Extended Prediction of North Indian Ocean Tropical Cyclones Using the ECMWF Variable Ensemble Prediction System - Monitoring and Prediction of Tropical Cyclones in the Indian Ocean and Climate Change - page 112

levels out to ten days along with 51 ensemble members (50 perturbed members
+ 1 control run) at TL639 spectral truncation (horizontal resolution ~32 km)
with 62 vertical levels. For days 11-15, the 51-member ensemble is processed
at a reduced TL319 spectral truncation (horizontal resolution about 63 km). To
represent the uncertainty in initial conditions, ensemble perturbations are
constructed using singular vectors which capture the fastest growing errors in
the first 48 hours (Buizza and Palmer, 1995). Stochastic perturbations are also
added during the model integration to account for the uncertainty in
parameterized physical processes. Five additional singular vectors are computed
and perturbed in the six grid spaces enclosing each TC using a diabatic, adjoint
version of the ECMWF global atmospheric model at TL42 spectral truncation
with 42 vertical levels (Barkmeijer et al., 2001). The 15-day VarEPS 00UTC
forecasts for the period 1 January 2007 to 31 December 2010 were obtained
for the North Indian Ocean domain, which included the region: 0-30°N; 40-
110°E and were at a horizontal resolution of 0.25° × 0.25° on a regular latitude-
longitude grid.
2.2 Tropical Cyclone Tracking Scheme
To isolate tropical cyclones in the VarEPS analysis and forecast fields, we use
a modified version of the Suzuki-Parker tracking scheme (Holland et al., 2010).
To increase the maximum intensity retrievals from the VarEPS, 10 m winds
are replaced with winds averaged in the lower troposphere (10 m, 925 hPa and
850 hPa). The tracking scheme processes each ensemble member for tropical
cyclones by first identifying candidate vortices that exhibit a local minimum
in mean sea level pressure. This initial set of vortices is filtered by removing
systems that do not have a maximum lower tropospheric wind speed greater
than 16 kts (8.2 m s -1 ) and an 850 hPa relative vorticity maximum greater than
1 × 10 -4 s -1 . The next component of the tracking scheme confirms that each
identified vortex possesses a warm core as defined using the Hart (2003) phase-
space method. The cyclone phase analysis quantifies the thermal structure by
assessing the lower to middle troposphere's thickness gradient across the
cyclone and the magnitude of the cyclone's lower troposphere and middle to
upper troposphere's thermal wind. After the tracking scheme has been
implemented for each ensemble member, any tracks that originate over land
are removed unless the ensemble tracks are within 300 n mi of an observed
tropical cyclone. Furthermore, all ensemble forecast tracks must have a lifetime
of at least one day.
3. Results and Discussion
3.1 Tropical Cyclone Formation and Pre-genesis Forecasts
Figure 1a shows the spatial distribution of tropical cyclone forecast tracks for
Severe Cyclone Nargis from the VarEPS initialized on 23 April 2008 00UTC,