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 OPERATIONAL USES OF THE LAYER SLICE METHOD

METEOROLOGIST JEFF HABY

The layer slice method employs looking at various layers of the troposphere and determining their (in)stability. In a bulk measure analysis the (in)stability of the troposphere is taken as a whole (such as LI). In a layer slice analysis, the troposphere is subdivided into distinct air masses and source regions for the air in that layer of the sounding.

The troposphere can be sliced by looking for rapid wind changes, rapid dewpoint changes, rapid temperature changes and rapid changes in cloud cover with height. When employing the slice method you are looking for fairly homogeneous layers of the troposphere (i.e. warm and moist boundary layer, Dry and cool mid-levels, moist between 400 and 500 millibars, upper level jet winds hauling like crazy).

Once you have divided the troposphere into homogeneous slices (usually you find from 2 to 3 slices) then assess the stability of each slice. See how the temperature changes from the bottom of the slice to the top of the slice and determine the distance from the bottom to the top of the slice (i.e. PBL has a depth of 1.5 km, temp at surface is 30 C, temp at 1.5 km level is 20 C, therefore the temperature lapse rate is (30-20) / 1.5 = 6.7 C/km. Find the lapse rate for each slice using this method. If the lapse rate is greater than 9.8 C/km, then that slice is absolutely unstable. If the lapse rate is less than 4 C/km, then that slice is absolutely stable. If the lapse rate is between 4 and 9.8 C/km, then that slice is conditionally unstable. Of course, a lapse rate of 8 C/km is much more conditionally unstable than a lapse rate of 5 C/km. Now you will be able to assess which regions of the troposphere are stable, unstable, and conditionally unstable.

Soundings have an indice called L57. This is good for estimating the stability or instability of the mid-levels of the troposphere. In a severe weather situation, the L57 (700 to 500mb lapse rate) will be steep indeed (i.e. 7.0+ degrees C per kilometer).

The most stable layers will be inversions, the temperature increases with height in these layers. With the presence of inversions, convection can not build from the surface into the mid and upper levels of the troposphere. Any convection would have to break the cap or develop as elevated convection (convection above the cap). Lifting that is initiated above the cap is usually not associated with thermodynamic thunderstorms, but rather dynamic lifting (such as cool season isentropic lifting and intense upper level divergence). CSI is elevated convection above the cap.

Next, look for hydrolapse(s). A hydrolapse occurs in the transition between slices. It marks the boundary between a moist and drier air mass. A wind shift usually accompanies the hydrolapse. The moist slice will have a wind direction from a moisture source, while the drier air will have a trajectory from a drier source such as a dry high elevation region or subsidence associated with a ridge of high pressure. The upper levels of the troposphere (above 500mb) are generally dry (low dewpoints) since temperatures are cold. Sometimes the mid and upper levels will be one continuous slice. Other times there will be a distinctly different wind direction and wind speed between the mid and upper levels (i.e. Mid level winds of 60 knts, with a jet streak in the upper levels with wind speeds of 120 knts).

Layers of clouds can be picked up from examining the Skew-T, clouds are present when the temperature and dewpoint just above the boundary layer are equal. In the mid and upper levels, if the temperature is within 5 degrees of the dewpoint, that is a good indication clouds exist at that level (this is why upper level station plots fill in the station plot circle when the dewpoint depression is 5 C or less). The calculation of dewpoint becomes more difficult as the rawinsonde climbs to low pressures and low temperature and for temperatures well below freezing the frost point is more relevant than the dewpoint. Therefore, on the sounding, the dewpoint will not necessarily equal the temperature in association with mid and upper level clouds.

Ask yourself the source region for air in each slice of the troposphere. Now you have a good composite view of the troposphere from the surface to the upper levels.

After finding the layer slices, also answer these questions: What is the potential for convective instability?, how strong is the CAP and what is the potential for the cap to break?; How strong is the speed and directional shear between slices?; Are the mid-levels unstable? How will thermal advections and lifting mechanisms impact the stability or instability of the slices throughout the day?

Bulk measures of the troposphere such as LI ignore inversions. The CAPE value also ignores inversions. Convection may not occur not matter how high the CAPE is. The cap must break for CAPE to be converted into KE (kinetic energy (a.k.a. Energy of motion)) in a thermodynamic convection situation. With the layer slice method you can answer if convection will occur at all in the first place and where in the troposphere it has the potential to occur. The layer slice method is critical to use for predicting winter or cool season precipitation. Elevated convection is very common in the cool season.

Use the soundings along with analysis charts to gain a complete understanding of thermal advections, moisture advections, lifting mechanisms, instability, the jet stream, wind shear, evaporational cooling potential and so forth that are important to today's forecast. Try your best to begin to work Skew-T's into your forecasting method if you have not already started.