  In a joint effort, MeteoSwiss and Deutscher Wetterdienst (DWD) address the
need for improving the initial state of the atmospheric boundary layer (ABL) by
exploiting ground-based profiling observations that aim to fill the existing
observational gap in the ABL. We implemented the brightness temperature
observations from ground-based microwave radiometers (MWRs) into our data
assimilation systems using a local ensemble transform Kalman filter (LETKF)
with RTTOV-gb (Radiative Transfer for TOVS, ground-based) as a forward
operator. We were able to obtain a positive impact on the brightness
temperature first guess and analysis as well as a slight impact on the ABL
humidity using two MWRs at MeteoSwiss. These results led to a subsequent
operational implementation of the observing system at MeteoSwiss. Furthermore,
we performed an extensive set of assimilation experiments at DWD to further
investigate various aspects such as the vertical localisation of selected
single channels. We obtained a positive impact on the 6h-forecast of ABL
temperature and humidity by assimilating two channels employing a dynamical
localisation based on the sensitivity functions of RTTOV-gb but also with a
static localisation in a single-channel setup. Our experiments indicate the
importance of the vertical localisation when using more than one channel,
although reliable improvements are challenging to obtain without a larger
number of observations for both assimilation and verification.
