Extended Range Tropical Cyclone Predictions for East Coast of India - Monitoring and Prediction of Tropical Cyclones in the Indian Ocean and Climate Change - page 134

large-scale conditions for tropical cyclone formation, which include low-level
cyclonic disturbances, warm SST, weak vertical shear, and moist unstable air.
Over the Atlantic and eastern Pacific sectors, the low-level cyclonic disturbances
usually manifest as tropical easterly waves (e.g., Carlson, 1969; Burpee, 1974;
Reed et al., 1977; Thorncroft and Hodges, 2001). While nearly 85% of the
intense (or major) hurricanes over the Atlantic have their origins as easterly
waves, only a small fraction of tropical easterly waves result in the formation
of a named tropical storm.
Easterly waves are ubiquitous in the trade wind region of the tropics, and
typically only a small percentage of these waves develop into tropical cyclones
(Frank, 1970; Ritchie and Holland, 1999). Role of easterly waves on cyclo-
genesis in North Indian Ocean (NIO) has been recognized since at least the
1930s: lower tropospheric westward travelling disturbances often serve as the
“seedling” circulations for a large proportion of tropical cyclones. They are
first seen usually in October to April. The waves have a period of about 3 or 4
days and a wavelength of 2000 to 2500 km. One should keep in mind that the
“waves” can be more correctly thought of as the convectively active troughs
along an extended wave train. Though these waves are generated frequently, it
appears that the number that is formed has no relationship to tropical cyclone
activity over the NIO each year. It is currently completely unknown, how
easterly waves change from year to year in both intensity and location.
2. Extended Range Probabilistic Prediction
of Tropical Cyclones
Recently, a number of commercial and non-commercial organisations have
started providing tropical cyclone related risk-analysis and these real-time risk-
analysis products can be accessed through web. Most of these analyses are
based on the high quality forecasts from global models. A seven-day lead
prediction of cyclone, with proven reliability, can be highly crucial for successful
cyclone disaster prevention and mitigation plan.
Dynamical prediction of tropical weather beyond a few days of lead time
is limited by what is known as “limit of predictability”, which arises mainly
due to two sources of errors: (1) the errors introduced by chaos or sensitive
dependence on the initial conditions; and (2) errors introduced because of
imperfections in the model, such as inadequacies in the formulation of physical
processes, and truncation errors introduced by numerical methods at each step
of model integration. One method to extend the limit of predictability of
dynamical models is to use a large ensemble of model predictions to enhance
the “signal” (the prediction) with respect to the “noise” (the uncertainties in
the model predictions). Ensemble forecasting is a numerical prediction method
that is used to attempt to generate a representative sample of the possible future
states of a dynamical system.