A Simple Technique to Infer the Missing Gravity Wave Drag in the Middle Atmosphere Using a General Circulation Model: Potential Vorticity Budget

Quart J Royal Meteoro Soc

Scheffler

Ruiz

et al. 2016

Oceanic and atmospheric global numerical models represent explicitly the large‐scale dynamics while the smaller‐scale processes are not resolved, so that their effects in the large‐scale dynamics are included through subgrid‐scale parametrizations. These parametrizations represent small‐scale effects as a function of the resolved variables. In this work, data assimilation principles are used not only to estimate the parameters of subgrid‐scale parametrizations but also to uncover the functional dependencies of subgrid‐scale processes as a function of large‐scale variables. Two data assimilation methods based on the ensemble transform Kalman filter (ETKF) are evaluated in the two‐scale Lorenz '96 system scenario. The first method is an online estimation which uses the ETKF with an augmented space state composed of the model large‐scale variables and a set of unknown global parameters from the parametrization. The second method is an offline estimation which uses the ETKF to estimate an augmented space state composed of the large‐scale variables and by a space‐dependent model error term. Then a polynomial regression is used to fit the estimated model error as a function of the large‐scale model variables in order to develop a parametrization of small‐scale dynamics. The online estimation shows a good performance when the parameter‐state relationship is assumed to be a quadratic polynomial function. The offline estimation captures better some of the highly nonlinear functional dependencies found in the subgrid‐scale processes. The nonlinear and non‐local dependence found in an experiment with shear‐generated small‐scale dynamics is also recovered by the offline estimation method. Therefore, the combination of these two methods could be a useful tool for the estimation of the functional form of subgrid‐scale parametrizations.

Section: Introductionmentioning

confidence: 99%

Estimation of the functional form of subgrid‐scale parametrizations using ensemble‐based data assimilation: a simple model experiment

Quart J Royal Meteoro Soc

Scheffler

Ruiz

et al. 2016

“…An estimation of this forcing term,

{\overset{X}{false}}_{GW}

, in the middle atmosphere from reanalyses was produced in Pulido and Thuburn [, ]. A useful framework to readily determine

{\overset{X}{false}}_{GW}

is from potential vorticity budget [ Pulido , ].…”

Section: Methodsmentioning

confidence: 94%

“…In the upper stratosphere and lower mesosphere, we expect that the forcing by gravity waves X GW dominates the residual mean meridional circulation. An estimation of this forcing term, X GW , in the middle atmosphere from reanalyses was produced in Pulido andThuburn [2006, 2008]. A useful framework to readily determine X GW is from potential vorticity budget [Pulido, 2014].…”

Section: 1002/2017jd026597mentioning

confidence: 99%

A climatology of Rossby wave generation in the middle atmosphere of the Southern Hemisphere from MERRA reanalysis

Rodas

2017

JGR Atmospheres

Self Cite

A climatological characterization of Rossby wave generation events in the middle atmosphere of the Southern Hemisphere is conducted using 20 years of Modern‐Era Retrospective Analysis for Research and Applications (MERRA) reanalysis. An automatic detection technique of wave generation events is developed and applied to MERRA reanalysis. The Rossby wave generation events with wave period of 1.25 to 5.5 days and zonal wave number from one to three dominate the Eliassen‐Palm flux divergence around the stratopause at high latitudes in the examined 20 year period. These produce an eastward forcing of the general circulation between May and mid‐August in that region. Afterward from mid‐August to the final warming date, Rossby wave generation events are still present but the Eliassen‐Palm flux divergence in the polar stratopause is dominated by low‐frequency Rossby waves that propagate from the troposphere. The Rossby wave generation events are associated with potential vorticity gradient inversion, and so they are a manifestation of the dominant barotropic/baroclinic unstable modes that grow at the cost of smearing the negative meridional gradient of potential vorticity. The most likely region of wave generation is found between 60° and 80°S and at a height of 0.7 hPa, but events were detected from 40 hPa to 0.3 hPa (which is the top of the examined region). The mean number of events per year is 24, and its mean duration is 3.35 days. The event duration follows an exponential distribution.

“…The model is based on PV in an icosaedral-hexagonal grid which is particularly suited to capture adequately planetary wave mean flow interactions as well as gravity wave drag effects on the stratosphere dynamics (Pulido, 2014). Whereas the model used in this work has a coarse resolution, it is able to give some of the most salient features of the 2002 SSW.…”

Section: Discussionmentioning

confidence: 99%

The Role of Gravity Wave Drag Optimization in the Splitting of the Antarctic Vortex in the 2002 Sudden Stratospheric Warming

Scheffler

Geophysical Research Letters

Rodas

2018

Self Cite

The impact of gravity wave drag on the Antarctic sudden stratospheric warming (SSW) in 2002 is examined through a mechanistic middle atmosphere model combined with a variational data assimilation system. Significant differences in the SSW representation are found between a model integration that uses reference gravity wave parameters and one that uses parameters estimated using data assimilation. Upon identical wave forcings at 100 hPa, the vortex breakdown may arise as either a vortex splitting event or a displacement vortex event depending on gravity wave parameters. A local enhancement of Rossby waves is found in the integration with estimated parameters, leading to a split SSW. The changes in the vortex breakdown are associated with changes in the vortex geometry caused entirely by modifying the gravity wave parameters. Gravity wave drag proved to play an instrumental role in preconditioning the stratosphere near a resonant excitation point that triggers the split SSW.