Radiosonde Penetration of an Undilute Cumulonimbus Anvil
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An example is presented of the serendipitous radiosonde penetration through the western edge of a rapidly growing undilute cumulonimbus anvil above 200 mb by an operationally released radiosonde balloon. The sounding is supportive of deep convection and contains a stable layer (13°C potential temperature increase) from 210 to 185 mb with a quasi-adiabatic mixed layer from there to 135 mb. The 185–135-mb layer has a wet-bulb potential temperature of 24°C which agrees to within 1°C of the subcloud-layer wet-bulb potential temperature. The wind perturbation of approximately 30 m s−1 within the mixed layer is larger than, but consistent with, relative outflow velocities estimated from satellite imagery and density current theory. Downstream soundings through decaying anvil debris 12 h later still show evidence of the initial convective thermal perturbation even as the tropopause attempts to reform in the vicinity of 200 mb. Implications for the tropospheric-stratospheric exchange of water vapor and the measurement of water vapor at cold temperatures are discussed. The present operational practice of not reporting moisture data at high levels in the troposphere when the ambient temperature is less than −40°C deprives users of potentially useful moisture information.