Radiatively Active Hydrometeors Frequencies from CloudSat-CALIPSO Data
for Evaluating Cloud Fraction in Global Climate Models
Abstract
This study derives radiatively-active hydrometeors frequencies (HFs)
from CloudSat-CALIPSO satellite data to evaluate cloud fraction in
present-day simulations by CMIP5 models. Most CMIP5 models do not
consider precipitating and/or convective hydrometeors but CESM1-CAM5 in
CMIP5 has diagnostic snow and CESM2-CAM6 in CMIP6 has prognostic
precipitating ice (snow) included. However, the models do not have snow
fraction available for evaluation. Since the satellite-retrieved
hydrometeors include the mixtures of floating, precipitating and
convective ice and liquid particles, a filtering method is applied to
produce estimates of cloud-only HF (or NPCHF) from the total
radiatively-active HF (THF), which is the sum of NPCHF, precipitating
ice HF and convective HF. The reference HF data for model evaluation
include estimates of liquid-phase NPCHF from CloudSat radar-only data
(2B-CWC) and ice-phase THF from CloudSat-CALIPSO 2C-ICE combined
radar/lidar data. The model evaluation results show that cloud fraction
from CMIP5 multi-model mean (MMM) is significantly underestimated (up to
30 %) against the total HF estimates, mainly below the mid-troposphere
over the extratropics and in the upper-troposphere over the midlatitude
lands and a few tropical convective regions. The CMIP5 cloud fraction
biases are reduced dramatically when compared to the cloud-only HF
estimates, but the area of overestimates expands from the tropical
convective regions to mid-latitudes in the lower and upper troposphere.
There is no CMIP5 standard output snow fraction available for comparison
against CloudSat-CALIPSO estimate. The implications of these results
show that hydrometeors frequency estimates from CloudSat-CALIPSO provide
a reference for GCM’s cloud fraction from stratiform and convective
form.