The data set contains daily files of microwave radiation measurements by the HATPRO microwave radiometer (see Rose et al., 2015, doi:doi:10.1016/j.atmosres.2004.12.005) during a surface-observation mode onboard the RV POLARSTERN during cruise PS131 (ATWAICE expedition, see Kanzow, 2023, doi:10.57738/BzPM_0770_2023). The instrument was installed at about 22 m height at the top deck on starboard site. Via a mirror construction the radiometers were observing the surface, i.e, ocean and sea ice, at a viewing angle of about 53° off nadir for 15 min each hour. The actual viewing angle could vary by a few degrees because of ship motion. Also, on six occasions the mirror position was changed to observe at different viewing angles. The data covers the range July 7, 2022 to August 12, 2022. The radiation measurements are given as brightness temperatures in seven K band channels (22.24 - 31.4 GHz), predominantly-vertical polarization at the viewing angle, and seven V band (51.26 - 58 GHz) channels, predominantly-horizontal polarization at the viewing angle. During the cruise the instrument was calibrated with liquid nitrogen on July 7 and July 30. The data is processed with the mwr_pro software (doi:10.5281/zenodo.7973552). Quality flags characterizing the instrument and retrieval performance are set and described. The brightness temperatures are provided for all available times so that it is up to the user to decide whether or not to use the values if quality flags are set. The sanity_receiver_band1, sanity_receiver_band2 and rain_flag are rather strict and data quality might still be satisfying despite the flag being set. However, we recommend to exclude data where visual_inspection_filter_band_1, visual_inspection_filter_band_2 or visual_inspection_filter_band_3 are set.

File descriptions: The data is uploaded as daily netCDF files following CF-1.6 convention. The dimensions and variables that are included in each files are:dimensions: time, nv, n_freq, n_freq2, n_wl_irpvariables(dimensions): time(time), time_bnds(time, nv), lat(time), lon(time), zsl(), freq_sb(n_freq), azi(time), ele(time), tb(time, n_freq), tb_bias_estimate(time, n_freq), freq_shift(n_freq), tb_absolute_accuracy(n_freq), tb_cov(n_freq2, n_freq), wl_irp(n_wl_irp), tb_irp(time, n_wl_irp), ele_irp(time), ta(time), pa(time), hur(time), flag(time), target_zenith(time)Here, we have two receivers (nv) with corresponding frequencies (n_freq and n_freq2)