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- Author or Editor: K. Laval x
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Abstract
We have investigated with our general circulation model the effect of an increase of surface albedo over the Sahel on climate. When albedo increases, the precipitation and evaporation rates are lower. Our experiment also has shown variations of zonal circulation over the Sahel: the Tropical Easterly Jet is stronger and easterlies at low altitude are weaker when the precipitation rate is higher. These results, in agreement with observations of the changes of circulation between dry and wet years, suggest that the albedo can play an important role in the triggering or the maintenance of a drought in the Sahel through the effects of circulation.
Abstract
We have investigated with our general circulation model the effect of an increase of surface albedo over the Sahel on climate. When albedo increases, the precipitation and evaporation rates are lower. Our experiment also has shown variations of zonal circulation over the Sahel: the Tropical Easterly Jet is stronger and easterlies at low altitude are weaker when the precipitation rate is higher. These results, in agreement with observations of the changes of circulation between dry and wet years, suggest that the albedo can play an important role in the triggering or the maintenance of a drought in the Sahel through the effects of circulation.
Abstract
The monsoon depressions that form over India during the summer are analyzed using simulations from the Laboratoire de Météorologie Dynamique general circulation model. This type of synoptic system often occurs with a frequency of one to two per month and can produce a strong Indian rainfall. Two kinds of analyses are conducted in this study. The first one is a subjective analysis based on the evolution of the precipitation rate and the pattern of the sea level pressure. The second one is an objective analysis performed using the TRACK program, which identifies and tracks the minima in the sea level pressure anomaly field and computes the statistics for the distribution of systems.
The analysis of a 9-yr control run, which simulates strong precipitation rates over the foothills of the Himalayas and over southern India but weak rates over central India, shows that the number of disturbances is too low and that they almost never occur during August, when break conditions prevail. The generated disturbances more often move north, toward the foothills of the Himalayas. Another analysis is performed to study the effect of the Tibetan Plateau elevation on these disturbances with a 9-yr run carried out with a Tibetan Plateau at 50% of its current height. It is shown that this later integration simulates more frequent monsoon disturbances, which move rather northwestward, in agreement with the current observations. The comparison between the two runs shows that the June–July–August rainfall difference is in large part due to changes in the occurrence of the monsoon disturbances.
Abstract
The monsoon depressions that form over India during the summer are analyzed using simulations from the Laboratoire de Météorologie Dynamique general circulation model. This type of synoptic system often occurs with a frequency of one to two per month and can produce a strong Indian rainfall. Two kinds of analyses are conducted in this study. The first one is a subjective analysis based on the evolution of the precipitation rate and the pattern of the sea level pressure. The second one is an objective analysis performed using the TRACK program, which identifies and tracks the minima in the sea level pressure anomaly field and computes the statistics for the distribution of systems.
The analysis of a 9-yr control run, which simulates strong precipitation rates over the foothills of the Himalayas and over southern India but weak rates over central India, shows that the number of disturbances is too low and that they almost never occur during August, when break conditions prevail. The generated disturbances more often move north, toward the foothills of the Himalayas. Another analysis is performed to study the effect of the Tibetan Plateau elevation on these disturbances with a 9-yr run carried out with a Tibetan Plateau at 50% of its current height. It is shown that this later integration simulates more frequent monsoon disturbances, which move rather northwestward, in agreement with the current observations. The comparison between the two runs shows that the June–July–August rainfall difference is in large part due to changes in the occurrence of the monsoon disturbances.
Abstract
In the broad context of the downscaling methods that are used to study climatic change impacts, the dependence of the surface hydrological processes simulated by the Organising Carbon and Hydrology in Dynamic Ecosystem (ORCHIDEE) land surface model, used in a stand-alone mode, on the spatial scale of the forcings is investigated over the Iberian Peninsula. These prescribed forcings are the outputs of a regional climate model, Pronóstico a Mesoescala (PROMES), with a high spatial resolution (20 km). In the first experiment, the PROMES outputs have been aggregated stepwise to the typical resolution of a general circulation model, and applied to ORCHIDEE, in order to analyze the impacts of the changing resolution on the simulated water balance. Then, subgrid-scale variability (SSV) for the different forcings has been progressively reintroduced. This second experiment is aimed at isolating the crucial elements of SSV that need to be preserved when a disaggregation is being performed.
The increase of interception loss when the spatial resolution goes beyond 100 km leads to unrealistic values of the interception loss ratio. In the northern humid region, the reduction of runoff frequency when the forcings are aggregated explains the decrease in runoff production, which can reach half the high-resolution runoff. These impacts drive the adjustment of the other hydrological components. The large increase of interception loss is compensated by a reduced transpiration in a dry climate, which induces a large change in soil moisture content, and by reduced runoff in humid regions. The second experiment underlines the dominant effect of precipitation SSV, and particularly the rainfall frequency, on the correct simulation of the water balance. The significant influence of the thermodynamic variables is also analyzed.
Abstract
In the broad context of the downscaling methods that are used to study climatic change impacts, the dependence of the surface hydrological processes simulated by the Organising Carbon and Hydrology in Dynamic Ecosystem (ORCHIDEE) land surface model, used in a stand-alone mode, on the spatial scale of the forcings is investigated over the Iberian Peninsula. These prescribed forcings are the outputs of a regional climate model, Pronóstico a Mesoescala (PROMES), with a high spatial resolution (20 km). In the first experiment, the PROMES outputs have been aggregated stepwise to the typical resolution of a general circulation model, and applied to ORCHIDEE, in order to analyze the impacts of the changing resolution on the simulated water balance. Then, subgrid-scale variability (SSV) for the different forcings has been progressively reintroduced. This second experiment is aimed at isolating the crucial elements of SSV that need to be preserved when a disaggregation is being performed.
The increase of interception loss when the spatial resolution goes beyond 100 km leads to unrealistic values of the interception loss ratio. In the northern humid region, the reduction of runoff frequency when the forcings are aggregated explains the decrease in runoff production, which can reach half the high-resolution runoff. These impacts drive the adjustment of the other hydrological components. The large increase of interception loss is compensated by a reduced transpiration in a dry climate, which induces a large change in soil moisture content, and by reduced runoff in humid regions. The second experiment underlines the dominant effect of precipitation SSV, and particularly the rainfall frequency, on the correct simulation of the water balance. The significant influence of the thermodynamic variables is also analyzed.
Abstract
In the Project for Intercomparison of Land-Surface Parameterization Schemes phase 2a experiment, meteorological data for the year 1987 from Cabauw, the Netherlands, were used as inputs to 23 land-surface flux schemes designed for use in climate and weather models. Schemes were evaluated by comparing their outputs with long-term measurements of surface sensible heat fluxes into the atmosphere and the ground, and of upward longwave radiation and total net radiative fluxes, and also comparing them with latent heat fluxes derived from a surface energy balance. Tuning of schemes by use of the observed flux data was not permitted. On an annual basis, the predicted surface radiative temperature exhibits a range of 2 K across schemes, consistent with the range of about 10 W m−2 in predicted surface net radiation. Most modeled values of monthly net radiation differ from the observations by less than the estimated maximum monthly observational error (±10 W m−2). However, modeled radiative surface temperature appears to have a systematic positive bias in most schemes; this might be explained by an error in assumed emissivity and by models’ neglect of canopy thermal heterogeneity. Annual means of sensible and latent heat fluxes, into which net radiation is partitioned, have ranges across schemes of30 W m−2 and 25 W m−2, respectively. Annual totals of evapotranspiration and runoff, into which the precipitation is partitioned, both have ranges of 315 mm. These ranges in annual heat and water fluxes were approximately halved upon exclusion of the three schemes that have no stomatal resistance under non-water-stressed conditions. Many schemes tend to underestimate latent heat flux and overestimate sensible heat flux in summer, with a reverse tendency in winter. For six schemes, root-mean-square deviations of predictions from monthly observations are less than the estimated upper bounds on observation errors (5 W m−2 for sensible heat flux and 10 W m−2 for latent heat flux). Actual runoff at the site is believed to be dominated by vertical drainage to groundwater, but several schemes produced significant amounts of runoff as overland flow or interflow. There is a range across schemes of 184 mm (40% of total pore volume) in the simulated annual mean root-zone soil moisture. Unfortunately, no measurements of soil moisture were available for model evaluation. A theoretical analysis suggested that differences in boundary conditions used in various schemes are not sufficient to explain the large variance in soil moisture. However, many of the extreme values of soil moisture could be explained in terms of the particulars of experimental setup or excessive evapotranspiration.
Abstract
In the Project for Intercomparison of Land-Surface Parameterization Schemes phase 2a experiment, meteorological data for the year 1987 from Cabauw, the Netherlands, were used as inputs to 23 land-surface flux schemes designed for use in climate and weather models. Schemes were evaluated by comparing their outputs with long-term measurements of surface sensible heat fluxes into the atmosphere and the ground, and of upward longwave radiation and total net radiative fluxes, and also comparing them with latent heat fluxes derived from a surface energy balance. Tuning of schemes by use of the observed flux data was not permitted. On an annual basis, the predicted surface radiative temperature exhibits a range of 2 K across schemes, consistent with the range of about 10 W m−2 in predicted surface net radiation. Most modeled values of monthly net radiation differ from the observations by less than the estimated maximum monthly observational error (±10 W m−2). However, modeled radiative surface temperature appears to have a systematic positive bias in most schemes; this might be explained by an error in assumed emissivity and by models’ neglect of canopy thermal heterogeneity. Annual means of sensible and latent heat fluxes, into which net radiation is partitioned, have ranges across schemes of30 W m−2 and 25 W m−2, respectively. Annual totals of evapotranspiration and runoff, into which the precipitation is partitioned, both have ranges of 315 mm. These ranges in annual heat and water fluxes were approximately halved upon exclusion of the three schemes that have no stomatal resistance under non-water-stressed conditions. Many schemes tend to underestimate latent heat flux and overestimate sensible heat flux in summer, with a reverse tendency in winter. For six schemes, root-mean-square deviations of predictions from monthly observations are less than the estimated upper bounds on observation errors (5 W m−2 for sensible heat flux and 10 W m−2 for latent heat flux). Actual runoff at the site is believed to be dominated by vertical drainage to groundwater, but several schemes produced significant amounts of runoff as overland flow or interflow. There is a range across schemes of 184 mm (40% of total pore volume) in the simulated annual mean root-zone soil moisture. Unfortunately, no measurements of soil moisture were available for model evaluation. A theoretical analysis suggested that differences in boundary conditions used in various schemes are not sufficient to explain the large variance in soil moisture. However, many of the extreme values of soil moisture could be explained in terms of the particulars of experimental setup or excessive evapotranspiration.
