18th Conference on Weather and Forecasting, 14th Conference on Numerical Weather Prediction, and Ninth Conference on Mesoscale Processes

Wednesday, 1 August 2001: 2:30 PM
Fine scale initialization and prediction of convective systems over French mountainous areas (formerly paper 4.2)
Didier Ricard, CNRM and Météo France, Toulouse, France; and V. Ducrocq and J. P. Lafore
Poster PDF (2.0 MB)
Both static and dynamic fine scale initializations are investigated for prediction of two convective events over mountainous area. These intense events occurred over the Massif Central (South East of France) during autumn 1995 and produced heavy rain amounts (more than 100 mm in 3-4 hours). In both cases, there was a leak synoptic forcing so a good description of mesoscale features is certainly needful for insuring a better prediction of these systems.

The high resolution simulations (horizontal mesh of 2.5 km) have been carried out with the 3D mesoscale non-hydrostatic model MESO-NH, allowing a two-way interactive grid-nesting fashion. With the intention of improving the numerical simulation of these convective cases, a particular care is focused on the model initialization. In this study, we assess different initialization methods: a static fine-scale physical one and several dynamic initializations.

For the static fine scale initialization, the meso-beta scale description of low-level conditions is improved by the use of a mesoscale surface data analysis as initial state. Moreover, a meso-gamma scale information about the presence of a developing convective system is supplied. Based on a simple cloud analysis from conventional radar and infrared satellite data, this step adjusts the humidity and hydrometeor fields of initial state. This improved initial state produces well developed systems and leads to better precipitation forecasts.

For dynamic initializations, we have developed a sequential data assimilation based on the nudging technique. The basic idea is to relax during a pre-forecast period some prognostic model variables (wind, temperature and humidity) towards their observed or analyzed values. For meso-beta scale, these variables are relaxed during a 3-h period towards mesoscale analysis. For meso-gamma scale, hydrometeor fields are relaxed during a shorter period towards some estimated values from conventional radar reflectivities and infrared satellite data.

Comparisons with observations and between the different initial states, by means of scores, backward and forward trajectories will be present at the conference as well as a discussion about the initialization strategy for fine scale numerical simulations.

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