Abstract
Geostationary Operational Environmental Satellite (GOES)-East and -West window channel radiances are directly assimilated using a 1D variational technique, providing surface skin temperature (Ts) estimates over all surface types (land, water, or ice) from a unique system. This is an important advantage over commonly used regression methods, such as split window. The physical nature of the method allows any combination of channels to be used, and adaptation to new sensors is straightforward. A full month (May 2001) of GOES-8 and -10 data is processed every 6 h; Ts estimates are obtained using radiances from imager channels 4 (11 μm) and 5 (12 μm). Imager channel 2 (3.9 μm) can also be used at night. Surface emissivity maps were constructed from available information based on surface type. The diurnal cycle is studied; its range is on the order of 0.7 K over the ocean. Over land, the diurnal range reaches 30 K for mountainous regions, such as the Rockies or Andes. A full GOES disk image can be processed in 4 min. The resulting retrieval error can be estimated locally. It is usually in the range of 0.5–1.0 K over ocean and 0.9–2.4 K over land. Differences between collocated GOES-8 and -10 retrievals are examined. These are as low as 0.23-K rms over ocean; over land they are in the range of 1.4–2.2-K rms with higher values in midafternoon because of higher emission anisotropy and local variability. The model background (6-h forecast) is found to underestimate the diurnal cycle over land by nearly 50%. A validation is done over land using surface data of upward broadband longwave radiation converted into equivalent skin temperature. These data confirm the range of the diurnal cycle and the model biases inferred from satellite data. Over oceans, the agreement between retrievals and ship, fixed-buoy, and drifting-buoy observations is 1.05, 0.90, and 0.65 K, respectively.
Corresponding author address: Louis Garand, Meteorological Service of Canada, Data Assimilation and Satellite Meteorology Division, 2121 Trans-Canada Highway, Dorval, QC H9P1J3, Canada. louis.garand@ec.gc.ca
