Atlantic Hurricane Season: The Saharan Air Layer and Vertical Wind Shear

By: 24hourprof , 1:23 PM GMT on August 29, 2013

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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.

Lee

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34. WunderAlertBot (Admin)
3:36 PM GMT on September 04, 2013
24hourprof has created a new entry.
33. 24hourprof
10:52 AM GMT on August 31, 2013
Quoting 32. georgevandenberghe:
Quoting Lee Grenci
" While tropical cyclones are indeed very interesting, a lot of good folks living along our East and Gulf Coasts are quite happy with the rather slow activity so far this season. And I don't blame them one bit! All it takes is the landfall of one strong hurricane to ruin peoples' lives."

*** end quote **

When the weather is "interesting" somewhere..
I want to be elsewhere.

Living in the DC area, our number is going to come up sooner or
later (Hopefully MUCH later!) and I'm not looking forward to it.
I'd rather tornado and severe weather activity around here stays boring also.

I'm at least not afraid of winter storms provided I don't have obligations to travel during them.




I hear you, George. There's a lot of hurricane season left, so we shouldn't let our guard down.
Member Since: October 24, 2012 Posts: 90 Comments: 798
32. georgevandenberghe
9:41 PM GMT on August 30, 2013
Quoting Lee Grenci
" While tropical cyclones are indeed very interesting, a lot of good folks living along our East and Gulf Coasts are quite happy with the rather slow activity so far this season. And I don't blame them one bit! All it takes is the landfall of one strong hurricane to ruin peoples' lives."

*** end quote **

When the weather is "interesting" somewhere..
I want to be elsewhere.

Living in the DC area, our number is going to come up sooner or
later (Hopefully MUCH later!) and I'm not looking forward to it.
I'd rather tornado and severe weather activity around here stays boring also.

I'm at least not afraid of winter storms provided I don't have obligations to travel during them.


Member Since: February 1, 2012 Posts: 17 Comments: 1630
31. 24hourprof
7:47 PM GMT on August 30, 2013
Quoting 25. daddyjames:
Thank you very much - its nice have a better understanding of the mechanisms, instead of just repeating the mantra.

I have not commented before, but have read your post. I agree with everyone else - you do a fantastic job presenting the material.


You're very kind. In a way, my readers are my students, so I have an obligation to give them my best. I certainly try!

Many thanks for posting!

Best,

Lee
Member Since: October 24, 2012 Posts: 90 Comments: 798
30. 24hourprof
7:45 PM GMT on August 30, 2013
Quoting 24. GatorWX:
Lee,

Thank you. Very nice informative post! I'm rather amateur, so the laymen-ism and graphics are much appreciated. I'll eagerly wait your analysis of the SAL. We've been discussing this frequently on Dr Master's blog. I think it's quite misunderstood and I'm interested in what you have to say about it. I'm curious if there are better graphics than the CIMSS one posted, perhaps two panel showing both height of SAL and concentration. There certainly seems to be a strong correlation to the phenomenon this year. Seems something is missing, even with the NHC, that we have not yet realized. I'm not sure if I'm correct, but this season, so far, has yet to meet many of the expectations regarding development. Conditions otherwise seemingly would support development. Strong trades and zonal shear seem to be the ultimate killer to a developing system. Now though, trades are nothing like they were early on and shear is quite marginal out there. I think understanding the elemental makeup, height and concentration should provide some clues, for me at least. The GOM and Western Caribbean don't seem to have had the same issues at all, unless affected but wind shear. Anyway, thanks again.

-Josh


Hi Josh,

Thanks for posting. Yes, it's been slow so far. But we're only nearing the peak of hurricane season (circa September 10, if I recall correctly). While tropical cyclones are indeed very interesting, a lot of good folks living along our East and Gulf Coasts are quite happy with the rather slow activity so far this season. And I don't blame them one bit! All it takes is the landfall of one strong hurricane to ruin peoples' lives.

And, yes, Josh, I certainly will try to add my insights to the tropical discussion.

Best,

Lee
Member Since: October 24, 2012 Posts: 90 Comments: 798
29. 24hourprof
7:40 PM GMT on August 30, 2013
Quoting 23. GeorgiaStormz:
You learn something new every day.

Thanks Mr Grenci.


Life is all about learning. Thanks so much for posting.

Best,

Lee
Member Since: October 24, 2012 Posts: 90 Comments: 798
28. 24hourprof
7:39 PM GMT on August 30, 2013
Quoting 23. GeorgiaStormz:
You learn something new every day.

Thanks Mr Grenci.


Please call me Lee. And, you're quite welcome. There are so many good folks here...it motivates me to do my best.

Cheers,

Lee
Member Since: October 24, 2012 Posts: 90 Comments: 798
27. 24hourprof
7:38 PM GMT on August 30, 2013
Quoting 22. Greg01:
Thank you Lee. Very informative as always, and very timely.


Many thanks for taking the time to post. Much appreciated.

Best,

Lee
Member Since: October 24, 2012 Posts: 90 Comments: 798
26. 24hourprof
7:37 PM GMT on August 30, 2013
Quoting 21. calkevin77:
Fantastic read. I understood that the Saharan dry air and dust have been major inhibiting factors to development this year but have had a hard time understanding the cause. This blog entry really does a great job of describing the mechanics to the cause. Its interesting to see how given the timing and position of MJO plus these conditions have truly led to a perfect storm of minimal activity...pun intended. Thanks again for the great insight.


Oh you're quite welcome. Many thanks for posting. And, yes, the MJO point you make is a good one.

Best,

Lee
Member Since: October 24, 2012 Posts: 90 Comments: 798
25. daddyjames
4:37 PM GMT on August 30, 2013
Thank you very much - its nice have a better understanding of the mechanisms, instead of just repeating the mantra.

I have not commented before, but have read your post. I agree with everyone else - you do a fantastic job presenting the material.
Member Since: June 25, 2011 Posts: 2 Comments: 3731
24. GatorWX
4:04 PM GMT on August 30, 2013
Lee,

Thank you. Very nice informative post! I'm rather amateur, so the laymen-ism and graphics are much appreciated. I'll eagerly wait your analysis of the SAL. We've been discussing this frequently on Dr Master's blog. I think it's quite misunderstood and I'm interested in what you have to say about it. I'm curious if there are better graphics than the CIMSS one posted, perhaps two panel showing both height of SAL and concentration. There certainly seems to be a strong correlation to the phenomenon this year. Seems something is missing, even with the NHC, that we have not yet realized. I'm not sure if I'm correct, but this season, so far, has yet to meet many of the expectations regarding development. Conditions otherwise seemingly would support development. Strong trades and zonal shear seem to be the ultimate killer to a developing system. Now though, trades are nothing like they were early on and shear is quite marginal out there. I think understanding the elemental makeup, height and concentration should provide some clues, for me at least. The GOM and Western Caribbean don't seem to have had the same issues at all, unless affected but wind shear. Anyway, thanks again.

-Josh
Member Since: January 1, 2008 Posts: 0 Comments: 2739
23. GeorgiaStormz
3:58 PM GMT on August 30, 2013
You learn something new every day.

Thanks Mr Grenci.
Member Since: February 11, 2012 Posts: 0 Comments: 9721
22. Greg01
3:52 PM GMT on August 30, 2013
Thank you Lee. Very informative as always, and very timely.
Member Since: July 13, 2012 Posts: 0 Comments: 124
21. calkevin77
3:32 PM GMT on August 30, 2013
Fantastic read. I understood that the Saharan dry air and dust have been major inhibiting factors to development this year but have had a hard time understanding the cause. This blog entry really does a great job of describing the mechanics to the cause. Its interesting to see how given the timing and position of MJO plus these conditions have truly led to a perfect storm of minimal activity...pun intended. Thanks again for the great insight.
Member Since: June 9, 2006 Posts: 0 Comments: 838
20. 24hourprof
10:08 AM GMT on August 30, 2013
Quoting 18. barbamz:
Lee, thanks from my side too for this excellent post. Now I got a deeper idea what's happening with these "collapsing thunderstorms" inside a tropical system, which will show up as yellow expanding low level bands on RGB-loops.


Excellent point!!!!!

And many thanks for taking the time to post. It is much appreciated.

Best,

Lee
Member Since: October 24, 2012 Posts: 90 Comments: 798
19. 24hourprof
10:08 AM GMT on August 30, 2013
Quoting 14. Astrometeor:
All of the above users I concur with, you truly do an amazing job writing these blog posts for us Lee, thank you.


You're way too kind, but I really appreciate you saying that.

You folks keep me going, and so my thanks right back at you.

Best,

Lee
Member Since: October 24, 2012 Posts: 90 Comments: 798
18. barbamz
10:06 AM GMT on August 30, 2013
Lee, thanks from my side too for this excellent post. Now I got a deeper idea what's happening with these "collapsing thunderstorms" inside a tropical system, which will show up as yellow expanding low level bands on RGB-loops.
Member Since: October 25, 2008 Posts: 52 Comments: 5707
17. 24hourprof
10:06 AM GMT on August 30, 2013
Quoting 13. nigel20:
Thanks for this very informative blog post! I really appreciate it.


You're welcome. I love to write and teach...but I don't miss the grading. :-)

Best,

Lee
Member Since: October 24, 2012 Posts: 90 Comments: 798
16. 24hourprof
10:05 AM GMT on August 30, 2013
Quoting 12. pottery:
Very nice. Thank you for this clear and thorough explanation.
It's most appreciated.

Here in Trinidad (11n 61w) we have often to 'suffer' through days of Sahara Dust which tend to raise temps and cut visibility.

It's not very nice, and I'm sure that Tropical Systems that are trying to spin up, feel the same way....


Thanks.

I grew up in a steel-mill town in western Pennsylvania in the 1950s (no pollution controls), and I remember, as a kid, dust and soot on the our car the morning after my dad cleaned the car (we parked along the curb).

So, in a small way, I understand the dust you write about.

Many thanks for your interest, and many thanks for being so kind.

Best,

Lee
Member Since: October 24, 2012 Posts: 90 Comments: 798
15. 24hourprof
10:02 AM GMT on August 30, 2013
Quoting 11. VirginIslandsVisitor:
Hi, Lee!

It is so nice to sit down and read something like this and actually be able to understand what you are saying and not feeling like the "village idiot" after all was said and done! I know I have a couple of questions but will get back to you on it after I've absorbed it one more time.

Thanks again, Lee

Lindy



Thanks a million, Lindy.

Feel free to ask any questions, and I'll do my best to answer.

Best,

Lee
Member Since: October 24, 2012 Posts: 90 Comments: 798
14. Astrometeor
7:50 AM GMT on August 30, 2013
All of the above users I concur with, you truly do an amazing job writing these blog posts for us Lee, thank you.
Member Since: July 2, 2012 Posts: 98 Comments: 9921
13. nigel20
2:54 AM GMT on August 30, 2013
Thanks for this very informative blog post! I really appreciate it.
Member Since: November 6, 2010 Posts: 11 Comments: 7884
12. pottery
12:43 AM GMT on August 30, 2013
Very nice. Thank you for this clear and thorough explanation.
It's most appreciated.

Here in Trinidad (11n 61w) we have often to 'suffer' through days of Sahara Dust which tend to raise temps and cut visibility.

It's not very nice, and I'm sure that Tropical Systems that are trying to spin up, feel the same way....
Member Since: October 24, 2005 Posts: 0 Comments: 24100
11. VirginIslandsVisitor
12:18 AM GMT on August 30, 2013
Hi, Lee!

It is so nice to sit down and read something like this and actually be able to understand what you are saying and not feeling like the "village idiot" after all was said and done! I know I have a couple of questions but will get back to you on it after I've absorbed it one more time.

Thanks again, Lee

Lindy

Member Since: July 30, 2011 Posts: 2 Comments: 605
10. 24hourprof
11:42 PM GMT on August 29, 2013
Quoting 7. LurknLearn:
Excellent blog! Once again explaining complex multi-faceted cause and effect in ways that a layman can comprehend.

Thanks!


Wow! I must be on a roll today! Thanks so much!

Best,

Lee

Member Since: October 24, 2012 Posts: 90 Comments: 798
9. 24hourprof
11:41 PM GMT on August 29, 2013
Quoting 6. beell:
Very nice, Professor.
Thank you.


You're so welcome.

Please feel free to call me Lee.

Best,

Lee
Member Since: October 24, 2012 Posts: 90 Comments: 798
8. 24hourprof
11:40 PM GMT on August 29, 2013
Quoting 5. tedauxie:
Lee:

I've been mostly lurking on Dr. Master's blog for about 5 years,  trying to learn weather science, (my training is in marine science) and this article taught me more about hurricane genesis and the SAL,  than I've learned in my 5 years of lurking.
 
And, thank you, so much, for teaching to the level of the beginning meteorologist.


Music to my ears. Thanks so much for letting me know.

Best,

Lee
Member Since: October 24, 2012 Posts: 90 Comments: 798
7. LurknLearn
11:02 PM GMT on August 29, 2013
Excellent blog! Once again explaining complex multi-faceted cause and effect in ways that a layman can comprehend.

Thanks!
Member Since: July 9, 2013 Posts: 0 Comments: 47
6. beell
10:19 PM GMT on August 29, 2013
Very nice, Professor.
Thank you.
Member Since: September 11, 2007 Posts: 141 Comments: 16271
5. tedauxie
7:21 PM GMT on August 29, 2013
Lee:

I've been mostly lurking on Dr. Master's blog for about 5 years,  trying to learn weather science, (my training is in marine science) and this article taught me more about hurricane genesis and the SAL,  than I've learned in my 5 years of lurking.
 
And, thank you, so much, for teaching to the level of the beginning meteorologist.
Member Since: August 25, 2009 Posts: 0 Comments: 24
4. 24hourprof
6:36 PM GMT on August 29, 2013
Quoting 3. georgevandenberghe:
The followup question is, why is there more of this type of air mass moving over the eastern tropical Atlantic this August compared with others? Are surface pressures anomalously high over the Mediterranean
Sea or is the Sahara warmer than usual leading to a stronger easterly midlevel Jet?


Quoting SomeoneHas2BeTheRookie
" You do an excellent job in explaining things in a way that even I can understand. Thanks, Lee!"

One of the hallmarks of a good scientist is the ability to explain his work and the broad outlines of his discipline, to educated people outside of his field. THis capability is underappreciated, thanks Lee from me also
George V.


You're quite welcome, George. You ask another GREAT question. I don't know offhand, but I'll look into it. Thanks again.

Lee
Member Since: October 24, 2012 Posts: 90 Comments: 798
3. georgevandenberghe
6:01 PM GMT on August 29, 2013
The followup question is, why is there more of this type of air mass moving over the eastern tropical Atlantic this August compared with others? Are surface pressures anomalously high over the Mediterranean
Sea or is the Sahara warmer than usual leading to a stronger easterly midlevel Jet?


Quoting SomeoneHas2BeTheRookie
" You do an excellent job in explaining things in a way that even I can understand. Thanks, Lee!"

One of the hallmarks of a good scientist is the ability to explain his work and the broad outlines of his discipline, to educated people outside of his field. THis capability is underappreciated, thanks Lee from me also
George V.
Member Since: February 1, 2012 Posts: 17 Comments: 1630
2. 24hourprof
5:11 PM GMT on August 29, 2013
Quoting 1. Some1Has2BtheRookie:
I am amazed that I could learn so much in one lesson. You do an excellent job in explaining things in a way that even I can understand. Thanks, Lee!

so SAL really isn't some angry dude kicking up a lot of dust in the desert? A variation of one of Johnny Cash's songs. :)


Thanks so much.

LOL!! at the SAL comment. I needed a good laugh today. Thanks again.

Lee
Member Since: October 24, 2012 Posts: 90 Comments: 798
1. Some1Has2BtheRookie
4:42 PM GMT on August 29, 2013
I am amazed that I could learn so much in one lesson. You do an excellent job in explaining things in a way that even I can understand. Thanks, Lee!

so SAL really isn't some angry dude kicking up a lot of dust in the desert? A variation of one of Johnny Cash's songs. :)
Member Since: August 24, 2010 Posts: 0 Comments: 4737

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Retired senior lecturer in the Department of Meteorology at Penn State, where he was lead faculty for PSU's online certificate in forecasting.

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