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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
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
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
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
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
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?
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
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
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
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
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
Heat Fluxes
SMEAR III SMEAR II
BLACK: observed. GREY: AROME
Data provided by Annika Nordbo and Ivan Mammarella
AROME Potential Temperature 900hPa
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
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
Ascent, potential temperature Simulated Radar reflectivity
22 UTC, B 00 UTC, B
22 UTC, A 00 UTC, A
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
Idealised Modelling with WRF
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.
Density Current
5 – 10 minutes : 20.5 ms-1
10 – 15 minutes: 15.3 ms-1
Force Balancelowest model level (995 hPa)
Blue: Potential temperature
Red: Pressure Gradient Force
Purple: Coriolis
Black: Acceleration
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
Potential temperature and surface pressure. Day 4.5. Parent domain
Potential Temperature and wind vectors. 20 km domain
Potential temperature and vertical motion
Force balance
Purple: Coriolis
Black: Acceleration
Blue: Potential temperature
Red: Pressure Gradient Force
LEVEL 1 ~ 975 h Pa LEVEL 7 ~ 805 h Pa
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
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
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
Thank you
You can look at more animations on my webpages
www.atm.helsinki.fi/~vsinclai
Force Balance: 5 hrs later
Force Balance across cold front