Boundary‐layer convection and diurnal variation of vertical‐velocity characteristics in the free troposphere

Abstract

AbstractDuring the field experiment SOMARE‐99 (Sounding System Multifrequency Atmospheric Radar Experiment 1999), the troposphere above the Harz Mountains in Northern Germany was observed with the SOUSY VHF radar, with four microbarographs, and with 26 radiosondes launched every 3 h from nearby the radar site. This paper examines the 72 h period between 0000 UTC 25 May and 0000 UTC 28 May. Time series, root‐mean‐square values, and power spectra of radar‐observed vertical‐wind, w, fluctuations in the free troposphere and of pressure, p, fluctuations at the surface are presented and discussed. A pronounced diurnal cycle was observed: standard deviations (from 1 h periods) were about 10 cm s−1 (w) and 2 Pa (p) at night and up to 60 cm s−1 and 6 Pa during daytime. This is seen as evidence for the relevance of boundary‐layer convection for short‐period gravity waves in the free troposphere. Frequency spectra of w show a power‐law drop‐off beyond the Brunt‐Väisälä frequency with logarithmic slopes close to ‐5/3 at moderate and strong wind speeds and down to ‐3 or ‐4 at weak winds (smaller than 5 m s−1), supporting earlier work on the effects of Doppler shift on Eulerian gravity‐wave spectra. The w spectra, which have been calculated from 8 h long w time series (sampling period 1 min), have a noise floor of 10−3 m2s−2Hz−1 or less. This corresponds to an uncorrelated noise with a standard deviation of 3 mm s−1 for the 1 min samples. The concept of gravity‐wave intermittency is introduced, in analogy to a well‐established concept in turbulence physics. It is demonstrated how this concept can be used to distinguish turbulence from gravity waves empirically.