25 June 2007 16th Conference on Atmospheric and Oceanic Fluid Dynamics

Potential vorticity dynamics of a tropopause polar vortex

Steven M. Cavallo

and Gregory J. Hakim

University of Washington

Department of Atmospheric Sciences

25 June 2007 16th Conference on Atmospheric and Oceanic Fluid Dynamics

Outline

• Tropopause polar vortices (TPVs)

• Ertel Potential Vorticity (EPV)

• Mechanisms for vortex intensity changes

• November 2005 TPV

Overview and observations

Potential vorticity analysis

PV budget

25 June 2007 16th Conference on Atmospheric and Oceanic Fluid Dynamics

What are tropopause polar vortices (TPVs)?

• Vortices that occur north and away from jet stream

• Based on tropopause

• Cold core

Vortices defined by closed contours of a materially conserved field such as PV

Changes in strength equivalent to changes in fluid properties within closed contours

Disturbances wave-like along jet stream

25 June 2007 16th Conference on Atmospheric and Oceanic Fluid Dynamics

Why do we care about TPVs?

• Formation and intensity changes are not well understood

• Are often long-lived

Can have lifetimes > 1 month

• Trigger surface cyclones, such as in the image above

Eventually drift into midlatitudes and jet stream, impacting the weather

25 June 2007 16th Conference on Atmospheric and Oceanic Fluid Dynamics

Ertel Potential Vorticity (EPV)

25 June 2007 16th Conference on Atmospheric and Oceanic Fluid Dynamics

What EPV changes can we expect with a cloud?

Latent heating Heating rate

25 June 2007 16th Conference on Atmospheric and Oceanic Fluid Dynamics

What EPV changes can we expect with a cloud?

Latent heating EPV changes

25 June 2007 16th Conference on Atmospheric and Oceanic Fluid Dynamics

What EPV changes can we expect with a cloud?

Radiation Heating rate

25 June 2007 16th Conference on Atmospheric and Oceanic Fluid Dynamics

What EPV changes can we expect with a cloud?

Latent heating EPV changes

25 June 2007 16th Conference on Atmospheric and Oceanic Fluid Dynamics

What EPV changes can we expect with a cloud?

Radiation + Latent heating

Net EPV changes (radiation > latent heating)

25 June 2007 16th Conference on Atmospheric and Oceanic Fluid Dynamics

What EPV changes can we expect with a cloud?

Radiation + Latent heating

Net EPV changes (radiation < latent heating)

25 June 2007 16th Conference on Atmospheric and Oceanic Fluid Dynamics

November 2005 TPV: Observations

GFS analysis tropopause pressure Coral Harbour, NT sounding

21 November 2005 at 00 UTC

25 June 2007 16th Conference on Atmospheric and Oceanic Fluid Dynamics

November 2005 TPV: Observations

GFS analysis tropopause pressure Coral Harbour, NT sounding

22 November 2005 at 00 UTC

25 June 2007 16th Conference on Atmospheric and Oceanic Fluid Dynamics

November 2005 TPV

WRF (V. 2.1.2) simulations:

• X = 30 km, Nz = 31

• 5-class microphysics, RRTM longwave radiation

• GFS initial conditions, boundaries updated every 3 hours

GFS Tropopause Analysis

25 June 2007 16th Conference on Atmospheric and Oceanic Fluid Dynamics

November 2005 TPV

WRF (V. 2.1.2) simulations:

• X = 30 km, Nz = 31

• 5-class microphysics, RRTM longwave radiation

• GFS initial conditions, boundaries updated every 3 hours

GFS Tropopause Analysis

25 June 2007 16th Conference on Atmospheric and Oceanic Fluid Dynamics

November 2005 TPV: Strengthening

PV Budget: Time mean vertical profiles for Siberia segment

All EPV components Diabatic components

25 June 2007 16th Conference on Atmospheric and Oceanic Fluid Dynamics

November 2005 TPV: Strengthening

Time-height: Diabatic components Diabatic components

PV Budget

25 June 2007 16th Conference on Atmospheric and Oceanic Fluid Dynamics

November 2005 TPV: Strengthening

Time-height: Radiation and cloud Diabatic components

PV Budget

25 June 2007 16th Conference on Atmospheric and Oceanic Fluid Dynamics

November 2005 TPV: Strengthening

Time-height: Latent heat + Radiation Diabatic components

PV Budget

25 June 2007 16th Conference on Atmospheric and Oceanic Fluid Dynamics

November 2005 TPV: Weakening

PV Budget: Time mean vertical profiles for Hudson Bay segment

All EPV components Diabatic components

25 June 2007 16th Conference on Atmospheric and Oceanic Fluid Dynamics

Summary

•TPV strengthening from cloud-top radiational cooling in November 2005 TPV

•TPV weakening processes not clear, but strengthening no longer appears to occur when latent heating effects dominate the radiational effects.

• Is radiation always the key driver? What dynamics control weakening?

Work is underway involving idealized experiments to isolate the dynamics responsible for TPV intensity changes.

25 June 2007 16th Conference on Atmospheric and Oceanic Fluid Dynamics

25 June 2007 16th Conference on Atmospheric and Oceanic Fluid Dynamics

What can we expect from a cloud?