Day 202: Update on Invest 97L

Posted by pmarsh on 21 Jul 2010 | 0 comments

For those of you who don’t routinely keep up with the happenings of the National Hurricane Center (NHC), you might be a little baffled by my choice of title tonight. “Invest 97L” is the official designation of the tropical (short)wave trough (orange dashed line below) I’ve been discussing on this blog the past few evenings; it is currently being monitored by the NHC for possible development into a tropical storm.

The NHC has downgraded the Invest 97L’s chances of development into a tropical depression/storm in the next 48 hours from 60% to 40%. The reason for this is that the (short)wave trough is still poorly organized. There are two main reasons for this. The first reason is that the (short)wave trough is currently interacting with Hispaniola. The increased friction in the low levels is disrupting air flow into the (short)wave trough just enough to prevent a surface low-pressure from developing.

The second, and probably more important, reason for the downgrade is the interaction of the tropical (short)wave trough with an upper-level low (yellow circle below) just northwest of the (short)wave trough. The airflow around the upper-level low is responsible for increasing the mid-to-upper level shear (green arrow below) atop the tropical (short)wave trough.

I know previously in this blog I’ve discussed how an increase in vertical wind shear is a good thing for the development of severe thunderstorms, so if there is a decent amount of wind shear affecting the tropical (short)wave trough, wouldn’t this increase the chance of development? In this case, no. Here’s why.

Tropical Storms and Hurricanes are giant atmospheric heat engines. They rely on a process known as “latent heat release” to maintain their vigor. In general, the more vigorous the thunderstorm, the more latent heat released into the atmosphere. If this latent heat release is confined to a small area, the pressure at the surface beneath the latent heat release will decrease. This is because as the air aloft heats, it expands. As the air expands, it exerts less pressure with the region it originally resided in, which results in lower pressure at the surface.

If we have an endless supply of warm, moist air (such as over the tropics), the potential exists for a positive feedback to occur. As a result of the lower pressure at the surface, surface air in the areas near the low pressure will begin to move into the low-pressure area in an attempt to increase the surface pressure. This increase in air moving into the same location results in surface convergence, which forces the air to rise. As long as the rising air is warm and moist (which it typically is in the tropics) it will create additional thunderstorms. These additional thunderstorms release latent heat into the atmosphere, which decreases the surface pressure even more, causing an increase in surface convergence… (and we’re off to the races).

This feedback will continue until you remove the supply of warm, moist air at the surface (i.e., hurricane moves over land or into colder waters) or you remove the latent heat from the middle-to-upper atmosphere. Here is where wind shear is detrimental to hurricane development — it blows the latent heat away from the center of the tropical storm/hurricane. The wind shear tilts the thunderstorms so they are no longer straight up and down (similar to the Leaning Tower of Pisa). Because the thunderstorm is no longer vertically oriented, the latent heat is not released over the center of the storm, preventing the positive feedback mechanism from fully taking hold.

In severe thunderstorm environments, we don’t have an endless supply of warm, moist air. As such, thunderstorms developing in this environment won’t have the benefit of the positive feedback loop described above to help maintain themselves. These thunderstorms must find a way to keep the air being drawn up into the thunderstorm (the updraft) warm and moist. If by chance the thunderstorm downdraft rains into the air being drawn into the storm, the air being drawn into the thunderstorm will cool. This cool air weakens the updraft (remember, warm air rises, cold air sinks!) to the point that the updraft can no longer hold all the water in the atmosphere it was previously holding. This water then collapses to the ground and the thunderstorm dies. In this case, if wind shear is present, the updrafts and downdrafts actually get tilted in such a way that the thunderstorm does not rain into the updraft and the thunderstorm can survive.