Towards Retrieving Cloud Top Entrainment Velocities from MISR Cloud Motion Vectors

Abstract. Although important, direct retrievals of entrainment rates in cloud-topped planetary boundary layer (PBL) remain elusive. Here we present a novel technique for retrieving cloud-top entrainment velocities using only Multi-angle Imaging Spectro-Radiometer (MISR) stereoscopic retrievals of cloud-motion vectors (CMVs) and cloud-top heights (CTHs). Mesoscale vertical air velocity at CTH is diagnosed from the continuity equation and then used to derive entrainment velocities from the PBL mass-budget equation. The uncertainties in the utilized CTHs and CMVs are propagated to derive systematic and random retrieval uncertainties. The algorithm is demonstrated through a case of marine stratocumulus deck off the California coast, with comparisons made against the output from European Center for Medium-range Weather Forecasting (ECMWF) reanalysis model (ERA5). MISR low-cloud CTH for this case were lower than the ERA5 reported PBL depth by 189 ± 87 m. These differences in cloud top heights partly modulate the differences in the ERA5 and MISR horizontal winds, with larger differences in meridional over zonal wind components. Average difference between ERA5 and MISR derived mesoscale vertical air motion at cloud top was 0.140.73 cm s-1, while the same for entrainment rate was -0.090.46 cm s-1. Fractional uncertainty is lower than 25 % when the retrieved mesoscale vertical air motion is stronger than ±0.04 cm s-1 and entrainment velocities are stronger than 0.03 cm s-1. These results showcase the ability to derive mesoscale vertical air motion and entrainment rates from MISR observations and motivate its extension to a generate a global climatology leveraging its full 23-year record (2000–2022).