|Title||All-sky assimilation of microwave humidity sounders|
|Year of Publication||2014|
|Authors||Geer, A, Baordo, F, Bormann, N, English, SJ|
|Secondary Title||Technical Memorandum|
A main aim of humidity, cloud and precipitation assimilation in global weather forecasting is to infer winds and other dynamical variables directly in the data assimilation system. This 'model-tracing' effect helps improve dynamical initial conditions and leads to improved forecasts. Recent improvements in radiative transfer modelling mean it is now feasible to assimilate microwave humidity sounding observations in all sky conditions over a large part of the globe, adding observations in many of the most interesting meteorological areas. The next operational version of the ECMWF system will assimilate all-sky humidity sounding observations from four Microwave Humidity Sounders (MHS) and one Special Sensor Microwave Imager / Sounder (SSMIS). These observations give significant improvements to dynamical forecast scores in the medium range in the midlatitudes, along with benefits to wind and temperature fields around the tropical tropopause. Single observation test cases and single-observing system experiments help isolate the mechanism that improves forecasts in the midlatitudes: 4D variational assimilation can indeed infer dynamical initial conditions from the humidity, cloud and precipitation features in the radiance observations. The benefit is greatest in the southern midlatitudes, where the storm-tracks provide ideal conditions for model-tracing. Here, the impact of all-sky humidity channels on upper-tropospheric winds approaches that of the microwave temperature sounding instruments, which can use geostrophic balance to infer winds in the assimilation system. All-sky assimilation has roughly doubled the impact on forecast scores and observation fits compared to clear-sky assimilation. For example, clear-sky assimilation of microwave humidity sounding observations improves day 5 forecasts in the southern hemisphere by about 3/4 hour; all-sky assimilation by about 1 1/2 hours.