Aircraft Measurements of the Subsynoptic-Scale Properties of the Monsoonal Onset-Vortex of 1979

Abstract

This paper reports an analysis of the uninterrupted 1 s resolution data on a constant mid-tropospheric level collected by the NCAR Electra on 16 June 1979 over the Arabian Sea. The Electra completely encircled a well-developed cyclone during this MONEX1 flight mission. It is found that the subsynoptic-scale vertical velocity field consists of a distinct mesoscale as well as a convective scale component. Their prevalent wavelengths are about 30 km and 6 km, respectively. The updrafts are typically 1?3 m s?1 and the downdrafts ?-1.0 m s?1. The meridional velocity spectrum is similar to that of the vertical velocity in the convective scale range. There is a significant upward flux of momentum associated with these motions. The moisture and liquid water fluxes axe also statistically significant. The fluctuations of the horizontal velocity components in the mesoscale range may be modeled by those of a Markov process with an autoregressive coefficient of 0.99. This is equivalent to having the spectra characterized by a power-law dependence on frequency (wave-number) with an empirical exponent of about ?2.0. A significant energy cascade process may be implied. The vertical flux of zonal momentum by those motions is found to be significant. The results suggest that the relationship between the vertical flux of momentum by a subsynoptic scale of motions and the synoptic velocity field is different for the case of a convective scale from that of a mesoscale. The temperature data of this MONEX flight are found to be unreliable in that they imply a negative buoyancy flux. Such temperature readings are believed to be due to wetting of the sensor by the cloud droplets.