Friday, June 24, 2011

Tropical Cyclone Meari and Extratropical Mischief

As discussed in yesterday's post, I'm quite interested in looking at tropical-extratropical interactions during this monsoon season.  I'm also interested in how tropical cyclones impact weather prediction in the midlatitudes, and today we have an interesting case to examine.

Over the past two days, tropical cyclone Meari has tracked north-northwestward from the maritime sub-continent toward China and is presently located between Taiwan and Japan.

The Joint Typhoon Warning Center forecasts Meari to move northward, making landfall on the Korean Peninsula at around 1200 UTC 26 June.
Source: Joint Typhoon Warning Center
As tropical cyclones move northward and interact with the midlatitude flow, they can cause all sorts of mischief with regards to weather prediction.  In particular, they are often accompanied by heavy rainfall and related diabatic heating.  Sometimes they undergo extratropical transition and redevelop as a extratropical cyclone.  Both of these processes can contribute to the building of an upper-level ridge and can lead to a process called downstream development in which the flow downstream becomes highly amplified.  Since these processes are highly nonlinear, this often leads to large uncertainty in medium-range forecasts.

As shown in the GFS loop below,  Meari is not expected to undergo extratropical transition (at least during this 5 day period).  It weakens following landfall, but as can be seen in the dynamic tropopause analysis (top panel), it does contribute to the development of a high amplitude ridge over Japan.

There is, however, another cyclone in this case, an extratropical cyclone that develops over the northwest Pacific and moves into the Bering Sea.  Downstream of this feature a pronounced upper-level ridge forms over the Gulf of Alaska and contributes to a sharp trough that develops off the Pacific Northwest coast.

There is great uncertainty in regards to the position and intensity of this trough.  If we look at the GFS ensemble, one can see wide ranging solutions.  In some, the trough moves onshore as an open wave, in others it closes off upstream of California.  This makes quite a bit of difference in the weather for folks in the Pacific Northwest!

Source: Penn State Meteorology E-Wall
At issue is how much this forecast uncertainty results primarily from processes in the mid and high latitudes, such as the midlatitude cyclone development, or processes related to tropical cyclone Meari.  I suspect the former may be quite important, but Meari could be contributing as well.  We'll see events where tropical cyclones have an even stronger impact on our forecast in the coming months.

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