Mesoscale Gravity Waves

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MesoscaleGravity Waves

• Brian Jewett

Department of Atmospheric Sciences Monday-Wednesda
y, 9/24,26/2007
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Why should you care?

• Gravity waves are - everywhere
• Mesoscale gravity waves are a special case...
• They can have severe weather associated with
  them
• Severe thunderstorms
• Snow bands -- locally heavy snowfall
• High winds

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Mesoscale gravitywave attributes

• or,
• Would you recognize one if you saw it?

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A Midwest example

• PUBLIC INFORMATION STATEMENT
• NATIONAL WEATHER SERVICE LINCOLN IL
• 155 PM CDT SUN APR 28
• ...STRONG WINDS PRODUCED DAMAGE IN PARTS OF
  CENTRAL ILLINOIS...
• A STRONG WIND FROM THE EAST AT 30 TO 40 MPH WITH
  GUSTS MORE THAN
• 50 MPH BLEW INTO PARTS OF CENTRAL ILLINOIS
  BETWEEN 10 AM AND NOON.
• THE WIND TOPPLED MANY TREES AND PRODUCED DAMAGE
  TO A FEW BUILDINGS.
• ROOF DAMAGE TO HOMES WAS REPORTED AROUND
  CHARLESTON/MATTOON BETWEEN 1120 AND 1145 AM. MUCH
  OF THE ROOF OF THE CHARLESTON HIGH SCHOOL
  GYMNASIUM WAS DAMAGED AS A RESULT OF 60 MPH WIND
  GUSTS.

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A Midwest example

• PUBLIC INFORMATION STATEMENT
• NATIONAL WEATHER SERVICE ST LOUIS MO
• 353 PM CDT SUN APR 28
• DAMAGING WINDS OCCURRED ACROSS PARTS OF THE ST.
  LOUIS METRO AREA ...
• THE AREA OF DAMAGING WINDS OCCURRED ALONG THE
  TRAILING (BACK) EDGE OF A SHOWER AND THUNDERSTORM
  COMPLEX. THIS TYPE OF DAMAGING WIND EVENT IS
  UNUSAL SINCE MOST DAMAGING WINDS FREQUENTLY OCCUR
  ALONG THE LEADING EDGE OF SEVERE THUNDERSTORMS...
• WINDS AT ST. LOUIS INTERNATIONAL AIRPORT REACHED
  71 MPH AT 938 AM CDT
• THIS MORNING. THE PRESSURE TRACE AT THE NATIONAL
  WEATHER SERVICE OFFICE IN WELDON SPRINGS FELL
  FROM 29.96 TO 29.70 INCHES WITHIN A 10 MINUTE
• PERIOD. THIS IS A SIGNIFICANT FALL WITHIN AN
  EXTREMELY SHORT TIME
• PERIOD.

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A Midwest example

• PUBLIC INFORMATION STATEMENT
• NATIONAL WEATHER SERVICE ST LOUIS MO
• 353 PM CDT SUN APR 28
• DAMAGING WINDS OCCURRED ACROSS PARTS OF THE ST.
  LOUIS METRO AREA ...
• THE AREA OF DAMAGING WINDS OCCURRED ALONG THE
  TRAILING (BACK) EDGE OF A SHOWER AND THUNDERSTORM
  COMPLEX. THIS TYPE OF DAMAGING WIND EVENT IS
  UNUSAL SINCE MOST DAMAGING WINDS FREQUENTLY OCCUR
  ALONG THE LEADING EDGE OF SEVERE THUNDERSTORMS...
• WINDS AT ST. LOUIS INTERNATIONAL AIRPORT REACHED
  71 MPH AT 938 AM CDT
• THIS MORNING. THE PRESSURE TRACE AT THE NATIONAL
  WEATHER SERVICE OFFICE IN WELDON SPRINGS FELL
  FROM 29.96 TO 29.70 INCHES WITHIN A 10 MINUTE
• PERIOD. THIS IS A SIGNIFICANT FALL WITHIN AN
  EXTREMELY SHORT TIME
• PERIOD.

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A Midwest example
Note the inverted pressure trough. This trough
was located north of the surface warm front.
Pressure wind analysis, 16z
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A Midwest example
3-hour Pressure, Wind change
The 3-hour pressure change helps identify the
wave.
The arrows depict the 3-hour vector wind change.
Note the relationship to the pressure falls.
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A Midwest example
This pressure trace was recorded at the UI
Atmospheric Sciences dept. Note the sharp dip
superimposed on a larger-time-scale pressure fall.
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Note the relationship between wind, pressure, and
(accumulated) precipitation
Wind max
Pressure min
Precip ends
04z
08z
12z
16z
20z
00z
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A Midwest Gravity Wave
1600z
WSR-88D (folded) velocities exceeded 70
kts. Note the direction!
Lincoln base velocity (knots), 1600 UTC
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Mesoscale gravity waves (MGWs)

• Wavelength 30-300 km
• Period 0.5 - 4.0 hours
• Phase speeds of 1535 m s-1
• Amplitude up to several mb
• May appear as packets or as singular waves
• Large amplitude waves may have long wavefronts,
  last gt 1 day
• Usually accompanied by clouds/precipitation

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Understanding rare phenomena

• MGWs have small temporal spatial scales
• traditional observing networks are inadequate
  wavesmost easily diagnosed from surface data
• unless MGW formation occurs in a region of
  comprehensive observations, the mechanism of wave
  genesis is difficult to identify with certainty
• our understanding is based largely on field
  experiment observations, and numerical modeling
• unfortunately, field experiment data is oftennot
  both detailed and comprehensive.

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Gravity Wave Examples
Ramamurthy et al. (1993)
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Gravity wave climatology
25-year distribution of U.S. IGW occurrences
(Koppel et al., 2000)
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Research questions

• What explains their climatology - why are they
  found almost exclusively east of the Rockies?
  From what height or location do they originate?
• What process or processes cause them?
• What are the genesis mechanisms?
• What modulates their intensity?
• Why are some waves (MGWs) long-lived?
• What is the maintenance process/mechanism?
• What explains the association with clouds
  precipitation?
• Can we forecast (or nowcast) mesoscale gravity
  waves, and if so, how?

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Gravity Wave Environment
Synoptic environment typical of MGW occurrences
(adapted from Koch OHandley 1997)
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Proposed genesis mechanisms

• Geostrophic adjustment
• Shearing instability
• Frontogenesis, frontal collapse
• Convection
• Orographic forcing

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Proposed genesis mechanisms

• Geostrophic adjustment (upper troposphere source)
• Unbalanced flow develops near and just
  downwindof a jet core
• Waves generated as jet max approachesdownstream
  ridge
• Flow adjusts to imbalance throughgeneration of
  gravity-inertia waves

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Proposed genesis mechanisms

• Shearing instability (upperair source)
• Richardson number ratio of stability to shear.
• High stable, low-shear flow
• Low near neutral, highly sheared.
• Critical level (u-cx0) can serve as generation
  level for waves when Rilt1/4, particularly
  w/stable region above.1
• Look for drop in vertical shear after wave
  passes.
• Important for non-supercell tornadoes2, wavy
  narrow cold frontal rainbands3, and ...
  mesoscale gravity waves?

1Gossard and Hooke 1975 Ramamurthy et al 1993
2Lee and Wilhelmson 1997
3Jorgensen et al 2003
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Proposed genesis mechanisms

• Frontogenesis, frontal collapse (upper/sfc
  source)
• Surface fronts tend to collapse to quite small
  scales
• Leading edge takes on density-current appearance
• Studies have shown gravity waves triggered by
  surface fronts
• Snyder (1993)

Jewett Wilhelmson 2006
Snyder et al 1993
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Proposed genesis mechanisms

• Convection (low/upperair source)
• Convection may trigger gravity waves ...
• Gravity waves may trigger convection ...
• Convection gravity waves maybecome and remain
  coupled throughwave-CISK
• What is the relationship to MGWs?

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STORM-FEST Case

• Observations

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STORM-FEST Gravity Wave

• During STORM Fronts Experiment Systems Test 14
  February 1992
• Long-lived MGW event wave crossed KS, MO, IL
• Problem MGW evident in surface data, but genesis
  mechanism unclear occurred in data-poor region
  before reaching dense observing network (mature
  wave structure well observed)
• Multiple mechanisms proposed for this event

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STORM-FEST Gravity Wave

• Publications concerning this case
• Rauber et al. (2001 MWR), Part I (origin and
  evolution)
• Yang et al. (2001 MWR), Part II (radar analysis)
• Trexler Koch (2000 MWR), profiler analysis
• Koch Siedlarz (1999 MWR), MGW environment
• Jin Koch (1998 WAF-Phoenix), predictability
• Koch OHandley (1997 WAF) operational
  detection, MM4

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STORM-FEST Data Network
Surface stations
Upperair stations
Rauber et al., Feb. 2001 MWR
Koch Siedlarz, 1999 MWR
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Synoptic setting

• MGW diagnosed in left-exit region of jet, north
  of surface warm front.
• Upper-air dry slot has overspread most of KS
  cloudy stable layer near surface.
• Dry air sweeping down Rockies from New Mexico
  into Texas panhandle.

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Wave diagnosis early evolution

• Up to reaching dual-Doppler domain

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Surface time series data
P (solid) U (dashed)(along directionof wave
movement)
Isochrones pu correlation.
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Satellite surface observations
Wave genesis.
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Satellite surface observations
Wave of depression moves towards dual-Doppler
domain..
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Satellite surface observations
Transition from wave of depression to wave of
elevation.
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Transition in surface P signature
Trexler and Koch, 2000 MWR
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Wave/rainband dry air evolution
Relative humidity.
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Wave/rainband RH, ?
RH
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Wave/rainband RH, ?
RH
?
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Sounding dry air ascends WF
Rauber et al., 2001 MWR
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11z radar, satellite, 298K pressure
Pressure
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Surface, radar, profiler data
14-23z traces
Time increasesright to left.
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Dual-Doppler analysis, 2136z, 2.5km
Left edge of plot isnorthwest side
ofdual-Doppler domain.
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Same data, but nowNorth UP on plot.
Data for next plotaveraged over Y-Y
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Radar sfc data

• Questions
• Well correlated pres, wind
• Strong surge of SW winds
• Clear convergence / divergence evident
• Make distinction small, freely-propagating
  gravity waves vs. sustained MGW
• Free or forced convection?

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Free or forced convection?
20z Topeka
Lifted 90mb
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Mesoscalegravity waves

• continued

Wednesday, Sep. 26, 2007
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Summary from Monday

• Key question MGW genesis

• Potential genesis mechanisms
• Geostrophic adjustment
• Shearing instability
• Convection
• Frontogenesis, frontal collapse

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Summary from Monday

• Key question MGW genesis
• As dry downslope flow ascended the KS warm front,
  a rainband formed and MGW genesis occurred.

• Genesis in southwest KS
• Wave of depression (GCK pressure)
• Cloud band radar echo forming
• At leading edge of surge of dry air
• North of surface warm front

RH
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Summary from Monday

• Key question MGW genesis
• As dry downslope flow ascended the KS warm front,
  a rainband formed and MGW genesis occurred.
• The wave matured as it crossed KS and moved into
  the dual-Doppler domain. The wave remained tied
  to the (forced) rain band and surge of dry air.

• MGW matures, crosses KS
• Wind surge in profiler, radar data
• Soundings showed dry air aloft
• Greater pressure perturbations
• Moved into dual-Dopper domain

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Summary from Monday

• Key question MGW genesis
• As dry downslope flow ascended the KS warm front,
  a rainband formed and MGW genesis occurred.
• The wave matured as it crossed KS and moved into
  the dual-Doppler domain. The wave remained tied
  to the (forced) rain band and surge of dry air.
• Analysis from dual-Doppler data near 21z shows
  pressure minimum behind rain band and beneath the
  surge of dry air (now at 3km MSL).

• Radar analysis sfc time series
• Strong winds at rear of rainband
• MGW detected behind/below band

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02/03z radar, satellite, 298K RH
RH
Rainband decouples from dry air mass between
00-02z.
Black dots are sounding sites.
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Wave ducting
Reflectinglayer
Critical level
Ductlayer
Vertical structure of a gravity wave (adapted
from Koch OHandley 1997) Above cross section.
At right idealized sounding.
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Wave maintenance after 23z

• Sounding from Kirksville (north central MO) at
  23z
• Deep stable layer
• Duct analysis supports wave longevity
• Still associated with convection - role less
  clear less data available

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Conclusions from Part I

• Rainband developed simultaneously with the wave
  as dry downslope air ascended over warm front.
• MGW and convection remained tied to dry air mass
  over KS. Orientation of rainband and MGW
  isochrones support this continued association.
• Genesis surface-based -- maintenance over KS
  w/rainband and dry air surge later (over MO/IL)
  aided by wave duct.

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Context

• Genesis and Maintenance
• How do jet-level mechanisms (geostrophic
  adjustment, shearing instability) produce a
  distinct MGW at the surface?
• Genesis here was surface-based. How common?
  Unknown.
• Note also
• How the authors take advantage of disparate data
  types, locations and times to study this
  phenomenon.

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Part II Finescale Structure

• Briefly!

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Pressure retrieval plan view

• Plan view of retrieved perturbation pressure at z
  1.5 km
• Overlay time-to-space conversion of Topeka
  pressure trace.
• Note min pressure _at_ sfc to rear of band.

High pressure in rain
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Cross section fields 2136z

• SW-NE Cross sections
• High-momentum dry air at rear of system
• Convergence axis near main updraft
• Buoyant region from 3-8km
• High pressure in rear precip region.

V
Divg
?
p
H

L
-
H
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Conceptual model from Part II
58
STORM-FEST Case

• Model Results
• Analysis

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MM5 Grid Configuration
MM5 3 grids innermost grid moved during run.
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MM5 - 20z Surface P
Perturbation pressure, wind, and locations of
sfc time series (red circles). Points C, D, E
and F were chosen to show the wave structure as
it passed at 1815, 1900, 2000 and 2100 UTC.
L
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MM5 Time Series
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MM5 20z Cross Section
L
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Transition to wave of elevation
L
H
q, P cross sections at 1815 and 2200 UTC
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Isochrones up correlation
Correlation exceeding 0.9
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Wave Precipitation
MM5 6-km fields illustrating the relationship
between the modeled gravity wave
and precipitation. Shown surface fronts,
550- mb rainwater (light shading), 850-mb q gt
293.5 K, surface rainwater (dark green) and wind
vectors (every 5th).
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Wave Precipitation
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Wave Precipitation
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Dry air gravity wave evolution
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Dry air gravity wave evolution
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Dry air gravity wave evolution
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Dry air gravity wave evolution
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Dry air gravity wave evolution
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Dry air gravity wave evolution
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Dry air gravity wave evolution
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Backward trajectory of air behind the modeled
rainband
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Dry air surge structure
Looking east/northeast, and down, from over the
Rockies
Another view development of downslope flow
0300 UTC
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Dry air surge structure
Looking east/northeast, and down, from over the
Rockies
0300 UTC
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Dry air surge structure
Looking east/northeast, and down, from over the
Rockies
0600 UTC
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Dry air surge structure
Looking east/northeast, and down, from over the
Rockies
0900 UTC
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Dry air surge structure
Looking east/northeast, and down, from over the
Rockies
1200 UTC
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Role of evaporative processes
Cross section of potential temperature (dotted
red lines, every 2.5K) and rainwater (every 0.05
g kg-1, solid) at 1800 UTC. Subsidence warming
is evident at this, the time of surface wave
genesis.
Full physics
82
Role of evaporative processes
Full physics
No evaporation after 12z
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Role of evaporative processes
Full physics
Evaporation restored
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Impact of cumulus parameterization (at 6 km)
Grell parameterization
No parameterization
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Conceptual Model Gravity Wave Genesis
Elevated precipitation
Downslope flow
Rainband formation
Evaporative cooling
Dry air surge
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Conceptual Model Gravity Wave Genesis
Subsidence above stable layer
Surface pressure falls
Depression of inversion
Wind field responds