Impact of Low-level Moisture and Convergence on Initiation and Development of Cumulonimbus

Abstract:

A lot of numerical studies showed that increase of moisture in atmospheric boundary layer lowers level of free convection (LFC) and increase convective inhibition (CIN), contributing to initiation and development of cumulonimbus. We examined observation data of a water vapor Raman lidar (RL) that offered clear time variation of vertical profile of water vapor mixing ratio (WVMR) near Tokyo International Airport as well as Doppler lidars, Doppler radars, ceilometers, and surface weather observations, to understand contribution of low-level moisture increase to initiation and development of cumulonimbus. Initiation and development of a cumulonimbus on 2 September 2018 near the airport were clearly observed by the two Doppler lidars and three Doppler radars. Cool and dry east-southeasterly sea breeze and relatively warm and wet northeasterly in-land wind formed sea breeze front (SBF) with strong convergence exceeding 5 x 10^-3 s^-1 near ground, initiating the cumulonimbus. At 10:35 UTC (19:35 in local time) the first radar echo of the cumulonimbus was observed by the radars and, then, at 11:00 UTC the cumulonimbus passed over the RL. The RL observation results indicate increase of WVMR at an altitude of 285 m from 09:00 UTC to 11:00 UTC. One interesting feature of the WVMR vertical profiles is that WVMR estimated at the time when the SBF passed over the RL (11:00 UTC) had an abrupt increase, indicating that the RL observed moist air near ground lifted by the SBF to higher altitudes. LFC and CIN at the RL station were estimated using vertical profiles of WVMR observed by the RL data as well as vertical profiles of pressure and temperature observed by a radiosonde released at Tateno station, which was about 60 km northeast from the RL station. While WVMR at an altitude of 285 m from above sea level increased from 14.9 g/kg at 09:00 UTC to 16.0 g/kg at 11:00 UTC, the LFC decreased from 1560 m at 09:00 to 1060 m at 11:00 and the CIN increased from -33 J/kg at 09:00 to -7 J/kg at 11:00. The 1.1 g/kg increase in WVMR in two hours at the low-level facilitated the convective initiation and development of the cumulonimbus. Since the abrupt increase of WVMR associated with SBF is about 0.4 g/kg in twelve minutes, which is 36 % of the increase in two hours (1.1 g/kg), the effect of lift of moist air by the SBF had considerable impact on the development of the cumulonimbus.