AOSS 414: Weather Systems Thunderstorm Environment and the Convective Cell Framework 11 April 2016 Positive Area Equilibrium Level 300 Positive 400 Area 500 Pressure (mb) Pressure 600 Level of Free Convection 700 800 LCL 900 1000 Td T Temperature (oC) (courtesy F. Remer) Convective Available Potential Energy (CAPE) Equilibrium Level 300 CAPE 400 500 Pressure (mb) Pressure 600 Level of Free 700 Convection 800 LCL 900 1000 Td T Temperature (oC) (courtesy F. Remer) EQUAL CAPE ≠ EQUAL BOUYANCY Normalized Convective Available Potential Energy (NCAPE) • CAPE (J kg-1) is sensitive to both the magnitude of the buoyancy and the depth of integration. • Vertical distribution of CAPE can play an important role in the nature of convective development NCAPE = CAPE / FCL (J kg-1 m-1 or J kg-1 mb-1) where FCL = depth of the Free Convective Layer ** Parcels experience greater accelerations when you have large amounts of buoyancy confined to a FCL of smaller depth. Entrainment • During convection, environmental air crosses cloud boundaries and dilutes the cloud air. • Net buoyancy and other properties of the cloudy air are thus moderated and the cloud is made less vigorous. – Think about the fact that environmental air is likely drier and thus less buoyant • The incorporation of environmental air into the cloud is called entrainment. Effects of entrainment Identical vertical CAPE profile, but sounding B is drier. Sounding A: Stronger updraft Sounding B: Stronger downdraft Entrainment of dry mid-level air: • Reduces buoyancy, primarily through cooling • Weakens updrafts • Strengthens downdrafts Negative Area Negative 200 Area Equilibrium Level 300 400 500 Pressure (mb) Pressure 600 Level of Free Convection 700 800 Negative LCL 900 Area 1000 Td T Temperature (oC) (courtesy F. Remer) Convective Inhibition Energy (CIN) • Is a measure of the negative area on a sounding between the surface and the LFC. – Is a vertically integrated quantity – Measures amount of energy necessary to move a parcel from the surface, through the layer that is warmer than the parcel. – For a parcel to reach the LFC, it must be forced upward with sufficient force to overcome the negative buoyancy experienced in the negative area. Modifications to Convective Inhibition (CIN) Three common mechanisms to overcome CIN: • Heating of from surface • Moistening • Synoptic scale lifting Thunderstorm Environment 2012 Dexter Tornado 15 March 2012 – 2119 UTC (5:19 EDT) This velocity couplet (green = inbound, red = output) continues to show good rotation in the storm, itself. Tornado is on the ground. Hodographs Hodographs do not always have a simple shape... Hodographs Wind vectors are plotted on polar stereographic chart using a common origin. • Special attention is often given to the lowest 6000 m of sounding •This represents surface to approximately 400-mb level, the mid-point of which (700-mb) is looked upon as the steering level for thunderstorms. Hodographs • Graphically, vector end points are connected. • The altitude (in this case in km) is often plotted on hodograph. • Vector from one endpoint to another is the shear vector for that layer, as it represents the vector difference between wind at two levels. Using Hodographs – Wind Shear SFC-6 km Vertical Shear Vector (kts) •Thunderstorms tend to become more organized and persistent as vertical shear increases. • Supercells are commonly associated with vertical shear values of 35 to 40 knots and greater through this depth (Surface to 6km). - Note: 40 knots ~ 20 m/s SFC-6km Vertical Wind Shear (kts) ~ 20 knots ~ 40 knots Markowski and Richardson (2010) Change in Shear with Height If shear vectors veer with height => Hodograph curves clockwise If shear vectors back with height => Hodograph curves counterclockwise This SENSE in the hodograph curves will impact whether we have a left or right moving supercell. Estimating Storm Motion 1. First, we estimate the average u-component of the wind using the average of only the surface and 6km u-components. 2. Next, estimate the average v-component of the wind using the average of all of the v-components for all wind vectors from surface through 6 km. 3. Compute resultant wind vector. - This represents mean storm motion direction and speed Thunderstorm Types • Single-cell Thunderstorms a. Typically have weak vertical wind shear i. Hodograph appears unorganized b. Are typically short-lived c. Severe weather is infrequent Courtesy of NWSFTC Thunderstorm Types • Multi-cell Thunderstorms a. Have stronger directional and speed shear than single cell thunderstorms i. Hodograph resembles a straight line b. Persist longer due to ability to support new cell growth c. Can produce large hail, strong winds and tornadoes Courtesy of NWSFTC Thunderstorm Types • Super-cell Thunderstorms a. Have strong directional and speed shear i. Hodographs have a curved appearance i. Clockwise turning hodograph favors a right-moving super-cell ii. Counterclockwise turning hodograph favors a left-moving super-cell b. May persist for hours, consist of one quasi-steady rotating updraft and produce large hail, strong winds and tornadoes Courtesy of NWSFTC Shear Impacts on Supercell Evolution …more about that later… Right moving supercell Left moving supercell Convective Cell Framework • Region having strong updraft – Perhaps in excess of 10 m/sec • Horizontal cross-section of O(10 to 100 km2). – Meso-beta and Meso-gamma scale • Often extends vertically through most of the troposphere. • Each updraft cell will have an area of precipitation associated with it that will be easily identifiable on radar. Convective Cell Framework • Research has shown that convective cells as observed on radar evolve into identifiable and repeated patterns. • Conceptual models have been developed based upon these observed patterns, with three predominant convective cell types being noted: – Single-cell thunderstorms – Multi-cell thunderstorms – Super-cell thunderstorms.