Retired senior lecturer in the Department of Meteorology at Penn State, where he was lead faculty for PSU's online certificate in forecasting.
By: 24hourprof , 1:23 PM GMT on August 29, 2013
When I arrived at Penn State in the early 1980s, Toby Carlson and I had several discussions about very warm air with very low relative humidity originating over the Sahara Desert and moving across the Atlantic between 5,000 and 15,000 feet during the summer and early autumn months (essentially, hurricane season). Toby's research on the origin of dusty air over the Atlantic intrigued me (check out Toby's paper published in 1972), especially his observation that Saharan dust contains distinctive minerals such as radon-222, which allows scientists to forensically trace the source of dusty air over the Atlantic Ocean back to Africa (here's the abstract of an early paper Toby published in Science circa 1970).
At any rate, when I read Dr. Masters' blog on the Saharan Air Layer (SAL) on Sunday, August 25, I jumped at the chance of contributing to his interesting discussion. I include the SAL analysis and the infrared satellite image from Meteosat at 12Z yesterday morning below, courtesy of the Cooperative Institute of Meteorological Satellite Studies. The yellows, oranges, and reds indicate various concentrations of Saharan dust, while cold cloud tops associated with showers and thunderstorms appear white on the infrared imagery.
The 12Z analysis of the Saharan Air Layer. Concentrations of dust are depicted by yellow, orange, and red. These data are superimposed on the 12Z infrared image from Meteosat. Courtesy of the Cooperative Institute for Meteorological Satellite Studies.
There are several consequences of Saharan Air moving across the Atlantic, including the negative impact that the SAL has on tropical cyclones (or developing easterly waves coming off the African Coast). I'll talk more about the SAL and easterly waves / tropical cyclones later in my blog, but, first, I believe it's important for readers to understand the prevailing weather pattern over Africa and the tropical Atlantic during late summer and early fall, when the frequency of easterly waves traveling westward from Africa typically increases.
The 12Z GFS temperature (red) and dew-point soundings over the Sahara Desert (23.84 degrees north, 15.42 degrees east) on August 28, 2013. Note the well-mixed layer extending from the desert floor to 600 mb (about 4 kilometers up). The wide separation of the temperature (red) and dew-point (green) indicates very low relative humidity ("low RH"). Courtesy of NOAA.
I'll start with the Saharan Air Layer, which, obviously, originates over the Sahara Desert from late spring to early fall. Considering its source region, it should come as no surprise that the SAL is very warm, very dusty, and has very low relative humidity. I randomly chose a point over the Sahara and generated the GFS model skew-T at 12Z yesterday morning (above). As you can see, the troposphere was well mixed from the earth's surface to about 600 mb (about 4000 meters). At times, the well-mixed layer can extend to 500 mb (roughly 5500 meters). Like clockwork, the SAL moves westward off the coast of Africa every few or several days during hurricane season. Of course, hot, desert air moving off the coast of Africa gets modified (low levels are cooled), so a reasonable general rule is that the SAL is largely found between 850 mb and 500 mb (1500 meters to roughly 6000 meters) as it moves westward over tropical Atlantic waters.
There is also vertical wind shear associated with the Saharan Air Layer. Let's explore this idea further by looking closely at conditions over Africa during summer. Given the major-league heating over the Sahara from late spring to early fall, it stands to reason that a summer monsoon (a seasonal reversal of wind direction) develops over central Africa (see this schematic, which displays the monsoonal regions of the world based on a monsoon index devised by S.P. Khromov in 1957.). Indeed, southerly winds over the Atlantic and Indian Oceans develop in response to the heating over the Sahara, carrying tropical moisture northward toward the southern edge of the great desert. To see what I mean, check out the GFS model analysis of streamlines representing 10-meter winds at 12Z on August 28 (below; larger image). I circled southerly winds associated with monsoonal flow...note that southerly winds in both areas have a westerly component (I'll refer to this westerly component in just a moment).
The 12Z GFS model analysis of 10-meter streamlines over northern and central Africa on August 28, 2013. Larger image. Courtesy of Penn State.
Meanwhile, scorching, hot Harmattan winds blowing from the northeast (revisit the GFS model analysis above) converge with the southerly monsoonal flow, creating a relatively strong temperature gradient stretching east-west across Africa. To see what I mean, check out (below; larger image) the 12Z GFS model analysis of 2-meter temperatures on August 28, 2013. Local meteorologists used to refer to this feature as the Intertropical Front (ITF).
The 12Z GFS model analysis of 2-meter temperatures on August 28, 2013. The convergence of hot desert winds and cooler, monsoonal winds creates a frontal zone sometimes called the Intertropical Front. Larger image. Courtesy of Penn State.
I'll add here that hot desert air ascends slantwise over the cooler (and more dense) tropical air hugging the earth's surface, paving the way for showers and thunderstorms. Check out the idealized cross section below, which nicely summarizes the discussion about monsoonal flow and the scorching, hot Harmattan winds).
An idealized cross section (larger image) showing the convergence of moist monsoonal winds and hot Harmattan winds, forming the Intertropical Front. (ITF). Hot Harmattan winds overrun cooler, denser moist air, setting the stage for recurrent showers and thunderstorms. Courtesy of, and copyright by, the Penn State online certificate program in weather forecasting.
The horizontal temperature gradient associated with the ITF decreases with increasing altitude above the earth's surface. To understand my point, check out the flash below. On the left, there are two soundings on the north and south side of the Intertropical Front...one in the hot, desert air mass and the other in the cooler tropical air mass. Note that temperature decreases faster in the hot desert air mass (essentially, dry adiabatic) than the cooler tropical air mass (here, the release of latent heat of condensation slows the rate of temperature decrease with increasing altitude. The bottom line is that the horizontal temperature gradient decreases steadily up to about 650 mb.
(Left) The temperature soundings in the hot, desert air mass over the Sahara (orange) and the cooler, more moist air associated with monsoonal flow (green). Because temperature decreases faster with increasing altitude in the hot air mass, the relatively large temperature gradient at the earth's surface decreases with increasing height, eventually going to zero at about 650 mb. (Right) The vertical variation of the zonal (east-west) component of the wind with altitude. Low-Level winds lose their westerly component, given the "reversed" north-south temperature gradient. Eventually, winds develop an easterly component, which increases with increasing altitude up to 650 mb, where the Mid-Level African Easterly Jet (MLAEJ) resides. Larger image. Courtesy of, and copyright by, the Penn State online certificate program in weather forecasting.
At about 650 mb (3500 meters), the horizontal temperature gradient goes to zero, marking the pressure altitude where the fastest easterly winds typically blow. Hold that thought for a moment. The plot on the right shows how the zonal (east-west) component of the wind changes with altitude. Remember when I said that the southerly monsoonal winds at ten meters have a westerly component? Well, you can see their footprint on the lower part of the plot. But the "reversed" temperature gradient causes the westerly wind component to decrease with height above the ground. Indeed, winds develop an easterly component in the lower troposphere, which then increases with increasing altitude until 650 mb (3.5 kilometers), where the temperature gradient goes to zero. At this altitude, easterly winds reach a maximum speed.
The 12Z GFS analysis of 650-mb isotachs (in knots) and 650-mb streamlines on August 28, 2013. The elongated maximum in easterly winds is the Mid-Level African Easterly Jet. Larger image. Courtesy of Penn State.
This maximum of easterly winds is the Mid-Level African Easterly Jet, which you can readily observe on the 12Z GFS model analysis of 650-mb isotachs (in knots) and 650-mb streamlines on August 28, 2013 (above; larger image). I note that this mid-level jet lies in the vicinity of the Intertropical Front.
In a similar fashion, Saharan air layers moving off the coast of Africa increase the easterly vertical wind shear over tropical seas, creating an unfavorable environment for the further development of easterly waves.
If wind shear doesn't do the trick, entrainment of air with low relative humidity will. Indeed, when an embryonic tropical wave entrains SAL air in the middle troposphere, chances are that it will not become a tropical cyclone. That's because mid-level air with low relative humidity entrained into the central-core thunderstorms causes stronger downdrafts to develop (entrainment promotes evaporational cooling that promotes sinking parcels of air to become more negatively buoyant...see the idealized schematic describing the impact of entrainment of air with low relative humidity below; larger image).
The impact of the entrainment of air with low relative humidity on thunderstorm downdrafts (in this context, "dry" means low relative humidity). Larger image. Courtesy of, and copyright by, the Penn State online certificate program in weather forecasting.
In turn, stronger downdrafts are able to penetrate into the boundary layer, injecting lower theta-e air (air that's more stable), which, acts to snuff out new convection in the vicinity of the central core. As a result, new convection is forced to develop farther away from the central core. Air pressure then decreases away from the core in response to the convective warming associated with these new thunderstorms. As a result, the pressure gradient decreases across the cloud cluster, thus weakening the already fragile system. The die is now cast and the system fizzles (or at least doesn't develop any further).
Here endeth my two cents.
Comments will take a few seconds to appear.