What are the causes of tropical cirrus longwave biases in global
storm-resolving simulations?
Abstract
Cirrus control the longwave radiative budget of the tropics. For the
first time, we quantify the variability in cirrus properties and
longwave cloud radiative effects (CREs) that arises from using different
bulk ice microphysical parameterizations within a single global
storm-resolving model. We run five-day meteorologically-nudged
simulations with four commonly-used microphysics schemes (M2005,
Thompson, P3 and SAM1MOM) and evaluate them with satellite products and
in situ observations. Tropical average longwave CRE varies over 20 W
m$^{-2}$ between schemes. Within the Thompson scheme, rapid
autoconversion of cloud ice to snow leads to deficient anvil cirrus even
with radiatively active snow. SAM1MOM, which uses saturation adjustment
for cloud ice, also has deficient anvil cirrus. M2005 and P3 simulate
cirrus with realistic frozen water path, and P3 best reproduces observed
longwave CRE. Even in those schemes, ice crystal number concentrations
commonly hit limiters and lack the observed variability and dependence
on frozen water content.