Convective Addiction

Upper Air

Posted on 04. Feb, 2010 by Ryan in Education, Upper Air Education

This portion of the website will be educate the user on how to interpret the weather and its effects above the surface. It is important to note that the atmosphere extends to space and has many different layers. However, we will only discuss the thermosphere, which is the closest layer to the earth. This whole section of the site is still under construction but will be updated with more information, pictures and help links.

There are many more complicated weather processes associated with the terms that will be discussed. However, it is important to note that I will talk about why these processes are important to a chaser, as this is not material aimed at your everyday forecaster.  This educational section is not aimed at seasoned chasers, nor is it intended to appeal to anyone who already has an extensive knowledge of weather forecasting.  Much of the information that will be covered in this educational section will be overviews of the various material (what material??), but additional reading and educational resources will be provided.
You should think of the atmosphere like a ruler, which is divided up and measured at different levels. Instead of inches, the atmosphere is measured in mill bars and decreases exponentially. If we started at the surface (sea surface) we would roughly be at 1000mb, and as we increase in height, the pressure would decrease to roughly 250mb at the top of the thermosphere. We typically look at the atmosphere at the mandatory pressure levels, which are listed on the graphic. Taking a deeper look at each of these levels and their significant contributions to the weather will be the focal point of this section (note: this is important to understand before applying severe weather indices).

Twice a day NWS offices, along with countries across the world, launch weather balloons (radiosonde) to sample the atmospheric pressure, temperature, humidity, and winds. The results of these balloon launches are individual soundings, like the one show below.

Picture Courtesy of NOAA

There are a couple uses for this data.  It can be used as a real time analysis of the atmosphere, or it can be ingested into supercomputers so that models can create weather forecasts. For our purposes we will discuss making real time decisions with soundings and forecast soundings. For starters, we will discuss the following four main pressure levels, other than the surface level, that should be analyzed when making forecasts: 850mb, 700mb, 500mb, and 250mb.

850mb

On a sounding, the 850mb height can help you gauge depth of moisture in the atmosphere and to check low-level atmospheric winds.

850mb is about 5,000 feet above the surface and generally sits just above the PBL. So in most cases, the complications that come with the PBL do not apply to the 850mb level. It’s especially crucial for one to take into account the PBL being a factor during times of strong heating. You may have realized that 5,000 feet is well above the surface in the central plains, yet the same is not true for many locations in the Rockies. In those locations, you need to go to a lower pressure level to get above the surface.

In many forecast discussions you will hear the term low-level jet or LLJ. The LLJ simply a fast moving streak of winds located about a mile above the surface. These winds create low-level divergence for storm development, shear for storm organization, and moisture transportation.

In general, there are two ways the low level jet is generated; the nocturnal LLJ and cyclone–induced LLJ.

1. The nocturnal jet develops due to differential heating close to the Rocky Mountains and in the Central Plains. The temperature gradient creates wind speeds, usually between 25 and 40 mph, and it is the strongest in the early morning hours.

2. A cyclone-induced LLJ is usually stronger than its nocturnal cousin. It develops in the warm-sector of a cyclone, to the east of the cold front. The temperature gradient of the air behind the cold front, coupled with the air in the warm sector, work in unison to create the jet. Wind speeds between 40 and 60 mph are not uncommon, especially when combined with the effects on the nocturnal LLJ.

The real value of the LLJ for severe weather development is two-fold, as it involves both moisture and shear. The LLJ brings north warm and moisture-rich air from the Gulf of Mexico. This helps to create a rather unstable environment for storm development.  The added wind speed also creates wind shear, which helps the storm maintain inflow and updraft structure.

Websites to find this info:

700MB

Notice the winds as you move from the surface to 700mb. Turning from out of the SE to the SW.

The next level, and the last that is considered in the low levels when analyzing the atmosphere, is the 700mb level, which is roughly 10,000 feet above the surface. Analyzing this level is important when forecasting vertical velocities.  However, it’s also necessary to analyze for severe weather development for two main reasons: capping and directional shear.

This sounding shows a 'cap' right below the 700mb. This cap prevented convection on this day before nightfall.

During many severe weather events, capping is an issue, because the presence of too strong or too weak of a cap can ruin an otherwise opportune atmospheric environment. This cap is typically in the elevated mixed layer (EML), which is warmer than the air below it. The EML originates from the Desert Southwest and it is a dry, warm layer of air that inhibits convection until heating or lift is strong enough to overcome this cap. On the other hand, if a cap is not strong enough to limit convection, then storms may develop too early to take advantage of maximum atmospheric instability.

One can track the cap over a large area by analyzing the isotherms on a 700mb chart. The temperatures and the cap represent a seemingly direct relationship; the higher the temperatures, the stronger the cap. By assuming that the lift and convective temperatures are relatively the same across a broad area, you can figure out where storms might potentially fire.

Another quick analysis that can pay off is to determine the amount of directional shear between the surface and 700mb. When you are looking at a traditional sounding of a Central Plains severe weather outbreak, it is good to see a strong southeasterly wind at the surface, but a strong southwest or westerly wind that’s advecting drier air into the storm at the 700mb level.

Websites to find this info:

500mb

Example of a 500mb chart with vorticity plotted.

The 500mb sits halfway through the atmosphere, so it is a quick way to view overall synoptic patterns in the atmosphere. It sits around 18,000 -20,000 feet above sea level, and it is well above the PBL and thus is considered to be in the ‘free’ atmosphere.  This makes it easy to spot the large-scale features such as troughs and ridges, but it also shows smaller-scale features such as shortwave troughs.

Following the height contours reveals many shortwaves rotating around an upper-level low in the Great Lakes. Click for higher detail.

Quickly identifying general temperature advection is easy at the 500mb level by identifying the flow in the atmosphere. Wherever the temperature contours cross the height contours is the point where either cold or warm air advection is occurring.

On top of the standard temperature, height and winds that are plotted on a 500mb chart, it will usually have vorticity plotted as well. The vorticity plot is not a traditionally analyzed map; it is a computer plotting. If you are looking for a current map without vorticity plotted, then it’s recommended that you consult a difax chart. The easiest way to think of vorticity is to remember that there are 2 types: Positive Vorticity (PV) and Negative Vorticity (NV).  Positive Vorticity leads to rising motions in the atmosphere when wind is flowing through the layer (Positive Vorticity Advection/PVA), and this is important for creating storms/lift in the atmosphere. This is probably the most direct way to look at vorticity and its atmospheric influences, but if you are looking for more insight, then you can use the following links, which do a great job of explain the processes further.

250mb

Example of a strong Jet streak during the May 3rd Central Plains Tornado Outbreak

You have most likely seen a map of the jet stream on The Weather Channel or your local news.  The quickest and easiest function of the jet stream is to analyze large scale temperature trends. But there are many other useful purposes for the Jet stream, including upper-air divergence and large-scale dynamic lift and sinking motion.

Finding the highest levels of the troposphere is not always easy to locate. In the mid-latitudes it can range from 300mb (30,000ft) in cold climates to 200mb (45,000 ft) in the warm, temperate climates. Since we are dealing with severe weather, we will assume that the upper-air jet is sitting near 200mb. This is where upper-level divergence takes place; think of it as the air vent for the storm. The strong winds literally force air upward within the column of air beneath it.

A practical model exists which shows the motions around a jet and a jet streak. It is commonly referred to as ‘4-jet cell max concept’ and it is used to identify large scale sinking and rising air motions. To actually have a meaningful jet streak, winds should exceed 120mph. If you were looking down the jet streak with the equator on your right and North Pole on your left (in Northern Hemisphere), you would identify 4 corners: right front, left front, right back and left back.

Without getting into more intricate laws of physics, rising motions are found on both the left front and right back quadrants. Sinking motions occur in the right front and left back quadrants. Identifying and forecasting the location of these quadrants can prove valuable in predicting the overall environment and initiation of a storm cell.

TROPOPAUSE
Above this level is the Tropopause, which 99.999% of the time does not directly play a part in our weather here in the Troposphere. However, severe storms can actually become so violent at times that they push through this layer of warmer air and low moisture with an ‘over-shooting top’

Websites for current upper-air: