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The capability of 4D‐Var systems to assimilate cloud‐affected satellite infrared radiances
Author(s) -
Chevallier Fr'ed'eric,
Lopez Philippe,
Tompkins Adrian M.,
Janisková Marta,
Moreau Emmanuel
Publication year - 2004
Publication title -
quarterly journal of the royal meteorological society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.744
H-Index - 143
eISSN - 1477-870X
pISSN - 0035-9009
DOI - 10.1256/qj.03.113
Subject(s) - satellite , radiosonde , geostationary orbit , cloud computing , radiance , meteorology , environmental science , cloud cover , remote sensing , data assimilation , infrared , atmospheric infrared sounder , numerical weather prediction , geostationary operational environmental satellite , troposphere , computer science , geology , physics , astronomy , optics , operating system
Four‐dimensional variational (4D‐Var) assimilation schemes assume the linearity of their forward model in the vicinity of prior information and usually do not properly handle variables that have finer temporal and spatial scales in the real world than in the forward model. Hence cloud‐affected satellite infrared radiances are discarded from numerical weather‐prediction 4D‐Var systems, despite the critical need of observations within the cloudy regions. This paper suggests the reappraisal of that choice, subject to achieving improvements in the numerical simulation of cloudiness. A new observation operator, that computes cloud‐affected infrared radiances from 4D‐Var control variables, namely atmospheric temperature, humidity, ozone, surface temperature and surface pressure, is presented. The vertical distributions of cloud cover and of cloud condensate are diagnosed in the operator itself. The goal of this paper is to assess the feasibility of using it to assimilate cloud‐affected infrared radiances, such as those from the narrow‐band Advanced Infrared Sounder on‐board the Aqua platform or those from the broad‐band Meteosat Visible and Infrared Imager. It is shown that there is a potential benefit in assimilating directly in 4D‐Var some of the upper‐tropospheric channels at 4.5, 6.3 and 14.3 µm in the presence of clouds, for instance the 6.3 µm channel on board all the geostationary satellites. The approach is illustrated with one‐dimensional variational retrievals collocated with radiosonde observations. © Royal Meteorological Society, 2004.