The Relationship Between Stratiform Clouds, Longwave Cloud–Radiation Forcing, and Column Moistening

Mounting evidence underscores the role of longwave cloud–radiation forcing (CRF) in accelerating the upscale growth of tropical convective systems, which manifests as accelerated tropical cyclone (TC) formation. This feedback is also hypothesized to be a key maintenance mechanism for the Madden–Julian Oscillation. Despite growing evidence for the importance of CRF in tropical convection and its upscale development, we neither understand i) the underlying mesoscale convective processes nor ii) the relative role of distinct cloud populations that drive this feedback. Here we leverage a novel microphysics-based cloud classification scheme applied to numerical model output to analyze the relative roles of distinct cloud populations in column radiative forcing and moistening. We leverage convection-permitting ensemble WRF simulations of Typhoon Haiyan (2013) and Hurricane Maria (2017) to do so, emphasizing the pre-genesis TC development period. In a separate presentation led by E. Luschen, we show that the most important drivers of radiative forcing on the scale of a convective cluster are stratiform and anvil clouds, due to both their strong local longwave cloud-heating and their large spatial coverage. Here we further examine the relationships between these cloud populations, their radiative forcing, and column moistening. We show that the strong radiative forcing due to stratiform and anvil clouds directly supports moistening both within the local column and at the scale of the overall convective system.