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The Saharan Air Layer and Hurricanes

posted on Tuesday, September 29th, 2009 at 5:52 pm
Saharan Dust, Image Credit: NASA

Saharan Dust, Image Credit: NASA

Meteorologists studying hurricanes are still trying to understand the Saharan Air Layer, or SAL, and its affect on hurricanes. On the one hand, scientists believe the SAL triggers the formation of easterly waves, and they know hurricanes form from easterly waves. But, on the other hand, the SAL is composed of hot, dry air. Hurricanes don’t like dry air. Studies have shown this. (Chris Landsea, NOAA) So, after the wave forms, the SAL can help kill the developing storm by injecting dry air into it.

Now, there’s a new question: What is the effect of the dust outbreaks that often occur and travel with the Saharan Air Layer? (The image above captures a dust outbreak in February 2000.)

After the record-breaking 2005 hurricane season, an international group of scientists came together to study these easterly waves coming off the coast of Africa. During the 2006 season, research aircraft based on the Cape Verde Islands, flew into these waves to study their development into tropical cyclones. As I tell you this, you may remember that the 2006 year was a bust for hurricane forecasters-it was an uneventful season. Many blamed a late developing El Nino. These researchers are blaming the SAL and the associated dust outbreak.

In the new issue of the Bulletin of the American Meteorological Society, authors Zipser et al. investigate, “The Saharan Air Layer and the Fate of African Easterly Waves”.

The SAL is composed of at least 3 layers:

1. The top layer: hot, dry air from the Saharan Desert 2. The mid-bottom layer: cool, moist air from the Atlantic coast. 3. Underneath this:  cool, dense marine air near the ocean surface.

Dust is not on my list. That’s because the SAL and the associated windy conditions can cause a dust outbreak, and then the dust will travel with the SAL. But, this is not always the case. The SAL is also associated with two other important features of hurricane forecasting; the easterly wave and the easterly jet. (The jet is an area of strong winds that is shown to negatively impact tropical storm development. – Chris Landsea, NOAA)

The majority of hurricanes in the Atlantic Ocean come from African easterly waves. (Keep in mind that about 60 waves form each year, and only a small number become tropical storms. – Chris Landsea, NOAA) In the article, Zipser et al. writes, African easterly waves “… originate over the African continent, with their initial growth fueled by the temperature contrast between the hot air over the Sahara desert and the relatively cooler, humid air to its south.” This difference in temperatures creates instability or rising air all summer long.  The rising, unstable air creates showers and thunderstorms. The thunderstorms then move off the coast of Africa toward the Atlantic Ocean, and then westward toward the United States, because the predominant wind flow in this area is east to west.

African Easterly Wave, Image Credit: NASA

African Easterly Wave, Image Credit: NASA

So, you have an easterly wave, simply put; an area of  showers and thunderstorms that persists as it is carried along with the predominant wind flow from east to west across the Atlantic Ocean. These are the tropical waves or areas of disturbed weather that TV forecasters are always pointing out on the screen saying, “This is an area of potential tropical development.” National Hurricane Center forecasters usually give these waves a low (less than 30%), medium (30-50%), or high (50% or greater) chance of developing.

In the images to your left, an easterly wave develops on the African continent, begins moving westward off the coast, and then starts to rotate.

In the first image, the big area of white clouds tells you there is a mass of unstable air that is forming showers and thunderstorms. (Really white images on satellite tell you you’re looking at high cloud tops, so most likely you are seeing thunderstorms.)

In the second image (6 hours later), the showers and thunderstorms begin to move westward toward the coast.

In the third image, the area of showers and thunderstorms, or African easterly wave begins to rotate. This tropical wave would later become Hurricane Irene in 2005. It did not affect any land areas.

Now that we’ve talked about what an easterly wave is, and how it is the starting point for a majority of hurricanes, let’s get back to the SAL and it’s impact on easterly waves.

In the year 2006, the year these scientists launched their research, the Saharan Air Layer filled the Atlantic. (It can grow as big as the continental United States.)

The images of the Saharan Air Layer in this blog are similar to what it looked like it 2006. At times, it stretched from the African Coast to the Caribbean Sea. In their research, Zipser et.al., found a couple of items of note. First, global computer models underestimate how dry the air is in the mid-levels in the SAL. Computers estimated the relative humidity is about 40-50% higher than it actually is. Remember, hurricanes do not like dry air. That data supported other papers that show the SAL can inhibit the development of a tropical storm or hurricane.

But, what was more interesting was what they witnessed from the dust. While flying through an easterly wave, instruments showed a, “… deep dust layer up to 4 km altitude.” (p. 1148) Researchers noted that this area of high dust concentration was “highly electrically active”, and it made them question whether the dust actually aided in the development of thunderstorms.

Unfortunately, the article has no conclusion regarding the impact of the dust and the development of tropical cyclones.  The authors hope that their data will aide researchers in the future who hope to answer the question conclusively.

-Dawn Brown

Saharan Air Layer, 2001, Image Credit: NOAA

Saharan Air Layer, 2001, Image Credit: NOAA

Frost and Freeze Advisories for West, Midwest

posted on Tuesday, September 29th, 2009 at 10:48 am
Temperature Forecaste (courtesy NOAA)

Temperature Forecast (courtesy NOAA)

Cold air is taking aim at the entire country, with frost and freeze advisories for both the West Coast and Great Lakes Region today. Watch the latest frost and freeze advisories from Eureka, California, Idaho Falls, Idaho, and Milwaukee, Wisconsin. A cold front is bringing cooler, drier air all the way to the deep South.

The graphic on your left shows the forecast lows for tonight across the country. Bundle up!

Watch for my blog later today on the Saharan Air Layer and hurricanes.

Dr. Jeff Masters has the latest on the devastation from Typhoon Ketsana. His blog is on the menu on your left.

-Dawn Brown

Both Coasts Cool Down!

posted on Monday, September 28th, 2009 at 9:41 am
Both Coasts Cool Down (courtesy Unisys Weather)

Both Coasts Cool Down (courtesy Unisys Weather)

A cold and windy storm is headed into the Great Lakes Region. Frost Advisories are posted for five states, including Michigan, Minnesota, North and South Dakota, and Nebraska. Click on the state name for your local TV forecast. Record heat across the Southwest will be replaced by temperatures 20-30 degrees cooler by Tuesday. Las Vegas will go from a high near 100 degrees today to a high of 75 by Wednesday! The upper air image from Unisys Weather above depicts the forecast for Tuesday night.

Still hot across portions of Arizona, New Mexico, Colorado and Utah. The tropics are quiet. I’m reading up on the latest issue of BAMS, the Bulletin of the American Meteorological Society. It has an artical regarding the Saharan Air Layer, SAL for short, and the development of tropical cyclones or hurricanes. For the last several years, scientists have been studying whether the SAL, and the dust that comes with it, keeps tropical disturbances from growing or intensifying into hurricanes. It’s an interesting read, I’ll let you know what I find out tomorrow, (I’m still deep into it.)

Have a great Monday.

-Dawn Brown

Last Update on TD 8

posted on Sunday, September 27th, 2009 at 12:05 am

The National Hurricane Center in Miami has issued its last advisory on Tropical Depression 8.

Saturday Soaking

posted on Saturday, September 26th, 2009 at 11:34 am

Stormy Saturday for some big college match-ups  today as flash flood warnings continue across the Ohio and Tennessee River Valleys. Click on the links for the latest flood warnings and forecasts in central Mississippi , Atlanta, Georgia, Lexington, Kentucky, and Chattanooga Tennessee.

Tropical Depression 8 (courtesy NOAA) Click to Enlarge

Tropical Depression 8 (courtesy NOAA) Click to Enlarge

Tropical Depression 8 is encountering dry air and wind shear. (See my previous post on how wind shear disrupted Invest 98L)

National Hurricane Center forecasters expect TD 8 to decay into a remnant low within the next 3 days. Some reliable computer models forecast a strengthening if the wind shear decreases. We’ll watch in for now. It still won’t affect the US.

-Dawn Brown

Tropical Depression 8

posted on Friday, September 25th, 2009 at 4:06 pm
Tropical Depression #8 (Courtesy NOAA) Click to Enlarge

Tropical Depression #8 (Courtesy NOAA) Click to Enlarge

Tropical Depression 8 has formed in the Atlantic Ocean, west of the Cape Verde Islands. National Hurricane forecasters think it may strengthen into Tropical Storm Grace before quickly weakening and then dissipating over the Eastern Atlantic.

If you want to track the storm, here’s a link to Tracking Hurricanes on my website, and the MYL Forums if you want to discuss the depression with other weather enthusiasts.

-Dawn Brown

Mop up the Mid-Atlantic, Invest 99L is Being Watched

posted on Friday, September 25th, 2009 at 12:18 pm
Rain Forecast Central US (Courtesy NOAA)

Rain Forecast Central US (Courtesy NOAA)

More rain is forecast for the Ohio and Tennessee River Valleys this afternoon. There’s not a whole lot going on this midday, but this weekend, the mop up will continue in many portions of the Southeast and Mid-Atlantic. Nashville forecaster’s have the latest on the potential for flash flooding there. Right now, there’s a pretty heave cell near Memphis.

For the latest on Invest 99L, click to Read More…

Wildfires Rage Again in California, Quiet in the Tropics…

posted on Thursday, September 24th, 2009 at 12:54 pm

Another wildfire rages about thirty miles from my hometown in Southern California. The fire started between Fillmore and Moorpark. Firefighters are calling it the Guiberson fire, after a street close to where it started. National Weather Service forecasters in Southern California have been warning people since late Sunday that hot, dry and windy conditions could spark a fire. Click here for the latest Los Angeles Weather reports regarding the high wind warnings that are still in effect. You know, I was worried once the television stations started warning people that the fire danger was sky high, some crazy would go out and start a fire. Today, it’s being reported that arson is suspected. We’ll see if that’s the official cause.

It is STILL raining in the midsection of the country. Click on the read more link for more on this and the tropics!

The Pacific is Up to “N”, More Snow in the Rockies?, Dr. Gray’s New Forecast

posted on Wednesday, September 23rd, 2009 at 10:49 am

Tropical Storm Nora has formed in the Pacific Ocean. It is well away from any landmass, and will not affect the US, including Hawaii. Cold air, especially for this time of the year, is trapped in the Rocky Mountains. More snow is possible there.  Denver forecasters have the latest. Dr. Gray, the renowned hurricane forecaster, whose specialty has been forecasting long range hurricane outlooks, (producing a forecast in December for the following summer) is now forecasting just the next two weeks. Click Read More…

Freeze Warnings! Snow! It’s Only the First Day of Fall.

posted on Tuesday, September 22nd, 2009 at 10:33 am

Freeze watches extended as far south as Albuquerque this morning. Cold air is plunging to the south across the Rocky Mountains on this first day of Fall. You can watch the local TV weather forecasts in Albuquerque, New Mexico and Denver, Colorado by clicking on the link below.

Freeze Watches in Albuquerque!

Snow in Colorado?

For the latest on weather across the United States and the tropics, click to Read More.

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