Underestimation of Diurnal Variations in ERA5 Temperature and Relative Humidity over Tropical Indian Ocean

Abstract

Significant upwelling in the equatorial ocean influences complex ocean–atmosphere interactions and contributes to diurnal variations in the lower troposphere. This study compares the temperature and relative humidity data from radiosonde observations over the tropical Indian Ocean with those from the ERA5, highlighting the underestimation of the diurnal variations in the ERA5. Radiosonde measurements were conducted at 3 h intervals for approximately 24 h, from 31 May to 1 June 2023, to investigate the diurnal variations in the lower troposphere at two fixed locations: (1) 65°E and 8°S in the upwelling region (Station 8) from 28 to 29 May 2023, and (2) 65°E and 4°S outside the upwelling region (Station 4). The radiosonde observations reveal pronounced diurnal variations in temperature and relative humidity between 950 and 650 hPa. The maximum diurnal range (maximum minus minimum) for temperature is observed above 800 hPa, with Station 8 exhibiting 4.7 °C and Station 4 exhibiting 2.7 °C. For relative humidity, Station 8 shows a diurnal range of 84%, while at Station 4, notable variations are observed only below 650 hPa, reaching 76%. However, the ERA5 underestimates the diurnal variations both within and outside the upwelling region. This underestimation is particularly evident between 850 and 750 hPa and is more pronounced within the upwelling region, where the diurnal range is larger. The diurnal ranges calculated from the ERA5 for 2004–2023 suggest that the reanalysis dataset exhibits limitations in capturing diurnal variations, particularly over the upwelling region. This report highlights the need for more in situ observations of the atmospheric variables to better represent diurnal variations in the tropical Indian Ocean.