Cold Fronts and their relationship to density currents: A case study and idealised modelling...
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![Page 1: Cold Fronts and their relationship to density currents: A case study and idealised modelling experiments Victoria Sinclair University of HelsinkI David.](https://reader036.fdocuments.us/reader036/viewer/2022062409/56649ce45503460f949b10e4/html5/thumbnails/1.jpg)
Cold Fronts and their relationship to density currents: A case study and idealised modelling experiments
Victoria SinclairUniversity of HelsinkI
David SchultzUniversity of Helsinki, FMI,
University of Manchester, UK
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Overview
• Previous work and some theory concerning cold fronts and density currents
• A Case Study– Observations– AROME simulation
• Idealised Modeling Experiments– 2D density current and 3D cold front– Quantify governing dynamics
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Can cold fronts be considered density currents?
Plenty of papers state that a cold front resembles a density current in appearance
Visual similarity does not equal dynamical similarity
Tower observations of a cold front, Colorado
Shapiro et al. 1985
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Density Current theory
0.5gh
c k
• Coriolis force can be neglected
• Equations exists which predict the speed of movement as a function of density difference and the depth
• Density currents have a low-level feeder flow behind the leading edge: the wind speeds behind the front (u) are greater than the speed that the gravity current moves at (c)
1
1
Du pfv
Dt x
Dv pfu
Dt y
XX
0u c
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Fronts Theory
• Fronts are often assumed to be balanced, at least in the cross front direction
• Acceleration term is assumed to be small.
1
1
Du pfv
Dt x
Dv pfu
Dt y
XX
• No formula to predict the speed that fronts move at
• Uncertainty remains as to what factors control the speed that cold fronts move at
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Questions
• What controls the speed that cold fronts move at?– Why do some cold fronts propagate – i.e. move faster
than the normal component of the wind?– Why do some cold fronts move slower than the
normal wind, and hence share a feature with gravity currents?
• When do cold fronts collapse to resemble density currents?
• Are collapsed cold fronts dynamically similar to density currents?
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Motivation
• Cold fronts that evolve into gravity current type features can produce hazardous weather
• The scale of a collapsed front means that even high resolution NWP models will not capture the structure and evolution well
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Case Study: synoptic evolution
• Developed as a frontal wave on pre-existing front• Mature front and is far from the parent low• Simulated event with AROME 33h1, 2.5km
12 UTC 29 Oct 00 UTC 30 Oct 00 UTC 31 Oct
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Shallow frontal zone 00:11 UTC
• Radial wind speeds from Kumpula Radar
• Cold air is confined to a shallow layer
• Resembles a density current
6 m/s
7 m/s
Image provided by Matti Leskinen
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Temperature at Kivenlahti
Observations AROME
black: 5 m red: 26 mblue: 48 mmagenta: 93 m
grey: 141 mgreen: 218 mbrown: 266 m orange: 296 m
black: 2 m blue: 38 mmagenta: 112 m
green: 200 m orange: 300 m
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Temperature at Kuopio
Observations AROME
black: 5 m red: 26 mblue: 48 mmagenta: 93 m
grey: 141 mgreen: 218 mbrown: 266 m orange: 296 m
black: 2 m blue: 38 mmagenta: 112 m
green: 200 m orange: 300 m
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Heat Fluxes
SMEAR III SMEAR II
BLACK: observed. GREY: AROME
Data provided by Annika Nordbo and Ivan Mammarella
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AROME Potential Temperature 900hPa
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Location of Cold Front from AROME
Averaged speed of front between 22:00 UTC and 02:00 UTC
Section B = 5.03 ms-1
Section C = 5.47 ms-1
Section A = 6.92 ms-1
Front is located objectively
Hewson (1998)
Jenker et al (2010)
Black: 18:00 UTC
Red: 20:00 UTC
Green: 22:00 UTC
Blue: 00:00 UTC
Purple: 02:00 UTC
Cyan: 04:00 UTC
B
B
A
C
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Wind Speeds from AROME
• Wind speeds decrease behind the front
• Unconvincing evidence of a “feeder flow”
920 hPa 990 hPa
u – c > 0 especially in south u – c ≈ 0
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Ascent, potential temperature Simulated Radar reflectivity
22 UTC, B 00 UTC, B
22 UTC, A 00 UTC, A
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Case Study Conclusions
• Shallow and narrow front– stable mid-troposphere– Stable BL may have prevented frontolysis by turbulent
mixing
• Dynamics differ to density current dynamics– No clear feeder flow
• Prefrontal boundary layer appears to affect structure
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Idealised Modelling with WRF
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Idealized Experiment
• WRF-ARW– Weather Research and Forecasting –
Advance Research WRF. V3.1– Non-Hydrostatic, range of physics options– Supported by NCAR
• First simulated a 2D density current at high resolution (100m grid spacing)
• Calculate force balance.
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Density Current
5 – 10 minutes : 20.5 ms-1
10 – 15 minutes: 15.3 ms-1
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Force Balancelowest model level (995 hPa)
Blue: Potential temperature
Red: Pressure Gradient Force
Purple: Coriolis
Black: Acceleration
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Simulate a Cold Front
• Model a full 3D baroclinic life cycle
• Include two nested domains over the cold front– horizontal grid spacing is 100km : 20km : 4km– All nests have 64 levels, model top at 100hPa
• Initial experiment has no moisture and no physical parameterizations
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Potential temperature and surface pressure. Day 4.5. Parent domain
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Potential Temperature and wind vectors. 20 km domain
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Potential temperature and vertical motion
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Force balance
Purple: Coriolis
Black: Acceleration
Blue: Potential temperature
Red: Pressure Gradient Force
LEVEL 1 ~ 975 h Pa LEVEL 7 ~ 805 h Pa
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Force Balance 5 hrs later
Blue: Potential temperature
Red: Pressure Gradient Force
LEVEL 1 ~ 975 h Pa LEVEL 7 ~ 805 h Pa
Purple: Coriolis
Black: Acceleration
Blue: Potential temperature
Red: Pressure Gradient Force
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Conclusions
• Idealised cold front does not visually resemble a density current, but does have many interesting features
• The force balance shows a three way balance near the cold front
• HYPOTHESIS– friction and turbulence will change force balance– Trailing part of cold front will be visually more similar
to density currents
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Future work
• Higher resolution (1km) simulation of cold front, include boundary layer scheme
• Different baroclinic life cycles
• Simulate 3D density current at comparable resolution to cold front case
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Thank you
You can look at more animations on my webpages
www.atm.helsinki.fi/~vsinclai
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Force Balance: 5 hrs later
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Force Balance across cold front