Comparisons of refractive index gradient and stability profiles measured by balloons and the MU radar at a high vertical resolution in the lower stratosphere

摘要

Many experimental studies have demonstrated that VHFStratosphere-Troposphere (ST) radar echo power is proportional to thegeneralized refractive index gradient squared M2 when using a verticallyoriented beam. Because humidity is generally negligible above thetropopause, VHF ST radars can thus provide information on the staticstability (quantified by the squared Brunt-V?is?l? frequencyN2) at stratospheric heights and this capability is useful for manyscientific applications. Most studies have been performed until now at avertical resolution of 150 m or more. In the present paper, results ofcomparisons between radar- and (balloon borne) radiosonde-derived M2 andN2 are shown at a better vertical resolution of 50 m with the MUradar (34.85° N, 136.15° E; Japan) by benefiting from the rangeresolution improvement provided by the multi-frequency range imagingtechnique, using the Capon processing method. Owing to favorable winds in thetroposphere, the radiosondes did not drift horizontally more than about 30 kmfrom the MU radar site by the time they reached an altitude of 20 km.The measurements were thus simultaneous and almost collocated. Very goodagreements have been obtained between both high resolution profiles of M2,as well as profiles of N2. It is also shown that this agreement canstill be improved by taking into account a frozen-in advection of the airparcels by a horizontally uniform wind. Therefore, it can be concluded that1) the range imaging technique with the Capon method really providessubstantial range resolution improvement, despite the relatively weak Signal-to-Noise Ratios (SNR) over the analyzed region of the lower stratosphere, 2)the proportionality of the radar echo power to M2 at a vertical scaledown to 50 m in the lower stratosphere is experimentally demonstrated,3) the MU radar can provide stability profiles with a vertical resolution of50 m at heights where humidity is negligible, 4) stable stratospheric layersas thin as 50 m or less have at least a horizontal extent of a few kmto several tens of kilometers and can be considered as frozenly advectedover scales of a few tens of minutes.