Non-orographic gravity waves in general circulation models

Transcription

1 Non-orographic gravity waves in general circulation models Erich Becker Leibniz-Institute of Atmospheric Physics (IAP) Kühlungsborn, Germany

2 (1) General problem and issues

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4 Assumed equilibirium state for January Simulated dynamically determined January climatology using the Kühlungsborn Mechanistic general Circulation model (KMCM) with high resolution T120/L190 and resolved GWs

5 Kühlungsborn Mechanistic general Circulation Model (KMCM) standard spectral dynamical core here: T120 or T210, 190 hybrid levels up to ~125 km mechanistic because Q rad c p idealized latent heating rates in the troposphere diagnostic turbulent diffusion scheme formulated and implemented in line with the conservation laws (Becker, 2003, MWR; Becker and Burkhardt, 2006, MWR) vertical and horizontal diffusion coefficients according to Smagorinsky s generalized mixing-length concept and, in addition, scaled by the Richardson criterion for dynamic instability of resolved gravity waves (Becker, 2009, JAS) ( T T (, p)) / E

6 Simulated wave driving during January zonal acceleration by the residual circulaton (m/d/s) Div (EPF) (m/s/d) GW drag from resolved mesoscale gravity waves (m/s/d) Div (qgepf) from planetary Rossby waves (m/s/d) maximum GW drag ~ 160 m/s/d in the summer MLT

7 Estimates of the GW drag from comprehensive GCMs with parameterized GWs The WACCM (Richter et al., 2008, JGR) employs a Lindzen-type GW paramerization: GW drag ~ 110 m/s/d in the summer MLT The extended CMAM (Fomichev et al., 2002, JGR) employs the Doppler-spread parameterization (Hines, 1997, JASTP): GW drag ~ 130 m/s/d in the summer MLT GW drag for July (m/s/d)

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9 Rocket-borne observations at Andenes (69ºN) of the (GW-induced) turbulent dissipation rate (Lübken, 1997, JGR) Sudden onset of mean dissipation > 10 K/d above 80 km in summer. Weaker but still significant Dissipation of ~1-10 K/d throughout the MLT in winter

10 Simulated sensible heat budget in the summer MLT (KMCM with resolved GWs) The large-scale dynamic cooling around and below the summer mesospause is largely balanced by direct GW-related effects, the dissipation is just one of which.

11 (2) PE with GWs and turbulence filtered out in single-column approximation

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14 2009, JAS)

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16 (3) Application in GW schemes

17 Interaction of GWs and turbulence in GW parameterizations Lindzen-type schemes: independent monochromatic GWs with individual diffusion coefficients for according to the saturation assumption; sum of individual GW fluxes and diff. coefficients applied to the resolved flow; GWs and resolved flow subject to different diff. coefficients Spectral schemes (Hines, Alexander-Dunkerton, Warner-McIntyre): continuous m-spectrum truncated and/or damped with increasing height due to some criterion (Doppler-spread critical-layer assumption, saturation condition); energy deposition scaled by tunable parameters or ignored; GW-induced turbulent diffusion affects the mean flow but not the GWs Medvedev-Klaassen scheme: continuous m-spectrum damped with increasing height due to parameterized nonlinear wave-wave interaction; straight-forward extension to include other damping mechanisms like molecular diffusion (Yiğit et al., 2008, JGR); no GW-induced turbulent diffusion coefficient for the resolved flow

18 Extending the Doppler-spread parameterization (DSP) by the consistent interaction of GWs and turbulence Conventional: Erosion of the spectrum causes energy desposition that induces turbulent diffusion which in turn acts only on the mean flow (Hines, 1997, JASTP). Consistent scale interaction: Erosion of the spectrum causes energy desposition that induces turbulent diffusion which in turn acts on both the mean flow and the unobliterated GWs diffusion feedback (Becker & McLandress, 2009, JAS).

19 Model sensitivity to the neglect of the diffusion feedback, using the extended DSP within the KMCM during July latitude latitude The diffusion feedback mainly induces an alternating zonal wind pattern in the tropics and winter subtropics. Potential importance in simulations of the SAO or the QBO (Becker & McLandress, 2009, JAS)

20 (4) GWs in high-resolution GCMs

21 Snapshots of the simulated vertical wind in the summer hemisphere KMCM T120L190: shortest resolved horizontal wavelength ~350 km KMCM T210L190: shortest resolved horizontal wavelength ~200 km For higher resolution, the simulated gravity waves have smaller spatial scales (and higher frequencies).

22 C o n c l u s i o n s GW parameterizations for comprehensive GCMs are based on the singlecolumn approximation and the assumption of a stationary GW kinetic energy equation. The latter is questionable with regard to GW-tidal interactions or intermediate spatial resolutions. GCMs generally agree on the strengths of the momentum flux and GW drag in the MLT. The accompanying thermal effects require additional formulations of 1) the pressure flux, 2) the wave heat (potential temperature) flux, and 3) the turbulent diffusion induced by GW breakdown. The overall pattern of the direct GW heating consists of heating around the mesopause and cooling above by about ± 3 10 K/d. In principle, all currently used GW parameterizations can consistenly be completed with the thermal effects and the scale-interactions of parameterized (GWs & turbulence) and resolved scales. With the caution of spectral biasing, the effects of extratopical nonorographic GWs in the MLT can be simulated explicitly in a mechanistic GCM, and probably also in comprehensive GCMs in the foreseeable future.

23 Parameterization of turbulent friction and dissipation in the KMCM c p v K S ( K ) v K 2 z 118Ri, F( Ri) 1 (1 9Ri), Ri t t z T diff diss l N 2 1 K h z / ( S v F( Ri), z v) 2 2 h ( K z ) ( Ri Ri 1 K h 0 0 Z Z v) l 2 h Ri Z S (1 F( Ri)) for (energy conservation, second p 100 hpa 3 free parameters: 2 mixing lengths and ; hydrodynamically consistent formulation Smagorinsky-type diffusion coefficients coefficients (Becker & Burkhardt, 2006) automatical adjustment to dynamic instability (Becker, 2009, JAS); discretization of the dissipation due to vertical momentum diffusion fulfills the no-slip condition: no exchange of mechanical energy between the atmosphere and the surface (Becker, 2003, MWR; Boville & Bretherton, 2003, MWR) 0 Z, S S T (ang. mom. conserv.) law)