Assimilation of Cloud and Rain-Affected Humidity Microwave Radiances in a Limited Area Model Over Arctic

This paper describes the work to implement and study the ECMWF all-sky radiance assimilation method in a regional numerical weather prediction model over the Arctic. The implementation is focusing on the humidity-sensitive microwave radiances from MHS sensors on NOAA and METOP satellites. To activate the all-sky observation operator RTTOV-SCATT, the hydrometeor variables from Harmonie-Arome forecasts are added in the interface, as well as the cloud fraction to calculate the contributions from clear and cloudy brightness temperature. Since the hydrometeors are absent in the control variables, they are set to zero in the minimization. The low-level observations from channel 5 are also assimilated, but only over sea-ice-free ocean. A non-cycling study to build the statistics for observation minus background statistics shows promising results for the distribution of model all-sky counterparts against satellite observations, where the density of all-sky cold temperatures fit better with observations than the clear-sky counterparts. The cycling experiments show that the impact of all-sky observation on upper-air forecasts is positive for the verifications against radiosonde observations for humidity fields, while it is neutral for temperature and wind. The outstanding result from this study is that the impact of all-sky assimilation is significantly positive on surface verifications over land, especially for humidity forecasts as well as surface pressure. This result is also confirmed over the ocean, where the fit to scatterometer 10m wind observations is improved in the all-sky experiments. Finally, we carried out two case studies for a period with two polar lows and a period with a large-scale low pressure system activity. The verifications against synop observations over land show that the positive impact of all-sky assimilation is largest for the case with the low pressure system, but is neutral for the polar low case.