Gravity waves and turbulence associated with cumulus convection observed with the UHF/VHF clear-air Doppler radars

Authors

  • Kaoru Sato,

  • Hiroyuki Hashiguchi,

  • Shoichiro Fukao


Abstract

Vertical wind disturbances are frequently observed during midsummer afternoons in association with cumulus convection. To examine the relation of these wind disturbances to atmospheric stability and turbulence intensity, we carried out intensive observations of wind, temperature and humidity over a large vertical region extending from the boundary layer to the lower stratosphere using UHF/VHF radars and radiosondes at Shiga, Japan (35°N, 136°E). The wind disturbances propagate quickly into the upper troposphere just after 1500 local standard time (LST) when the level of free convection (LFC) descends below the top of the mixed layer, while before that, the disturbances are confined in the mixed layer. However, strong turbulence is largely confined to the region below 3 km even after 1500 LST, suggesting that the height of 3 km corresponds to the top of cumuli, while the wind disturbances above are due to gravity waves generated in association with cumulus convection. In addition, stratospheric disturbances were examined for 12 cases of midsummer disturbances using routine observational data taken with a VHF radar in a height region of 5–24 km. Wind disturbances are observed in the lower stratosphere when turbulence as well as wind disturbances are strong in the whole troposphere. The stratospheric disturbances are dominant in two regions below and above a height of 18 km where the background horizontal wind is weak. Long-period disturbances are observed only in the lower region. This fact suggests that the height of 18 km is a critical level for the long-period gravity waves. The existence of a strong turbulent layer around 18 km is consistent with this inference.

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