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AOSS 414: Weather Systems Introduction to Mesoscale Weather

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AOSS 414: Weather Systems Introduction to Mesoscale Weather AOSS 414: Weather Systems Atmospheric Stability and Thunderstorm Environment 04 April 2016 Scales of Meteorological Phenomena Convective Storms • Convective storms occur on a range of scales – Meso-α scale ~ 200 to 2000 km • Large hurricanes • Squall lines – Meso-β scale ~ 20 to 200 km • Large, Individual convective storms – Meso-γ scale ~ 2 to 20 km • Most thunderstorms and large cumulus clouds Fujita (1986) Categories of Convective Storms • Single-cell Thunderstorms • Multi-cell Thunderstorms • Super-cell Thunderstorms • Mesoscale Convective Complex (MCCs) – Organized region of multi-cell thunderstorms – Often acquires an oval shape as seen on satellites • Mesoscale Convective Systems (MCSs) – Squall lines, Derechos Convective storms owe their existence to positive buoyancy in the atmosphere. (COMET) Thunderstorm environment dictates the structure, impact and longevity of buoyancy-driven convection. Thunderstorm Environment • Plays a key role in determining strength, type and thus longevity of convection. – Example: vertical wind shear Thunderstorm Environment • Important Considerations – Atmospheric Stability/Instability* – Availability of Low Level Moisture* – Vertical Wind Shear * – Environmental Lifting Mechanism* – Low level thermal advection (warm advection) * = Factors that are easily analyzed using a Skew-T/Log P diagram Thunderstorm Environment • Atmospheric Stability/Instability – Three traditional static stability categories – Convective (Potential) Instability – Also important: Understanding the processes by which atmospheric stability is modified • Availability of Low Level Moisture – Increasing low level moisture is important ingredient in developing potential instability • One measure: Precipitable Water – PW is a vertically integrated measure of water vapor – 25 mm needed to support T-storms in eastern Texas – 10 mm needed to support T-storms in eastern Colorado Static Stability • Absolutely Stable – Stable for both dry- and moist-adiabatic ascent • Conditionally Stable – Stable for dry-adiabatic ascent, but unstable for moist-adiabatic ascent • Absolutely Unstable – Unstable for both dry- and moist-adiabatic ascent Convective Instability A parcel that is statically stable, may become “convectively unstable” if a lifting mechanism is present to force the layer rise and cool adiabatically (ex., cold frontal lifting) Within a convectively unstable layer, the equivalent potential temperature decreases with height. Atmospheric Stablity • Until now, we have mainly been interested in the “sense” of the static stability: – Stable – Neutral – Unstable • Next…. – We will start to look at parameters which provide a measure of the magnitude of this stability or instability. Thunderstorm Environment • Atmospheric Stability/Instability – Three traditional static stability categories – Convective (Potential) Instability – Also important: Understanding the processes by which atmospheric stability is modified • Availability of Low Level Moisture – Increasing low level moisture is important ingredient in developing potential instability – Abundant low-level moisture, when lifted, can lead to significant release of latent heating • Has impacts on buoyancy • Has impacts on small scale pressure perturbations Theta-E, Θe (Equivalent Potential Temperature) Application: A ridge of Theta-E indicates a region of warm and moist air that can provide fuel for developing mesoscale convective systems. Thunderstorm Environment • Vertical Wind Shear – Thunderstorms tend to become more organized and persistent as vertical shear (ΔV/ΔZ) increases – Too strong of shear will destroy balanced circulations within supercells – The surface through 6-km above ground level shear vector denotes the change in wind throughout this height. – Weak convection is commonly associated with vertical shear values of less than 25 knots. – Supercells are commonly associated with vertical shear values of 25 to 40 knots and greater through this depth. Thunderstorm Environment • Environmental Lifting – Surface frontal boundary – Dryline – Outflow boundary from past convection – Low level thermal advection • Think rising motion here – Divergence in upper levels • Think about relationship with four quadrants of a jet streak Thunderstorm Environment • Environmental Lifting – Surface frontal boundary – Dryline – Outflow boundary from past convection – Low level thermal advection • Think rising motion here – Divergence in upper levels • Think about relationship with four quadrants of a jet streak Drylines • In the U.S., generally found in the Southern Plains – Separates moist air flowing off the Gulf of Mexico from dry air flowing off semi-arid high plateau regions of Southwestern US and Mexico • Do not necessarily have classic characteristics of a front – Complex nature of surface energy budget (as influenced by soil moisture and low level moisture) can result in distinct diurnal variation in the sign of the temperature gradient from day to night. Drylines • Typically moves eastward during the day, regresses westward at night – Different sections of the dryline may move at different speeds and directions • The normal convention of placing the surface front on the “warm side” of the temperature gradient does not apply. • For analysis of dry a dryline, the 9g/kg isohume or the 55°F isodrosotherm is recommended as a first guess over the U.S. Southern Plains. Drylines • Can occur at any time of year, but it is typically confined to late spring and early summer. • Typically oriented north-south and parallel to topography. • Can trigger convection due to enhanced convergence at surface. • On 70% of the days that a dryline is present, new radar echoes occur within 400 km of the dryline. Dryline – 11 May 1970 Dryline – 11 May 1970 Other lifting mechanisms: Outflow boundary from past convection Outflow boundary from a thunderstorm has characteristics not unlike a cold front. Thunderstorm Environment • Low level thermal advection (warm advection) – Associated with ascending motions – Associated with directional shear that is favorable for super-cell development – Helicity = related to rotational nature of flow – However, if warm air advection occurs at mid-levels (ex: 700-mb), this could suppress convection • 700mb temperatures greater than 14C will typically result in a strong “cap” at mid-levels and suppress convection Convective Condensation Level • The Convective Condensation Level (CCL) is the height to which a parcel of air, if heated sufficiently from below, will rise until it is just saturated. – This is typically the level associated with the base of afternoon cumuliform clouds – To locate the CCL, draw a line upward from the surface dew point, parallel to the mixing ratio lines, until it reaches the temperature curve. • The CCL is the height of this intersection. NOTE: When there is a large variation in moisture near the surface, one should use assign the average moisture content of the lowest 100 mb to the surface level. Convective Condensation Level 200 ) 300 mb ( 400 500 Pressure 600 700 CCL 800 900 1000 Td Temperature (oC) (courtesy F. Remer) •For the KILN sounding, the CCL is approximately 800 mb. Convective Temperature • The Convective Temperature – Surface temperature that must be reached to start the formation of cumuliform clouds – To determine the convective temperature, locate the CCL and then proceed down along the dry adiabat until it intersects the surface pressure. Convective Temperature 200 300 400 500 Pressure(mb) 600 700 CCL 800 900 Tc 1000 Td Temperature (oC) (courtesy F. Remer) For the KILN sounding, the convective temperature is approximately 29°C. You can use CCL and CT to predict time for the onset of summertime convection, as well. METARS – DTW – 01 April 2009 Previously…. Lifting Condensation Level (LCL) • The Lifting Condensation Level (LCL) is the level at which a parcel of air reaches saturation when lifted dry adiabatically – That is, via forced ascent – To determine the LCL • From the surface dew point, draw a line upward that is parallel to the constant mixing ratio lines. • From the surface temperature, draw a line upward along the dry adiabat. • The point of intersection of the two lines is the LCL. Lifting Condensation Level (LCL) 200 ) 300 mb 400 500 Pressure( 600 700 800 LCL 900 1000 Td T Temperature (oC) (courtesy F. Remer) For the KILN sounding, the LCL is at approximately 850 mb. Level of Free Convection (LFC) • The Level of Free Convection (LFC) is the height at which a parcel, lifted dry adiabatically until saturation and then moist adiabatically thereafter, would first become warmer (less dense) than the surrounding air. – To locate the LFC: • Determine LCL • Follow moist adiabat upward from LCL until parcel becomes warmer than the environmental temperature. – This point is the LFC. Level of Free Convection (LFC) 200 ) 300 mb 400 500 Tp> Te Pressure( Level of Free 600 Convection 700 Tp< Te 800 LCL 900 1000 Td T Temperature (oC) (courtesy F. Remer) For our KILN sounding, the LFC is found at approximately 760 mb Level of Free Convection (LFC) • The Level of Free Convection (LFC) is the height at which a parcel, lifted dry adiabaitcally until saturation and then moist adiabatically thereafter, would first become warmer (less dense) than the surrounding air. – To locate the LFC: • Determine LCL • Follow moist adiabat upward from LCL until parcel becomes warmer than the environmental temperature. – This point is the LFC. (from Sturtevant 1994) Equilibrium Level • The Equilibrium Level (EL) is the height in the upper troposphere
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