Koen De Ridder

Land surface processes and the potential for convective precipitation

The impact of the land surface on the potential for precipitating convection is examined. To do so, an analytical expression that relates boundary layer equivalent potential temperature to the surface evaporative fraction is derived by means of the slab model of the convective boundary layer. Analysis of the resulting expression shows that the equivalent potential temperature increases with the evaporative fraction under most circumstances, except for very dry atmospheres, and that this increase is enhanced in the presence of a low-level moist layer.

Heat wave hazard classification and risk assessment using artificial intelligence fuzzy logic

The average summer temperatures as well as the frequency and intensity of hot days and heat waves are expected to increase due to climate change. Motivated by this consequence, we propose a methodology to evaluate the monthly heat wave hazard and risk and its spatial distribution within large cities. A simple urban climate model with assimilated satellite-derived land surface temperature images was used to generate a historic database of urban air temperature fields. Heat wave hazard was then estimated from the analysis of these hourly air temperatures distributed at a 1-km grid over Athens, Greece, by identifying the areas that are more likely to suffer higher temperatures in the case of a heat wave event. Innovation lies in the artificial intelligence fuzzy logic model that was used to classify the heat waves from mild to extreme by taking into consideration their duration, intensity and time of occurrence. The monthly hazard was subsequently estimated as the cumulative effect from the individual heat waves that occurred at each grid cell during a month. Finally, monthly heat wave risk maps were produced integrating geospatial information on the population vulnerability to heat waves calculated from socio-economic variables.

QUANTITATIVE ESTIMATION OF SKIN SOIL MOISTURE WITH THE SPECIAL SENSOR MICROWAVE/IMAGER

A physically based retrieval scheme is described toinfer near surface moisture content of bare soilpatches from time series of the Special SensorMicrowave/Imager (SSM/I) Polarisation DifferenceTemperature (PDT) at 19.35 GHz. The PDT is modelled asa function of soil moisture using Fresnels relationsfor specular reflection at the air-soil boundary,together with a dielectric mixing model for themoisture-dependent soil dielectric constant. Theproblem of the specification of parameters describingatmospheric and soil roughness effects is avoided byconsidering temporal variations of the PDT. Theretrieval method is applied to the FIFE site for atwo-week period in summer, during which heavy rainfalloccurred on a dry upper soil. Although differences insensing depths made a rigorous validation impossible,remotely sensed soil moisture showed a high degree ofconsistency with ground measurements. An erroranalysis shows that the method is capable of providingan accuracy of approximately 20% of moisture contentat field capacity. Finally, it is demonstrated thatthe method is relatively insensitive to soil texturalcharacteristics when moisture content is scaled withfield capacity.A physically based retrieval scheme is described to infer near surface moisture content of bare soil patches from time series of the Special Sensor Microwave/Imager (SSM/I) Polarisation Difference Temperature (PDT) at 19.35 GHz. The PDT is modelled as a function of soil moisture using Fresnels relations for specular reflection at the air-soil boundary, together with a dielectric mixing model for the moisture-dependent soil dielectric constant. The problem of the specification of parameters describing atmospheric and soil roughness effects is avoided by considering temporal variations of the PDT. The retrieval method is applied to the FIFE site for a two-week period in summer, during which heavy rainfall occurred on a dry upper soil. Although differences in sensing depths made a rigorous validation impossible, remotely sensed soil moisture showed a high degree of consistency with ground measurements. An error analysis shows that the method is capable of providing an accuracy of approximately 20% of moisture content at field capacity. Finally, it is demonstrated that the method is relatively insensitive to soil textural characteristics when moisture content is scaled with field capacity.

The Impact of Vegetation Cover on Sahelian Drought Persistence

The sensitivity of the potential for convective precipitation to land surface characteristics in the West African Sudano-Sahelian zone is studied. Simulations are performed with a one-dimensional version of a mesoscale atmospheric model coupled to a detailed land surface model to investigate the influence of the land surface on rain infiltration, surface evapotranspiration, and their impact on the convective available potential energy. The simulations show that the presence of a densely vegetated surface acts as a catalyst in the hydrological cycle, creating a positive feedback and enhancing precipitation recycling. It is argued that this is due to the relation between the characteristic drying-out time of the soil and the return frequency of the rain-triggering African easterly waves.The sensitivity of the potential for convective precipitation to land surface characteristics in the West African Sudano-Sahelian zone is studied. Simulations are performed with a one-dimensional version of a mesoscale atmospheric model coupled to a detailed land surface model to investigate the influence of the land surface on rain infiltration, surface evapotranspiration, and their impact on the convective available potential energy. The simulations show that the presence of a densely vegetated surface acts as a catalyst in the hydrological cycle, creating a positive feedback and enhancing precipitation recycling. It is argued that this is due to the relation between the characteristic drying-out time of the soil and the return frequency of the rain-triggering African easterly waves.

Evaluation of satellite-derived products for the characterization of the urban thermal environment

Abstract. Knowledge of the air and land surface temperature and their temporal and spatial variations within a city environment is of prime importance to the study of urban climate and human–environment interactions and to monitoring environmental changes due to urbanization. We present a number of air and land surface temperature products that have been produced, archived, evaluated, and analyzed for 10 European cities within the framework of the European Space Agency–funded “Urban Heat Islands and Urban Thermography” project. We evaluate in what way these products are suited to explore the urban thermal dynamics and how products with different temporal and spatial resolution can provide a complementary view, both for thermal patterns as well as heat waves. Level of confidence was evaluated through quantitative, qualitative, and user-based analyses.

Comprehensive Parametrization of Surface-Layer Transfer Coefficients for Use in Atmospheric Numerical Models

A new non-iterative bulk parametrization for surface-layer transfer coefficients for momentum and heat is presented. It is applicable for a wide range of aerodynamic and thermal roughness lengths, and includes the effect of the roughness sublayer. As a consequence, the non-iterative method is suitable for every surface type, especially for urban surfaces for which existing non-iterative parametrizations fail. The analytical approximation compares very well with an iterative approach. Our method can be easily implemented in atmospheric numerical models that already employ a non-iterative approach.

Improved Algorithms for Advection and Vertical Diffusion in Aurora

Modules for advection and diffusion in AURORA were re-designed, resulting in a more accurate and faster version of the Eulerian Dispersion Model (EDM). In this new version, advection is treated by means of the recently developed algorithm by Walcek (2000), which is positive definite, mass conservative, accurate, and fast. The superior capacity of this scheme to preserve sharp concentration gradients was demonstrated. Vertical diffusion is treated with the well-established Crank-Nicholson scheme. The comparison of simulated concentration profiles with an analytical solution demonstrated the correct functioning of those schemes.

Air-quality modelling in the Lake Baikal region

In this paper, we assess the status of the air quality in the Lake Baikal region which is strongly influenced by the presence of anthropogenic pollution sources. We combined the local data, with global databases, remote sensing imagery and modelling tools. This approach allows to inventorise the air-polluting sources and to quantify the air-quality concentration levels in the Lake Baikal region to a reasonable level, despite the fact that local data are scarcely available. In the simulations, we focus on the month of July 2003, as for this period, validation data are available for a number of ground-based measurement stations within the Lake Baikal region.

Assessing Seasonality in the Surface Urban Heat Island of London

AbstractThis paper assesses the seasonality of the urban heat island (UHI) effect in the Greater London area (United Kingdom). Combining satellite-based observations and urban boundary layer climate modeling with the UrbClim model, the authors are able to address the seasonality of UHI intensity, on the basis of both land surface temperature (LST) and 2-m air temperature, for four individual times of the day (0130, 1030, 1330, and 2230 local time) and the daily means derived from them. An objective of this paper is to investigate whether the UHI intensities that are based on both quantities exhibit a similar hysteresis-like trajectory that is observed for LST when plotting the UHI intensity against the background temperature. The results show that the UrbClim model can satisfactorily reproduce both the observed urban–rural LSTs and 2-m air temperatures as well as their differences and the hysteresis in the surface UHI. The hysteresis-like seasonality is largely absent in both the observed and modeled 2-m ...

Remote sensing of parameters that regulate energy and water transfer at the land-atmosphere interface

Abstract The exchange of energy and water between the land surface and the atmosphere is dominated by two parameters: the fractional green vegetation cover, and the wetness of the bare soil patches. An account is given of the way satellite remote sensing can be employed to infer these parameters, and of their integration into a land surface model to simulate the surface energy and water balance. Fractional green vegetation cover is obtained by means of the AVHRR Normalized Difference Vegetation Index, adopting a simple linear relation between this vegetation index and fractional green vegetation cover. Soil moisture is retrieved using the SSM/I Polarization Difference Temperature, employing the dependence of the dielectric constant of the soil on water content. A demonstration is given of the implementation of these two parameters in a land surface model, running the latter for the temperate grassland area of the FIFE field experiment for a two-week period in summer. A comparison with observations shows that the error on the simulated energetic fluxes is of the same order as the estimated experimental error of ground-based flux measurements.