Implications of retrieval bias in bi-spectral derived cloud effective radius on estimates of aerosol-cloud interaction using airborne and satellite remote sensing measurements from CAMP2Ex

The longest spaceborne record of cloud optical and microphysical properties comes from the bi-spectral approach. It simultaneously retrieves cloud optical depth and cloud effective radius (Re) from visible/near-infrared and shortwave infrared radiances. Previously, using field data collected during NASA's Cloud, Aerosol and Monsoon Processes Philippines Experiment (CAMP2Ex), bias estimates in Terra MODIS's bi-spectral Re were evaluated against airborne remotely sensed Re from the Research Scanning Polarimeter (RSP) and in situ measurements sampled in cumuliform cloud fields, confirming the existence of large overestimates (on average, ~7 mm) in bi-spectral Re for the tropical western Pacific reported in our earlier studies. Here, we investigate the impact of the observed overestimates in bi-spectral Re retrievals on estimates of aerosol-cloud interaction in the CAMP2Ex domain.

Using Re retrievals from various techniques, cloud droplet number concentration (Nd) and the aerosol first indirect effect (IE) are estimated from both satellite and airborne perspectives. Our results show that removing the ~7 μm overestimate in MODIS Re results in a 400% increase in derived Nd values, on average, when compared to the original MODIS derived Nd. The bias-adjusted MODIS Nd (removing the estimated Re bias) distribution has a median value of ~140 cm-3, which is similar to the in situ measured Nd median value of ~111 cm-3. Despite sampling and wavelength differences, the original MODIS Nd and RSP bi-spectral derived Nd distributions both have median Nd values of ~35 cm-3. The RSP polarimetric derived Nd distribution has a median Nd value of ~227 cm-3. IE is inferred from the partial derivative of ln(Nd) versus ln(AOD), where AOD is aerosol optical depth. Airborne IE estimates range from 0.42 to 0.51 for RSP polarimetric derived Nd and 0.07 to 0.28 for RSP bi-spectral derived Nd. Satellite IE estimates range from 0.58 to 0.66 for the original MODIS Nd and 0.38 to 0.62 for the bias-adjusted MODIS Nd. Overall, adjusting for the overestimate in MODIS Re results in slightly lower IE estimates, but much less frequent small Nd values (in the 10-15 cm-3 range). The good agreement between MODIS bias-adjusted Nd and in situ measured Nd shows potential in using satellite-derived Nd to evaluate model parameterization in cumulus cloud fields.