An evaluation of the liquid cloud droplet effective radius derived from MOMS, airborne remote sensing, and in situ measurements from CAMP2Ex

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 R-e 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" (CAMP(2)Ex), which, in part, targeted the objective of providing the first detailed evaluation of R-e retrieved across multiple platforms and techniques in a cumulus and congestus cloud region. Our evaluation consists of cross-comparisons of R-e 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 R-e 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 R-e samples of 9.6 (10.2 +/- 4.0) mu m, 11.0 (13.6 +/- 11.3) mu m, and 10.4 (10.8 +/- 3.8) mu m, respectively. These values are far lower than the values of 15.1 (16.2 +/- 5.5) mu m and 17.2 (17.7 +/- 5.7) mu m from the bi-spectral retrievals of RSP and MODIS, respectively. Similar results are observed when R-e is segregated by cloud-top height and in detailed case studies. The clouds sampled during CAMP(2)Ex consist of mostly small (mean transect length similar to 1.4 km) and low clouds (mean cloud-top height similar to 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 R-e 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 R-e. For shallow clouds that dominated the liquid cloud cover for the CAMP(2)Ex 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 R-e retrievals in cumulus cloud fields.