Effect of Northward Ageostrophic Winds Associated with a Tropical Cyclone on PRE Rainfall Enhancement

As a complement work to the authors' previous studies (Saito 2019; Saito and Matsunobu 2020), we examined the pre-typhoon rainfalls (PRE) ahead of a typhoon. The influence of moistening in the upper atmosphere induced by the northward ageostrophic winds on PRE precipitation was examined by a sensitivity experiment using a cloud resolving model. The simulation showed a large impact of the water vapor in the upper atmosphere on the precipitation. In the sensitivity experiment where the moisture in the middle and upper layers was reduced, the water vapor reduction area was advected northward, and snow in the middle and upper layers decreased, reducing the rain below the melting level. The intrusion of drying air into the upper atmosphere reduced the thickness of the moist absolutely unstable layer (MAUL), and the maximum intensity of convective updrafts decreased by about 10% in the test experiment. The increase of rain in PRE was primarily caused by the deep northward water vapor transport which yielded a large amount of condensation in the middle and upper layers, and change of moist instability in the upper atmosphere enhanced the convective updrafts.

References:

Saito, K., 2019: On the northward ageostrophic winds associated with a tropical cyclone. SOLA, 15, 222-227.

Saito, K. and T. Matsunobu, 2020: Northward ageostrophic winds associated with a tropical cyclone. Part 2: Moisture transport and its impact on PRE. SOLA, 16, 198-205.

Saito, K., T. Matsunobu, and T. Oizumi, 2022: Effect of upper-air moistening by northward ageostrophic winds associated with a tropical cyclone on the PRE enhancement. SOLA, 18, 81-87.