A method to estimate clear sky mesoscale vertical motion from geostationary satellite imagery

Mesoscale vertical velocity is a key element to understand links between clouds, radiation and circulation. However, in situ measurements of this variable remain sparse and costly.

Here, we present a method to estimate clear-sky free tropospheric mesoscale vertical motion from rapid-scan geostationary satellite radiance measurements in the water vapour absorption band. Subsidence is indeed associated with drying and a decrease of emission level height (and conversely for ascendance). Under basic physical assumptions, the associated temporal changes in brightness temperature can be quantitatively related to vertical velocity.

The retrieval method is evaluated against in situ observations from EUREC4A and OTREC field campaigns that sampled respectively the winter trades and the intertropical convergence zone. Although suffering from significant error bars (+/-4hPa/hr), retrievals are able to reproduce the general temporal evolution and spatial patterns of mid-tropospheric mesoscale vertical motion.

The retrieval method is further evaluated using kilometer-scale simulations associated with a radiative transfer code. Basic climatological features are captured such as the distribution of mesoscale vertical velocity or its influence on cloud cover.

Despite notable drawbacks, the method is able to provide time-continuous estimations of vertical velocity in clear sky regions of the whole tropical belt at the scale of the pixel of the satellite imager. First results suggest that mesoscale (20-200km) vertical velocity structures are ubiquitous in the tropical free troposphere.

These observations could prove valuable for studying dynamical links between deep convection and its environment, especially in association with the new generation of satellites, that will provide measurements of in-cloud convective mass flux and clear-sky time-continuous water vapour and temperature profiles.