Deep Convection: Initiation

Mesoscale M. D. Eastin

Deep Convection: Initiation



Convective Initiation

• Big Picture• Boundary Layer Basics• Boundary Layer Convection• Initiation on the Mesoscale


Convective Initiation: Where and when will moist convection develop?

Importance:

• Quantitative Precipitation Forecasts (QPF)• Severe Weather Forecasting• Hydrology (flash flooding, stream levels)• Aviation Forecasting (microbursts)


Palmer Divide

Cheyenne Ridge

DCZ

Storm Initiation Locations during 3-monthstudy of the Denver Convergence Zone (DCZ)

From Wilson andSchrieber (1986)


Atmospheric Structure

• Potentially Unstable:

• Deep convection is not automatic• Contain some CAPE (positive area), but often located above CIN (negative area)• Air must be physically lifted to the LFC Convection must be initiated, or “triggered”

by “local” regions of enhanced ascent

Mechanisms that can trigger deep convection:

• Synoptic-scale fronts• Mesoscale fronts (drylines and gust fronts)

How do we get convection away from fronts? What initiates the first gust-front producing storm? Do all “boundaries” produce deep convection?


CIN

CAPE

LFC


The Boundary LayerStructure and Evolution

Definition: The part of the atmosphere directly influenced by the Earth’s surface that responds to surface forcing (i.e. friction and energy fluxes) within a time scale of ~1 hour or less

Sub-layers:

• Surface Layer • Mixed Layer (ML)• Entrainment Zone (EZ)• Stable Boundary Layer (SBL)• Residual Layer (RL)

Common parameters used to study and locate the boundary layer:

• Temperature (T)• Mixing ratio (w)• Potential Temperature (θ)

• Remember: Both θ and w are conserved (constant) for dry adiabatic processes

pcR

p

pT

0



In the absence of frontal forcing (i.e., under high pressure systems):

• Evolves in a well-defined manner• Daily cycle is very pronounced and regular

Surface Layer: Lowest ~100 m AGL Layer where heat and moisture are exchanged between land and air

Strong vertical gradients in winds, temperature, and moisture Stability often super-adiabatic (daytime)



Late Afternoon After Sunset Before Sunrise Early Morning Mid-Morning

Potential Temperature

Temperature



Mixed Layer: Located above the surface layer during the dayDepth ~1000 m

Overturning thermals regularly transport (or “mix”) heat and moisture from the surface layer to the entrainment zoneMixing often strongest ~1-2 hours after solar noonHeat and moisture are conserved (θ and w are constant)

Stability often dry-adiabatic





Temperature


The Boundary LayerConvection in the Mixed Layer

Vertically point airborne cloud radar (95Ghz)

Radar echo due to insects in NW Oklahoma

+ = Top of BL



Entrainment Zone: Located above the mixed-layer during the day Depth ~100-200 m

Transition layer between the well-mixed convective boundary layer and the free atmosphere Strong vertical gradients in temperature and moisture Often contains a temperature inversion (source of CIN) Stability often absolutely stable (prevents cloud growth) Strong inversions will prevent deep convection





Temperature



Stable Boundary Layer: Depth ~100-500 m Radiational cooling of the land creates a stable cold layer

Layer deepens as night progress (no vertical mixing) Strong temperature inversion at top

Residual Layer: Remnant mixed layerCapping Inversion: Remnant entrainment zone





Temperature



• Daytime sounding from Amarillo, Texas

• Can you identify each of the regions just discussed?

Surface Layer Mixed Layer Entrainment Zone



• Night time sounding from Amarillo, Texas

• Can you identify each of the regions just discussed?

Stable Boundary Layer Residual Layer


Boundary Layer ConvectionWhy does it occur?

• Transport heat and moisture from the surface to the free atmosphere

• Two common scenarios for boundary layer convection:

Daytime Solar Heating Cold Air Advection


What is the result of the convection?

Shallow clouds often occur from noon to late afternoon

• “Popcorn” or “Fair-Weather” cumulus

• Clouds can appear random, but are often organized into distinct structures

• “Cloud Streets”• “Open/Closed Cells”

Boundary Layer Convection


Horizontal Convective Rolls (HCRs):

• Due to daytime solar heating of land• Mixed-layer thermals organized into bands• Horizontal helices oriented nearly parallel to the ambient flow• Produce cloud streets• Commonly seen in satellite and radar imagery prior to the onset of deep convection (useful to forecasters)

From Houze (1993)




• Typical aspect ratio (horizontal to vertical scale) is 3:1 but can vary from 2:1 to 10:1

• Typical updrafts are 1-3 m/s

Updrafts often contain higher values of T, θ, and w compared to adjacent downdrafts

• Result from a combination of buoyancy and vertical wind shear within the boundary layer

Most often occur in strong shear, moderate heat flux environments (the same environment most severe weathers occurs in…)




• If updrafts contain higher T, θ, and w then there should be less negative area (CIN) to overcome and more positive area (CAPE) available for deep convection

• On radar, higher reflectivity cells often correspond to the “deeper” convection along the bands that are more likely to reach their LFC and “trigger” the first deep convection (helpful for short–term forecasts)




• Along-band periodicity is often observed in the shallow clouds

• Called “pearls on a necklace”

• Believed to be caused by gravity waves propagating along the temperature inversion of the entrainment zone

From Christian (1987)



Open Cell Convection

• Due to advection of cold air over a warm surface (either land or water)• Common late fall thru early summer (over land)• Cell has hexagonal structure (aspect ratio 10:1)• Descending motion at the core• Updrafts on edge are ~1 m/s Form in weak shear environments• Well observed by satellites (visible)• Difficult to detect on radar (looks like random “noise”) Can trigger deep convection

Visible Satellite Image of Labrador Sea



Closed Cell Convection

• Often occurs over cold surfaces (e.g. stratocumulus off California coast)• Forced by strong radiational cooling at cloud top• Cell has hexagonal structure• Ascending motion at the core

Form in weak shear environments with minimal surface fluxes Rarely triggers deep convection

Visible Satellite Image



Non-homogeneous Surface Conditions:

• Acts to modulate (or slightly alter) the convection generated by both solar heating and/or cold air advection

• Strong gradient in surface properties

Low albedo High albedo

Low soil moisture High soil moisture

Small friction Large friction

Low soil thermal capacity High soil thermal capacity

Low elevation High elevation

Urban Rural



Convective Initiation on the MesoscaleGiven:

• A synoptic-scale environment conducive to deep convection (e.g. ample CAPE)• Some CIN which must be “overcome” to permit deep convection

Required:

• “Boundaries” are needed to provide mesoscale regions of forced ascent• Not all boundaries produce deep convection• Deep convection is often not uniform along a given boundary

Possible Boundaries:

• Synoptic fronts and troughs• Dry Lines• Coastal fronts and sea breezes• Gust fronts• Topographically induced fronts• Boundary Layer Thermals• Horizontal Convective Rolls• Open Cell Convection• Non-homogeneous surface conditions


Convective Initiation on the MesoscaleOften Needed and/or Occurs:

• Changes to the thermodynamic profile (i.e. lower CIN and increase CAPE)

Possible Processes:

• Differential horizontal temperature advection• Persistent synoptic-scale ascent (unrelated to boundaries) – (a)• Low-level moistening – (b)• Low-level warming – (c)


Convective Initiation on the MesoscaleForecasts:

• Are getting better, but we still have much to learn about convective initiation• The best forecasters continuously monitor thermodynamic (i.e. stability = soundings) and kinematic (i.e. wind shear) changes along ALL boundaries


Summary

• Definition• Importance• Contributing Factors

• Boundary Layer (basic structure, diurnal evolution)

• Boundary Layer Convection• Physical processes• Horizontal convective Rolls• Open cell Convection• Closed Cell Convection• Heterogeneous surface conditions

• Convective Initiation on the Mesoscale (requirements)



ReferencesChristian, T. W., 1987: A comparative study of the relationship between radar reflectivities, Doppler velocities, and clouds

associated with horizontal convective rolls. M.S. thesis, Department of Atmospheric Sciences, University of California, Los Angeles, 94 pp

Houze, R. A. Jr., 1993: Cloud Dynamics, Academic Press, New York, 573 pp.

LeMone, M., 1973: The structure and dynamics of horizontal vorticities in the planetary boundary layer.J. Atmos. Sci., 30, 1077-1091.

Stull, R. B., 1988: An Introduction to Boundary Layer Meteorology. Kluwer Academic Publishers, Boston, 666 pp.

Weckworth, T. M., J. W. Wilson, R. M. Wakimoto, and N. A. Crook (1997): Horizontal convective rolls: Determining theenvironmental conditions supporting their existence and characteristics. Mon. Wea. Rev., 125, 505-526.

Wilson, J. W., and W. E. Schreiber, 1986: Initiation of convective storms at radar observed boundary-layer convergence lines. Mon. Wea. Rev., 114, 2516–2536.

Deep Convection: Initiation

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