Clark
Meteorologist
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Learning Series: Dry air, the SAL, and wind shear
Mon Aug 07 2006 07:50 PM
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Today, with the tropics quiet (relatively speaking), I'd like to provide a bit of information toward tropical cyclone forecasting, specifically with regards to the effects of dry air and the Saharan air layer (SAL) upon developing (or already developed) tropical disturbances. We'll call this a special in-season form of the "Learning Series" instituted during the off-season.
Generally, the presence of a dry mid-level environment is viewed as a negative factor for both developing and developed tropical cyclones. In the tropics, this generally results from large-scale subsidence throughout the atmosphere on the periphery of the subtropical ridge of high pressure. The presence of the SAL, as carried westward across the Atlantic by tropical waves, is viewed as an even stronger negative influence given that it typically combines the effects of subsidence-forced drying as well as a stable and dry mid-level environment from the deserts of northern Africa. What the dry air does is promotes the existence of cool downdrafts within any convection that is present, while tropical cyclones tend to exist in rather moist environments where such downdrafts are suppressed.
However, these two factors alone are generally not enough to keep tropical systems from developing. They may slow development to some degree, especially during the organizational stages, but tend to not keep formation from happening (if all other conditions are favorable). This is because a developing disturbance, especially once it has closed off a circulation at low/mid-levels, tends to be relatively isolated from the external environment. While all low pressure systems have, by definition, some inflowing component toward the center, this is relatively weak compared to that of an open wave or to a sheared storm. More on this in a minute. Convection that is associated with a closed low/disturbance generally serves to moisten its environment and provide the growing disturbance with conditions conducive to development.
Note that yes, in regions where a tropical wave is ill-defined in the first place or lacks some sort of focusing mechanism, dry air and/or the SAL will be enough to keep tropical systems from developing. However, it can be argued that such disturbances aren't likely to develop even under ideal conditions. It does also take more persistance from convection and potentially longer-lasting or more frequent convective bursts to ultimately get the disturbance to close off and become a viable feature in such environments. But, this is generally only a delaying factor and not a negating factor toward development.
Instead, shear is the big problem, and not just because it is occasionally found in regions where such dry air exists. Wind shear provides a mechanism by which dry air can be ingested into a tropical cyclone's circulation -- it erodes and tilts the convection associated with the tropical system, leading to disorganization. Dry, stable air is now able to enter the storm's circulation, providing a sort of cascade effect against further development and toward enhanced weakening. Continued shear or dry air entrainment/ingestion can lead to the dissolution of the storm. (A brief note on entrainment: simply put, this is the drawing in of air into something, whether generic like a cloud or specific like a hurricane. Thanks to Ed for commenting on how to make this a bit clearer.) This is exactly what happened to Chris: it developed in spite of the surrounding dry/stable environment as all other factors were favorable such SSTs and shear, but once the upper level winds became non-conducive to development due to the cut-off low that formed to its NE, this unfavorable environment engulfed the storm, essentially killing it off.
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