From: Mark A. Lander <mlander@uog9.uog.edu>
To: tropical-storms@tstorms.org
Subject: [Tropical-storms] TC on the Mei-yu

TC group,

    Tropical Depression 07W (as per the JTWC) has formed along the Mei-Yu
trough in the northern part of the South China Sea (see attached Fig.).

The Mei-Yu trough has been very active for the past couple of weeks,
with several TC-like lows (including the recent TD 06W) forming along it
and moving ENE.
    Eastward movement in low latitude is typical behavior for those TCs
that form along the Mei-yu.  TCs associated with a reverse-oriented
monsoon trough in the western North Pacific also tend to exhibit eastward
motion at low latitude.
    Although the Mei-yu trough often gets quite active this time of year
during most years, the lows that form on it don't always get designated as
TCs.
    The year 1982 was a banner year for TCs on the Mei-Yu, when Skip, Val
and Tess formed in quick succesion along the Mei-Yu trough during the
period 29 June-04 July.

    Regards,  Mark Lander

John McBride

These discussion groups are fantastic in terms of broadening our horizons and learning about the large scale
structure of the tropics.  I have been quiet this week as I have spent from 8.30 am to 5.45 pm each day attending
a Bureau of Met Weather Services Technical Conference.  Yesterday afternoon I raced away from the
meeting for ten minutes to look at the charts on my office PC.

Since I dived into real-time monitoring several
months ago, I have developed models in my head of the structure of the ITCZ, the monsoon trough etc., whereby
I look for very large scale lines of low-level cyclonic relative vorticity associated mainly with -partialU/partial-y in the trades
or monsoon westerlies.  I pulled up yesterday's satellite image and low-level vorticity map; and thought to myself, "What in God's name is that?"

On my web-page, I have put the satellite image for last night (1200 UTC) and the corresponding GASP large-scale 850 hPa
vector arrow plot superimposed on a relative vorticity analysis (this being a new NMOC Melbourne operational product).

Looking first at the 850 analysis there is a broad band of Indian Ocean monsoon westerlies that continue across IndoChina
and into the northern part of the South China Sea.  Cyclonic relative vorticity is shaded yellow, we see there is a band of
cyclonic maximum through the monsoon current, and then continuing across Indo-China, through the South China Sea, into
the Pacific north of Japan, on across the chart to the far north east corner at 40N, 180E.  This was the feature I was looking
at yesterday, but didn't understand.  Looking at the satellite image,

it looked tropical/monsoonal back over India and in the south China Sea, but seemed to blend with a frontal-type structure out east of China.

I know Johnny Chan, Mark Lander and so on live and work up in that region, and are familiar with this beast; but today I raced away from
the Conference for ten minutes, logged on to my email, saw the subject-heading of Mark's message containing the word, "Mei-yu"...... ........Light bulbs
lit up  above my head:  "So that's the Mei-yu front!!!".    I've been in monsoon meteorology a long time now, seen the words Mei-yu in the literature,
sat half asleep through talks on the topic  at conferences, but there is nothing beats seeing it, on the web, today, now!!!

For completeness, I have also put up the 200hPa vector wind and coloured background vorticity on my webpage.  It
also is interesting: the equatorial upper level easterlies across the Indian Ocean (which I consider a fundamental
component of monsoon structure) are now well developed.  The low-level monsoon trough (yellow line across the Indian
Ocean on the 850 chart) is underlain by the subtropical ridge: a purple line of anticylonic vorticity on the 200hPa chart.
On the 850 chart the monsoon trough blends continuously into the Mei-Yu; while at upper levels, the subtropical
ridge also continues across, at all locations the upper-level anticylonic line (blue) overlaying low-level cyclonic line (yellow).

Hmm.... well... I like it anyway

cheers

John mcBride

Johnny Chan

Mark:

I think you have used the term "Mei-Yu" trough too loosely.  This term
generally refers to the trough that brings the summer monsoon rain to
central China, especially the lower reaches of the Yangtze River and
Taiwan.  The trough that is now over the northern part of the South China
Sea is more appropriately referred to as the "monsoon trough" as suggested
by Ramage and Sadler.

On TD07W, it is interesting to note that even up to 00UTC this morning (7
June), neither RSMC Tokyo or the Taiwan Central Weather Bureau has issued
warnings but Hong Kong Observatory and Guangzhou Meteorological Bureau both
have.

Johnny Chan

Mark Lander

Hi Johnny,

   There is a problem in calling the trough that now extends out from the
northern South China Sea to the south of Japan "the monsoon trough" since
it exists simultaneously with a second trough at low latitudes in the
western North Pacific.  No climatology of the monsoon trough shows it
extending NE from the south China Sea to the south of Japan, but rather,
SE into the Philippine Sea.  The two perhaps merge into the same trough
system from about longitude 120E and westward).  I like to reserve
"monsoon trough" for the extension into the Philippine Sea towards Guam,
and Mei-Yu trough for the northern trough going NE to the south of Japan.
In the upper air there are also two corresponding separate ridge systems:
the sub-tropical ridge that runs out to the east along ~25 N (give or take
several degrees depending on the season), the TUTT trough along 20 N,
and the other ridge system along about 10 N that Sadler called the
"sub-equatorial ridge".  The subtropical ridge often overlies (or is a bit
south of) the "Mei-Yu" trough, while the sub-equatorial ridge overlies
the low-latitude western North Pacific monsoon trough.  The TUTT is atop
trades about mid-way between the axis of the surface sub-tropical ridge
and the western North Pacific monsoon trough.

    Also, I think that the rains brought to Taiwan in April and May
by the higher latitude trough are said to be due to the "Mei-Yu" front (or
trough).  It may later move north to bring central China its summer rains.
The terminology for things out here is definitely in need of some
refinement.  At least the various troughs are no longer just lumped
together as the "ITCZ".

    On another subject, looking at TC 07W this morning (0000 UTC 08 June),
one could easily get a T3.0 or T 3.5 out of the VIS and IR imagery.  Now
it has a ragged eye on the evening VIS.  Roger Edson has posted the
appropriate imagery.  No need of anything fancy -- a solid T 3.0 (45 kt)
on conventional imagery!  The synoptic data and radar confirm it.

Regards,  Mark Lander