Remote Sensing and Radiative Forcing of Inhomogeneous Trade-Wind Cumuli

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The core objective of the project is to quantify the large-scale radiative forcing of shallow trade-wind cumuli as a function of the cloud macrophysical and microphysical properties, the cloud spatial organization, and the mesoscale vertical motion. We will combine macrophysical, microphysical, and radiative properties of trade-wind cumuli obtained from airborne remote sensing cloud observations and in situ irradiance measurements aboard the High Altitude and Long Range Research Aircraft (HALO) during the ElUcidating the Role of Cloud-Circulation Coupling in ClimAte (EUREC4A) campaign east of Barbados in February 2020.To retrieve the relevant cloud and radiation data, we will extend the instrumentation of HALO by (i) a multi-wavelength thermal infrared (IR) imager, and (ii) pairs of upward and downward looking, hemispheric broadband pyranometers and pyrgeometers. These broadband radiometers will provide solar and terrestrial irradiance measurements to quantify the atmospheric radiation budget at flight level. The thermal IR imager will map the cloud top brightness temperatures at different thermal IR spectral bands with high spatial (5 m) and temporal (20 Hz) resolution. The instruments were not operated on HALO yet. Therefore, a crucial part of the proposed work plan is related to extensive tests and calibrations of the new instruments and developing tools for handling and post processing the data.The thermal IR imager will be used to develop an IR-based cloud product, providing maps of cloud top temperature, cloud liquid water path and cloud effective droplet size. The maps will be analysed statistically to obtain the cloud fraction, degree of clustering, and cloud size distributions. The data will be correlated with atmospheric parameters (temperature/humidity profiles, background aerosol, large-scale divergences). The observations of the broadband radiometers will be analysed in combination with the maps of cloud properties derived from the thermal IR imager. Quantifying the cloud radiative forcing for scenes of trade-wind cumuli with different cloud fraction, degree of clustering, and cloud top temperatures will indicate how sensitive the cloud radiative forcing is with respect to the macroscopic properties and organization of trade-wind cumuli. Parameterizing this sensitivity provides a tool to evaluate the representation of trade-wind cumuli in numerical weather prediction models and global climate models.