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Transcript of Mesoscale Instabilities
Mesoscale InstabilitiesJames T. MooreCooperative Institute for Precipitation SystemsSaint Louis UniversityDept. of Earth & Atmospheric Sciencesmoore@eas.slu.edu
COMET-RFC/HPC Hydrometeorology Course 01-114-21 November 2000
Four Types of Mesoscale Instabilities Which Can Result in Banded PrecipitationElevated Convective InstabilityConditional Symmetric InstabilityWeak Symmetric StabilityFrontogenesis
In each case the mean relative humidity must be sufficiently high (e.g., > 80%) and a source of lifting available to realize the instability.
Dry over Moist Moist over Dry Very Dry over Moist e/z = 0 e/z > 0 e/z < 0 Convectively Neutral Convectively Stable Convectively UnstableResults of lifting an initially stable, unsaturated layer to saturation given various gradients of moisture. Initial lapse rate is isothermal and 50 mb deep. Dry adiabats are thin solid lines, saturation adiabats are dash-dot lines. Adapted from Hess (1959) Understanding Convective Instability
Elevated Convective InstabilityTypically develops north of quasi-stationary or warm frontsAssociated with moderate southerly low-level jets ( > 15 m s-1) oriented nearly normal to the frontal boundaryUsually there is a strong north-south gradient of e In many cases the CAPE based on the lowest 100 mb is nearly zero while the max e CAPE is over 100 J kg-1 (winter) or 1000 J kg-1 (summer) Usually associated with strong vertical wind shear; especially veering with height
Elevated Convective Instability (cont.)Thunderstorms may form directly north of boundary or several hundred km north of boundaryWhere thunderstorms form depends upon:Relative humidity of incoming air streamSlope of isentropes in vicinity of frontal zoneMagnitude of the moisture convergence
Elevated Convective InstabilityTrier and Parsons, 1993; MWR, vol. 121, 1078-1098
Conditional Symmetric Instability: Synoptic CharacteristicsTypically a cool season phenomenaWind profile: speed increasing with height and weak directional veering with height; indicative of strong baroclinicityThermodynamic profile: nearly saturated and close to the moist-adiabatic lapse rate. Parcel motion will be neutral to moist ascent. Lapse rate is NOT conditionally unstableOften found in the vicinity of a extratropical cyclone warm front, ahead of long-wave troughs in regions of strong, moist, mid-tropospheric southwesterly flowLarge scale forcing for upward vertical motion is usually present
Conditional Symmetric Instability: Synoptic Characteristics (cont.)Soundings reveal a deep, moist layer that is convectively stable with a moist-adiabatic lapse rateOn satellite or radar imagery CSI is exhibited by multiple bands of clouds/precipitation oriented parallel to the mid-tropospheric thermal wind (or thickness lines); sometimes the bands have a component of motion toward the warm airThese heavier precipitation bands may be embedded (obscured) by other lighter precipitationWarm frontal rain/snow bands are often good candidates for being associated with CSI
Conditional Symmetric Instability: Physical CharacteristicsWidth of the bands is approximately 100 km; length of the bands is approximately 100-400 km; time scale of the bands is approximately 3-4 hTypical CSI vertical motions are on the order of tens of cm s-1 to a few m s-1 and thus, usually DO NOT produce lightning/thunder (need > 5 m s-1 to produce lightning)However, these mesoscale bands of precipitation can be intense and result in significantly higher rain/snow fall totals than the surrounding areaCSI is characterized by inertial stability and convective stability but, when realized, results in slanted or tilted mesoscale circulations which convert inertial energy into buoyant energy
Conditional Symmetric Instability: Physical Characteristics (cont.)The atmosphere can contain regions of CSI and convective instability (CI), but since CI has a faster growth rate (tens of minutes) relative to CSI (a few hours), it will dominate.CSI is favored to occur in regions of:High vertical wind shear Weak absolute vorticity (values near zero)Weak convective stabilityHigh mean relative humidityLarge scale ascentOften these conditions are found in the entrance region of an upper-level jet streak during the cool season
Schematic illustration of moist slantwise convective updrafts and downdrafts; slanted updrafts are narrow, saturated and intense, while downdrafts are diffuse, unsaturated and weak. From Emanuel (1984) Dynamics of Mesoscale Weather Systems, NCAR Summer Colloquium Lecture Notes, 11 June 6 July 1984, p. 159.
Conditional Symmetric Instability: Theory
Conditional Symmetric Instability: Theory
Understanding Conditional Symmetric Instability: Cross section of e and Mg taken normal to the 850-300 mb thickness contours
Conditional Symmetric Instability: Theory
Equivalent Potential Vorticity (EPV)When EPV < 0 conditional symmetric instability is present.However, EPV is also < 0 when there is convective instability you need to see if the lines of e are folded , I.e., where e decreases with height to separate areas of CI from areas of CSI. CI will dominate.Schultz and Schumacher (1999, MWR) suggest using es (saturated e instead of regular e) to assess CSI.
Nicosia and Grumm (1999, WAF) Conceptual Model for CSIDifferential moisture advection northeast of the surface low (in the previous diag.) leads to a steepening of the e surfaces.Mid-level frontogenesis increases the north-south thermal gradient, thereby increasing the vertical wind shear. In this case the easterlies increase below while the westerlies increase above which increases the differential moisture advection, increasing the e surfaces slope.
EPV Tendency Equation from Nicosia and Grumm (1999, WAF)
Figure from Nicosia and Grumm (1999,WAF). Zone of EPV reduction occurs where the mid-level dry tongue jet overlays the low-level easterly jet (or cold conveyor belt), north of the surface low. In this area dry air at mid-levels overruns moisture-laden low-level easterly flow, thereby steepening the slope of the e surfaces.
Nicosia and Grumm (1999, WAF) Conceptual Model for CSIAlso.since the vertical wind shear is increasing with time the Mg surfaces become more horizontal (become flatter). Thus, a region of CSI develops where the e surfaces are more vertical than the Mg surfaces.In this way frontogenesis and the development of CSI are linked.
Problems in Diagnosing CSI OperationallyTemporal Resolution: CSI is resolved in 3-5 h while current data collection is every 12 h.Spatial Resolution: Precipitation bands are meso- scale with lengths of 100-200 km and widths of 50-100 km.Geostrophic assumption is not always valid (e.g., in regions of cyclonic curvature or within ULJ exit/entrance regions).When the shear vector turns with height, the inertial stability criteria is no longer valid for some portions of the cross section; Mg is not strictly conserved.
Frontogenesis: Shear TermShearing Advection changes orientation of isotherms and contracts themCarlson, 1991 Mid-Latitude Weather Systems
Frontogenesis: Confluence TermCold advection to the northWarm advection to the southCarlson, 1991 Mid-Latitude Weather Systems
Frontogenesis: Tilting TermAdiabatic cooling to north and warming to south increases horizontal thermal gradientCarlson, 1991 Mid-Latitude Weather Systems
Frontogenesis: Diabatic Heating/Cooling TermfrontogenesisfrontolysisT constantT increasesT increasesT constantCarlson, 1991 Mid-Latitude Weather Systems
Frontogenetical CirculationAs the thermal gradient strengthens the geostrophic wind aloft and below must respond to maintain balance with the thermal wind.Winds aloft increase and cut to the north while winds below decrease and cut to the south, thereby creating regions of div/con. By mass continuity upward motion develops to the south and downward motion to the north a direct thermal circulation.This direct thermal circulation acts to weaken the frontal zone with time and works against the original geostrophic frontogenesis.
Frontogenetical CirculationQ vectorsDirect Thermal CirculationConfluent Flow
Frontogenetical CirculationFrontogenetical circulations typically result in one band of precipitation which is parallel to the frontal zone.The strength of this circulation is modulated by the ambient static stability.Grumm and Nicosia (1997, NWD) found in their studies that a weakly stable environment in the presence of frontogenesis lead to one transient band of heavy precipitation.However, they also found that frontogenesis in the presence of greater stability resulted in classic CSI bands of precipitation.
Frontogenesis and Weak Symmetric StabilityEmanuel (1985, JAS) has shown that in the presence of weak symmetric stability the frontogenetical circulation is changed.The upward branch of the vertical circulation becomes contracts and becomes stronger. The strong updraft is located ahead of the region of maximum geostrophic frontogenetical forcing.The distance between the front and the updraft is typically on the order of 50-200 kmOn the cold side of the frontogenetical forcing EPV >>0 and the downward motion is broader and weaker than the updraft.
Frontogenetical CirculationFrontogenetical Circulation + WSSEmmanuel (1985, JAS)
Sanders and Bosart, 1985: Mesoscale Structure in the Megalopolitan Snowstorm of 11-12 February 1983. J. Atmos. Sci., 42, 1050-1061.
Nolan-Moore Conceptual ModelMany heavy precipitation events display differe