Implementation of All-sky Assimilation of Microwave Humidity Sounding Channels in Environment Canada’s Global Deterministic Weather Prediction System

Cloud-affected microwave humidity sounding radiances were excluded from assimilation in 4D-EnVar system of the Global Deterministic Prediction System (GDPS) at Environment and Climate Change Canada (ECCC). This was due to the inability of the current radiative transfer model to consider the scattering effect from frozen hydrometeors at these frequencies. In addition to upgrading the observation operator to RTTOV-SCATT, quality control, bias correction, and 4D-EnVar assimilation components are modified to perform all-sky assimilation of Microwave Humidity Sounder (MHS) channels 2-5 observations over ocean in the GDPS. The input profiles to RTTOV-SCATT are extended to include liquid cloud, ice cloud, and cloud fraction profiles for the simulation and assimilation of MHS observations over water. There is a maximum 35% increase in number of channel 2 assimilated MHS observations with smaller increases for channels 3-5 in the all-sky compared to the clear-sky experiment, mostly because of newly assimilated cloud-affected observations. The stddev of difference between the observed GPSRO refractivity observations and the corresponding simulated values using the background state was reduced in lower troposphere below 9 km in the all-sky experiment. Verifications of forecasts against the radiosonde observations show statistically significant reductions of 1% in stddev of error for geopotential height, temperature, and horizontal wind for all-sky experiment between 72- and 120- hr forecast ranges in troposphere in Northern Hemisphere domain. Verifications of forecasts against ECMWF analyses also show small improvements in zonal mean of error stddev for temperature and horizontal wind for all-sky experiment between 72- and 120-hr forecast ranges. This work is planned for operational implementation in the GDPS in Fall 2023.