Pro Met Questions

[Comanche]

What layer generally is considered the one that steers a TC??

http://cimss.ssec.wisc.edu/tropic/real- ... s-dlm.html

[MWatkins]

I'm not a "pro met" but I have been forecasting and learning about the tropics since college (incidentally at OU as a meteorology major)...so hope you don't mind me taking a swing at this one.

The steering layer is strongly linked to the strength of the cyclone. Weaker cyclones, like tropical depressions and weak tropical storms don't extend as far up in the atmosphere as their stronger counterparts, so they tend to be steered by the lower level flow (850MB to 700MB).

The deeper, stronger systems are thought to be steered by the deep layer flow, which can be inferred by looking at a 500MB chart.

A very common mistake people make when looking at models is looking at the surface chart and drawing conclusions on where a hurricane might go. It tells you a little, but one can gain much more information about the enviornment by looking at a 500MB chart. These deep layer features like upper lows (that may not reflect at the surface) are the players that determine where a TC may go.

Here is a 500MB chart from the latest GFS run:

http://www.nco.ncep.noaa.gov/pmb/nwprod ... 0_000m.gif

and at 700MB:

http://www.nco.ncep.noaa.gov/pmb/nwprod ... 0_000m.gif

Compare the differences in pressure heights...the maps are much different, reflecting the various features in the maps.

Hope this helps!

MW

[Derek Ortt]

was just discussed at the HRD map discussion today

It is more of a function of shear than it is intensity. TCs with little shear are steered by the deep layer mean flow while sheared storms by the shallow. The reason is that in a sheared TC, the circulation is shallow. In a low shear TC, the circulation extends well into the atmosphere

[MWatkins]

was just discussed at the HRD map discussion today

It is more of a function of shear than it is intensity. TCs with little shear are steered by the deep layer mean flow while sheared storms by the shallow. The reason is that in a sheared TC, the circulation is shallow. In a low shear TC, the circulation extends well into the atmosphere

Isn't this a bit of chicken vs. egg? I mean, how many strongly sheared category 3 hurricanes are getting pushed around in the low level flow? Not many...if any.

Plus...a sheared TC is not vertically stacked...hence the low level center will move along with the low level flow. I find it hard to believe that a TC producing little deep convection (hence, not extending up past 500MB) is going to be driven around by the mean layer flow whether or not it is strongly sheared. Weak remnant TC's in the pacific get driven around by the low level flow once they stop producing deep convection because they move over cooler water.

Glad you were talking about it in the HRD map discussion...but I respectfully disagree that steering level is directly correlated to shear. It's related...but not caused...

MW