Flying the TRMM satellite in a GCM:

By incorporating the Tropical Rainfall Measurement Mission (TRMM) satellite orbital information into the Colorado State University General Circulation Model (CSU GCM), we are able to "fly" a satellite in the GCM, and sample the simulated atmosphere in the same way as the TRMM satellite samples the real atmosphere. The TRMM sampling errors at seasonal-mean and seasonal-mean diurnal time scales can then be inferred by comparing the satellite-sampled and fully-sampled simulated atmospheres.

The TRMM orbit samples each 4degx5deg grid box in the tropics and subtropics about once per day, but at a different local time every day. The sampling errors in outgoing longwave radiation (OLR) monthly- and seasonal-means are less than 10 W m-2 (6%) for each 4degx5deg grid box. The OLR monthly- and seasonal-means are not sensitive to diurnal undersampling due to the TRMM orbit. However, this is not the case for total precipitation. Diurnal undersampling could produce errors as large as 18% in the Tropics and 30% in the subtropics, for the zonally averaged monthly-mean rain rates. Great care should be taken when applying rainfall error estimates derived from one region to others.

The TRMM orbit samples each 4ox5o grid box in the tropics and subtropics about 3 times for each hour of the day within a single season. The seasonal-mean diurnal cycles of precipitation and OLR are not well sampled for any one grid box. By either accumulating the satellite data for a long enough period, or averaging the data over a large area with a relatively uniform diurnal signal, the diurnal cycles of precipitation and OLR can be well sampled..

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Selected results