Archive for the ‘Synoptic weather’ Tag

FLORIDA’S RAINY SEASON – 2009

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A related post, “Why Is Florida So Humid” has been added.

It can be found here:

https://cloudman23.wordpress.com/2009/07/04/why-is-florida-so-humid/


About a third of the way into May I noticed that television weather reports and a few of my acquaintances were starting to suggest that “perhaps” Florida’s rainy season had begun.   To be sure, before the middle of May many parts of Florida had been experiencing very significant rainfall events, some of those places on a daily basis.  One of those places was northeast Citrus County where I live.  However, I doubted that those rainfall events signaled the beginning of the “real” rainy season because my experience living much further south in Florida had conditioned me to considered the “true” rainy season to be that time when precipitation was due almost entirely to mesoscale systems, namely sea breezes and sea breeze convergence within the peninsula.  And – unless the views were severely obscured by buildings or dense stand of trees, at those times one can detect evidence of thunderstorms within hearing and/or seeing distance on almost a daily basis.

Florida’s rainfall this May was almost entirely due to weather systems of a much larger magnitude than the mesoscale – systems that show up on the national weather maps (middle-latitude cyclones with their associated frontal weather, et. al.).  Those systems, along with anticyclones (rotating highs) are often referred to as synoptic systems.

I’ve always found it interesting that the majority of our annual precipitation in peninsular Florida occurs (on the average) as a result of weather systems far smaller in magnitude than either the mid-latitude synoptic systems or the tropical synoptic systems such as hurricanes and tropical storms.

Here are three graphic illustrations of the synoptic nature of our May events followed three more images of today’s weather (June 2, 2009) over the Florida peninsula.  Comments labeled A through F  follow each illustration:

5-17-09loop2238z

70 minute loop begins 5:28 pm EST, May 17, 2009

MOST ALL IMAGES IN THIS WEB-LOG ENLARGE

WITH A LEFT CLICK OR TWO

AND

IN MOST CASES IN ORDER TO ACTIVATE LOOPS

YOU MUST FIRST ENLARGE.

A.  In this 70 minute loop (starting at 5:28 PM EST on May 17th notice the cold front that shows up well along a line from eastern Tennessee down to southern Mississippi.  If one were to see only the Florida peninsula portion of this image I can see how he/she might immediately assume that this was a sea breeze convergence day.  But as you can see, this is pre-cold frontal weather being drawn northward.  Not to say the warmer land surface and some convergence did not play a role, it is nonetheless clear that the weather is dominated by the synoptic scale.

5-17-09seLoop0328

B.  This 70 minute loop of the same system shows very nicely the pre-frontal nature of Florida’s rainfall by virtue of the fact that it has moved on in accordance with the general motion of the cyclone across the United States from west to east.  This loop starts at 11:28 PM EST on May 17th.

5-26-09 2100z SurfC.  Here is an impressive array of alternating lows and highs of the synoptic scale on May 26.  At this time the movement of the lows was almost perfectly synchonized in the diurnal mode so that each day, with the help of the intense heating of the peninsula, we got significant rainfall in my neighborhood (latitude 29˚North by longitude 80.4 West – to the nearest 10th of a degree).  Notice the lows centered off the Georgia coast, south-central Alabama, and Texas – all three with associated troughs.  Each of those provided my neighborhood a great deal of rain and certainly cramped my style as I was attempting to spend a lot of time outdoors landscaping and doing my annual manicuring of my woods.  But – because of three years of drought here, I was thanking the Great Guy In The Sky for each and every drop and respecting His audible commands to stay safely indoors in the form of lightning hits that were uncomfortably close.

I was surprised to learn recently that the National Weather Service Forecast Office has declared May 11 to be the beginning of the 2009 “rainy season” of Florida.  This is a full 9 days ahead of May 20, the mean starting date.  Who am I to disagree with the experts?  It matters not in the real world I suppose – only in the academic world in which people like me often get lost.  The bottom line is that we need the rain and no matter whether May’s events were “true, traditional” rainy season events or not, they were a blessing.

Now lets take a look at weather over the peninsula a little earlier today.

6-2-09 sea breeze

UNLIKE THE FIRST TWO IMAGES –

THIS IS A STILL – NOT A LOOP.

D.  Today, June 2, 2009, the radar shortly before 3 pm EST is showing precipitation as a result of sea breeze “fronts” along both sides of the peninsula.  I suspect convergence is occurring in the south part as shown by the beginning of development over some of the glades south of Lake Okeechobee.  This is more like a Florida “rainy season” day as I have learned to know it but even today – a synoptic system is providing a noticeable influence (see next two images). For those of you who live in my neighborhood, the Crystal River winds at the time of this observation were 7 mph from the west and that is ample to bring in moist air which is rising over the heated land to form the showers that are appearing on this radar image.

6-2-09 628pEST rad ed

E. Later today the thunderstorms became more intense and in the still radar image above you can see a decided concentration toward the western side of the peninsula.

6-2-09 333p ESTsurf

F.  And here is a synoptic map showing the low (with its associated fronts) that is influencing Florida’s weather today.  There is a “rule of thumb” in meteorology that the air ahead of a front moves more or less parallel to that front.  If you will simply extend in your mind’s eye the warm front further toward Florida you will realize that there is a force over most of Florida tending to make smaller weather systems (like mesoscale thunderstorm complexes) move toward the WNW.  Apparently the winds aloft are not strong enough to counteract that.

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Here are some interesting statistics for two locations in Florida providing some geographical contrasts along the peninsula.

Ocala averages almost 50” of rainfall per year of which nearly two-thirds falls in May through October.

Homestead (south of Miami) averages nearly 60” per year of which over three-fourths falls in May through October.

Here are the actual numbers (statistical means):

Ocala (in Central Florida) 49.68” annual     31.10” May through October = 62.6%

Homestead (south of Miami) 58.20” annual    45.70” May through October = 78.5%

For further information about Florida’s rainy season  here is a safe link in the pdf format from NOAA.

http://www.srh.noaa.gov/images/mfl/news/Rainy_Season_Beginning_2009.pdf

Yours Truly,

Tonie A. Toney

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THE CORIOLIS EFFECT IN THE REAL WORLD – A TUTORIAL – PART 1


Some physicists prefer to use the term “Coriolis Effect” over Coriolis Force claiming that it is only an apparent force.  I tend to agree with that.

The Coriolis force is something that just about everyone in school learns about at one time or another.  To be sure, it is a topic in secondary school earth science and physics courses.  A low percentage of students enroll in the latter but a very large number are exposed to the former partly because in many school systems earth and/or environmental science is required.  Non-science majors in college enroll in earth and/or environmental sciences partly because it is perceived to be far easier than some of the other options – e.g. physics or chemistry.

The crux of the Coriolis force with regard to earth is that because our planet is rotating – objects and fluids in motion tend to deflect to the right in the northern hemisphere and to the left in the southern hemisphere.  The larger the circulation system the more there is likely to be an obvious response to the force.  Physicists, by the way, tell us that it is but an “apparent” force and that it is more accurate to call it the Coriolis “effect” which I intend to do from here on. There is no need to debate the term here but if you want to learn more about the Coriolis effect I suggest you use both terms in your search.

Earth’s period of rotation is once per day.  The rotational direction is from west to east.  If you looked at earth from “above” the north pole you would discover that the rotation is counterclockwise, and if you looked at the earth from “below” the south pole you would find a clockwise rotation.  If you have difficulty envisioning that “reversal” I recommend that you pick up an item and rotate it watching the rotation from one end of the axis.  Then continue rotating it in the same direction – don’t stop – but view it from the other end of the axis.  You should observe the reversal; from one end it will be counterclockwise and from the other end it will be clockwise.

SPECIAL NOTE: One of the greatest myths or misconceptions in physics is that the Coriolis effect determines the direction of rotation of water down a toilet or other drain.  That is absolutely untrue.  If you live in the United States and observe the direction the water moves down a toilet in your dwelling, then, crate it up and ship it to New Zealand and have someone install it there, upon flushing the water would go down the same way.

Next, look at the demonstrations shown on Quick Time at the following site.  Before you go there take note of this.  The first boy, wearing the blue headgear is rotating clockwise when the playground device is viewed from atop the axis of rotation so his setup is analogous to the southern hemisphere.  The other two boys (one with red headgear and the other bare-headed) are rotating counterclockwise so their setup is analogous to the northern hemisphere.

http://ffden-2.phys.uaf.edu/213.web.stuff/ET_AL_web_site/movies.html

Hopefully you saw that the first boy’s ball went to HIS left as would be expected in the southern hemisphere (clockwise) and the other two experienced the opposite (to THEIR right) as would be expected for the northern hemisphere (counterclockwise).  You might want to scroll down a little further on that page and you will find a Quick Time animation of a ball deflecting to the right on a rotating table.  The rotation will not be apparent because the camera was fixed above the table and rotating at exactly the same period.  Since the ball deflects to the right you should correctly deduce that the rotation of the table was counterclockwise like the rotation of earth from the northern hemisphere point of view.

There are many examples of the Coriolis effect here on earth.  Cold air masses in the northern hemisphere rotate clockwise because of the right turn of the air which, after sinking toward the surface flows outward from the domal system’s high pressure core; this is a great example of an anticyclone.  But my favorite example of the Coriolis phenomenon, surprisingly, is not an atmospheric example.  It is the manner in which most of the water being carried by the oceanic gyres turns right in the northern hemisphere, especially when it reaches a continental margin and left in the southern hemisphere especially when it reaches a continental margin.  Observe the image below where I have removed all but the gyre components of oceanic surface circulation.

SPECIAL NOTE:  Though not discussed here, it is the general circulation of the atmosphere at or near the surface that creates these gyres and general circulation is guided by the Coriolis effect.  If you wish to learn more about the “general circulation” of the atmosphere, other terms are global circulation, planetary circulation, and large macroscale circulation.

Left click image to enlarge.

Have you noticed – I have not explained the earth’s Coriolis effect!  I have described it, I have linked you to visual evidence, I have described a meteorological example and shown you an oceanic example via a very generalized map of the 5 oceanic gyres.  But I have not provided an explanation other than indicating that it is caused by rotation of the earth. If you have stuck with me to this point, I want to entice you with an “issue” that has often remained unaddressed/overlooked by some teachers and learners of meteorology.  That is this:  If the Coriolis effect is an important influence in large scale weather systems, and since hurricanes are synoptic scale (a type of macroscale) system, why do hurricane winds turn left in the northern hemisphere and right in the southern hemisphere?  THAT WILL BE THE TOPIC OF MY NEXT TUTORIAL POST AND IT WILL BE COMING SOON.  Now, let’s look at a hurricane.

If you want a head start on understanding a hurricane’s circulation, read this repeat of a 9-9-2008 post using hurricane Ike as an example