Friday, April 25, 2014

The Basics of a severe weather sounding.

Severe weather season is quickly approaching. So, here is another installment on severe weather  parameters and indices. This installment will deal with SKEW-T soundings.

I did a similar one quite a while back. But I think that one was a bit too complex. So I will do this one that I hope is easier to understand.

If you want to read that one, you can find it here.

                                                A 18Z sounding taken the other day.

When you look at a sounding you're looking at a vertical profile, so the bottom of the chart is the surface, and the top of the chart is up near the top of the atmosphere.

The black numbers on the far left side of the Skew- T diagram are all pressure values measured in millibars. They go from 1000mb to 100mb, 1000mb is very close to the average sea-level pressure, 500mb would be in middle of the atmosphere, and 100mb is very close to the top of the atmosphere.

The red numbers on the left side of the diagram are altitude values in kilometers. SFC means the surface altitude where the balloon was launched.

The black numbers on the bottom of the diagram are temperature values in degrees Celsius. Note the temperature lines skewed (tilted up) to the upper right .

The red line represents the temperature of the atmosphere, while the green line represents the dewpoint. (the dewpoint line is always on the left and the temperature line is always on the right.)

On the 18z skew T you can see the surface dewpoint is 51 and the surface temperature is 80. You can also see the wind barbs on the right side of the diagram. Each wind symbol represents the wind speed and direction at that level of the atmosphere. This allows you to see the shear (twist) in the air.

Don't know if you know how to read a wind barb. But, the end of the staff points in the direction the wind is blowing and the barbs show wind speed.

A full blacked pennant or triangle is 50 knots of speed.

A full line on the wind barb is 10 knots of speed.

 A half line is 5 knots of speed.

 Now look at the winds. The surface wind is out of the south southwest at 20 knots, but the wind direction changes to the southwest and speed increases with height. A clockwise wind direction change is called veering, when it the other way it's called backing. Twisting in the atmosphere is a major component in promoting severe thunderstorm development.

In the upper right hand side is a diagram called an hodograph. A hodograph shows the change of wind speed and direction with height (vertical wind shear), though the layers of the atmosphere. The wind barb data next to the SKEW-T is plotted on the hodograph and a line is drawn connecting the different layers. It makes it easier to see the twisting action going on.  
Note the temperature (red line) decreases with height from the surface up to about 925 mb. The temperature then briefly increases with height (This is called a temperature inversion), before going on to decrease with height. The dewpoint follows a pattern that looks similar.  All thunderstorms require low level moisture, instability and a trigger. Severe thunderstorms also require decent wind shear - speed and/or directional change of the winds with height.

The sounding immediately shows the moisture profile. The better thunderstorm setups will have plenty of moisture in the lowest few km of the atmosphere with dry air aloft. But not too much low level moisture - you want to see the temperature and dew point lines with a few degrees of separation . If the temp/DP lines are very close together or the same values , it will be very cloudy and raining.

The brown dotted line near the temperature line is called the Theoretical Air Parcel Plot (TAPP) is a representation of how a parcel of air may rise from near the surface. Where the brown dotted line is to the right of the temperature trace it is unstable. the further to the right of the Temp trace the  line is the more instability. And if brown dotted line is to the left of the temp trace then things are relatively stable.

 If the surface layer moisture increases or decreases more than expected. Increasing or decreasing the dew point makes a much bigger difference to overall instability than a changing surface temperature. You can also see the complete wind shear profile at a glance. Wind speeds will typically increase the higher up you go. The better thunderstorm setups will have winds backing (anticlockwise) with height in the lower few km. For example N-NE at the surface, N at 925hPa, NW at 850hpa and W by 700hPa is a typical scenario along the east coast. This turning helps establish good updraft/downdraft separation and hopefully a supercell. Of course it is not that simple.

On the SKEW-T you can see where they have labeled the LCL. The LCL will be where cloud bases will initially form - ie. the first cumulus clouds. As more and more convection occurs the air will mix - and cloud bases will develop lower down. Numerous thunderstorm indices can be determined on the Sounding. The for now I will only deal with Lifted Index (LI) and Convective Available Potential Energy (CAPE). The higher the CAPE and lower the LI means there is generally a better shot at severe weather.

On the sounding you provided you can see that CAPE is around 1700 and LI is at -7....showing a moderate severe set up...but nothing that great. The CAP (the temperature inversion) is a relatively warm layer of air that may delay or completely suppress the formation of thunderstorms. On the sounding it can be seen where the temperature stays the same or often increases with height. You can see there is a CAP starting just below 800mb. This is a moderate CAP. You can see another CAP at 550hPa. These higher CAPS do not necessarily stop thunderstorms but may slow updraft strength.

If you want to go into more detail on CAPE. use this link 

Regarding storm height, this is easily read form a sounding. The level at which the Theoretical Air Parcel Plot line (TAPP) crosses the temperature trace is known as the Equilibrium level (EL). This is the level where storms anvil out. Note though, a strong thunderstorm can overshoot this level resulting is storm tops above the EL. For this reason, on any given day, the taller storms are most likely to be the strongest but storm height is largely dependent on the height of the EL.

A few examples of severe soundings:

When looking for severe potential certain shapes can get your attention.  

One of these is called the Inverted V.

Here is an example of an Inverted V.

When you see an inverted V it means there is a good chance for severe weather capable of strong downdrafts and gusty winds.

Another is called a loaded Gun sounding, or sometimes called a goal post sounding.
Here's an example

When you see this type of sounding, it means there is tons of instability, but there is a strong cap in place that is keeping the lid on. It's like a pressure cooker, it keeps the pressure on until the lib blows. When this happens you can see very explosive thunderstorm development.

Here are two soundings that show the  environment that was in place for the severe weather outbreaks over Oklahoma in May of 2013. One is the sounding for the Newcastle/Moore and the  El Reno Tornadoes.

                        Taken on May 20th, 2013, just 3 hours before the Newcastle/Moore EF5  

                                      Sounding of the May 31,2013 El Reno Tornado
That's the basics

As always I will answer any question you may have........

Friday, April 4, 2014

What is CAPE.

One term you will hear when listening to NOAA Weather Radio, Myself, and other weather outlets during severe storm season is CAPE. Here is a little write up about what CAPE is and how it pertains to severe weather.

This is a very intricate subject, and involves a lot of math and science. I will try to stay away for most of the math and science, as much as I can.  This will be quick lesson in plain speak. When you're done with it, you should have a good understanding of what CAPE is.

CAPE is an acronym that stands for Convective Available Potential Energy.  The amount of CAPE is a measure of just how much energy is available convection.

In meteorology, convection is the vertical movement of heat and moisture in an unstable atmosphere.   Clouds are a visible signal that convection is taking place. There is a process called dry convection, but that is beyond the scope of this post. For our purpose in this write up, convection has to do the updrafts needed for thunderstorm development. Another term for this type of convection is called  moist convection.

CAPE represents the amount of buoyant energy available to speed up a parcel vertically in an updraft. The potential energy available for convection is expressed mathematically using a standard measurement of energy represented as Joules Per Kilogram (J/Kg).  

one more thing I think I need to make clear.

One Joules is the amount of energy expended in moving an object through one meter when it is opposed by a force of one  Newton ( a Joules is equal to one Newton-meter).  If you remember your high school science , you will recall that energy is stored work, Work being the application of a force through a distance. Power is the rate of flow of energy, or the rate at which work is done. Stored energy becomes working energy when it is used.

CAPE is a measure of instability in the atmosphere. Therefore, the higher the value the greater the potential for severe weather.

CAPE Values:
0001-1000 J/kg: Marginally (Weakly) Unstable
1000-2500 J/kg: Moderately Unstable
2500-3500 J/kg: Very Unstable
3500 J/kg and higher: Extremely Unstable
CAPE values of 5000 -6000 J/Kg  show staggering instability.

There are no threshold values above which severe weather becomes imminent. So, you can still have values of CAPE and have no thunderstorms.

Without diving into the laws of thermodynamics, this will be basic, but you will still have a good understanding.

An air parcel is a small volume of air that has the same general temperature, air pressure, and amount of moisture. There is no exact definition of the size of the parcel, but for our purpose   a cubic foot of air will work.
A few more acronyms:

Lifting Condensation Level (LCL) is altitude at which clouds begin in a rising parcel of air. LCL can be said to be the level to which an unsaturated air parcel can be lifted adiabatically before it becomes saturated.
Level of Free Convection (LFC): the pressure level at which the lifted parcel becomes warmer than its environment, and therefore becomes positively buoyant.

Level of Free Convection (LFC): the pressure level at which the lifted parcel becomes warmer than its environment, and therefore becomes positively buoyant.

Equilibrium Level (EL): the pressure level at which the lifted parcel becomes colder that its environment, and therefore becomes negatively buoyant.

Equilibrium Level (EL): the pressure level at which the lifted parcel becomes colder than its environment, and therefore negatively buoyant.

Convective inhibition (CIN) is a numerical measure That shows an area in the atmosphere that hinders the updraft necessary to produce convective weather.  Most of you have heard it more often called the Cap on the evening weather segment. The stronger the Cap the more difficult it is to get convective thunderstorms.

CAPE can be visualized on a thermodynamic diagram by lifting a parcel dry adiabatically until it becomes saturated this is the LCL. From there, lifted until the parcel temperature crosses the environmental temperature the LFC. The area enclosed by the two curves between the LCL and the LFC is the CIN, or negative area. From the LFC up to where the parcel temperature again crosses the environmental temperature and becomes cooler than the environment This is the EL. This area often is called positive area, but normally just called CAPE.

A Skew-T

Here is a link to a post I did, that went into a little detail on the Skew-T.
How to read a skew-t-log-p

Types of CAPE:

MLCAPE: Mean Layer CAPE  this is sometimes referred to as Mixed Layer CAPE. This type of
CAPE is calculated using a parcel consisting of mean layer values of temperature and moisture from the lowest 100 mb above the ground level.

MUCAPE: Most Unstable CAPE:
This type of CAPE is calculated using a parcel from a pressure level in the lowest 300 mb that will give you the most unstable CAPE there is.

SBCAPE: Surface-Based CAPE
This type of CAPE is calculated using a surface based parcel.

Other Types of CAPE

DCAPE: Downdraft CAPE used to estimate the potential strength of rain-cooled downdrafts

NCAPE: Normalized CAPE is CAPE that is divided by the depth of the buoyancy layer.

NCAPE is very important.   NCAPE is found by taking CAPE and dividing it by the distance between the LFC and EL in meters.

There are two aspects of CAPE that are important in trying to forecast severe weather. The size and distribution of the area of CAPE, describes the potential strength of the updrafts.

The width of the area of CAPE shown on the sounding, is important as it describes the potential strength of the speed of the updraft needed for thunderstorm development. 


Skinny CAPE

A larger number indicates what we call Fat CAPE.. Fat CAPE indicates the possibility of stronger updrafts as compared to Skinny CAPE.

Well  I most likely went in a bit too deep, but I wanted to give you a thorough understanding of what CAPE is and how we use it.    As always I will answer any questions you may have.

I will add other segments to this series that will talk more about instability, and other severe weather indices, such as:  Helicity, LI, BRN, EHI.

Other post you might like.

The thunderstorm lifecycle

Types of thunderstorms

Types of thunderstorms part 2

Severe thunderstorm structure
The Tornado

Hope you enjoyed this........and maybe came away with a better understanding of the subject.