MODIS comparisons with northeastern Pacific in situ stratocumulus microphysics

摘要

Vertical sounding measurements within stratocumuli during two aircraft field campaigns, Marine Stratus/stratocumulus Experiment (MASE) and Physics of Stratocumulus Top (POST), are used to validate Moderate Resolution Imaging Spectroradiometer (MODIS) cloud optical thickness (COT), cloud liquid water path (LWP), and cloud effective radius (re). In situ COT, LWP, and r_e were calculated using 5m vertically averaged droplet probe measurements of complete vertical cloud penetrations. MODIS COT, LWP, and r_e 1 kmpixels were averaged along these penetrations. COT comparisons in POST showed strong correlations and a near 1:1 relationship. In MASE, comparisons showed strong correlations; however, MODIS COT exceeded in situ COT, likely due to larger temporal differences between MODIS and in situmeasurements. LWP comparisons between two cloud probes show good agreement for POST but not MASE, giving confidence to POST data. Both projects provided strong LWP correlations but MODIS exceeded in situ by 14–36. MODIS in situ r_e correlations were strong, but MODIS 2.1 μm r_e exceeded in situ re, which contributed to LWP bias; in POST, MODIS r_e was 20–30 greater than in situ re. Maximum in situ r_e near cloud top showed comparisons nearer 1:1. Other MODIS r_e bands (3.7 μm and 1.6 μm) showed similar comparisons. Temporal differences between MODIS and in situ measurements, airplane speed differences, and cloud probe artifacts were likely causes of weaker MASE correlations. POST COT comparison was best for temporal differences under 20 min. POST data validate MODIS COT but it also implies a positive MODIS r_e bias that propagates to LWP while still capturing variability.