Stratospheric Sudden Warming from a Potential Vorticity Perspective Rainer Bleck, Shan Sun, Stan Benjamin, John Brown NOAA Earth System Research Lab Boulder, Colorado April 2017

Stratospheric Sudden Warming from a Potential Vorticity Perspective PV and q coordinate Rainer Bleck, Shan Sun, Stan Benjamin, John Brown NOAA Earth System Research Lab Boulder, Colorado April 2017

Do events in the stratosphere affect the troposphere? A question that, even after 70 years, won’t go away: Do events in the stratosphere affect the troposphere? More specifically: Is correct simulation of sudden stratospheric warmings (SSWs) beneficial for NWP?

Supporting arguments: SSW is accompanied by significant weakening of polar vortex Altered wind profile can alter reflection of planetary waves (Charney & Drazin) There is evidence of downward propagation of planetary waves after SSW (Baldwin & Dunkerton, Shaw & Perlwitz, Hitchcock & Simpson)

In this talk: Review of twice-weekly 32-day predictions during 14 N.H. winters, performed with a near-isentropic coordinate global NWP model with adequately resolved stratosphere (http://fim.noaa.gov). Ensemble size: 1☹️ Review of Eliassen-Palm flux formulation in isentropic coordinates. Hovmöller-type diagrams of SSW predictability. Inspection of individual SSW events in the context of upward wave coupling (before) and downward wave coupling (after); North Atlantic Oscillation index.

Mean flow Wave forcing

Questions: Are SSW events “harbingers” of tropospheric circulation anomalies? If so, can we isolate a physical signal? For example,… Does SSW affect vertical propagation of planetary waves? Specifically, is there evidence (on a case-by-case basis) of downward wave propagation following SSW events?

Coarse (64 lyrs) fine (74 lyrs) Sample Eliassen-Palm flux vectors in isentropic 60km FIM simulations Contoured field: vertical EP flux component (i.e. form drag) 2 vertical resolutions: Coarse (64 lyrs) fine (74 lyrs) winter Vertical model coordinate (K) Vertical model coordinate (K) summer avg. pressure avg. pressure

Red: wave uplink Blue: wave downlink Another example of Eliassen-Palm flux vectors. Contoured field: vertical EP flux component (layer form drag) Red: wave uplink Blue: wave downlink

Next slide: Hovmöller-style diagrams of SSW predictions Abscissa: model initialization time (twice weekly throughout the winter season) Ordinate: forecast lead time (0-32 days) Slanted dashed lines are isolines of equal verification time Contours lining up with dashed lines indicate successful predictions 2 contoured fields: max. 10mb temperature north of 600N to show all warmings mean zonal 10mb wind at 600N to indicate major warmings Brown line near bottom: time series of NAO index

FIM uses an icosa-hedral grid FIM uses an icosa-hedral grid. “G7” means 7 recursive refinements of the icosahedron (=> 60km mesh size) warming (literally) Winter season 2012/2013 10mb zonally averaged zonal velocity at 60 N “major” warming

Next slide: identify major form drag events Sum up form drag values shown in earlier cross sections (pos. and neg. separately) Identify the 20 strongest wave uplink and 20 strongest wave downlink events found in all forecasts during a given winter season. Mark events in Hovmöller diagrams as gold- and blue- colored diamonds (for up- and downlink events resp.)

Winter season 2012/2013 gold: uplink events blue: downlink events

uplink events downlink events Winter season 2012/2013

Another example: Winter season 2008/2009 downlink events Another example: Winter season 2008/2009 uplink events

last example: Winter season 2000/2001 uplink events downlink events Unusual: NO warming despite strong wave uplink event and zonal flow reversal

Conjecture: a distorted polar vortex is a prerequisite for SSW (the other ingredient being an upward-propagating wave) Exhibit 1: symmetric vortex => no SSW despite strong form drag (winter 2000/01) pole

Exhibit 2: stratosphere 3 weeks before SSW (winter 2012/13) Conjecture: a distorted polar vortex is a prerequisite for SSW (the other ingredient being an upward-propagating wave) Exhibit 2: stratosphere 3 weeks before SSW (winter 2012/13) pole

Anomaly correlation of Umean (solid) and Tmax (dashed) predictions at 10hPa during winter seasons 1999/2000 to 2013/2014, plotted against forecast lead time (days). Analysis was limited to cases where observed Umean < 5m/s or observed Tmax > -15C, respectively, at verification time.

Discussion EP flux derivation in q coordinates straightforward Simple interpretation of vertical component as layer form drag (Andrews et al. 1987) Model does good job predicting SSW 2 weeks ahead Polar vortex loses symmetry long before SSW (Plumb 1981) SSW usually preceded by upward planetary wave propagation often robustly simulated up to 4 weeks ahead SSW usually followed by downward wave propagation (Shaw & Perlwitz 2013, Hitchcock & Simpson 2014 ) downward wave typically much weaker than upward wave In case-by-case inspection (14 winters), no apparent impact on North Atlantic Oscillation index

Unused slides

The wave/mean flow interaction terms can also be expressed very simply as advection of perturbation potential vorticity by the perturbation meridional wind….