Isallobaric Component

[Andrew Sorce]

Not sure if there are any other fans of high wind events, but for some reason the late fall events always get me a little excited. I have a decent understanding of the dynamics that create a high wind events, but sometimes I feel like I don't completely understand what the isallobaric component is. I know many of you are fantastic and explaining these things...so...have at it.

Give the latest Buffalo forecast discussion a read if your bored, flood watches, seiches, lakeshore flooding, high winds, low topped supercells, some minor Lake effect. I'm loving it.

 

[nickgrillo]

"Isallobaric Component" was likely just referencing to the augmentation of wind speed due to pressure falls (the velocity corresponds to the allobaric gradient). This term is regularly used in NWS discussions -- especially during high-wind events associated with deepening surface cyclones.

 

[Jeff Snyder]

An isallobar is a line of constant pressure change (just as an isotherm is a line of constant temperature, etc). One of the ageostrophic wind components deals with isallobars (and it's called, fittingly, the isallobaric wind):

Visallobaric = - 1 / (rho*f^2) grad (dP/dt)

The important thing to take away from that equation is that the isallobaric wind is proportional to the gradient of pressure changes. So, imagine there are very strong surface pressure falls over Ohio and very strong pressure rises over Indiana, yielding a gradient in pressure change that points to the west, and a isallobaric wind vector that points to the east (accounting for the negative sign). In other words, winds will turn to "point" from the region of max pressure rises to the region of max pressure falls.

Now, the atmosphere will try to adjust to this response. We know that the geo wind tendency (dV/dt) is normal and to the left of the ageo wind. So, in the case above, the isallobaric wind is towards the east, and the adjustment to the geostrophic wind is to the north (note that this takes time -- it does not happen instantaneously!).

OK, so quick overview: Isallobaric wind (a component of the ageostrophic wind, which means that, since we don't allow the geo wind to be divergence, it can be a source for vertical motion if continuity is called) points from max pressure rises to max pressure falls. Around an area of max pressure falls, then, the isallobaric wind will yield convergence near that area of max pressure falls (and divergence in the area of max pressure rises, where the isallobaric winds will point "away" from it). So, if there is a concentrated area of very strong pressure falls next to an area of pressure rises (in other words, an isallobaric couplet of sorts, with a strong gradient in the pressure tendecies), there should be enhanced convergence (and implied vertical motion) nearly collocated with the area of max pressure falls (and divergence / implied subsidence / in the area of max pressure rises).

 

[Andrew Sorce]

Alright, that makes sense to me, its the kind of thing you understand, somewhat intuitively, if your really into weather; but sometimes you need to hear it explained in different ways. Anyway, before its too late, check out the 36 hour 12z GFS. Directly over WNY (my area) you see the relative deep low, and immediatly behind it there is strong surface ridging. I presume this is the reason for the widespread high wind warnings. A similarly deep low, without the strong pressure rises following on its heas would probably not produce the same intensity..correct?

 

[samuel stone]

correct, it is all relative, a pressure center 0f 998 milibars surrondend by only gentle presseure rises of up to oh say... 1008 milibars would not produce as much wind as a weaker preasure center of 1004 milibars surrounded by preasure highs of 1024 milibars. It all depends on how steep the gradient.

 

[Jeff Snyder]

correct, it is all relative, a pressure center 0f 998 milibars surrondend by only gentle presseure rises of up to oh say... 1008 milibars would not produce as much wind as a weaker preasure center of 1004 milibars surrounded by preasure highs of 1024 milibars. It all depends on how steep the gradient.

Just to avoid any confusion, the isallobaric wind pertains to pressure changes at any given location (well, technically the spatial configuration of those changes). It sounds like Sam is describing how wind speed is related to pressure gradients on a whole. The isallobaric wind is a component of the ageostrophic wind that is proportional to the gradient of pressure change, not to the gradient of pressure itself. In other words, it's the gradient of pressure tendencies (the quasi-horizontal change in pressure changes with respect to time), in contrast to the gradient of pressure (the quasi-horizontal change in pressure). The isallobaric wind is not a function of the actual pressure gradient (though the pressure gradient adjusts to the pressure tendency fields), which means that there can be a very strong isallobaric wind even if the actual pressure gradient at some given time is very weak (or vice versa -- a weak isallobaric wind in the presence of very strong pressure gradient).