Download Mesoscale Atmospheric Systems Surface fronts and frontogenesis

Mesoscale Atmospheric Systems
Surface fronts and frontogenesis
28 February 2017
Heini Wernli
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Last year during the lecture course ...
Cold frontal passage over N Switzerland
Radar
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Measurements CHN
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Temperature (degC)
Frontal passage in
Mainz on
26 March 2010
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Pressure (hPa)
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Windspeed (m/s)
Winddirection
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Rapid surface frontogenesis
over the US
surface isotherms (F)
and wind barbs
12 UTC 20 Jan 1959
00 UTC 21 Jan 1959
Carlson 1998, Fig. 13.1
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Concept of
frontogenesis due to
confluent flow
Confluence occurs in flows with
strong deformation
(see later)
Note: confluence ≠
convergence!
Bluestein (in Rao 1985, Fig. 9.7)
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Vertical cross-section across front
isentropes, normal wind velocity
Sanders 1955
Carlson 1998, Fig. 13.2
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What is a front?
front = “elongated” zone of “strong” temperature gradient
-  elongated:
length ~1000 km; width ~100-200 km
-  strong:
~5 K (100 km)-1
i.e., an order of magnitude larger than
typical background baroclinicity
~5 K (1000 km)-1
Other definitions (less appropriate):
-  temperature discontinuity
-  boundary between tropical and polar air masses
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Importance of fronts
•  ubiquitous weather phenomenon in extratropics
•  strongly varying meteorological conditions across front
•  severe weather associated with frontal passage (heavy
rain, thunderstorms, strong winds)
•  upper-level fronts (see later) often associated with clearair turbulence and stratosphere-troposphere exchange
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Kinematics: frontogenesis function
Early concepts by
Petterssen (1936)
Miller (1948)
Basic idea:
Consider
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=?
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(1)
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Theory of frontogenesis
Horizontal version of (1) in quasi-geostrophic limit:
g
(2)
is the Q-vector
and E = Dθ/Dt is the diabatic heating (latent heating in clouds,
radiation)
Caveat: These equations describe evolution of horizontal temperature
gradient along motion of fluid parcel – but not for the front itself!
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Radiative effects on frontogenesis
Frontogenesis with stratus
cloud in cold air
Frontolysis with stratus cloud
in warm air
Carlson 1998, Fig. 13.9
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Theory of frontogenesis
Consider adiabatic limit (E=0) of surface frontogenesis
(i.e., consider only horizontal gradient of θ):
where
D
total deformation = √ (α12+α22)
δ
angle between dilatation axis and isentropes
Note: there is a deformation term and a divergence term (which
is zero in the q.g. limit)
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Effect of deformation on fronts
frontogenetic (b=δ < 45°)
frontolytic (b=δ > 45°)
Bluestein (in Rao 1985, Fig. 9.9)
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Quasi-geostrophic frontogenesis
Structure of front as evolved from QG deformation frontogenesis
Unrealistic aspects:
- frontal zone does not tilt with height
-  regions of static instability are produced
Remark about lecture notes:
•  Chap. 1.1 and 1.2: what we
discussed so far
•  Chap 1.3-1.5: a detailed
mathematical excursion using
semi-geostrophic theory
•  Chap 2: what we do next
Stone 1966 (in Rao 1985, Fig.9.12)
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Fronts in baroclinic waves
- fronts develop typically within growing baroclinic waves
-  cold fronts are typically 2-dimensional, warm fronts 3-dim
Idealized experiment of baroclinic instability: baroclinic zone (i.e.,
upper-level jet) and finite amplitude upper-level perturbation (i.e.,
positive PV anomaly)
Hoskins et al. 1985
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Idealized experiments of fronts developing in intensifying
extratropical cyclone
Day 0
initial perturbation:
upper-level trough
(induces weak surface
pressure minimum)
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north-south temperature gradient
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Simultaneous development of surface cyclone and fronts
Day 1
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Simultaneous development of surface cyclone and fronts
Day 2
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Simultaneous development of surface cyclone and fronts
Day 3
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Simultaneous development of surface cyclone and fronts
Day 4
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Quasi-geostrophic frontogenesis: the dynamical picture
Equations for vorticity, vertical motion and the horizontal
temperature gradient
g
g
g
where the material derivative is only along the geostrophic flow
g
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Frontogenesis in evolving extratropical cyclone (dry, E=0)
Q-vectors and divQ at the surface on day 2
Wernli et al. 1998
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Frontogenesis in evolving extratropical cyclone
Q-vectors and divQ at the surface on day 4
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Frontogenesis in evolving extratropical cyclone
Strong difference between warm and cold fronts with respect to
correlation of ζ (gray colors) and divQ (dashed contours): WHY?
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Frontogenesis in evolving extratropical cyclone
Flow of air parcels in regions of cold, warm, and bent-back warm front;
dashed lines show regions of large divQ
parcel positions
on day 2
day 3
day 4
-  cold frontal air parcel moves with front
-  warm frontal air parcels move along front and cross max divQ region
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Temporal evolution of vorticity along trajectories
vorticity
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