TROPICAL CYCLONE trajectory forecast products

• TROPICAL CYCLONE trajectory forecast requirements
• BUFR definition for TROPICAL CYCLONE trajectory forecast products

The ECMWF Tropical cyclone forecast products are designed to provide both deterministic and probabilistic information on movement and intensity of individual tropical cyclones. The system is vitally dependent on observations from various tropical cyclone centres around the world. In other words, the TC forecasts does not take genesis into account - TCs can however die in the process if they are not forecasted to stay strong enough.

Once observations are available, the movement of a TC is automatically tracked, both in the deterministic and the EPS forecasts. The tracking algorithm is based on extrapolation of past movement and the mid-tropospheric steering flow to obtain a first guess position. The actual position is determined by searching for MSLP and 850 hPa vorticity extremes around the first guess position. Also thickness maximum, wind speed and orography are evaluated. More details can be found in the following Tech. Memo http://www.ecmwf.int/publications/newsletters/pdf/102.pdf. Tropical cyclone products are presented in the FM 94-IX Ext. BUFR binary representation.

The calculations of perturbations in the Tropics

To improve the tropical- cyclone (TC) prediction in terms of the spread in the cyclone tracks and intensity a specific set of perturbations for the Tropics was introduced in the EPS (Puriet al, 2001). Tropical singular vectors are specifically designed to take in account perturbation growth due to diabatic processes.

To benefit from tropical diabatic singular vectors in TC ensemble forecasting, it is necessary to define target areas in the vicinity of TC locations. This area is defined as a 30x40 latitude-longitude degree box around all the TCs laying in the tropical strip 25S - 25N and classified at least as strong as Tropical Storm. Even though the number of TC in the tropics may vary from day to day, to limit the numerical cost a maximum number of target area is set to four merging the closest target area. Considering that weather systems originating in the Caribbean area may influence medium-range European forecast, this area (0N - 20N, 100W - 60W) is always targeted.

Tropical Cyclone products are disseminated separately from other EPS products and have their own dissemination schedule (Table 5.15 below).

TROPICAL CYCLONE trajectory forecast requirements

The following command language parameters are specific for the TROPICAL CYCLONE trajectory forecast requirements. Other parameters are listed under the 4.2.6 above.

BUFR definition for TROPICAL CYCLONE trajectory forecast products

ECMWF's TROPICAL CYCLONE trajectory forecast products are presented in the FM 94-IX Ext. BUFR binary representation form. Products are coded into BUFR Edition 3. They contain the following:

• Table 5.12 BUFR table reference for TROPICAL CYCLONE products