Liquid water path derived from passive microwave measurements over open ocean during the HALO-(AC)³ Campaign in Spring 2022

This data set contains files of vertically integrated cloud liquid water (liquid water path (LWP)) between the surface and the High Altitude and LOng range (HALO) research aircraft for each research flight of the HALO-(AC)³ campaign carried out in March and April 2022. The LWP has been retrieved from the brightness temperature measurements of the microwave radiometers (passive part) of the HALO microwave package (HAMP). The microwave radiometers measure radiation emitted by the surface, atmospheric gases (mainly water vapour and oxygen) and liquid hydrometeors at 25 channels in the frequency range between 22 and 190 GHz. Due to the uncertainties of the emitted signals of ice-covered parts of the ocean the retrieval is confined to open ocean segments of each research flight, covering the North Atlantic sector of the Arctic between Norway, Svalbard and Greenland. For retrieving LWP, we designed a Multilayer Perceptron Neural Network that has been trained with 12 years of ERA5 data randomly selected between 1 March and 30 April 2001 and 2020 at 15 grid points in the ice-free parts of the Fram Strait. The ERA5 data has been forward simulated with the Passive and Active Microwave radiative TRAnsfer tool (PAMTRA, Mech et al. 2019, doi:10.5194/amt-12-5019-2019) with artificial noise to obtain synthetic HAMP measurements. To avoid biases of LWP, we applied a clear-sky offset correction: Clear-sky periods are identified according to 1) HAMP radar reflectivity thresholds (at least 5 range bins with reflectivity factor Ze > -40 dBZ), 2) 30-second-maxima of the standard deviation of 30-second rolling mean brightness temperatures of the HAMP microwave radiometers must be < 0.5 K for the frequencies between 22 and 130 GHz. We also verified clear-sky periods using the specMACS images (https://doi.org/10.1594/PANGAEA.965546). Clear sky periods must be longer than 30 seconds to be used as base for the LWP clear sky offset correction. After having identified the clear-sky periods, a 30-second rolling mean of LWP is computed. The values the rolling mean LWP takes during the clear-sky periods are used as clear-sky offset. In between clear-sky periods, the LWP offset is determined by linear interpolation if the time between clear-sky periods is less than 6 hours.