Akhilesh
Gupta (to
Tropical Storms list)
Date:
Thu, 8 May 2003 10:31:38 +0800
Hi,
I
am a new entrant to TC group-an operational TC forecaster from India for
NIO
region.
Satellite imageries of today show deep convection with no organization over the South east Bay of Bengal. An east-west shear zone is seen in wind field extending from 15degN/80degE to 3degN/100degE with embeded weak circulation around 5 deg N/90 degE. A number of models run in India e.g.,Global>> T-170, T-80; Meso/Regional Eta, MM5, FSU, QLM suggest slight development of vortex but movement towards west-north-west and thus crossing Srilanka and moving to Arabaian sea in 5-7 days global model forecast. We have checked NCEP and COLA forecasts which also suggest similar results as available from our models. However, ECMWF prediction suggests rapid development and northwest movement. By day-5-7 , it is seen as quite intense vortex crossing Indian eastcoast around 13-15 deg lat. The atmosphere appears to be favourable with sufficiently high SST and weak vertical shear. There appears to a westerly surge over the region south of initial vortex. My personal assessment is that it is initial vortex problem as all our model analyses keep vortex at much lower latitude (4-6 deg). I am not sure about ECMWF analysis. We haven't introduce synthetic vortex as yet because currently there is no defined vortex. Right now there is cloud cluster with Dvorak's intensity scale T-1 located around 4 deg N/92 degE. Could anyone suggest what is the consensus forecast? How far should we depend on ECMWF products particularly when none other models suggest any such thing?
We have operational responsibility for entire NIO region and therefore issue of early alert to disaster managers assumes considerable importance. As per climatology, this is one of the most favourable periods for severe cyclones formation in NIO but motion climatology suggests there was not a single case of vortex of TC or above intensity crossing Srilanka.
Akhilesh
Gupta
--------------------------------------------------------------------------------
Dr
Akhilesh Gupta
Director
& Scientist F
National
Centre for Medium Range Weather Forecasting (NCMRWF)
Department
of Science & Technology
Mausam
Bhavan Complex, Lodi Road, New Delhi-110 003
John McBride
Gedday all.
I have been purposely quiet lately; but I can't resist responding to Ahkilesh's post, because I am so pleased we have a correspondent from the Indian sub-continent. This is a step towards our dream for the internet: tropical forecasters and researchers from around the world in conversation with one another; and so there being no longer an information gap between researchers and forecasters. I only wish I had something more perceptive to say about the incipient development in the north Indian Ocean, at around 5N 90 E.
On my web-page (see reference below: follow link to synoptic discussion, today's date), I have put up the Eumetsat images for the past two days and the current (1200 UTC 8 May) GASP 850 hPa analysis.
As you can see on these images,
despite the fact we are still in early May, the switch from a southern
summer to a northern summer pattern has taken place. This is evidenced
by the low level southerly flow along the African coast is southerly
and by the zone of "monsoon westerlies" across the Indian Ocean between
the equator and about 5 N. Also, looking at the satellite images,
the ITCZ in terms of a planetary scale east-west extending line of convection
is located (in Indian Ocean longitudes) in the northern Hemisphere, along
about 10N. Further east in the Maritime continent, things are more
messy. The ITCZ (in terms of a confluence zone between Trades from
each hemisphere) is close to the equator; and the convection covers quite
a wide latitudinal span and is also centred on the equator.
Looking at Matt's diagnostics
( http://www.bom.gov.au/bmrc/clfor/cfstaff/matw/maproom/OLR_modes/h.6.ALL.EQ.html)
(archived copy here), the
convection over Indonesia is considered to be an MJO event, and was located
over the western Indian ocean a week ago.
Going by last year's events, Cyclogenesis at this transition season time of year tends to be related to the n=1 equatorially-trapped Rossby wave. Indeed, looking at the GASP analysis there is a clear Rossby wave pattern located at about 90E, with an equatorial westerly maximum and a cyclonic vortex on either side.
Since I have gone into the computer and pulled out charts, I also had a look at tropical cyclone Manou threatening the east Madagascar coastline. At the surface/850 this is extremely interesting, being a long-way from the ITCZ, in the opposite hemisphere in fact. The cyclonic vorticity (purple on the GASP analysis) seems to be associated with strong shear in the trades as they are deflected around Madagascar as they flow into the cross equatorial current along the African coastline.
At upper levels (200 hPa) see chart on my web-page, there is a very large scale anticylone sitting above Manou. The anticyclone looks like the southern member of a twin-vortex n=1 Rossby wave pair; but it is further west and of a much larger scale than the Rossby wave sitting at 90 E at low-levels.
This mis-match in scale and location of the waves at upper and lower levels has been discussed often through last year; but it is rarely as dramatic as in thsi event. Interestingly, beside the Hovmoiler diagnostic of the MJO referred to above, Matt the boy wonder also plots plan views of the OLR anomalies associated with each wave-type. His current animation is archived here. You can see he has diagnosed a westwards travelling anomaly, symmetric about the equator and therefore an n=1 Rossby wave) with the convection at about the latitude of Madagascar and located over Madagascar round about now.
So, the Rossby-wave dynmaics for both these events seems definite; but the mismatch in location and scale between 850 and 200 hPa needs pondering over.
Also, I must add, as pointed out by Roger Edson and others, theoretically interesting as this is, it is also deadly serious, in that Madagscar is heavily populated (something like 16 million people) and has already suffered severe flooding associated with Tropical cyclone Fari in January this year http://www.reliefweb.int/w/rwb.nsf/vCD/Madagascar?OpenDocument&StartKey=Madagascar&ExpandView
cheers
John McBride
Roger Edson
Hi,
The current (08May00UTC)
QuikSCAT pass suggests circulations at both 8N 93E and 9S 93E with a very
nice westerly surge of 25-30kt
along the equator at these longitudes.
Suggest you pick the model that shows the circulation initializing in the most correct location...
Satellite imagery, however,
indicates moderate upper level shear over both areas, in particular the
southern hemisphere circulation.
The Bay of B area looks a little better, but still needs a bit to get
going.
Good luck.
Roger
Akhilesh Gupta
Hi,
VORTEX in the NIO REGION
Thanks Roger, Julian, John for valuable inputs!
Latest satellite imageries
indicate intense convection with spiral bands over south bay of Bengal.
We are locating the vortex around 6N/91E. IMD declared the system as Tropical
Depression on dvorak^Òs scale T-1.5. and also issued early
Alert for the system.
Now our models are responding the initial vortex. Most of our models (global
T-80/T-170) show the system is moving close to Indian east coast south
of Chennai (13 N/81E) by Day 4 (13th) with severe cyclonic storm
intensity. NCMRWF forecast
(www.ncmrwf.gov.in) suggests its further eastward movement to emerge
into the Arabian Sea in Day 5 forecast. NCEP /COLA predictions are quite
similar except they do not show movement towards Arabian sea. ECMWF shows
system is skirting the Indian coast after day 3. The vortex in the South
Indian Ocean at 8S/92E is right now well organized but NCMRWF forecast
suggest weakening and SEward movement in Day3-Day5. NIO circulation appears
to be intensifying at the expense of weakening of SIO one. SST is
quite favourable (> 29deg) over South BOB but environment appears to be
not that conducive (slightly high Vertical shear). There are
questions! What could be the chances of intensification? Particularly
into a hurricane? Right now vortex lies more than 1000 km away from
coast. Climatology does not suggest crossing south of Chennai.
Akhilesh
--------------------------------------------------------------------------------
Dr Akhilesh Gupta
Director& Scientist
F
National Centre for Medium
Range Weather Forecasting (NCMRWF)
Johnny Chan
Subject: Onset of the Summer Monsoon over the South China Sea
Dear All:
Following McBride's discussion,
I have made a short comment on how the tropical cyclone over the
Bay of Bengal may be a precursor of the onset of the South China
Sea summer monsoon. Since McBride has not set up a monsoon
discussion list, I decided to use this list to initiate the discussion.
I hope those who are not interested in monsoons would not be too
upset. Please find
the discussion in http://aposf02.cityu.edu.hk/~mcg/discussion/index.htm
and click on today's date.
Johnny Chan
**************************************************************
* Prof. Johnny C. L. Chan
* Laboratory for Atmospheric
Research
* Dept. of Physics &
Mat. Sci., City University of Hong Kong
* Tat Chee Ave., Kowloon,
Hong Kong, CHINA
