Characteristics of instabilities in the mesopause region over Maui, Hawaii

Abstract

Characteristics of convective and dynamical instabilities in the mesopause region (between 85 and 100 km) over Maui, Hawaii (20.7°N, 156.3°W) are investigated using 19 nights, ∼133 hours of high-resolution wind and temperature data obtained by the University of Illinois Na wind/temperature lidar during the Maui Mesosphere and Lower Thermosphere (Maui MALT) campaigns. The mean probabilities of convective and dynamical instabilities are observed to be ∼3 and 10%, respectively, but there is considerable night-to-night variation. At any given time the probability that an unstable condition is found at some altitudes in the 85–100 km range is ∼90%. The Maui MALT data exhibit a distinct trend for N² to increase with wind shear and vice versa. This correlation has important implications in the understanding of the development of instabilities. The night of 11 April 2002 is studied in detail in order to investigate the spatial and temporal structures of N², wind shear, and convective and dynamical instabilities. A close linkage between instability and the mesosphere inversion layers (MILs) is identified. Most of the convectively and dynamically unstable regions are located above the MILs, with a tendency for dynamical instability to develop below convective instability. It is found that the vertical variations of N² are often correlated with those of wind shear, but with a phase shift such that the maxima and minima of N² are located ∼0.5–1 km below those of wind shear. Because of this shift, dynamical instability tends to develop in the region above the maximum wind shear, where relatively small N² is observed to be associated with large wind shear. We also found that the wind shear is dominated by the contribution of the meridional wind, especially when the wind shear is strong. Possible mechanisms for the observed features are discussed.