Novel method of drizzle formation observation at large horizontal scales using multi-wavelength satellite imagery simulation

The observations of on-board satellite imaging radiometers are representative of a far-reaching two-dimensional cloud top properties, however with a cutback in the capacity of profiling the cloud vertically. A combination of simulated radiances calculated at the top of the cloud in the near-infrared (IR) and thermal infrared part of the spectra, is used as a proxy to estimate in-cloud droplet growth stage and ongoing precipitation intensity at the water cloud base. We present a drizzle observational technique that is built on simulated satellite imaging radiometry via the EarthCARE SIMulator (ECSIM). A period of 40 hours of the modeled cloud field evolution (using Dutch Atmospheric Large Eddy Simulator DALES) for the case study during the Atlantic Stratocumulus Transition Experiment campaign (ASTEX) is used to create a series of cloud scenes of a transitioning Sc into a Sc topped Cumulus (Cu) fields. Drizzle appears throughout the cloud evolution, evaporating on its way to the surface, depleting the cloud droplets at the cloud base. Longwave radiation model from ECSIM is applied to the ingested three-dimensional cloud scenes of a Stratocumulus evolution. The cloud top brightness temperatures are calculated using a three-dimensional, Monte Carlo, long-wave Radiative Transfer Model (RTM). The simulated Brightness Temperatures Difference (BTD) between the channels 3.9 and 11μm is then used to highlight the cloud top droplet size spatial variability, during the production of drizzle near the cloud base. The observed correlation of the BTD with the droplet size variability is used to interpret the conditions at which the precipitation at the cloud base is triggered or went through a change in the intensity. Tracking the process of evolution of the cloud droplet into a precipitating drop is likely due to the sensitivity of the 3.9μm channel to the particle size and cloud phase, near the cloud top. A comparison of the observed BTD with the vertically averaged cloud droplet size from the imported cloud scene is done. It is used to examine if a single pixel value of BTD from the cloud top (as retrieved via RTM) can be representative of the inhomogeneous microphysical vertical structure of a Sc deck and the potential precipitation intensity at the cloud base. A range in BTD for the two infra-red channels, between 0 and 2K is correlated to the presence of effective radius of the cloud droplets larger than 15μm, treated as drizzle drops in this paper.