Adjoint sensitivity of the forecast to TOVS observations

Abstract

AbstractThe adjoint sensitivity to observations which is based on the adjoint operator of the variational assimilation process is used here on ten cases of the Fronts and Atlantic Storm‐Track EXperiment (FASTEX), conducted in January and February 1997. It is used as a diagnostic tool allowing one to indicate which TIROS‐N (Television Infrared Observation Satellite) Operational Vertical Sounder (TOVS) channels have an influence on the forecast of mid‐latitude lows. In order to study the effects of the observations on the modification of the forecast from the guess, a particular cost function has been chosen: the energy of the difference between the forecast derived from the guess and the one resulting from the analysis. The first part of the paper deals with the sensitivity to the assimilated TOVS observations for the intensive observation period 17 of FASTEX. Then, the influence of TOVS data is compared with one of the other conventional datasets assimilated at the same time. After that, these results are generalized in an overview of the ten studied cases. This study highlights that the Microwave Sounder Unit and clear sky or partly cloudy High‐resolution Infra‐Red Sounder have the larger influence of the TOVS observations on the modification of the forecast. However, the other conventional data have a larger absolute contribution than the TOVS ones. Copyright © 2002 Royal Meteorological Society.