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
Deep Convection: Initiation
Convective Initiation: Where and when will moist
convection develop?
Importance:
Storm Initiation Locations during 3-month
study of the Denver Convergence Zone (DCZ)
• Quantitative Precipitation Forecasts (QPF)
• Severe Weather Forecasting
• Hydrology (flash flooding, stream levels)
• Aviation Forecasting (microbursts)
Cheyenne Ridge
DCZ
Palmer Divide
Mesoscale
From Wilson and
Schrieber (1986)
M. D. Eastin
Deep Convection: Initiation
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
CAPE
LFC
Mechanisms that can trigger deep convection:
CIN
• 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?
Mesoscale
M. D. Eastin
The Boundary Layer
Structure 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)
p 
  T  0 
 p 
R
cp
• Remember: Both θ and w are conserved (constant) for dry adiabatic processes
Mesoscale
M. D. Eastin
The Boundary Layer
Structure 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:
Mesoscale
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)
M. D. Eastin
The Boundary Layer
Structure and Evolution
Late Afternoon
After Sunset
Before Sunrise
Early Morning
Mid-Morning
Potential
Temperature
Temperature
Mesoscale
M. D. Eastin
The Boundary Layer
Structure and Evolution
Mixed Layer:
Mesoscale
Located above the surface layer during the day
Depth ~1000 m
Overturning thermals regularly transport (or “mix”) heat and moisture
from the surface layer to the entrainment zone
Mixing often strongest ~1-2 hours after solar noon
Heat and moisture are conserved (θ and w are constant)
Stability often dry-adiabatic
M. D. Eastin
The Boundary Layer
Structure and Evolution
Late Afternoon
After Sunset
Before Sunrise
Early Morning
Mid-Morning
Potential
Temperature
Temperature
Mesoscale
M. D. Eastin
The Boundary Layer
Convection 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 Layer
Structure 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 Layer
Structure and Evolution
Late Afternoon
After Sunset
Before Sunrise
Early Morning
Mid-Morning
Potential
Temperature
Temperature
Mesoscale
M. D. Eastin
The Boundary Layer
Structure 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:
Capping Inversion:
Mesoscale
Remnant mixed layer
Remnant entrainment zone
M. D. Eastin
The Boundary Layer
Structure and Evolution
Late Afternoon
After Sunset
Before Sunrise
Early Morning
Mid-Morning
Potential
Temperature
Temperature
Mesoscale
M. D. Eastin
The Boundary Layer
Structure 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 Layer
Structure 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 Convection
Why does it occur?
• Transport heat and moisture from the surface to the free atmosphere
• Two common scenarios for boundary layer convection:
Daytime Solar Heating
Mesoscale
Cold Air Advection
M. D. Eastin
Boundary Layer Convection
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”
Mesoscale
M. D. Eastin
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)
Mesoscale
M. D. Eastin
Boundary Layer Convection
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…)
Mesoscale
M. D. Eastin
Boundary Layer Convection
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)
Mesoscale
M. D. Eastin
Boundary Layer Convection
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)
Mesoscale
M. D. Eastin
Boundary Layer Convection
Open Cell Convection
Visible Satellite Image of Labrador Sea
• 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
Mesoscale
M. D. Eastin
Boundary Layer Convection
Closed Cell Convection
Visible Satellite Image
• 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
Mesoscale
M. D. Eastin
Boundary Layer Convection
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
Mesoscale
M. D. Eastin
Convective Initiation on the Mesoscale
Given:
• 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 Mesoscale
Often 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 Mesoscale
Forecasts:
• 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
Deep Convection: Initiation
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)
Mesoscale
M. D. Eastin
References
Christian, 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 the
environmental 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.
Mesoscale
M. D. Eastin