Mesoscale Instabilities

James T. Moore

Cooperative Institute for Precipitation Systems

Saint Louis University

Dept. of Earth & Atmospheric Sciences

moore@eas.slu.edu

COMET-RFC/HPC Hydrometeorology Course 01-1

14-21 November 2000

Four Types of Mesoscale Instabilities Which Can Result in Banded Precipitation

• Elevated Convective Instability• Conditional Symmetric Instability• Weak Symmetric Stability• Frontogenesis

• 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 Unstable

Results 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 Instability

• Typically develops north of quasi-stationary or warm fronts

• Associated with moderate southerly low-level jets ( > 15 m s-1) oriented nearly normal to the frontal boundary

• Usually 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 boundary

• Where thunderstorms form depends upon:– Relative humidity of incoming air stream– Slope of isentropes in vicinity of frontal zone– Magnitude of the moisture convergence

Elevated Convective Instability

Trier and Parsons, 1993; MWR, vol. 121, 1078-1098

Conditional Symmetric Instability: Synoptic Characteristics

• Typically a cool season phenomena• Wind profile: speed increasing with height and weak

directional veering with height; indicative of strong baroclinicity

• Thermodynamic profile: nearly saturated and close to the moist-adiabatic lapse rate. Parcel motion will be neutral to moist ascent. Lapse rate is NOT conditionally unstable

• Often found in the vicinity of a extratropical cyclone warm front, ahead of long-wave troughs in regions of strong, moist, mid-tropospheric southwesterly flow

• Large 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 rate

• On 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 air

• These heavier precipitation bands may be embedded (obscured) by other lighter precipitation

• Warm frontal rain/snow bands are often good candidates for being associated with CSI

Conditional Symmetric Instability: Physical Characteristics

• Width 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 h

• Typical 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 area

• CSI 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 stability– High mean relative humidity– Large scale ascent– Often 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 CSI

• Differential 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 CSI

• Also….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 Operationally

• Temporal 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 Term

Shearing Advection changes orientation of isotherms and contracts them

Carlson, 1991 Mid-Latitude Weather Systems

Frontogenesis: Confluence Term

Cold advection to the north

Warm advection to the south

Carlson, 1991 Mid-Latitude Weather Systems

Frontogenesis: Tilting Term

Adiabatic cooling to north and warming to south increases horizontal thermal gradient

Carlson, 1991 Mid-Latitude Weather Systems

Frontogenesis: Diabatic Heating/Cooling Term

frontogenesis

frontolysis

T constant T increases

T increases T constant

Carlson, 1991 Mid-Latitude Weather Systems

Frontogenetical Circulation

• As 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 Circulation

Q vectors

Direct Thermal Circulation

Confluent Flow

Frontogenetical Circulation

• Frontogenetical 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 Stability

• Emanuel (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 km

• On the cold side of the frontogenetical forcing EPV >>0 and the downward motion is broader and weaker than the updraft.

Frontogenetical Circulation

Frontogenetical Circulation + WSS

Emmanuel (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 Model

• Many heavy precipitation events display different types of mesoscale instabilities including:– Convective Instability (CI; e decreasing with

height)– Conditional Symmetric Instability (CSI; lines

of e are more vertical than lines of constant absolute geostrophic momentum or Mg)

– Weak Symmetric Stability (WSS; lines of e

are nearly parallel to lines of constant absolute geostrophic momentum or Mg)

Nolan-Moore Conceptual Model for CI-CSI-WSS

Nolan-Moore Conceptual Model

• These mesoscale instabilities tend to develop from north to south in the presence of strong uni-directional wind shear (typically from the SW)

• CI tends to be in the warmer air to the south of the cyclone while CSI and WSS tend to develop further north in the presence of a cold, stable boundary layer.

• It is not unusual to see CI move north and become elevated, producing thundersnow.

Nolan-Moore Conceptual Model• CSI may be a precursor to elevated CI, as

the vertical circulation associated with CSI may overturn e surfaces with time creating convectively unstable zones aloft.

• We believe that most thundersnow events are associated with elevated convective instability (as opposed to CSI).

• CSI can generate vertical motions on the order of 1-3 m s-1 while elevated CI can generate vertical motions on the order of 10 m s-1 which are more likely to create charge separation and lightning.

Emanuel’s Conceptual Model of CSI-associated Vertical Circulation

Emanuel (1985, JAS)

Parting thoughts on Banded Precipitation• Schultz and Schumacher (1999, MWR) review paper on

CSI suggests that we use es (i.e., a saturated equivalent potential temperature) to assess CSI instead of just e since the air should be saturated for CSI.

• Numerical experiments suggest that weak positive symmetric stability (WSS) in the warm air in the presence of frontogenesis leads to a single band of ascent that narrows as the symmetric stability approaches neutrality.

• Also, if the forcing becomes horizontally widespread and EPV < 0, multiple bands become embedded within the large scale circulation; as the EPV decreases the multiple bands become more intense and more widely spaced.

• However, more research needs to be done to better understand how bands form in the presence of frontogenesis and CSI.