Structure of the 
shear-line polar low 
south of Svalbard

Denis Sergeev, Ian Renfrew, Thomas Spengler

This presentation is available online: 

https://slides.com/denissergeev/deck

Objectives

  • To investigate the structure of the shear-line polar low (PL)
  • To validate the model against airborne and satellite observations

Motivation

  • One of a dozen of PLs observed with instrumented aircraft
  • Very few shear-line cases in PL studies
  • Archetype of many mesoscale vortices forming in cold-air outbreaks near Svalbard

Polar low evolution

Synoptic overview 

Mesoscale structures

Outline

Data & methods

ACCACIA field campaign

https://arcticaccacia.wordpress.com/

March-April 2013

  • In-situ measurements

    • Standard equipment of FAAM aircraft

    • 11 dropsondes

    • Cloud probes (CDP, 2D-S, Nevzorov)

  • Satellite data

    • Cloud cover: AVHRR

    • Cloud composition: CloudSat

    • Surface wind: ASCAT

Observations

FAAM aircraft
Photo by Rhiannon Davies

UK Met Office Unified Model (MetUM) vn10.2
Horizontal grid 1300x1300 km, grid spacing 2.2 km (500 m)
Vertical grid 70 levels, up to 40 km, incl. 16 below 1 km
Time step 60 s
Parameterizations
PBL scheme
  • Unstable conditions: non-local closure with entrainment fluxes
  • Stable conditions: SHARPEST scheme [Brown et al., 2008]
Microphysics Single-moment 3-phase [Field et al., 2013]
Experiment set-up
Start time 12:00 UTC 25 March (~ 24h before the flight)
Initial and boundary conditions MetUM global run

The big picture

Synoptic

overview

  • Stationary large-scale depression
  • “Merry-go-round”
  • Intense cold-air outbreak in W part
  • Reverse-shear conditions in the lower troposphere

 

  • Upper-level PV anomaly >4 PVU

  • SST-T500 ≈ 50 K

  • MCAO index > 7

  • Sharpening baroclinic zone

PL precursors

500 hPa

850 hPa

Polar low life cycle

Initial stage

  • Convergence of two branches of CAO
  • High positive relative vorticity filament in the lee of Svalbard

Decaying stage

  • The PL travels SE around the synoptic low

  • The core disintegrates into smaller disturbances

  • Remnants of the vortex get absorbed by a new stronger cyclone


Mature stage

  • Instability waves merge into the dominant mesocyclone - the PL of interest

6 h

14 h

25 h

Surface wind patterns

  • PL diameter is 100-150 km
  • Wind speed - up to 25 m/s
  • Stronger winds - at crests of the shear-line waves

ASCAT

Model output

Flying through the shear line

Vertical structure

Mesoscale structure
of the shear line and polar low

Close-up of the surface layer

  • Sharp gradient in wind velocity
  • Peaks in total humidity and cloud particles

 

Model performance

  • Excellent match in wind gradients
  • Temperature front is slightly steeper than observed

 

Surface heat fluxes

  • Surface heat fluxes repeat the wind speed pattern

  • Total heat transfer reaches 500 W m-2

  • Sensible heat flux is about twice larger than latent heat flux

  • SHF overestimated, LHF - underestimated

Cloud structure

derived by CloudSat radar

Credit: www.turbosquid.com

Summary

  • Synoptic conditions: merry-go-round pattern, CAO behind a low

  • PL genesis: wrap-up of a vorticity filament fuelled by shear-instability waves

  • Mesoscale features:

    • Well-defined shear line (~25 m/s over 50 km) and convective cloud wall

    • The eye-like centre with well-mixed mostly dry boundary layer

    • Clouds are mostly mixed-phase

  • MetUM performance:

    • Wind gradients captured perfectly

    • Near-surface atmosphere is not sufficiently mixed (error of ~3 K)

    • SHF overestimated, LHF underestimated (20-30 W m-2)

    • Too many ice crystals, too few liquid droplets

 

Overall, the model does a great job, which lays ground to future exploration of the dynamics of this shear-line PL

Current work:

dynamical diagnostics

Vorticity budget

Horizontal advection

Vertical advection

Vorticity budget

Stretching term

Tilting term

Title Text

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