| Summary: | Shallow trade wind clouds pose one of the largest uncertainties in climate models. Due to the difficulties in assessing these clouds with routine observations the next-generation aircraft remote-sensing for validation campaign with the German High Altitude and LOng range research aircraft (HALO) took place in December 2013. Here we take advantage of the synergy of the HALO active and passive microwave package as well as spectrally resolved solar radiation (SR) measured by HALO-SR to characterize shallow clouds in the Caribbean. Based on a cloud mask developed from HALO-SR, about 12 000 cloudy profiles within ~4100 individual clouds could be detected with about 70 % of them having a length of less than 2 km. Corresponding measurements with passive microwave measurements reveal that these small clouds also contain little water with 36% of the clouds showing a liquid water path (LWP) of less than 50g · m<sup>-2</sup>. We show that these small and thin clouds are difficult to characterize with satellite observations by the special sensor microwave imager/sounder due to its coarse resolution. Moderate imaging spectroradiometer measurements are able to identify the smaller clouds but suffer in terms of LWP when clouds start precipitating, which is the case for about 7% of the clouds as detected by the airborne 35 GHz radar.
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