An evaluation of the liquid cloud droplet effective radius derived from MODIS, airborne remote sensing, and in situ measurements from CAMP2Ex
Authors: Fu Dongwei , Di Girolamo Larry , Rauber Robert M. , McFarquhar Greg M. , Nesbitt Stephen W. , Loveridge Jesse , Hong Yulan , van Diedenhoven Bastiaan , Cairns Brian , Alexandrov Mikhail D. , Lawson Paul , Woods Sarah , Tanelli Simone , Schmidt Sebastian , Hostetler Chris , Scarino Amy Jo 
Published: June 2022

Abstract. The cloud drop effective radius (Re) of the drop size distribution
derived from passive satellite sensors is a key variable used in climate
research. Validation of these satellite products has often taken place under
stratiform cloud conditions that favor the assumption of cloud horizontal
homogeneity used by the retrieval techniques. However, many studies have noted
concerns with respect to significant biases in retrieved Re arising from cloud
heterogeneity, for example, in cumulus cloud fields. Here, we examine data
collected during the 2019 “Cloud, Aerosol and Monsoon Processes Philippines
Experiment” (CAMP2Ex), which, in part, targeted the objective of
providing the first detailed evaluation of Re retrieved across multiple
platforms and techniques in a cumulus and congestus cloud region. Our
evaluation consists of cross-comparisons of Re between the Moderate Resolution
Imaging Spectroradiometer (MODIS) onboard the Terra satellite, the Research
Scanning Polarimeter (RSP) onboard the NASA P-3 aircraft, and in situ
measurements from both the NASA P-3 and Learjet aircraft that are all taken in
close spatiotemporal proximity to the same cloud fields. A particular advantage
of our approach lies in the capability of the RSP to retrieve Re using a bi-spectral
MODIS approach and a polarimetric approach, which allows for the evaluation of
bi-spectral and polarimetric Re retrievals from an airborne perspective using
the same samples. Averaged over all P-3 flight segments examined here for warm clouds, the
RSP polarimetric method, the in situ method, and the bias-adjusted MODIS method of Fu et al. (2019) show a comparable median (mean ± standard deviation) for the Re samples of
9.6 (10.2 ± 4.0) µm, 11.0 (13.6 ± 11.3) µm, and 10.4
(10.8 ± 3.8) µm, respectively. These values are far lower than the values of
15.1 (16.2 ± 5.5) µm and 17.2 (17.7 ± 5.7) µm from the
bi-spectral retrievals of RSP and MODIS, respectively. Similar results are
observed when Re is segregated by cloud-top height and in detailed case
studies. The clouds sampled during CAMP2Ex consist of mostly small
(mean transect length ∼ 1.4 km) and low clouds (mean cloud-top
height ∼ 1 km), which had more numerous small clouds than the
trade wind cumuli sampled in past field campaigns such as Rain in Shallow
Cumulus over the Ocean (RICO) and the Indian Ocean Experiment (INDOEX). The
overestimates of Re from the RSP bi-spectral technique compared with the polarimetric
technique increased as cloud size and cloud optical depth decreased.
Drizzle, cloud-top bumpiness, and solar zenith angle, however, are not
closely correlated with the overestimate of bi-spectral Re. For
shallow clouds that dominated the liquid cloud cover for the CAMP2Ex
region and period, we show that 3-D radiative transfer and cloud heterogeneity,
particularly for the optically thin and small clouds, appear to be the
leading cause of the large positive biases in bi-spectral retrievals.
Because this bias varies with the underlying structure of the cloud field,
caution continues to be warranted in studies that use bi-spectral Re
retrievals in cumulus cloud fields.