Quantifying cloud base updraft speeds of marine stratocumulus from cloud top radiative cooling

Marine stratocumulus clouds play a significant role in the Earth’s radiation budget. The updrafts at cloud base (Wb) govern the supersaturation and therefore the activation of cloud condensation nuclei, which modifies the cloud and precipitation properties. A statistically significant relationship betweenWb and cloud top radiative cooling rate (CTRC) is found from the measurements of the Department of Energy’s Atmospheric Radiation Measurement Mobile Facility on board a ship sailing between Honolulu and Los Angeles. A similar relation was found on Graciosa Island but with greater scatter and weaker correlation presumably due to the island effect. Based on the relation, we are able to estimate the cloud base updrafts using a simple formula: Wb= 0.44 × CTRC+ 22.30 ± 13, where the Wb and CTRC have units of cm/s and W/m, respectively. This quantification can be utilized in satellite remote sensing and parameterizations ofWb in general circulation models.