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"From its chamber comes the whirlwind, and cold from the scattering winds." - Job 37:9.

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Saturday, May 7, 2011

IN The Mood For Clouds, A Shower, A Storm? Let's go PWAT CHASING!!!


Throughout previous posts, and in the current Convective/Wet Season in Florida in progress, I've used this term, PWAT. But what is that, and why is precipitable water value expressed in these discussions?  

Namely, because it is important in a variety of ways. It can define days that will have clouds (or none) and at what level(s) of the atmosphere they will be for starters. It also determines if there will be showers or storms. But it does not stand alone. Other parameters include the vertical thermal profile of the atmosphere (temperatures near the ground and aloft), the amount of instability in the lower levels and conversely, the amount of stability above the lower levels.

Technical precipitable water (PWAT)—(Or precipitable water vapor.) The total atmospheric water vapor contained in a vertical column of unit cross-sectional area extending between any two specified levels, commonly expressed in terms of the height to which that water substance would stand if completely condensed and collected in a vessel of the same unit cross section.

The total precipitable water is that contained in a column of unit cross section extending all of the way from the earth's surface to the “top” of the atmosphere. Mathematically, if x(p) is the mixing ratio at the pressure level, p, then the precipitable water vapor, W, contained in a layer bounded by pressures p1 and p2is given by

where g is the acceleration of gravity. In actual rainstorms, particularly thunderstorms, amounts of rain very often exceed the total precipitable water vapor of the overlying atmosphere. This results from the action of convergencethat brings into the rainstorm the water vapor from a surrounding area that is often quite large. Nevertheless, there is general correlation between precipitation amounts in given storms and the precipitable water vapor of the air massesinvolved in those storms.

AUTHOR'S NOTE: So what if you're not so hot at, or are even familiar with, the mathematical principles of "Integration" as shown in the example above by that vertical squirrelly line running up and down with p1 and p2 shown?  All that means in the equation, is that they are using the moisture content from one pressure level (p1) to the next higher level in the atmosphere (p2)..of whatever chosen levels desired. For instance, to even have rain  in Florida I'd look at the surface to 700mb (10000 ft, the standard height/level) for starters, and to simplify it. There has to be 'some' moisture at 700mb, but it doesn't have to be a fact, lower moisture at or near that level will ENHANCE storm strength because the more moist (and more buoyant) air below that level will rise faster into the drier and colder air above the warm/moist air below, inducing rising air currents/lift/convective type clouds/showers/thunderstorms. The 'w= 1/g' portion of the equation is the upward component (g being gravity) 1/g is the inverse of the downward pull of gravity.  In sum, we are looking at the mixing ratio (which involves moisture( integrated from one level of the atmosphere to another.

1.  A little easier now (I never say, "simply" because nothing is ever THAT simple). 

What is PW?

PW stands for Precipitable Water. It is a parameter which gives the amount of moisture in the troposphere (which is the part of the atmosphere that contains weather as we know it).

2. How is PW determined?

PW is determined by taking all the mass of water vapor in the troposphere and depositing it on the earth's surface (or as shown in that equation up to, 'integrating' the moisture content through two chosen levels of the atmoshere).

The depth of moisture that would be on the earth's surface is the PW value. The mass of water vapor is determined by the dewpoint (saturation mixing ratio) of the air integrated over the troposphere. Higher dewpoints lead to higher PW values, especially if the relatively high dewpoints extend through a significant vertical depth. The scale below gives an indication of the moisture content of the troposphere via PW. Note, these are TOTAL precipiciptable water values in the 'weather region' which can be quite high up there. 

*****Author's Note: THIS is where I think some folks mistake a lower PWAT value as meaning it will not rain. It is NOT the total PWAT that matters nearly so much as how and where that moisture is distributed vertically, as well as how much square footage on the ground it encompasses, as well as what type of conditions surround the area of concern!!! 

Conversely, a very HIGH PWAT does not guarantee rain if the entire atmosphere is a big soupy mess. It just means it will be very cloudy and in the case of a Florida summer, very humid and sticky. I've seen this over and over again when frontal boundaries go stationary over the Central Peninsula. Almost a guaranteed Forecast Bust if one is calling for big rain chances area wide. Often on these days, the forecast calls for a good chance of rain that never materializes (not always though). These days can be very 'conditional' though, since we can never know all the time what is going on in the atmosphere aloft in regards to the atmospheric profile. THAT is a problem.

0.50 inches or less = very low moisture content
0.50 to 1.25 inches = low moisture content
1.25 to 1.75 inches = moderate moisture content
1.75 to 2.00 inches = high moisture content
2.00 inches or above = very high moisture content

3. (Operational) significance of PW:

Flooding potential: A forecast area has a climatological normal PW for a certain time of year. In cases where the PW value is 2 to 3 or greater times more than the climatological value, flooding becomes more likely when (I'll add, "IF") a heavy precipitation event occurs.

!*****Lightning: In a high CAPE environment, high PW will lead to storms that produce an abundant amount of lightning.

PERSONAL NOTE: In Florida, not necessarily true. Our highest lightning days can be when PW is 'moderate'. But again, it depends on how the moisture is distributed. We need an area in the mid-levels that is NOT saturated, odd as it may sound. Additionally, when air aloft is colder then 'normal' (that is another discussion FOR SURE)...this produces more ice particles (hail) aloft which, in theory, when they clash well aloft, generate more lightning whether or not the hail reaches the ground does not matter. NOTE: In the full bore summer time, the most prolific lightning makers will produce  small pea or smaller hail mixed in with the rain. Originally, that hail was likely much larger before reaching your noggin.  The statement about the 'clashing hail...causing friction' is also very much in question in my mind. There is a lot more to it than that). Also, I think the hail does not only go up and down until falling, but also moves horizontally in and out of swirling currents. Still a lot to be researched.

Updraft velocity: PW is the most significant contribution to water loading. ****Water loading reduces updraft strength since gravity tries to push the precipitation mass downwards-- against the momentum of the updraft. High PW also produces a heavier downdraft. The updraft being reduced is especially evident in a weak shear environment where the downdraft locates very near the updraft, thus destroying the updraft.*****

***This was my point previously. Water loading, TOO MUCH PWAT kills updrafts and makes it cloudy and clammy, with some showers storms around which are of short duration and do not lend will for active lightning as well.****

Hail: High PW tends to reduce hail size since the updraft velocity is reduced. Again, we need a drier level to induce the moist, buoyant air below to RISE through the dry layer.  Moisture content even higher does not have to be very much, in fact..but 'some' is needed. Bone dry won't work in other words.  The most active storms will have several thin drier layers distributed throughout the vertical column (the troposphere/boundary layer) promote more rising, the moistening, then more rising again into cold cold cold air way up there.

Hail is another story altogether. It is fairly common knowledge that hail rises and falls in the updrafts, gaining more and more layers of ice until it is too heavy for the storm's supporting updraft. This is SO over simplified.  Hail can fall far from the updraft in supercell thunderstorms where no rain exists...namely and mostly in the Plains. In Florida, when it starts to hail in the summer it is usually in the last stages of the storm. Lightning will be most prevalent just before we know it to be over our heads and through the falling of it..but shortly thereafter the storm will collapse under its own weight.

Convective wind gusts: A high PW often occurs when the troposphere is fairly saturated. This can reduce convective wind gusts since convective wind gusts require dry mid-level air to add to their significance. 

Get it? Big wet blobs (preferably very cold ones, colder than what one would expect based on morning sounding data) will accelerate DOWN ward through dry air, just as they will upward during storm formation. Convective wind gusts can be microbursts, wet microburts, and downdrafts. Extensive study has been made concerning these terms...I originally included a link to a study (of many) that was made and published, but not so sure the individual would like the paper mentioned in this post. If you would like the URL (web link)...flash and email or Facebook message this way.

FORECAST: No issues today. Sunny, but guess what..some riches low level PWAT air might move into Eastern Portions of North Central late today bringing in some late afternoon/early evening clouds. Let the PWAT FUN BEGIN!

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