Why does the RR not assimilate surface temperatures over land?
Land surface temperature observations cannot be assimilated successfully in the 3DVAR scheme used for the regional reanalysis. The background error contains implicit synoptic scale balance information, but knows nothing about boundary layer dynamics. Because of large diurnal variation of temperature over land, which exists at any time of year, observation – background residuals are regularly quite large. The balance relationship in the 3DVAR erroneously connects these large surface residuals to the wind field, which has a relatively large vertical correlation length (around 250-300mb). As a result, the large surface residuals have an undesirable impact on lower-tropospheric temperatures and lower- to mid-tropospheric winds, instead of being limited to the boundary layer. Newer versions of 3DVAR with more sophisticated background error definition will eventually eliminate this problem, but no mature version existed at the time the reanalysis was started.
