Multi-model Ensemble Based Extended Range Forecast of Tropical Cyclogenesis over the North Indian Ocean - Monitoring and Prediction of Tropical Cyclones in the Indian Ocean and Climate Change - page 201

and the Arabian Sea. With the genesis of a depression, IMD begins issuing
forecast advisories (RSMC, 2012) valid for 72 hrs. A guidance of TC genesis
with longer lead time is also useful. Hence, IMD is issuing the guidance on
cyclogenesis on experimental basis since 2010 on extended range scale (up to
two weeks) over the NIO based on multi-model ensemble (MME) applied to
ECMWF coupled model and the NCEP's Climate Forecast System (CFS)
coupled model. Considering all the above, an attempt is made to evaluate the
MME forecast of TC and also the associated rainfall activity in the extended
range time scale (up to two weeks) over the NIO for the cyclones during post-
monsoon season of 2011.
2 Coupled Models Output Considered for Preparing
Multi-model Ensemble Forecast
In recent years, it is found that the forecast errors from numerical models in
different temporal scales can be reduced with ensemble prediction techniques
and secondly through the combination of the forecasts from multiple models
(Krishnamurti et al., 1999; Wang et al., 2004). For providing the outlook for
cyclogenssis potential IMD utilizes products from two well known coupled
models viz., the monthly forecasting system of ECMWF coupled model and
the NCEP CFS coupled model. The details of these models forecasts, and the
methodology of multi-model ensembles, are discussed.
2.1 ECMWF Monthly Forecast System
The ECMWF monthly forecasting system (Vitart, 2004) used here is based on
32-day coupled ocean-atmosphere integrations set up at ECMWF. This system
has run routinely since March 2002. The atmospheric component of the model
used in this study has horizontal and vertical resolution of T159 L40 (1.125° ×
1.125°) with 40 levels in the vertical. The ECMWF monthly forecasting system
is based on fully coupled ocean-atmosphere integrations forced by persisted
SSTs. The oceanic component is the same as that for the current ECMWF
seasonal forecasting system. It is the Hamburg Ocean Primitive Equation
(HOPE) model (Wolff et al., 1997). The coupling between atmosphere and
ocean is done through the Ocean Atmosphere Sea Ice Soil (OASIS) coupler
(Terray et al., 1995). The atmospheric fluxes of momentum, heat, and freshwater
are passed to the Ocean every hour. The atmospheric and land surface initial
conditions are obtained from the ECMWF operational atmospheric analysis/
reanalysis system, whereas the oceanic initial conditions originate from the
oceanic data assimilation system used to produce the initial conditions of the
ECMWF seasonal forecasting system.
The ECMWF monthly forecasting system has an ensemble size of 51
members. One forecast, called the control, is run from the operational ocean
and atmosphere ECMWF analyses. The 50 additional integrations, the perturbed