Contrasting potential vorticity structures in two summer extratropical cyclones Oscar Martínez-Alvarado NCAS-Atmospheric Physics Sue Gray John Methven Department of Meteorology University of Reading Mesoscale Group Department of Meteorology University of Reading 2 June 2015

UTC 18 July UTC 15 August 2012 Radar rainfall rate Both cyclones produced heavy prolonged frontal precipitation, implying strong diabatic processes >32 Rain rate (mm hr -1 )

UTC 18 July UTC 15 August 2012 MSLPSummerWinter 930 – 970 hPa – 990 hPa – 1010 hPa – 1030 hPa1510 Adapted from Čampa and Wernli (2012) Synoptic conditions Yet their synoptic structure and development were very different

Objective To compare the evolution of these two contrasting summer cyclones in terms of: Potential temperature modification Heating and cross-isentropic vertical motion Potential vorticity modification Changes in circulation and vorticity 4

Method Tracers of potential temperature and potential vorticity Both variables are conserved under adiabatic frictionless flow POTENTIAL TEMPERATURE is an excellent marker of cross-isentropic motion and air mass origin. POTENTIAL VORTICITY provides information about the wind and potential temperature fields. The results are from 21-h hindcast simulations using the Met Office Unified Model (MetUM) 5

Description of tracers 6 (1) These tracers tell us how important different processes are for the formation of structures in cyclones

7 Upper-level (320-K) structure – 18 July 2012 BLACK: 2-PVU contour (tropopause) stratospheric tropospheric warm conveyor belt outflow K θ 0 Potential temperature that the air had at the start of the simulation A B

8 BLACK: 2-PVU contour (tropopause) stratospheric tropospheric warm conveyor belt flow K C D θ 0 Potential temperature that the air had at the start of the simulation Upper-level (320-K) structure – 15 August 2012

9 Vertical structure BOLD BLACK: 2-PVU THIN BLACK: θ AB θ 0 on 18 July 2012 K 380 K warm conveyor belt flow Low-pressure centre θ 0 on 15 August 2012 K 380 K warm conveyor belt flow Low-pressure centre CD

10 Mass redistribution 18 July August 2012 Averaged over a 1000-km radius circle Median 98 th percentile In each θ 0 bracket

11 Mass redistribution 18 July August 2012 Averaged over a 1000-km radius circle Median 98 th percentile In each θ 0 bracket

12 Integral interpretation: Circulation The absolute circulation on an isentropic surface is given by The PV diabatic tracers induce contributions to the circulation on isentropic surfaces Where Potential vorticity can be written as

13 Absolute vorticity 18 July 2012 Averaged over a 1000-km radius circle f 15 August 2012 f

14 Absolute vorticity 18 July 2012 Averaged over a 500-km radius circle f 15 August 2012 f

15 PV impermeability theorem (Haynes and McIntyre 1987) Potential vorticity PV equation in flux form Potential vorticity flux The potential vorticity flux J does not have a cross-isentropic component

16 PV impermeability theorem Isentropic coordinates PV substance PV equation in flux form PV flux In this coordinates

17 PV diabatic tracers Potential vorticity can be written as Evolution equations in flux form Where the fluxes are now given by These fluxes no longer lie on isentropic surfaces

18 PV tracers: Cross-isentropic fluxes The fluxes’ cross-isentropic components are now given by It can be shown that Therefore, The PV diabatic tracers are not PV per se, but contributions to PV

19 Absolute vorticity 18 July August 2012 Averaged over a 500-km radius circle

The evolution of diabatically-generated PV in two contrasting summer extratropical cyclones has been characterised. One cyclone exhibited relatively shallow lower- and upper- level PV anomalies In contrast, the second cyclone exhibited very strong and well- developed PV anomalies leading to a strong PV tower Diabatic tracers have been used to show that the convection scheme was much more active in the case with a strong PV tower; its effects are present in the heating production, cross-isentropic vertical motion and the changes in vorticity within the cyclone The boundary layer and the cloud microphysics schemes were important in producing low-level anomalies in both cyclones 20 Conclusions