Research studies in progress at RSMC La Réunion : microwave imagery database related on tropical cyclones ; estimation of tropical cyclone genesis probability over the south-west indian ocean with ECMWF ensemble prediction forecast

1- Microwave imagery database related on tropical cyclones

Dvorak Technique is currently based on EIR and VIS imagery to analyse and estimate tropical cyclones intensity. Since the advent of the microwave instruments, new data and products are available and have been of major interest in tropical cyclone monitoring, mainly in intensity changes.

In order to improve the Dvorak technique in its visual analyse concept, the idea was to try and include microwave imagery.

To deal with this project, in situ observations were required to make comparisons with microwave imagery. These observations are the RECON of NOAA/NHC (aircraft measurements) and the microwave imagery comes from NRL/Monterey.

This first part of the project has suffered insufficient samples and pointed a problem up : storm life was not considered. A pattern related to a specific intensity could not emerge.

The next idea was to build a microwave database to overcome these problems. In this database, AODT data from CIMSS are available as well.

We will show the way this database can be used, and in which ways microwave data can contribute to estimate the TC intensity.

2- Estimation of Tropical Cyclone Genesis Probability over the South-West Indian Ocean with ECMWF Ensemble Prediction Forecast

The ECMWF ensemble prediction forecast is based on 50 perturbed analysis that influence both the physics and the dynamics of the model ; this leads to a large and probabilistic information. In this project, the goal is to build a composite operational and objective estimation of the cyclogenesis potential based on the ECMWF ensemble prediction forecast by giving prominence to the areas where the dynamic , thermodynamic and energetic conditions (model) are favourable for a tropical cyclogenesis.

A basic approach of this issue brings to the fore 4 parameters : the 850 hPa vortex, the CAPE index, the 700 hPa relative humidity and the DGP (Daily Genesis Parameter, defined as z850-z200). A detailed analysis of the ECMWF ensemble model 'reaction' to the 2004/2005 cyclone season cyclogenesis brings out characteristic thresholds for each of the 4 parameters. The operational composite tropical cyclone genesis probability is then computed by mixing the 4 parameters : over a specific grid point, the genesis index represents the probability that the 4 parameters exceed simultaneously their respective threshold.

This tropical cyclone genesis probability has been tested by the operational RSMC La Réunion tropical cyclone forecasters during the 2006/2007 cyclone season. An objective control based on the 35 cyclogenesis confirms the detection skill of this index : the forecast probability is about 80% from 24-hour to 72-hour range, and remains above 50% up to 168-hour range ; still considering the 2006/2007 cyclone season over the South-West Indian Ocean, the 'false alarm' rate is about 30% from tau 24 to tau 120. Besides, this tropical cyclone genesis probability signal often comes before a significant model predicted MSLP deepening.