Rhône-AGG) database reveals that the baseline GSWP-2 precipitation forcing is drastically overestimated over the Rhône river basin. Hydrological simulations driven with each dataset and using the ISBA land surface model and the MODCOU river routing model are also compared. The simulated river discharges are validated against a dense network of river gauges and are generally less realistic when using the GSWP-2 instead of the Rhône-AGG precipitation forcing. Secondly, the GSWP-2 precipitation forcing is compared with three alternative data sets (GPCP-2, CRU-2, CMAP) at the global scale. Moreover, the results of a global sensitivity study to the precipitation forcing conducted with six land surface models are shown. The TRIP river routing model is used to convert daily runoff from all models into river discharges, which are compared at 80 gauging stations distributed over the globe. In agreement with the regional evaluation, the results reveal that the baseline GSWP-2 precipitation forcing is generally overestimated over the mid and high latitudes, which implies systematic errors in the simulated discharges. This study reveals that the empirical wind corrections applied to the GSWP-2 precipitation forcing are exaggerated, whereas the GPCP satellite adjustments seem to be useful for simulating realistic annual mean river discharges over the East Siberian river basins.     

A GCM-based assessment of the global moisture budget and the role of land-surface moisture reservoirs in processing precipitation

The performance of the Canadian Land Surface Scheme (CLASS) when coupled to the CCCma third generation general circulation model is evaluated in an AMIP II simulation. Our primary aim is to understand how CLASS processes moisture and to compare model estimates of moisture budget components with observations. The modelled mean annual precipitation and runoff, and their latitudinal structures, compare well with observations although some discrepancies remain in the simulation of regional values of these quantities. The amplitude and phase of the first harmonic of the precipitation annual cycle also compares well with observations although less well over regions of sparse precipitation and/or high topography. In the model, the canopy plays a major role in processing moisture at the land surface indicating the importance of vegetation in climate. The canopy intercepts a large fraction of the precipitation and provides the medium for returning much moisture back to the atmosphere as evapotranspiration. Though important locally, the snow moisture reservoir plays a relatively minor role in the global moisture budget. It acts primarily as a storage and delay mechanism with winter precipitation released to the ground reservoir on melting. The ground moisture reservoir also plays a major role and processes a similar amount of moisture as the canopy, although in a different manner. The globally averaged model runoff compares well with observation-based estimates, although the model partitioning into surface runoff and drainage does not agree particularly well with the single available observation-based estimate. How moisture is processed at the land surface serves as a basis for model intercomparison and for understanding the modelled moisture budget and its variation and changes with climate change. Only the most basic quantities (precipitation, runoff, and partitioning of runoff into surface runoff and drainage) may be compared with observation-based estimates, however, and the establishment of more complete moisture budget remains an important need.     

Estimates of continental-scale soil wetness and comparison with the soil moisture data of Mintz and Serafini

 Soil wetness, in both its global distribution and the seasonal change, has been mainly estimated by the water balance approach using the bucket model which regards the soil wetness as soil moisture. The soil moisture data of Mintz and Serafini is one of the representatives examples, however, this method has problems since it does not incorporate the effects of flooding, snow accumulation on the ground, and so on. In this study, we use the Amazon and Volga river basin to carry out a case study to evaluate these problems. In the Amazon river basin, the annual range of the entire terrestrial water storage, about 400 mm, can be mainly explained by the rising and falling of the water level, and flooding around river channels, although soil moisture data of Mintz and Serafini is almost constant throughout the year. In the Volga river basin, snow accumulates on the ground producing 80 mm of water equivalent during winter, however the soil moisture data of Mintz and Serafini is almost saturated in winter. Received: 30 October 1996 / Accepted: 4 June 1997     

长江流域水分收支以及再分析资料可用性分析

首先利用实测资料定量计算了长江流域水分收支的各分量,包括降水、径流、蒸发、水汽辐合等,分析其季节循环、年际变化以及线性趋势变化。结果表明,多年平均该流域是水汽汇区,主要来自平均流输送造成的水汽辐合,而与天气过程密切相关的瞬变波则主要造成流域的水汽辐散。蒸发所占比例接近于径流,对流域水分循环十分重要。大部分要素的季节变化和年际变化都很大,只有蒸发和大气含水量的年际变化较小。降水和平均流输送造成的水汽辐合一般在6月达到年内最大,12月达到年内最小,而径流和大气含水量则一般滞后1个月于7月达到年内最大,1月降为年内最小。1958—1983年,夏半年降水略微增加,冬半年略微减少,各月实测径流为弱的增长趋势,但均不显著,年平均蒸发亦无显著的趋势变化。然后将实测资料同ECMWF及NCEP/NCAR再分析资料作进一步对比分析,以检验两套再分析资料对长江流域水分循环的描述能力。在量值上,NCEP/NCAR再分析资料中的降水、蒸发、径流均比实测偏大很多,大气含水量及由平均流输送所造成的水汽辐合则偏小很多;ECMWF再分析资料中的降水量、径流量基本上与实测接近,蒸发量偏大,大气含水量及由平均流输送所造成的水汽辐合偏小,但比NCEP/NCAR再分析资料要接近实测。另外,该两套再分析资料均可以较好地描述长江流域水分收支的季节循环和年际变化,而且同样是ECMWF再分析资料与实测资料的一致性更好。但是两套再分析资料在1958—1983年均存在十分夸张的线性趋势变化,尤其是ECMWF再分析资料。     

Snowpack and runoff response to climate change in Owens Valley and Mono Lake watersheds

Precipitation from the Eastern Sierra Nevada watersheds of Owens Lake and Mono Lake is one of the main water sources for Los Angeles’ over 4 million people, and plays a major role in the ecology of Mono Lake and of these watersheds. We use the Variable Infiltration Capacity (VIC) hydrologic model at daily time scale, forced by climate projections from 16 global climate models under greenhouse gas emissions scenarios B1 and A2, to evaluate likely hydrologic responses in these watersheds for 1950–2099. Comparing climate in the latter half of the 20th Century to projections for 2070–2099, we find that all projections indicate continued temperature increases, by 2–5 °C, but differ on precipitation changes, ranging from ?24 % to +56 %. As a result, the fraction of precipitation falling as rain is projected to increase, from a historical 0.19 to a range of 0.26–0.52 (depending on the GCM and emission scenario), leading to earlier timing of the annual hydrograph’s center, by a range of 9–37 days. Snowpack accumulation depends on temperature and even more strongly on precipitation due to the high elevation of these watersheds (reaching 4,000 m), and projected changes for April 1 snow water equivalent range from ?67 % to +9 %. We characterize the watershed’s hydrologic response using variables integrated in space over the entire simulated area and aggregated in time over 30-year periods. We show that from the complex dynamics acting at fine time scales (seasonal and sub-seasonal) simple dynamics emerge at this multi-year time scale. Of particular interest are the dynamic effects of temperature. Warming anticipates hydrograph timing, by raising the fraction of precipitation falling as rain, reducing the volume of snowmelt, and initiating snowmelt earlier. This timing shift results in the depletion of soil moisture in summer, when potential evapotranspiration is highest. Summer evapotranspiration losses are limited by soil moisture availability, and as a result the watershed’s water balance at the annual and longer scales is insensitive to warming. Mean annual runoff changes at base-of-mountain stations are thus strongly determined by precipitation changes.     

以气候适宜降水量为基础的水分距平的计算方法

介绍了一种以气候适宜降水量为基础的水分距平的计算方法。通过统计一个地区逐月的水分平衡分量，可以计算出该地在气候上适宜的蒸散、补水、径流、失水和降水量，从而得到可以衡量水分盈亏的水分距平值d。这个水分距平作为衡量水分异常的指标可以更好地表现同一个地区同一时期不同年份的水分盈缺情况。应用这种方法计算了北京1961～2000年逐月水分距平值，并分析了1月、4月、7月和10月份的逐年水分距平变化。     

Effects of projected climate change on the hydrology in the Mono Lake Basin, California

The Californian Mono Lake Basin (MLB) is a fragile ecosystem, for which a 1983 ruling carefully balanced water diversions with ecological needs without the consideration of global climate change. The hydroclimatologic response to the impact of projected climatic changes in the MLB has not been comprehensively assessed and is the focus of this study. Downscaled temperature and precipitation projections from 16 Global Climate Models (GCMs), using two emission scenarios (B1 and A2), were used to drive a calibrated Soil and Water Assessment Tool (SWAT) hydrologic model to assess the effects on streamflow on the two significant inflows to the MLB, Lee Vining and Rush Creeks. For the MLB, the GCM ensemble output suggests significant increases in annual temperature, averaging 2.5 and 4.1 °C for the B1 and A2 emission scenarios, respectively, with concurrent small (1–3 %) decreases in annual precipitation by the end of the century. Annual total evapotranspiration is projected to increase by 10 mm by the end of the century for both emission scenarios. SWAT modeling results suggest a significant hydrologic response in the MLB by the end of the century that includes a) decreases in annual streamflow by 15 % compared to historical conditions b) an advance of the peak snowmelt runoff to 1 month earlier (June to May), c) a decreased (10–15 %) occurrence of ‘wet’ hydrologic years, and d) and more frequent (7–22 %) drought conditions. Ecosystem health and water diversions may be affected by reduced water availability in the MLB by the end of the century.     

Closing the Mackenzie basin water budget,water years 1994/95 to 1996/97

Abstract

A particularly elusive science objective for the Mackenzie Global Energy and Water Cycle Experiment (GEWEX) Study (MAGS) has been to close the atmospheric moisture budget and rationalize it against the surface water budget at annual or even monthly timescales. The task, while not difficult in principle, is complicated by two factors. First is the importance of basin snow‐cover, soil and water‐body storage in the surface water budget. Month‐to‐month changes in these components are frequently greater than the atmospheric flux terms, for example, during spring snowmelt. Furthermore, there is approximately a six‐week lag before local changes are evident in the discharge at the mouth of the basin. Second, the coarse resolution of all of the supporting data may add significant systematic errors. For example, the two radiosonde soundings per day available to the project are unlikely to account adequately for all the moisture generated locally through evapotranspiration during the summer convective season.

This analysis will directly address these two main issues by applying hydrologic and atmospheric computations to assess the storage question, and by using additional soundings at a single site to sample the diurnal signature in atmospheric moisture caused by evapotranspiration. Resulting modifications to the atmospheric moisture and surface water budgets then allow near closure of the MAGS monthly water budget within acceptable error limits.     

The temporal variability of soil wetness and its impact on climate

The temporal variability of soil wetness and its interactions with the atmosphere were studied using a general circulation model of the atmosphere. It was found that time series of soil wetness computed by the model contain substantial amounts of variance at low frequencies. Long time-scale anomalies of soil moisture resemble the red noise response of the soil layer to white noise rainfall forcing. The dependence of the temporal variability of soil moisture on potential evaporation and precipitation is discussed.     

Sensitivity of a GCM simulation to subgrid infiltration and surface runoff

A subgrid parameterization of infiltration and surface runoff was evaluated using a land surface model coupled to an atmospheric general circulation model. Averaged over 5 year simulations, the subgrid parameterization resulted in significantly less infiltration of water into the soil compared to a simulation without subgrid hydrologic processes. As a result, the soils were drier, latent heat flux decreased, and surface air temperature increased. These results are consistent with other studies of subgrid hydrologic parameterizations, which also resulted in drier soils, decreased latent heat flux, and warmer surface temperatures. Several river basins were studied in detail. In the Amazon and Lena basins, the subgrid parameterization resulted in better annual runoff compared to observed annual river flow; surface air temperature was unchanged in the Amazon and better, compared to observations, in the Lena. In the Ob, Yenisey, and Amur basins, the subgrid parameterization resulted in too much annual runoff; July surface air temperature was unchanged or worse (Amur). Annual runoff for the Mississippi basin was better with the subgrid parameterization, but July surface air temperature was worse. These results suggest the utility of subgrid hydrologic parameterizations vary among river basins depending on the relative importance of Horton and Dunne runoff and the geologic factors affecting runoff generation.     

Downscaling the hydrological cycle in the Mackenzie basin with the Canadian regional climate model

Abstract

The Canadian Regional Climate Model (CRCM) has been nested within the Canadian Centre for Climate Modelling and Analysis ‘ second generation General Circulation Model (GCM), for a single month simulation over the Mackenzie River Basin and environs. The purpose of the study is to assess the ability of the higher resolution CRCM to downscale the hydrological cycle of the nesting GCM. A second 1‐month experiment, in which the CRCM was nested within analyzed fields of a global data assimilation system, was also performed to examine the sensitivity of the basin moisture budget to atmospheric lateral boundary forcing.

We have found that the CRCM can produce realistic lee cyclogenesis, preferentially in the Liard sub‐basin, along with associated circulation and precipitation patterns, as well as an improved rainshadow in the lee of the Rocky Mountains compared to the GCM. While these features do quantitatively affect the monthly average climate statistics, the basin scale moisture budgets of the models were remarkably similar, though some of this agreement is due to compensating errors in the GCM. Both models produced excessive precipitation compared to a recent climatology for the region, the cause of which is traced to lateral boundary forcing. A second experiment, identical to the first except that the CRCM was forced with analyzed fields at the lateral boundaries, produced a qualitatively different basin moisture budget, including a much more realistic precipitation field. Errors in the moisture budget of the first experiment appear to be associated with the poor representation of the Aleutian Low in the GCM, and do not appear to be strongly connected to (local) surface processes within the models. This suggests that an effective strategy for modelling the hydrological cycle of the Mackenzie Basin on the fast climate timescale ‐ a major requirement of the Mackenzie GEWEX Study ‐ will involve nesting the CRCM within analyzed (or re‐analyzed) atmospheric fields.     

Global validation of the ISBA sub-grid hydrology

Over recent years, many numerical studies have suggested that the land surface hydrology contributes to atmospheric variability and predictability on a wide range of scales. Conversely, land surface models (LSMs) have been also used to study the hydrological impacts of seasonal climate anomalies and of global warming. Validating these models at the global scale is therefore a crucial task, which requires off-line simulations driven by realistic atmospheric fluxes to avoid the systematic biases commonly found in the atmospheric models. The present study is aimed at validating a new land surface hydrology within the ISBA LSM. Global simulations are conducted at a 1° by 1° horizontal resolution using 3-hourly atmospheric forcings provided by the Global Soil Wetness Project. Compared to the original scheme, the new hydrology includes a comprehensive and consistent set of sub-grid parametrizations in order to account for spatial heterogeneities of topography, vegetation, and precipitation within each grid cell. The simulated runoff is converted into river discharge using the total runoff integrating pathways (TRIP) river routing model (RRM), and compared with available monthly observations at 80 gauging stations distributed over the world’s largest river basins. The simulated discharges are also compared with parallel global simulations from five alternative LSMs. Globally, the new sub-grid hydrology performs better than the original ISBA scheme. Nevertheless, the improvement is not so clear in the high-latitude river basins (i.e. Ob, MacKenzie), which can be explained by a too late snow melt in the ISBA model. Over specific basins (i.e. Parana, Niger), the quality of the simulated discharge is also limited by the TRIP RRM, which does not account for the occurrence of seasonal floodplains and for their significant impact on the basin-scale water budget.     

Introduction of a sub-grid hydrology in the ISBA land surface model

In atmospheric models, the partitioning of precipitation between infiltration and runoff has a major influence on the terrestrial water budget, and thereby on the simulated weather or climate. River routing models are now available to convert the simulated runoff into river discharge, offering a good opportunity to validate land surface models at the regional scale. However, given the low resolution of global atmospheric models, the quality of the hydrological simulations is much dependent on various processes occurring on unresolved spatial scales. This paper focuses on the parameterization of sub-grid hydrological processes within the ISBA land surface model. Five off-line simulations are performed over the French Rhône river basin, including various sets of parameterizations related to the sub-grid variability of topography, precipitation, maximum infiltration capacity and land surface properties. Parallel experiments are conducted at a high (8 km by 8 km) and low (1° by 1°) resolution, in order to test the robustness of the simulated water budget. Additional simulations are performed using the whole package of sub-grid parameterizations plus an exponential profile with depth of saturated hydraulic conductivity, in order to investigate the interaction between the vertical soil physics and the horizontal heterogeneities. All simulations are validated against a dense network of gauging measurements, after the simulated runoff is converted into discharge using the MODCOU river routing model. Generally speaking, the new version of ISBA, with both the sub-grid hydrology and the modified hydraulic conductivity, shows a better simulation of river discharge, as well as a weaker sensitivity to model resolution. The positive impact of each individual sub-grid parameterization on the simulated discharges is more obvious at the low resolution, whereas the high-resolution simulations are more sensitive to the exponential profile with depth of saturated hydraulic conductivity.     

极端强降水下小河流域泥石流灾害评估研究

基于马尔康红苕沟流域的DEM、地貌特征、土地利用类型、土壤类型和土壤湿度程度等地理信息数据和分钟降水资料,根据SCS模型的产流-汇流原理和泥石流配方法计算模拟了2015年6月30日红苕沟暴雨-泥石流灾害规模。模拟结果表明,与泥石流实地勘查资料相比较,SCS模型的模拟精度在95%以上,说明SCS模型径流模拟和泥石流配方法是可以应用于类似红苕沟这样无降水观测资料的小河流域泥石流灾害的评估方法。应用此方法计算红苕沟极端强降水触发的泥石流总量是2015年6月30日发生泥石流总量的5倍。      

Changes of time mean state and variability of hydrology in response to a doubling and quadrupling of CO2

This paper examines the subject of hydrologic variability and its changes in two separate integrations of a coupled ocean-atmosphere general circulation model developed at the Geophysical Fluid Dynamics Laboratory/NOAA assuming a 1% per year increase to a doubling and quadrupling of CO₂, respectively. Changes in time mean state and variability of precipitation, runoff and soil moisture are evaluated using monthly and seasonal mean data derived from these integrations. Various statistical tests are then performed on the resulting time mean and variability changes. The patterns of hydrologic change for these three quantities are similar to those obtained from previous studies. In northern middle to higher latitudes for the time means, the changes include increases in monthly mean precipitation, increases in monthly mean runoff during the fall, winter and spring seasons and decreases of monthly mean soil moisture during summer. Many of these changes are found to be statistically significant at the 5% significance level for both the time mean and variability especially for the results where CO₂ is quadrupled such as monthly mean precipitation. Significant changes also include increases of runoff variability during spring, winter and spring and increases of soil moisture variability during the summer season. These results support statements made in previous IPCC reports that increasing greenhouse gases can lead to more severe and frequent floods and droughts depending upon season and latitude. This study also indicates that the approaches to equilibrium of these two integrations, and the resulting hydrologic changes, take place over time scales of hundreds of years in agreement with several previous investigations.     

A non-hydrostatic modeling study of surface moisture effects on mesoscale convection induced by sea breeze circulation

Summary Convection and subsequent precipitation induced by the sea breeze circulations are often observed in the Florida peninsula during summer. In this study, the mechanisms of initiation and maintenance of the convective clouds and precipitation are investigated. A fully-compressible fine resolution non-hydrostatic mesoscale numerical model is used in this study. Surface energy and moisture budget were included in this model to simulate the diurnal cycle of ground surface temperature and wetness. The model also has a sophisticated boundary layer and explicit cloud physics. A sounding obtained from Orlando, Florida at 1110 UTC 17 July 1991 as part of the Convection and Precipitation Electrification (CaPE) experiment is used for initialization. The initial data for the model is kept in geostrophic and thermal wind balance. Several sensitivity tests were conducted to investigate the effects of different treatments of ground surface moisture and temperature on the model forecast of the convection and precipitation induced by the sea breeze circulations. The simulations agree reasonably well with the observations when both surface energy and moisture budget were included in the model to predict ground surface temperature and wetness. The surface moisture has a significant impact on the formation, strength, sustenance, and the location of convection and precipitation induced by the sea breezes.With 17 Figures