Meteorological data are to be collected for surface measurements of wind speed and direction, vector-averaged wind speed, temperature, barometric pressure, relative humidity, vapor pressure and total precipitation. Vertical profiles of temperature, relative humidity, pressure, wind direction and speed will also be measured.
Radiometric data will involve measurements of spectrally resolved atmospheric infrared irradiance; direct, total horizontal and diffuse horizontal components of solar spectral and broadband irradiance; and spectral upwelling and downwelling of solar and longwave irradiances at the surface and at 10 and 25-m towers. These measurements will be used to calculate the surface albedo.
Surface flux and surface characterization data include measurements of latent and sensible heat fluxes, net radiation flux to the surface, land use/land cover and soil temperature. Water vapor data include atmospheric (column-integrated precipitable and liquid water) and soil moisture.
Some of these data can be collected using ground-based stations. These typically consist of various monitors mounted on towers. However, the limited coverage of fixed stations reduces the confidence in applying these results to larger areas. They are also relatively expensive to set up and maintain, especially in a hostile environment like the desert. For this reason, unmanned aerial vehicles (UAVs) will be used to collect information over a much wider area. Satellite monitoring using existing satellites will complete the coverage and serve as a backup.
Historically, satellite monitoring of the surface has been hampered by the fact that satellites have to look down through more than a hundred miles of atmosphere and thus, surface albedos as well as other radiometric measurements can be subject to interference by dust particles and clouds. There is little that can be done about this problem other than to carefully compare the data with that from observations taken closer to the surface, i.e., by ground stations or UAVs. The problem here is that most satellite sensors have a spatial resolution of 1 Km for meteorological satellites to tens of Kms for meteorological research satellites. While this may not be important for some models, which require degradation to 250 Km grids (147), it is a serious problem when trying to compare data with that from a few fixed stations.
The recently launched series of satellites that make up the Earth Observation System seems to have solved the resolution problem (148). Terra, launched in 1999, includes among its instrument package MODIS, the Moderate-Resolution Imaging Spectroradiometer which can measure a 2300 Km wide section of reflected solar radiation with a resolution of 250 m, while ASTER, the Advanced Spaceborne Thermal Emission and Reflectance Radiometer can resolve objects as small as 15-90 m (149).
Because these satellites may be dedicated to certain areas, the Km resolution of meteorological satellites may have to be sufficient to verify ground-station measurements. For example, Meteosat has produced surface albedos for the Sahara and Sahel with a resolution of 4 Km every 10 days (118). Regardless, due to the size of the coverage areas, satellite monitoring will be a key element.
