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Observations of atmospheric density currents using a tethered balloon‐borne turbulence probe system
Author(s) -
Alapworth
Publication year - 2000
Publication title -
quarterly journal of the royal meteorological society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.744
H-Index - 143
eISSN - 1477-870X
pISSN - 0035-9009
DOI - 10.1002/qj.49712656911
Subject(s) - turbulence , current (fluid) , front (military) , atmospheric sciences , meteorology , wind speed , thermal , mechanics , inlet , environmental science , geology , physics , oceanography
A tethered kite balloon with multiple instrumented probes clamped to the tethering cable is used to investigate atmospheric density‐current fronts passing through three sites at varying distances from the coast. The system allows high‐frequency measurements of the three components of wind, temperature and humidity to be made simultaneously at up to ten levels as the front passes through. This enables a two‐dimensional visualization of the current and its circulation to be constructed. As a result the structure of some of the wide variety of atmospheric density currents is revealed by direct observation. The internal circulations and the effects of varying environments on the currents and their movement are described, together with turbulence statistics. One current was associated with what may have been a coastal trapped disturbance, and in one case two opposing sea breezes appear to have collided inland. Two currents penetrating into surface inversions were dissociated from the surface. One of these was travelling downwind and formed a strong near‐surface jet. Associated wave and bore structures were found to be travelling in layers above the currents in two cases. In one case environmental air was found to intrude into the head region and convect internally, while, in another, air from the head region extruded into the environmental flow. The measurement array enabled the different terms of the dynamic equations to be evaluated and their relative contribution assessed. A remarkable correlation is found between thermal and dynamic variables. A study of the internal flux balance of daytime density currents shows that the internal flow plays a significant part in maintaining the capping inversion of the current. Eddies were observed at interfacial regions in which Richardson numbers were significantly greater than 0.25, confirming some modelling studies. Frontal slopes in most cases are in accord with theoretical values but in one downwind case the slope is considerably less than predicted. The inland currents are found to be deeper than coastal currents which may explain the increase in speed observed later in the day. Water vapour frontal structures are generally less marked than thermal fronts.