Abstract

This paper presents experimental evidence showing the ubiquitous presence in the lower atmosphere (at least up to 25 km) of very strong (positive) temperature gradients within very thin layers. The presence of such “sheets” in the temperature field of the free atmosphere was frequently hypothesized in order to account for the aspect sensitivity of VHF radar measurements. Owing to their high vertical resolution (20 cm) and to the fast-response thermometers used, the in situ balloon measurements discussed in this paper constitute the first direct evidence of their true existence. Statistical study of the properties of the sheets results in the following typical values: thickness 3–20 m, temperature increase 0.2–0.8 K, gradient 30–100 K/km. The sheets are frequently observed in groups, associated with and taking part in regions of high static stability. Local measurements using two pairs of sensors one meter apart indicate that the sheets are not flat and horizontal. Sometimes, clear evidence of ongoing or recent mixing, despite the strong local static stability, can account for such distortions. Observations of the same sheets by two thermometers 65 m apart (vertically) indicate vertical distortions of the sheets up to 10 m and horizontal extensions larger than 100 m. The possible contribution of the observed sheets to the VHF radar vertical reflectivity is estimated and compared with simultaneous reflectivity profiles measured by the PROVENCE radar. Without any adjustment, these profiles compare favorably, both in shape and in level, thus suggesting that the sheets can account for a significant fraction of the VHF radar vertical echoes.

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