A potential application of height-dependent cloud emission and scattering indices for identifying CrIS clear channels above clouds

Abstract

Satellite data assimilation of hyperspectral Cross-track Infrared Sounder (CrIS) clear channels in regions with but not affected by low-level clouds requires a set of height-dependent cloud emission and scattering indices (CESIs) that are sensitive to cloud top pressures. In this study, three CESIs are generated by pairing CrIS longwave and shortwave channels at three altitudes around 212 (CESI-5), 452 (CESI-9) and 1085 hPa (CESI-19). The training dataset for CESIs is the European Centre for Medium-Range Weather Forecasting analysis. CESI-5 is strongly (weakly) affected by those ice clouds with cloud-top pressures (CTPs) above (below) ∼300 hPa; CESI-9 is strongly (weakly) affected by those ice clouds with CTPs above (below) ∼600 hPa; and CESI-19 is highly correlated with the ice cloud optical depth (ICOD) obtained from the Atmospheric Infrared Sounder (AIRS) retrieval product. CESIs have latitudinal and scan-dependent biases that need removal. Verified with AIRS ICOD data, CESI-19 is greater than 5 K (6.5 K) if an ice cloud is present. This 5-K threshold gives a probability of correct typing of 85.3% (86.5%), a false alarm rate of 9.5% (5.1%) and a leakage rate of 4.5% (5.0%) during the daytime (nighttime). CESI-5 (CESI-9) is greater than −3 K (−1 K) if an ice cloud is present above 200 hPa (between 200 and 300 hPa). The case of Typhoon Maria (2018) demonstrates the potential of using height-dependent CESIs to identify above-cloud CrIS clear channels.