Retired senior lecturer in the Department of Meteorology at Penn State, where he was lead faculty for PSU's online certificate in forecasting.
By: Lee Grenci , 3:36 PM GMT on September 04, 2013
With the conversation still focusing on the lack of Atlantic hurricanes this season, I have noted the media's somewhat loose language describing "dry" and "moist" air and their impact on tropical cyclogenesis (tropical development). First, we're not talking about dry or moist air near the earth's surface, which viewers don't realize because the media rarely offers any quantification. Second, I note that relative humidity is the more correct scientific term to use in the context of tropical cyclogenesis and the presence or the absence of the Saharan Air Layer (see 18Z SAL analysis on September 3 below), which I wrote about in my last blog. By the way, tropical cyclogenesis refers to the development of (or the intensification of) a tropical cyclone.
The SAL analysis at 18Z on September 3, 2013. The presence of the SAL near a tropical wave coming off Africa at the time dramatically reduced any possibility of development in the following 48 hours. Courtesy of CIMSS.
For starters, I point out that one of the criteria for the development of a tropical cyclone is high mid-level relative humidity (of course, there are several other ingredients in the recipe for a tropical cyclone).
When thunderstorms entrain air whose relative humidity is low, evaporational cooling is enhanced aloft, which, in turn, promotes stronger downdrafts. These stronger downdrafts then penetrate into and stabilize the boundary layer, discouraging further development (see the complete explanation in my last blog).
On the other hand, high relative humidity in the middle troposphere limits evaporational cooling and thereby avoids the issue of thunderstorms developing stronger downdrafts (and the associated tropical wave eventually fizzles out).
An idealized closed system with a layer of water along the bottom. The gray "spheres" indicate molecules of water vapor. At this time, the system is in an equilibrium state (saturation), with the rate of evaporation equaling the rate of condensation. The relative humidity is 100%. Courtesy of A World of Weather: Fundamentals of Meteorology.
In general, net evaporation rates increase with decreasing relative humidity. That's because more water molecules can evaporate into the air before the evaporation rate equals the condensation rate. To understand my point, check out the image above, which shows an idealized closed system with a layer of water at a fixed temperature along the bottom of the box. At this point, the air inside the closed box is saturated...the evaporation rate equals the condensation rate, and the relative humidity is 100%. Now imagine that I remove half the water vapor molecules (the grayish "spheres" in the air inside the box). Upon removal, the evaporation rate exceeds the condensation rate (in other words, there's net evaporation) and this state will continue until the system gradually reaches equilibrium (saturation).
And so it is when the middle troposphere over the tropical Atlantic has low relative humidity. In these conditions, thunderstorms associated with tropical waves (disturbances) from Africa develop relatively strong downdrafts when they entrain mid-level air with low relative humidity. This explanation is one of the reasons why the Saharan Air Layer has such a negative impact on tropical waves and tropical cyclones (vertical wind shear associated with the SAL is another reason...revisit my last blog).
The 00Z upper-air station models (500 mb, more specifically) on September 4, 2013 (8 P.M. EDT on September 3). The number in the upper left of the each station (in red) is the temperature in degrees Celsius, and the number below it is the dew-point depression, in degrees Celsius. Larger image. Courtesy of NCEP.
The media's recent use of the words "dry" and "moist" as they relate to tropical waves coming off the coast of Africa are so vague that they are, in my opinion, essentially meaningless. Indeed, such vapid usage does not indicate the altitude of the air in question. These rather "loose" references annoy me because, in the middle troposphere, distinguishing between "dry" and "moist" air boils down to an exercise in futility. To see what I mean, check out (above; larger image) the upper-air station models (at 500 mb) over the western Atlantic at 00Z last evening (8 P.M. EDT on September 3). The red number is the 500-mb temperature in degrees Celsius. The green number below it is the dew-point depression (also expressed in degrees Celsius). Over the western Caribbean, where the National Hurricane Center indicated this morning that there was a 30% chance of development into a tropical cyclone within 48 hours (see tropical weather outlook right here), note that the 500-mb dew points were all lower than 0 degrees Celsius (keep in mind that the dew point equals the air temperature minus the dew-point depression). I'm sorry, but dew points lower than 0 degrees Celsius hardly could qualify as "moist" in my book, especially in the context of tropical systems.
This is the same kind of rubbish I frequently hear during winter, when the media states, unequivocally, that the subtropical jet stream carries "lots of tropical moisture" into the Western states. Really? There's a lot of moisture at 200 mb??? Really?? The standard pressure altitude of the subtropical jet stream is 200 mb, which lies at roughly 12000 meters (12 kilometers). Look at the temperature and dew-point depressions on the display of 200-mb station models over the western Atlantic last evening (00Z on September 4). All the dew points were lower than minus 60 degrees Celsius. Hardly any water vapor at all. Yet the media continues to describe the subtropical jet stream as carrying a lot of moisture. Annoying...Lee gives a deep sigh.
Hopefully, you've gained an appreciation for why the media's usage of the words, "dry" and "moist," in the context of the development of tropical waves from Africa, is, at the very least, too vague, and, at its worst, constitutes questionable science.
The tropical outlook issued by the National Hurricane Center around 18Z on September 3, 2013. At the time, the presence of mid-tropospheric air with low relative humidity worked against development in the following 48 hours. Courtesy of NHC.
My ranting aside, NHC on its tropical weather outlook yesterday afternoon (see above; larger 18Z infrared satellite image from Meteosat), noted that the wave over the Cape Verde Islands had little chance of developing in the next 48 hours. I looked at the GFS model analysis of a skew-T near the Cape Verde Islands at 18Z yesterday (see below; review the basics of skew-T diagrams). Note the relatively wide separation of the temperature (red) and dew-point (green) soundings in the middle troposphere (700 mb to 400 mb, with the single exception at 500 mb). This indicates relative humidity sufficiently low that, if entrained into the updrafts of thunderstorms, it would cause enough evaporational cooling to produce stronger downdrafts that would be counterproductive to tropical cyclogenesis (i.e., development). No wonder NHC gave it a 0% chance of development.
The 18Z GFS model sounding in the vicinity of the Cape Verde Islands on September 3, 2013. Courtesy of NOAA.
So I guess I'm admitting that using "dry" and "moist" air in the context of tropical waves is too vague and sidesteps the correct science. And, yes, I slip up sometimes and do the same thing, but I always try to correct my mistakes.
Here endeth the lesson.
The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.
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