ATMS 316 Mesoscale Meteorology

1
ATMS 316- Mesoscale Meteorology

• Packet4
• Interesting things happen at the boundaries, or
  at the interface
• Cold air, warm air

http//www.ucar.edu/communications/factsheets/Torn
adoes.html
2
ATMS 316- Mesoscale Meteorology

• Outline
• Background
• Cold fronts
• Introduction
• Data analysis
• Synoptic-scale analysis
• Synoptic-scale frontogenesis
• Mesoscale structure
• Mesoscale frontogenesis
• Conclusions

3
ATMS 316- Background

• Turbulence and fluxes of heat, momentum, and
  moisture
• The surface is quite often the most important
  source and sink of important atmospheric
  properties (heat, momentum, moisture)
• How do these properties get transported?
  Turbulence (a.k.a. friction)
• Scales
• 200 m, BL turbulence
• 20 m, surface-layer turbulence
• 2 m, inertial subrange turbulence
• 2 mm, fine-scale turbulence

Wallace Hobbs, p. 381-389
4
ATMS 316- Background

• Turbulence and fluxes of heat, momentum, and
  moisture
• Example, heat flux (W m-2)
• FH is the kinematic heat flux K m s-1

Wallace Hobbs, p. 381-389
5
ATMS 316- Background

• Turbulence and fluxes of heat, momentum, and
  moisture
• Turbulence closure problem
• Always more equations than unknowns
• Parameterize approximate remaining unknowns as a
  function of the knowns

Wallace Hobbs, p. 381-389
6
ATMS 316- Background

• Turbulence and fluxes of heat, momentum, and
  moisture
• Turbulence closure problem
• Local, first-order closure
• K-theory
• Gradient-transfer theory
• Eddy-diffusivity theory
• Mixing length theory

Wallace Hobbs, p. 381-389
7
ATMS 316- Background

• Turbulence and fluxes of heat, momentum, and
  moisture
• Turbulence closure problem
• zeroth-order closure
• Similarity theory
• Mean flow state is parameterized directly
• turbulent fluxes are related to simple scaling
  parameters (friction velocity)

Wallace Hobbs, p. 381-389
8
ATMS 316- Background

• Turbulence and fluxes of heat, momentum, and
  moisture
• Bulk aerodynamic formulae, surface fluxes

Wallace Hobbs, p. 381-389
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ATMS 316- Background

• Fronts, a review

http//www.bsmo.co.uk/newsfeatures/beginnersguides
/guides/coldfrontsimulation/1.htm
10
Baroclinic zone

• A region of strong thermal contrast
• Significant horizontal temperature gradient

N
E
T1
Baroclinic zone
T2
T3
T4
T5
11
Observed StructureCarlson 1991

• A zone of stronger
• Temperature, moisture, and vertical motion
  gradients normal to the frontal boundary on the
  cold side of the front
• Frontal gradients that appear discontinuous from
  those of the synoptic-scale background
• Relative minimum in pressure
• Relative maximum of vorticity along the front
• Zone of confluence along the front
• Strong vertical and lateral (cyclonic) wind shear
• Rapid changes in cloud cover and precipitation

12
Temperature Discontinuities
0th order
T4
T5
T3
T4
T2
T3
z
T1
T2
N
T1 gt T2 gt T3 gt T4 gt T5 gt T6
1st order
T6
T5
T4
T3
T2
T1
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Frontal Surface
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Transition (Frontal) Zone
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Frontogenesis

• Defined as
• Total derivative we are following a parcel.
• Frontogenesis does not imply a strengthening
  front.
• Frontolysis does not imply a weakening front.
• Response is dependent upon changes in a parcels
  temperature gradient not changes in the frontal
  temperature gradient.

Schumacher
16
Miller (1948) frontogenesis

• Three dimensional equation (from Bluestein,
  1993)

Terms 1, 5, 9 Diabatic terms. Terms 2, 3, 6, 7
Horizontal deformation terms Terms 4, 8
Tilting terms Terms 10, 11 Vertical
deformation terms Terms 12 Vertical
divergence term To focus on changes in the
horizontal thermal gradient, well ignore terms
9-12.
Schumacher
17
Diabatic terms

• Term 1 Term 5
• 12Z 21Z
• ?/?x(dQ/dt) gt 0, ??/?x gt 0 gt F gt 0
• Important with differential cloud cover, snow
  cover gradients, and land/water interfaces.
• Example Sea breeze front, coastal front.

Schumacher
18
Tilting Terms

• Term 4 Term 8

?w/?y gt 0, ??/?z gt 0, ??/?y lt 0 gt F gt 0
?w/?y lt 0, ??/?z gt 0, ??/?y lt 0 gt F lt 0 From
Bluestein, 1993
Schumacher
19
Tilting terms

• Needs to be considered when
• Above the surface
• Near mountains
• Change in slope of terrain
• Change in wind speed along the terrain
• Example
• Front approaching Appalachians results in
  prefrontal downslope which can increase the
  cross-front temperature gradient.

Schumacher
20
Horizontal convergence/confluence terms

• Term 2 Term 7

• ?u/?x lt 0
• ??/?x gt 0
• F gt 0
• Important around most fronts, troughs and outflow
  boundaries.

Schumacher
21
Horizontal shear

• Term 3 Term 6

• ?v/?x gt 0
• ??/?y lt 0
• ??/?x gt 0
• F gt 0
• Important around fronts, outflow boundaries, and
  troughs.

Schumacher
22
ATMS 316- Cold Fronts

• Structure of a Cold Front Over the Ocean
• Nicholas A. Bond and Robert G. Fleagle
• Quarterly Journal of the Royal Meteorological
  Society, 1985
• p. 739-759

23
ATMS 316- Cold Fronts

• Introduction
• Purpose
• Determine the kinematic and thermodynamic
  structure of a cold front
• Determine the magnitude and distribution of
  advective processes, condensational heating, and
  turbulent mixing in the vicinity of the front

24
ATMS 316- Cold Fronts

• Introduction
• Fronts first discussed by J. Bjerknes (1919)
• Dynamics of cold fronts is complex
• Not yet entirely understood
• Dramatic weather associated with cold fronts

25
ATMS 316- Cold Fronts

• Introduction
• Observational studies
• Radar-based
• Precipitation structures
• Wind structures
• Meteorological tower

26
ATMS 316- Cold Fronts

• Introduction
• Dynamical studies of fronts and frontogenesis
• Sawyer (1956)
• Eliassen (1959, 1962)
• Hoskins and Bretherton (1972)
• Model-based
• Blumen (1980)
• Keyser and Anthes (1982)

27
ATMS 316- Cold Fronts

• Introduction
• Storm Transfer and Response Experiment (STREX)
• Gulf of Alaska
• Autumn of 1980
• Two research aircraft and a research ship (Papa)
  in addition to usual buoy and coastal obs

http//www.illywhacker.com/images/maps/map1003.gif
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ATMS 316- Cold Fronts

• Introduction
• Study was unique
• Carried out over the open ocean
• Minimal topographic influence
• Weak ocean surface temperature gradients
• High resolution observations

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ATMS 316- Cold Fronts

• Data analysis
• Synoptic scale
• 25 soundings
• 11 obs from ships of opportunity
• Mesoscale
• High resolution flight-level measurements by
  research aircraft

http//www.omao.noaa.gov/newsevents.html
30
ATMS 316- Cold Fronts

• Data analysis
• Synoptic analysis
• 15, 16 Nov 1980 case study
• 115 km horizontal grid spacing
• Objective analysis of four fields at 50 mb
  intervals from the surface to 500 mb

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ATMS 316- Cold Fronts

• Data analysis
• Synoptic analysis
• Separate analyses in pre- and post-frontal
  regions
• Avoid smoothing gradients in analyses
• Geopotential heights calculated using hypsometric
  equation
• Ageostrophic components found by subtracting
  geostrophic components from objectively analysed
  winds

32
ATMS 316- Cold Fronts
cross front (x-direction)

• Data analysis
• Synoptic analysis
• To improve effective resolution in cross-frontal
  direction, observations were collapsed onto a
  single plane normal to the front
• Variations along the front were considered less
  significant

along front (y-direction)
33
ATMS 316- Cold Fronts

• Data analysis
• Mesoscale analysis
• Based on four crossings by aircraft at 49oN
• Surface observations made from the ship
  Oceanographer were transformed to a coordinate
  system relative to the front at 0000 UTC 16 Nov.

(flight and dropsonde data indicated by dots)
34
ATMS 316- Cold Fronts

• Data analysis
• Mesoscale analysis
• How to align aircraft flight data
• Align maximum vertical velocities from 300, 950,
  and 2100m altitude P-3 crossings
• 4500m crossing was made to conform to 2338 UTC 15
  Nov. dropsonde data

35
ATMS 316- Cold Fronts

• Data analysis
• Mesoscale analysis
• Observations projected onto a vertical
  cross-section normal to the front (x-direction)
• Assumed variations in fields along the front were
  negligible
• Concentrations of energy at horizontal scales of
  250 m and 2-3 km

36
ATMS 316- Cold Fronts

• Data analysis
• Mesoscale analysis
• 700 m horizontal resolution, 12.5 mb (100 m)
  vertical resolution
• Differences in aligned fields between aircraft
  and dropsonde data
• 0.5oC, 1 m s-1 cross-front velocity, and 4 m s-1
  along-front velocity

37
ATMS 316- Cold Fronts

• Synoptic-scale analysis
• Coldest air south of 986 mb low centre
• Geostrophic stretching deformation at 45oN
  contributing to frontogenesis
• Stationary ridge along west coast of Canada

SLP, 1000-700 mb DZ 00 UTC 14 Nov 1980
38
ATMS 316- Cold Fronts

• Synoptic-scale analysis
• Substantial development of frontal system
• New rapidly deepening low at 51oN 148oW
• Two open waves along cold front

SLP, 1000-700 mb DZ 00 UTC 15 Nov 1980
39
ATMS 316- Cold Fronts

• Synoptic-scale analysis
• Front reaches its maximum strength
• Northernmost open wave deepens 40 mb to 965 mb
• Southernmost open wave deepens 25 mb to 981 mb
• Strongest thermal gradients in region of
  intensive meteorological measurements
• Little temp. gradient along-front

SLP, 1000-700 mb DZ 00 UTC 16 Nov 1980
40
ATMS 316- Cold Fronts

• Synoptic-scale analysis
• Front is oriented nearly vertically from surface
  to this level
• Strong southerly flow ahead of front
• Weak temp. gradients in cold air west of 100
  km-wide frontal zone

850 mb
(surface front shown)
41
ATMS 316- Cold Fronts

• Synoptic-scale analysis
• Broader frontal zone than at 850 mb
• Frontal zone displaced 1o longitude to the west
  of 850 mb front
• Temp. change across frontal zone (12 K) is a
  maximum at this level

700 mb
(surface front shown)
42
ATMS 316- Cold Fronts

• Synoptic-scale analysis
• Wind speeds greater than 50 m s-1 just ahead of
  the surface (sfc) front
• Isotherms parallel to sfc front with maximum
  packing found 200 km behind sfc front
• Short trough-ridge spacing ? large positive
  vorticity advection above sfc front

500 mb
(surface front shown)
43
ATMS 316- Cold Fronts

• Synoptic-scale analysis
• Narrow band of high cyclonic vorticity nearly
  vertical from sfc to 650 mb
• Largest horizontal temp. gradient just behind or
  coincident with vorticity max.
• Frontal zone ahead of vort. max. above 600 mb

44
ATMS 316- Cold Fronts

• Synoptic-scale analysis
• Strong convergence (conv) limited to lowest 150
  mb (max. at 950 mb)
• Divergence (div) predominated above 600 mb in
  frontal zone
• Low-level conv capped by small region of div at
  700 mb

45
ATMS 316- Cold Fronts

• Synoptic-scale frontogenesis
• Frontogenesis ? increasing temp. gradient across
  front, vertical shear in the along-front wind
  must increase to maintain thermal wind balance
• Accelerations in along-front wind are accompanied
  by ageostrophic motions in the cross-front plane

http//www.aos.wisc.edu/aalopez/aos101/wk12.html
46
ATMS 316- Cold Fronts

• Synoptic-scale frontogenesis
• Secondary circulation equation for a 2D front

sum of terms involving ageostrophic motions
sum of frontogenetical processes
forcing ageostrophic motions
47
ATMS 316- Cold Fronts

• Synoptic-scale frontogenesis
• Secondary circulation equation for a 2D front

frontogenesis due to geostrophic motions
frontogenesis due to the effect of friction
frontogenesis due to cross-front gradients in
diabatic heating
48
ATMS 316- Cold Fronts

• Synoptic-scale frontogenesis
• LHS of Eq (1)
• Ageostrophic flow CCW about positive center
• Maximum in frontal zone at 700 mb results largely
  from second term on LHS of Eq (1)
• Minimum ahead of front at 900 mb results largely
  from third term on LHS of Eq (1)

49
ATMS 316- Cold Fronts

• Synoptic-scale frontogenesis
• RHS of Eq (1) geostrophic forcing
• Correspondence between Figs 6 7 indicates
  degree to which secondary circulations can be
  attributed to geostrophic forcing
• Absence of strong geostrophic forcing near the
  surface isotherms parallel to front in Fig 2(c)

50
ATMS 316- Cold Fronts

• Synoptic-scale frontogenesis
• RHS of Eq (1) effect of turbulent stresses
• Estimated from the results of Fleagle and Nuss
  (1985)
• Assumed a linear stress profile from the surface
  to zero at 900 mb

http//faculty.eas.ualberta.ca/jdwilson/jdw46.html
51
ATMS 316- Cold Fronts

• Synoptic-scale frontogenesis
• RHS of Eq (1) cross-front gradient in diabatic
  heating, considered two processes
• Turbulent flux of sensible heat in the boundary
  layer
• Taken from Fleagle and Nuss (1985)
• Condensation or evaporation
• Determined from the change in mixing ratio
  following a parcel

http//www.emc.ncep.noaa.gov/gmb/noor/au99op/dh306
0S.gif
52
ATMS 316- Cold Fronts

• Synoptic-scale frontogenesis
• RHS of Eq (1) sum
• Friction term is dominant at the low-level
  negative max. at 900 mb
• Condensational heating and cooling above the
  boundary layer accounts for the greater value of
  the positive center at 700 mb over the
  corresponding center in Fig. 7

53
ATMS 316- Cold Fronts

• Synoptic-scale frontogenesis
• RHS of Eq (1) sum
• Boundary layer turbulent heat fluxes had a
  minimal effect on the synoptic-scale secondary
  circulation
• 40 underestimate of intensity of negative
  maximum in circulation at 900 mb (Fig. 6)

Potential error sources obs. errors, poor data
coverage, bad parameterizations, finite
difference errors, poor assumptions, along-front
winds not in complete geostrophic balance
54
ATMS 316- Cold Fronts

• Synoptic-scale frontogenesis
• General agreement between Figs. 6 8
• indicates that the dominant features of the
  synoptic-scale ageostrophic circulation at a
  strong front can be accounted for by the
  geostrophic forcing, plus the diabatic and
  frictional effects

55
ATMS 316- Cold Fronts

• Mesoscale structure
• Flight-level pressure measurements indicate
  significant variations within a few kilometers of
  the front
• Appear to be non-hydrostatic
• Correlated with large vertical accelerations
• Assumptions required in applying Eq (1) are not
  valid on the mesoscale near the front

56
ATMS 316- Cold Fronts

• Mesoscale structure
• Flight-level pressure measurements indicate
  significant variations within a few kilometers of
  the front
• Assumptions required in applying Eq (1) are not
  valid on the mesoscale near the front
• Restrict mesoscale analyses to a kinematical and
  thermodynamical description rather than to a
  dynamical equation

57
ATMS 316- Cold Fronts

• Mesoscale structure
• Equivalent potential temperature (qe) is used to
  represent a quasi-conservative thermodynamic
  variable
• Air in the cross section is nearly saturated
• Zero point on horizontal scale is set at the
  intersection of the surface with the 307 K qe
  contour

58
ATMS 316- Cold Fronts

• Mesoscale structure
• Strongest horizontal gradients of q and qe were
  at 900 mb level
• General pattern of qe suggests a gravity current
  (a.k.a. density current), propagation speed
• calculation 14 to 18 m s-1
• measured 17 m s-1

59
ATMS 316- Cold Fronts

• Mesoscale structure
• Coordinate system moving with the front
  (front-relative)
• Greatest inflow at low levels from warm side
• Conv drops off rapidly with height above the nose
  of the front
• Div found between 850 and 700 mb directly above
  sfc conv

cross-front wind
60
ATMS 316- Cold Fronts

• Mesoscale structure
• Max. velocities greater than 40 m s-1 in warm air
  near 850 mb
• Less prominent prefrontal low-level jet than in
  other studies
• Relatively low velocities (30 m s-1) above and
  to the left of the nose of the front

along-front wind
61
ATMS 316- Cold Fronts

• Mesoscale structure
• Max. updraft velocity at 875 mb, wider than in
  other studies
• Downdraft of 1 m s-1 found 2 km behind the main
  updraft
• Large discrepancy between estimated (using
  kinematic method) and measured vertical
  velocities

measured vertical velocities (m s-1)
62
ATMS 316- Cold Fronts

• Mesoscale structure
• Large discrepancy between estimated (using
  kinematic method) and measured vertical
  velocities, caused by ??
• Change in velocity field between successive
  crossing of the front
• Mesoscale variation in the parallel wind
  component in the along-front direction
• Frontal precipitation cores??

http//www.atmos.washington.edu/houze/improve2_su
mmaries/011128-011129images/ppi_rf_0111281803.gif
63
ATMS 316- Cold Fronts

• Mesoscale structure
• qe and streamlines relative to front

64
ATMS 316- Cold Fronts

• Mesoscale structure
• Within the boundary layer, air flows toward the
  front from both the cold and warm sides
• Warm air approaches more rapidly than the cold
  air
• Converging cold and warm streams form an updraft
  within and east of region of greatest horizontal
  gradient of qe

65
ATMS 316- Cold Fronts

• Mesoscale structure
• Converging cold and warm streams form an updraft
  within and east of region of greatest horizontal
  gradient of qe
• Mass of water vapor carried upward from boundary
  layer by the updraft is equivalent to
  precipitation of 1 mm h-1 extending over a
  west-east span of 580 km

66
ATMS 316- Cold Fronts

• Mesoscale structure
• Mass of water vapor carried upward from boundary
  layer by the updraft is equivalent to
  precipitation of 1 mm h-1 extending over a
  west-east span of 580 km
• Frictional convergence accounted for nearly the
  entire updraft mass flux

67
ATMS 316- Cold Fronts

• Mesoscale structure
• Above 875 mb the updraft undergoes strong
  horizontal div
• Part of warm air flows eastward and continues
  slowly upward
• Another part flows westward along sloping frontal
  surface
• Gravity waves generated by Kelvin-Helmholtz
  instability in and above frontal zone?

68
ATMS 316- Cold Fronts

• Mesoscale structure
• Two vortices in cold air behind front
• Implies divergence in along-front direction
• Cold air between vortices
• Result of diabatic effects
• Variations in qe in the along-frontal direction

69
ATMS 316- Cold Fronts

• Mesoscale structure
• Converging boundary layer flow convective
  instability stronger on cold air side
• Features similar to other frontal studies
• Browning and Harold (1970)
• Carbone (1982)
• Shapiro (1984)

70
ATMS 316- Cold Fronts

• Mesoscale structure
• Is qe conserved by individual air parcels?
• If so, streamlines would be everywhere parallel
  to qe isotherms
• Tend to be parallel in Fig. 13
• Regions where they are not parallel

71
ATMS 316- Cold Fronts
freezing level

• Mesoscale structure
• Regions where streamlines are not parallel to qe
  isotherms
• Just below the freezing level
• Cooling due to melting of ice crystals Carbone
  1982?
• Near two cold air vortices
• Turbulent mixing
• Changes with respect to time
• Variations in the y direction

72
ATMS 316- Cold Fronts

• Mesoscale structure
• Regions where streamlines are not parallel to qe
  isotherms
• Turbulent mixing
• Not strong enough to account for all of the
  Lagrangian change in qe, especially in the
  regions of greatest gradients of qe
• Other potential causes
• Radiation, cooling by melting, warming or cooling
  by falling rain, obs and analysis errors,
  along-front variation (advection)

73
ATMS 316- Cold Fronts

• Mesoscale structure
• Streamlines and qe isotherms are similar in their
  major features
• Each of the fields provides confirmation of the
  other
• Although (1) effects of change w/ time and (2)
  variation along the front have not been
  explicitly accounted for, they do not appear to
  have invalidated this analysis

74
ATMS 316- Cold Fronts

• Mesoscale frontogenesis
• Frontogenesis (in the Lagrangian sense) for the
  case of uniformity along the front

confluence
tilting
diabatic processes other than condensation and
evaporation
describes the changes experienced by a parcel
and does not yield an estimate of the local
change in the strength of the front
75
ATMS 316- Cold Fronts

• Mesoscale frontogenesis
• RHS Eq. (7), first term
• No obvious pattern (noisy)
• Third order derivatives

( frontogenesis)
76
ATMS 316- Cold Fronts

• Mesoscale frontogenesis
• RHS Eq. (7), second term
• Large positive values at 900 mb
• Forced by maximum wind conv in boundary layer
  below 900 mb, on warm side of largest temperature
  gradient

( frontogenesis)
77
ATMS 316- Cold Fronts

• Mesoscale frontogenesis
• RHS Eq. (7), third term
• Negative on cold side of updraft core
• Positive on warm side of updraft core
• Sign is reversed at lower levels where the
  atmosphere was convectively unstable
• Frontolysis in very stably stratified frontal
  inversion at 875 mb

( frontogenesis)
78
ATMS 316- Cold Fronts

• Mesoscale frontogenesis
• RHS Eq. (7), sum
• Confluence and tilting terms contribute equally
  to strong frontogenesis in the boundary layer
• Tilting term is responsible for strong
  frontolysis above 900 mb
• Turbulent mixing is the only strongly frontolytic
  process in the boundary layer near the front

( frontogenesis)
boundary layer mixing may be the primary process
which limits the sharpness of the front
79
ATMS 316- Cold Fronts

• Mesoscale frontogenesis
• Comparison to other studies

80
ATMS 316- Cold Fronts

• Conclusions
• Synoptic-scale analysis
• Friction, condensational heating, and geostrophic
  forcing are all important in creating the
  ageostrophic circulation near the front
• Friction is important in the boundary layer

81
ATMS 316- Cold Fronts

• Conclusions
• Mesoscale analysis
• Vertical mass transport was highly concentrated
  within a zone about 2 km wide at the leading edge
  of the front
• Gradients of temperature and humidity and
  associated frontogenetical processes were also
  concentrated near the leading edge of the front

82
ATMS 316- Cold Fronts

• Conclusions
• Mesoscale analysis
• Frictional convergence in the boundary layer
  accounted for 80 of the observed updraft
• Disclosed frontal features similar to gravity
  currents in the laboratory
• Extension of the cold, dense air ahead of the
  cold air at the surface
• Vertical undulations of the frontal surface
  behind the surface front

83
ATMS 316- Cold Fronts

• Conclusions
• Mesoscale analysis
• Low-level frontal zone 2 km wide, width largely
  determined by
• Confluent flow
• Turbulent mixing
• Tilting has a secondary effect in the surface
  layer stronger impact above the surface layer

84
ATMS 316- Cold Fronts

• Which scenario?
• Scenario1 synoptic scale forcing alone
• Scenario2 synoptic scale dominates mesoscale
  forcing
• Scenario3 weak synoptic scale forcing

http//www.jeffsweather.com/archives/2006/02/
17 February 2006 Cold front passage