Thunderstorm Anatomy and Dynamics - WeatherAnswer.com

1 Thunderstorm Anatomy AND DYNAMICSAn OverviewPrepared by LCDR Bill NisleyMR 3421 Cloud PhysicsNaval Postgraduate School Monterey, credits: Thunderstorm Cell and Mammatus Clouds by: Michael Bath; Tornado by Daphne Zaras / NSSL; Supercell Thunderstorm by: AMOS; Wall Cloud by: Greg MichelsPAGE 2 OF IntroductionThe purpose of this paper is to present a broad overview of the various cloud structuresdisplayed during the life cycle of a Thunderstorm and the atmospheric Dynamics associatedwith each. Knowledge of atmospheric Dynamics provides for a keener understanding of thephysical processes related to the why and how certain cloud features form.

2 Accordingly,observation of cloud features presents visual queuing of changes in the Thunderstorm Formation and Stages of DevelopmentThunderstorm development is dependent on three basic components: moisture, instability,and some form of lifting Moisture As air near the surface is lifted higher in the atmosphere and cooled, available water vaporcondenses into small water droplets which form clouds. As condensation of water vapor occurs, latentheat is released making the rising air warmer and less dense than its surroundings (figure 1).

3 The addedheat allows the air (parcel) to continue to rise and form an updraft within the developing cloud In general1, low level moistureincreases instability simply bymaking more latent heat availableto the lower mid level moisture candecrease instability in theatmosphere because moist air isless dense than dry air andtherefor is unable to evaporateprecipitation and cloud droplets aseffectively as drier of precipitation at or below cloud level cools the air within precipitation downdrafts.

4 Thismakes the air more dense and increases downdraft velocity and Sources of Moisture. Air masses Maritime polar air massesoriginating over the Northern Pacific Oceanbring large quantities of moisture to thenorthwestern , while dry continentalpolar air from Canada frequently interactswith moist sub-tropical air from the Gulf ofMexico (figures 2 and 3). Advection Low level convergence as aresult of differential heating produces muchof the Thunderstorm activity along the GulfCoast and Florida while mid level advectionof tropical moisture associated with low leveljet streams decreases stability aloft (figure3).

5 1 Baker, P., , NWS, San Angelo, 1999) Figure 1 Positive buoyancy / instability as a result ofcondensation and release of latent heat. (fm WW2010) Figure 2 Surface Analysis VT 11/1900z APR 99 PAGE 3 OF 13 Figure 3 GOES 9 - IR 11/1945z APR Instability Defined as: The tendency for air parcelsto accelerate when they are displaced from theiroriginal position, especially the tendency to accelerateupward after being lifted. Instability is a prerequisitefor severe weather - the greater the instability, thegreater the potential for severe , 3 Sources of increased instability include: Warm temperatures in the lowest 5,000ft andcool temperatures in the middle and upperatmosphere (10-35 kft).

6 Increased low level moisture the highmoisture content of maritime tropical airresults in high wet-bulb potential temperatures which is marginally unstable and convectiveclouds readily form consistent with afternoon heating of near-surface layers4. Generally, the greater the instability, the stronger the Thunderstorm updraft. Additionally, strongvertical wind shear may compensate for weak instability. Strong wind shear can lead to updraftrotation and rapid intensification and also helps direct precipitation fallout away from thethunderstorm updraft, allowing for longer duration thunderstorms .

7 Surface heating and radiational cooling. Over land vs over water Land heats andcools approximately 3 times faster than areas near large bodies of water Florida,daytime surface heating of the land results inconvective currents which sets up a seabreeze. This convergence of air from both theGulf of Mexico to the west and Atlantic Oceanto the east results in 90-100 days withthunderstorms per year over Central 4 shows the effect of convergence andsubsequent destabilization of the air columnas it is forced aloft.

8 The resulting afternoonthunderstorms persist until the solar insolationdecreases enough to dissipate the thermalgradient between the land and water surfaces,causing the sea breeze to dissipate. Whilethe land surface undergoes radiational coolingafter sunset, the ocean surface retains muchof the heat gained during the day andconsequently, an off shore flow or land breeze often results. Off shore flow combined withsynoptic or monsoonal flow sets up low level convergence at sea which frequently provides thelifting required for the formation of thunderstorms at night.

9 Additionally, as the air above the watersurface emits longwave radiation it forms a cooler layer above the warmer water. Since the airparcels nearest the ocean surface will be warmer than the layer above, an absolutely unstablesituation will exist (figure 5). 2 Branick, M., A Comprehensive Glossary of Weather Terms for Storm Spotters, NWS, Norman, OK3 A Thunderstorm is defined as severe if it produces a tornado, 3/4 inch diameter or larger hail, and/or wind gusts 50 kts (58mph) or Kessler, Edwin.

10 , Thunderstorm Morphology and Dynamics , University of Oklahoma Press, 1983, Kessler, p. 15. Figure 4 Destabilization of an initially stable columndue to lifting associated with horizontal convergence.(Kessler, p67)PAGE 4 OF 13 At higher altitudes, radiational cooling ofsaturated parcels in the upper portion ofclouds results in conditional instability. If thisoccurs in cumulus clouds (and assumingthere is convection or updrafts alreadypresent), the increased instability will mostlikely lead to enhanced cloud the upper level instability leads to the liftingof parcels beyond the level of freeconvection (LFC), thunderstorms are likelyto Lifting Lift is needed to overcome convective inhibition,sometimes referred to as a "cap.