Introduction

Polewards breaking Rossby waves (RWB) are frequently observed over Northern Europe during Winter. The following figures illustrate the climatological background of this phenomenon.

Climatological zonal wind at 300 hPa for December from ERA40 data. RWB over Europe takes place in 40 % of the Winter days. A sufficiently strong polar vortex induces a forward breaking (in pole direction).

Four types of RWB may be distuingueshed - depending on on the dominant direction breaking direction (poleward - equatorward or upstream - downstream; Peters & Waugh, 1996)

Schematic presentation of four different types of RWB (green contour) and the most probable position of jet streaks (green arrows) and associated IGWs (red wavy arrows).

RWB are connected with large-scale transports of air masses - the associated wind jets and convection induce the emission of inertia-gravity waves (IGW).

ERA40 (1999-2003) zonal wind profiles over Kühlungsborn (black solid line: mean value; black dashed line: standard deviation; colored lines: four selected LEWIZ campaigns). In most of the cases, tropospheric jets at about 10 km altitude are accompanied with strong wind in the stratosphere.

IGWs and RWs transport energy and momentum and influence the dynamics of the atmosphere (extreme weather events, polar stratospheric clouds, ... and the circulation in the mesosphere). General circulation models require a parameterisation of not resolved IGWs. The following figures illustrate the connection between RWB and IGW.

ECMWF forecasts for the RWB event at 12:00 UTC 17 December 1999 of the Ertel potential vorticity in EPV units (= 10^-6 Km²/s*kg) at 330 K (roughly mid-latitude tropopausel level). A downstream poleward breaking Rossby wave is indicated.

In connection with poleward breaking Rossby waves (Peters and Waugh, 1996, J. Atmos. Sci. 53: 3013-3031) tropospheric wind jets occur on the polar side of a ridge. Downstream of that jet in special regions of troposphere and stratosphere observations and simulations show the appearance of inertia-gravity waves. In the literature (for example O'Sullivan & Dunkerton, 1995, J. Atmos. Sci. 52: 3695-3716; Guest et al., 2000, J. Atmos. Sci. 57, 737 - 752.) the forcing of these waves and their interaction with the polar vortex is associated with many physical processes und not clear yet. The contribution of inertia-gravity waves to the exchange and transport processes in the middle atmosphere is another aspect of the problem.

ECMWF forecasts of the horizontal divergence (a measure for the IGW amplitude) at 300 hPa. A region of increased IGW activity is indicated.

Aim

Observe, simulate and model the connection between RWB and IGW.

Problems

generation of IGWs by the tropospheric jet
interaction of jet-generated IGW with orography and convection
climatology of RWB and IGW
parameterization of IGW effects for GCMs

Methods

Observations
- radiosondes (including ozone)
- VHF radar
- re- and analysis products (DWD-GME, ECMWF-IFS, -ERA40)
- assimilated products (ELDAS, SMHI-MESAN)
Models
- non-hydrostatic mesoscale model (MM5)
- contour advection
- forecasts (DWD-GME)

Projects

RWB characteristics & climatology

classification and statistics of breaking Rossby waves --> AFO2000-LEWIZ
cirrus clouds as indicators for air mass advection by the Rossby wave --> LEWIZ-cirri

IGW generation

IGW generation in the exit region of tropsopheric jet streaks
- conceptual model --> Pilot-LEWIZ
- aggregated field campaigns --> AFO2000-LEWIZ

Scetch of jet-generated inertia-gravity waves

interaction between convection and inertia-gravity waves --> PIGW

Scetch of generation of inertia-gravity waves by convection

contributions from mountain- and jet-generated inertia-gravity waves --> Matrix-LEWIZ
observations of IGW from the surface up into the mesosphere with radiosonde, radar, LIDAR and falling spheres launched at Andenes--> ROMA/LEWIZ

Scetch of orographically excited inertia-gravity waves

IGW impact

IGW patterns in surface wind and precipitation --> PIGW

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