AOSS 414: Weather Systems

Atmospheric Stability and 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 and large cumulus

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 . Thunderstorm Environment

• Plays a key role in determining strength, type and thus longevity of convection. – Example: vertical shear Thunderstorm Environment • Important Considerations

– Atmospheric Stability/Instability*

– Availability of Low Level Moisture*

– Vertical *

– Environmental Lifting Mechanism*

– Low level (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 – 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 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 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 perturbations

Theta-E, Θe (Equivalent )

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 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 , 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.

• 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) Pressure 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. – 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 ( 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 ( 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)

• The Equilibrium Level (EL) is the height in the upper troposphere where the parcel of saturated air, rising due to its positive buoyancy, first becomes cooler than its environment.

– To locate the Equilibrium Level (EL), locate the LFC and then continue to lift parcel moist adiabatically until the parcel first becomes cooler than the environment.

– This level is the EL. Equilibrium Level

200 Tp< Te Equilibrium T = T p e Level 300

400 Tp> Te

500 Pressure (mb) Pressure 600 Level of Free Convection 700 Tp< Te 800 LCL 900 1000 Td T Temperature (oC) (courtesy F. Remer) For our KILN sounding, the EL is at approximately the 210 mb level.

Positive Area

• If the path of a vertically moving parcel is warmer than ambient temperature:

– The area between the path and temperature curve a “positive area”

• If parcel starts at bottom of positive area and accelerates upward – Positive area is proportional to kinetic energy gained

• If parcel starts at top of positive area and accelerates downward – Positive area is proportional to kinetic energy lost Positive Area

Equilibrium Level

) 300 mb Positive 400 Area

500 Pressure ( 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 (courtesy F. Remer) Convective Available Potential Energy (CAPE) • Used to evaluate convective potential of the atmosphere for a given thermodynamic structure or sounding.

– Is a vertically integrated quantity – Measures cumulative buoyant energy (positive area) from LFC to EL.

• In general, the larger the value of CAPE, the greater the buoyant energy.

– There are various forms of CAPE:

• based upon parcel starting at the surface* • based upon parcel starting at the pressure level that will result in the most unstable CAPE value possible • based upon mean temperature and conditions in lowest 100-mb of sounding

* This is the value reported, unless otherwise stated. Negative Area

Negative 200 Area Equilibrium Level

) 300 mb 400

500 Pressure ( Pressure 600 Level of Free Convection 700 800 Negative LCL 900 Area 1000 Td T (courtesy F. Remer) Negative Area

• If the path of a vertically moving parcel is cooler than ambient temperature:

– The area between the parcel path and environmental temperature curve is deemed a “negative area”

• If parcel starts at top of negative area and accelerates downward – Negative area is proportional to kinetic energy gained

• If parcel starts at bottom of negative area – and is in motion, the negative area is proportional to kinetic energy lost as parcel ascends 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. Thunderstorm Environment

• In this example, note:

• Lack of CIN • Deep and wide area of CAPE • Penetration of convection beyond equilibrium level. Thunderstorm Environment

• Positive area (if present) is representative of the Convective Available Potential Energy or CAPE.

• The net impact of CAPE (our measure of positive buoyant energy) is influenced by: • Water loading • Entrainment

•Sounding A: • CAPE is concentrated in lower half • Surface air parcel accelerates at a greater rate • Produces stronger updraft • There is less falling into lowest portion of updraft as it is carried aloft •This reduces downward drag of water loading

• Entrainment

• During convection, environmental air crosses 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. Entrainment

• Air may be drawn in from below the cloud base.

• Air may be drawn in by turbulence at the top of the cloud.

• Air may be drawn in laterally to satisfy mass continuity. – Example: Increasing mass flux with height, aka mass flux divergence. Entrainment

• If entrainment is actively occurring, the result of mixing environmental air with cloud air is that the within the cloud will be somewhere between the moist adiabatic and environmental lapse rates.

• Mixing of moist cloudy air and drier air from outside the cloud impacts cloud drop size distributions and their chemical content. Entrainment

• Entrainment is not a continuous and homogeneous process, but rather occurs in pulses that are intermittent in both space and time.

Source: Cloud Dynamics by Houze (1993) 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 Modifications to Convective Inhibition (CIN)

Capping Inversion: • Measured by CIN • Prevents parcels from reaching the LFC • Additional mechanism needed to initiate convection

Three common mechanisms to overcome CIN:

• Heating of from surface • Moistening • Synoptic scale lifting 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)

Markowski and Richardson (2010)