Mesoscale M. D. Eastin
Deep Convection: Initiation
Mesoscale M. D. Eastin
Deep Convection: Initiation
Convective Initiation
• Big Picture• Boundary Layer Basics• Boundary Layer Convection• Initiation on the Mesoscale
Mesoscale M. D. Eastin
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)
Deep Convection: Initiation
Palmer Divide
Cheyenne Ridge
DCZ
Storm Initiation Locations during 3-monthstudy of the Denver Convergence Zone (DCZ)
From Wilson andSchrieber (1986)
Mesoscale M. D. Eastin
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?
Deep Convection: Initiation
CIN
CAPE
LFC
Mesoscale M. D. Eastin
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
Mesoscale M. D. Eastin
The Boundary LayerStructure and Evolution
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)
Mesoscale M. D. Eastin
The Boundary LayerStructure and Evolution
Late Afternoon After Sunset Before Sunrise Early Morning Mid-Morning
Potential Temperature
Temperature
Mesoscale M. D. Eastin
The Boundary LayerStructure and Evolution
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
Mesoscale M. D. Eastin
The Boundary LayerStructure and Evolution
Late Afternoon After Sunset Before Sunrise Early Morning Mid-Morning
Potential Temperature
Temperature
Mesoscale M. D. Eastin
The Boundary LayerConvection in the Mixed Layer
Vertically point airborne cloud radar (95Ghz)
Radar echo due to insects in NW Oklahoma
+ = Top of BL
Mesoscale M. D. Eastin
The Boundary LayerStructure and Evolution
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
Mesoscale M. D. Eastin
The Boundary LayerStructure and Evolution
Late Afternoon After Sunset Before Sunrise Early Morning Mid-Morning
Potential Temperature
Temperature
Mesoscale M. D. Eastin
The Boundary LayerStructure and Evolution
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
Mesoscale M. D. Eastin
The Boundary LayerStructure and Evolution
Late Afternoon After Sunset Before Sunrise Early Morning Mid-Morning
Potential Temperature
Temperature
Mesoscale M. D. Eastin
The Boundary LayerStructure and Evolution
• Daytime sounding from Amarillo, Texas
• Can you identify each of the regions just discussed?
Surface Layer Mixed Layer Entrainment Zone
Mesoscale M. D. Eastin
The Boundary LayerStructure and Evolution
• Night time sounding from Amarillo, Texas
• Can you identify each of the regions just discussed?
Stable Boundary Layer Residual Layer
Mesoscale M. D. Eastin
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
Mesoscale M. D. Eastin
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
Mesoscale M. D. Eastin
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)
Boundary Layer Convection
Mesoscale M. D. Eastin
Horizontal Convective Rolls (HCRs):
• 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…)
Boundary Layer Convection
Mesoscale M. D. Eastin
Horizontal Convective Rolls (HCRs):
• 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)
Boundary Layer Convection
Mesoscale M. D. Eastin
Horizontal Convective Rolls (HCRs):
• 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)
Boundary Layer Convection
Mesoscale M. D. Eastin
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
Boundary Layer Convection
Mesoscale M. D. Eastin
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
Boundary Layer Convection
Mesoscale M. D. Eastin
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
Boundary Layer Convection
Mesoscale M. D. Eastin
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
Mesoscale M. D. Eastin
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)
Mesoscale M. D. Eastin
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
Mesoscale M. D. Eastin
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)
Deep Convection: Initiation
Mesoscale M. D. Eastin
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.
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