Weather deteriorated rapidly over Mauritius during the night of 12 January 2013, with violent thunderstorms and heavy showers spreading over the island.
22 October 2020
12 January 2013
Around 17:00 UTC (21:00 LT) lightning was observed to the east of the island, over the sea. The lightning was spectacular, as it was quite frequent. Rain subsided from 05:00 UTC (09:00 LT) the next day.
by Philippe Veerabadren (Mauritius Meteorological Services )
On ECMWF upper air charts of 12 January (00:00 Z model run), a low was forecast to develop to the northeastern part of Mauritius, on the 500 hPa level. In higher levels up to 200 hPa the model was showing a deep trough oriented in the NW–SE direction and off the northeastern coast of the island.
The mid-tropospheric low seemed to have been cut off from a similar deep trough that was present at 500 hPa earlier. This low was seen to be embedded in a cold pool.
The 12:00 UTC ECMWF model run of the same day forecast the low, with its cold mid-tropspheric air, to cross Mauritius on Saturday night into Sunday morning.
The cooling by about four degrees at 500 hPa brought about a change in the lapse rate and contributed towards static instability of the atmosphere.
Models also expected the CAPE to reach a peak of 800 Joules/Kg at the time of crossing. Further model output of 18:00 UTC on 12 January, showed convergence at 700 hpa and divergence at 200 hPa and a highly negative vertical velocity between 600 hPa and 150 hPa.
The model was predicting high humidity from surface to reasonably high in the troposphere and even expecting saturation from 500 to 200 hpa at 18:00 UTC on the 12th.
The moisture could be attributed to clouds advection, which could be seen from the satellite images at that time (Figures 1 and 2). The models were accurate and all these ingredients came together to produce the violent thunderstorms.
Night Microphysics RGB with 850 hPa relative humidity (ECMWF analysis), 12 Jan 18:00 UTC (left) and 13 Jan 00:00 UTC (right). Only the 70% contour line is plotted (in white) to separate moist-tropical air from dry-polar air.
The Airmass RGB with 500 hPa Geopotential (ECMWF analysis), 12 Jan 18:00 UTC (left) and 13 Jan 00:00 UTC (right), shows the position of the Upper Level Low.
Further details about the event can be found in Philippe's research
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