Jérôme Colin

Quantifying the impact of cloud cover on ground radiation flux measurements using hemispherical images

Linking observed or estimated ground incoming solar radiation with cloud coverage is difficult since the latter is usually poorly described in standard meteorological observation protocols. To investigate the benefits of detailed observation and characterization of cloud coverage and distribution, a fieldwork campaign has been set up in order to collect data about cloud cover conditions and daily evolution to directly analyse their impacts on solar radiation fluxes. To do so, daytime hemispherical images have been collected at a very high frequency, simultaneously to ground measurements of solar radiation fluxes in a scientific station close to Lake NamCo, China. After calibration, one of the main tasks was the classification of those hemispherical images and the extraction of meaningful indices to describe the cloud cover, such as cloud fraction or cloud cover distribution. The classification is based on automatic detection of threshold on the red channel histogram. The results show that several cloud indices could be successfully derived from the hemispherical images, even if very thin clouds can be difficult to detect. The indices are then correlated to the measured solar radiation values and the impact of cloud cover on surface radiation fluxes were analysed. This analysis highlights that, more than the cloud fraction, the cloud distribution in the hemisphere is of importance when modelling radiation fluxes in the solar domain.

Correction for the Impact of the Surface Characteristics on the Estimation of the Effective Emissivity at Fine Resolution in Urban Areas

Most of the methods used to retrieve land surface temperature (LST) from thermal infrared (TIR) satellite data in urban areas do not take into account the complexity of the surface. Cities are characterized by high surface roughness and one of the main constraints to estimate LST over those areas is the difficulty to define an effective emissivity for a given pixel at a given scale. When working with mixed pixels, the emissivity used to estimate the LST is an effective emissivity composed of the emissivities of each basic element constituting the pixel. In urban areas, the surface geometry has a strong impact on this effective emissivity. Its estimation from TIR satellite data must be carried out considering multiple surface reflections and diffusions within the urban canopy in order to retrieve accurate LST values. The objective of this study is then to evaluate the impact of the surface geometry within the pixel on effective emissivity estimation and to propose a method to derive an effective emissivity corrected for those effects. Emissivity can be derived at 90 m of spatial resolution from the TIR data acquired by ASTER. To evaluate the impact of the geometry at the scale of an ASTER pixel, several urban canyon configurations are designed to develop and test the correction method. The basic principle behind the method is to accurately estimate the downwelling TIR radiation received by a pixel integrating contributions from both the atmosphere and the scene inside this pixel and then derive the corrected effective emissivity from ASTER data using the TES (temperature emissivity separation) algorithm. First, the total downwelling TIR radiation is estimated from the geometric characteristics of the scene, using morphological indicators and integrating the non-isothermal behavior of the pixel thanks to 3D thermo-radiative model simulations. The validation of those estimations for each canyon configuration provides a maximum RMSE (Root Mean Square Error) value of 2.2 W·m−2. The validation performed over a district extracted from the 3D numerical model of Strasbourg (France) shows a RMSE of 2.5 W·m−2. Once the method to estimate the total downwelling TIR radiation is validated, LSE and LST maps are retrieved from an ASTER image over three districts of Strasbourg, showing that accounting for the surface geometry highlights thermal behavior differences inside districts, and that the impact of the geometry seems more influenced by building height than street width or building density.

Modélisation de l’îlot de chaleur urbain à Strasbourg

Simulation of the urban heat island at Strasbourg The mesoscale meteorological models include new surface schemes that have been design to simulate the urban behaviour. These schemes made them able to simulate the constraint exerted by the town on the low level layers of the atmosphere. When they are activated, they allow to study the behaviour of the urban boundary layer dynamics or the interaction that exists between the town and the rural areas. But, all these models have their own weaknesses and it is necessary to verify the quality of their results. It is the reason why, in a first step, it is necessary to do a simulation on a real case, for which validation data are available. The aim of this paper is to Modélisation de lîlot de chaleur urbain à Strasbourg 22 present a simulation of the urban heat island at Strasbourg (450 000 inhabitants). The simulation is done with the Méso-NH model of Météo-France and the Laboratoire d’Aérologie of the Paul Sabatier University (Toulouse). It is done for anticyclonic summer days for which the weather conditions permits the development of the phenomenon (low winds and maximum incoming radiation). The simulation extends on 5 days (from august 13 to 17, 2002). The first day (august 13) was still concerned by some clouds and is considered as an initialisation day. The simulation procedure consists to initialise the meteorological fields and to constrain them every 6 hours with the reanalysis of the European Prevision Centre (CEPMMT). The quality of the simulation was verified with data acquired during a measurement campaign that took place in 2002 in the frame of a research project called (RECLUS). The simulation results were compared to the measurements for some fundamental variables. On the whole, the results are satisfactory. Consequently, it is possible to use the simulation to investigate the urban climate better than with the measurements alone (especially with regard to the spatial information). After one simulation day, when the weather conditions become ideal (the august 14), the urban heat island is fully developed during all the nights. The simulation can be used to do an analysis of the processes and their consequences on the urban atmosphere. As a conclusion, this study confirms the usefulness to obtain realistic simulations of the urban climate. The results are helpful in fundamental research (to understand the mechanisms) and also in the field of the applied research (especially for the need of urban planning).