The capability of 4D‐Var systems to assimilate cloud‐affected satellite infrared radiances

Abstract

SUMMARYFour-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.

Experimental 1D + 4D‐Var assimilation of CloudSat observations

“…An attempt at exploiting visible and infrared cloudy satellite radiances in four-dimensional variational assimilation (4D-Var) has been made by Vukićević et al (2004) using a mesoscale model. On the global scale, the capability of 4D-Var assimilation systems to assimilate cloud-affected satellite infrared radiances using observations from the narrow-band Advanced Infrared Sounder (AIRS) has been investigated by Chevallier et al (2004). Benedetti and Janisková (2008) performed some experiments for assimilation of cloud optical depths retrieved from the Moderate Resolution Imaging Spectroradiometer (MODIS) in the European Centre for Medium-Range Weather Forecasts (ECMWF) 4D-Var system.…”

Assimilating humidity pseudo‐observations derived from the cloud profiling radar aboard CloudSat in ALADIN 3D‐Var

“…Hence, assimilation systems often do not exploit satellite observations in cloudy conditions, that are all the more important for correctly forecasting severe weather events (McNally, 2002). Several attempts are currently being made to address the RRTMs limits in cloudy conditions (see, for instance, Chevallier et al, 2004).…”

Analysis and forecast impact of the main humidity observing systems

“…In the tropics, the supply of low-level humidity affects the intensity of the tropical convection, and hence the intensity of the Hadley circulation. ECMWF's current development programme for radiance assimilation in cloudy and precipitating conditions (Marécal and Mahfouf, 2003;Chevallier et al, 2004;Moreau et al, 2004;Andersson et al, 2005;Bauer et al, 2006a, b) relies on an accurate assimilation procedure for humidity.…”