Sahany, S., V. Venugopal and R. S. Nanjundiah
Meteorology and Atmospheric Physics, Vol. 109, Numbers 3-4, doi: 10.1007/s00703-010-0099-3, 115-128.
Abstract
The performance of the Advanced Regional Prediction System (ARPS) in simulating an extreme rainfall
event is evaluated, and subsequently the physical mechanisms leading to its initiation and sustenance are explored.
As a case study, the heavy precipitation event that led to 65 cm of rainfall accumulation in a span of around 6 h
(1430 LT–2030 LT) over Santacruz (Mumbai, India), on 26 July, 2005, is selected. Three sets of numerical experiments
have been conducted. The first set of experiments (EXP1) consisted of a four-member ensemble, and was carried out
in an idealized mode with a model grid spacing of 1 km. In spite of the idealized framework, signatures of heavy rain-
fall were seen in two of the ensemble members. The second set (EXP2) consisted of a five-member ensemble, with a
four-level one-way nested integration and grid spacing of 54, 18, 6 and 1 km. The model was able to simulate a
realistic spatial structure with the 54, 18, and 6 km grids; however, with the 1 km grid, the simulations were domi-
nated by the prescribed boundary conditions. The third and final set of experiments (EXP3) consisted of a five-member
ensemble, with a four-level one-way nesting and grid spacing of 54, 18, 6, and 2 km. The Scaled Lagged Average
Forecasting (SLAF) methodology was employed to construct the ensemble members. The model simulations in
this case were closer to observations, as compared to EXP2. Specifically, among all experiments, the timing of maxi-
mum rainfall, the abrupt increase in rainfall intensities, which was a major feature of this event, and the rainfall
intensities simulated in EXP3 (at 6 km resolution) were closest to observations. Analysis of the physical mecha-
nisms causing the initiation and sustenance of the event reveals some interesting aspects. Deep convection was
found to be initiated by mid-tropospheric convergence that extended to lower levels during the later stage. In addition,
there was a high negative vertical gradient of equivalent potential temperature suggesting strong atmospheric insta-
bility prior to and during the occurrence of the event. Finally, the presence of a conducive vertical wind shear in
the lower and mid-troposphere is thought to be one of the major factors influencing the longevity of the event.
Citation Sahany, S., V. Venugopal and R. S. Nanjundiah, 2010: The 26 July 2005 Heavy Rainfall Event over Mumbai: Numerical Modeling Aspects. Meteorology and Atmospheric Physics, Vol. 109, Numbers 3-4, doi: 10.1007/s00703-010-0099-3, 115-128.