Abstract
Abstract. All-sky assimilation of infrared (IR) radiances has not yet become operational at any weather forecasting centre, but it promises to bring new observations in sensitive areas and avoid the need for cloud detection. A new all-sky IR configuration gives results comparable to (and in some areas better than) clear-sky assimilation of the same data, meaning that operational implementation is now feasible. The impact of seven upper-tropospheric water vapour (WV) sounding channels from the Infrared Atmospheric Sounding Interferometer (IASI) is evaluated in both all-sky and clear-sky approaches. All-sky radiative transfer simulations (and the forecast model's cloud fields) are now sufficiently accurate that systematic errors are comparable to those of clear-sky assimilation outside of a few difficult areas such as deep convection. All-sky assimilation brings 65 % more data than clear-sky assimilation globally, with the biggest increases in midlatitude storm tracks and tropical convective areas. However, all-sky gives slightly less weight to any one observation than in the clear-sky approach. In the midlatitudes, all-sky and clear-sky assimilation have similarly beneficial impact on mid- and upper-tropospheric dynamical forecast fields. Here the addition of data in cloudy areas is offset by the slightly lower weight given to the observations. But in the tropics, all-sky assimilation is significantly more beneficial than clear-sky assimilation, with improved dynamical short-range forecasts throughout the troposphere and stratosphere.
Highlights
Infrared (IR) radiances from geostationary and polar-orbiting satellites are widely assimilated at operational weather forecasting centres, mostly for their sensitivity to temperature and water vapour
The cloud overlap scheme has been documented by Matricardi (2005), where it is known as the “streams” method, but here we describe it as “multiple independent column” radiative transfer, which helps avoid confusion with the streams used in some scattering radiative transfer solvers
The seven water vapour (WV) channels from MetopA and Metop-B Infrared Atmospheric Sounding Interferometer (IASI) are activated in either clear-sky or allsky and these experiments are compared to a “No WV7” control that contains all other observations except the seven WV channels
Summary
Infrared (IR) radiances from geostationary and polar-orbiting satellites are widely assimilated at operational weather forecasting centres, mostly for their sensitivity to temperature and water vapour. Many forecasting centres use infrared observations above a diagnosed cloud top (e.g Pavelin et al, 2008; McNally, 2009; Pangaud et al, 2009; Lavanant et al, 2011) Some of these approaches assimilate channels with significant visibility of the cloud top but only if the cloud is overcast. The method of Pavelin et al (2008) assimilates scenes with fractional cloud but in practice only from channels with less than 10 % visibility of the cloud top, restricting the amount of data that can be gained Because these approaches hold the cloud information constant in the assimilation system, it is not possible to use cloud directly to improve the model initial conditions
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
