Performance of the ECMWF Operational Analyses over the Tropical Indian Ocean

Abstract

The quality of the ECMWF operational analyses is evaluated against independent upper-air sounding data collected during the Joint Air–Sea Monsoon Interaction Experiment (JASMINE; April–May 1999) and the Indian Ocean Experiment (INDOEX; February–March 1999). Statistics of the difference between observations and analyses are compiled for temperature, humidity, and wind speed. The results show that the analyzed temperature has a cold bias between 1000 and 750 hPa. However, in the upper troposphere, a warm bias occurs between 350 and 150 hPa, while a cold bias is seen above 150 hPa. Compared to the observations, the analyzed humidity is lower between 1000 and 950 hPa and higher between 950 and 750 hPa. The analyzed wind speeds are close to observations over much of the troposphere, except near the tropopause, where they are overestimated by 1–2 m s−1 in the analyses. The low-level (1000– 750 hPa) biases in moisture and temperature in the ECMWF analyses over the Indian Ocean are similar to those reported for the tropical Pacific Ocean in past studies. The occurrence of a cold and dry bias in the lowest 50 hPa indicates reduced convective available potential energy, which will render difficult the initiation and development of convection in numerical models initialized with these analyses. The moisture biases arise most likely because of the poor fit to humidity observations by the four-dimensional variational data assimilation scheme. This suggests that a better fit to humidity observations will yield an improved water vapor climatology over the Arabian Sea and Indian Ocean.