DIRECT EVIDENCE OF SHEETS IN THE ATMOSPHERIC-TEMPERATURE FIELD

This paper presents experimental evidence showing the ubiquitous prese nce in the lower atmosphere (at least up to 25 km) of very strong (pos itive) temperature gradients within very thin layers. The presence of such ''sheets'' in the temperature field of the free atmosphere was fr equently hypothesized in order to account for the aspect sensitivity o f VHF radar measurements. Owing to their high vertical resolution (20 cm) and to the fast-response thermometers used, the in situ balloon me asurements discussed in this paper constitute the first direct evidenc e of their true existence. Statistical study of the properties of the sheets results in the following typical values: thickness 3-20 m, temp erature increase 0.2-0.8 K, gradient 30-100 K/km. The sheets are frequ ently observed in groups, associated with and taking part in regions o f 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 st rong local static stability, can account for such distortions. Observa tions of the same sheets by two thermometers 65 m apart (vertically) i ndicate vertical distortions df the sheets up to 10 m and horizontal e xtensions larger than 100 m. The possible contribution of the observed sheets to the VHF radar vertical reflectivity is estimated and compar ed with simultaneous reflectivity profiles measured by the PROVENCE ra dar. 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 VBF radar vertical echoes.