Radiosonde observations of equatorial atmosphere dynamics over Indonesia: 1. Equatorial waves and diurnal tides

Abstract

This paper describes the characteristics of the mean winds, equatorial waves with periods ranging from 4 to 20 days, and diurnal tides determined by analyzing the profiles of wind velocity, temperature, and humidity obtained every 5–7 hours in the height range up to about 35 km with a height resolution of 150 m during an observation campaign conducted February 27 to March 22, 1990, in East Java, Indonesia. The structures of the mean winds in the troposphere and lower stratosphere seemed to be affected by the Australian monsoon and the quasi‐biennial oscillation, respectively. Frequency spectra indicated that the equatorial waves as well as the diurnal tides were dominant below about 25 km, while gravity waves with periods shorter than 4 days became more significant above 25 km. A 7‐day oscillation showing an antiphase relation between the eastward and northward components and exhibiting large amplitudes was observed in the lower troposphere. The time‐height variations of the activity of this 7‐day oscillation were clearly correlated with a region of high relative humidity. Perturbations in the zonal wind and temperature with wave periods varying from 15 to 17 days were also enhanced in the troposphere, while Kelvin waves with periods of about 7 and 20 days were detected in the lower stratosphere, and activity near the tropopause was conspicuously enhanced. We found that the 20‐day Kelvin wave greatly modified the structure of the tropopause, such as the minimum temperature, the tropopause height, and the values of the Brunt‐Väisälä frequency squared N2, which further suggests the effects of Kelvin waves on the transportation of tropospheric water vapor into the stratosphere and on the downward mixing of stratospheric minor constituents into the troposphere. The observed profiles of the diurnal oscillation were compared with those of a numerical model assuming only migrating tides, which reasonably agreed above about 25 km. Below 25 km, however, the observed amplitudes were 1–1.5 m/s, exceeding those of the model about 10 times. Moreover, the phase profiles involved fluctuations with small vertical scales, suggesting interference by many nonmigrating tides with short vertical wavelengths.