Gravity wave activity and dynamical effects in the middle atmosphere (60–90km): observations from an MF/MLT radar network, and results from the Canadian Middle Atmosphere Model (CMAM) (original) (raw)
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Annales Geophysicae, 2004
We report the sensitivity of the Berlin Climate Middle Atmosphere Model (CMAM) to different gravitywave (GW) parameterisations. We perform five perpetual January experiments: 1) Rayleigh friction (RF) (control), 2) non-orographic GWs, 3) orographic GWs, 4) orographic and non-orographic GWs with no background stress, and 5) as for 4) but with background stress. We also repeat experiment 4) but for July conditions. Our main aim is to improve the model climatology by introducing orographic and non-orographic parameterisations and to investigate the individual effect of these schemes in the Berlin CMAM. We compare with an RF control to determine the improvement upon a previously-published model version employing RF. Results are broadly similar to previously-published works. The runs having both orographic and non-orographic GWs produce a statistically-significant warming of 4-8 K in the wintertime polar lower stratosphere. These runs also feature a cooling of the warm summer pole in the mesosphere by 10-15 K, more in line with observations. This is associated with the non-orographic GW scheme. This scheme is also associated with a heating feature in the winter polar upper stratosphere directly below the peak GW-breaking region. The runs with both orographic and non-orographic GWs feature a statistically-significant deceleration in the polar night jet (PNJ) of 10-20 ms −1 in the lower stratosphere. Both orographic and non-orographic GWs individually produce some latitudinal tilting of the polar jet with height, although the main effect comes from the non-orographic waves. The resulting degree of tilt, although improved, is nevertheless still weaker than that observed. Accordingly, wintertime variability in the zonal mean wind, which peaks at the edge of the vortex, tends to maximise too far polewards in the model compared with observations. Gravity-planetary wave interaction leads to a decrease in the amplitudes of stationary planetary waves 1 and 2 by up to 50% in the up
Atmosphere-Ocean, 2003
An overview of the parametrization of gravity wave drag in numerical weather prediction and climate simulation models is presented. The focus is primarily on understanding the current status of gravity wave drag parametrization as a step towards the new parametrizations that will be needed for the next generation of atmospheric models. Both the early history and latest developments in the field are discussed. Parametrizations developed specifically for orographic and convective sources of gravity waves are described separately, as are newer parametrizations that collectively treat a spectrum of gravity wave motions. The differences in issues in and approaches for the parametrization of the lower and upper atmospheres are highlighted. Various emerging issues are also discussed, such as explicitly resolved gravity waves and gravity wave drag in models, and a range of unparametrized gravity wave processes that may need attention for the next generation of gravity wave drag parametrizations in models.
The Seasonal Cycle of Gravity Wave Drag in the Middle Atmosphere
Journal of Climate, 2008
Using a variational technique, middle atmosphere gravity wave drag (GWD) is estimated from Met Office middle atmosphere analyses for the year 2002. The technique employs an adjoint model of a middle atmosphere dynamical model to minimize a cost function that measures the differences between the model state and observations. The control variables are solely the horizontal components of GWD; therefore, the minimization determines the optimal estimate of the drag. For each month, Met Office analyses are taken as the initial condition for the first day of the month, and also as observations for each successive day. In this way a three-dimensional GWD field is obtained for the entire year with a temporal resolution of 1 day. GWD shows a pronounced seasonal cycle. During solstices, there are deceleration regions of the polar jet centered at about 63° latitude in the winter hemisphere, with a peak of 49 m s−1 day−1 at 0.24 hPa in the Southern Hemisphere; the summer hemisphere also shows a ...
Effects of Latitude-Dependent Gravity Wave Source Variations on the Middle and Upper Atmosphere
Frontiers in Astronomy and Space Science, 2021
Atmospheric gravity waves (GWs) are generated in the lower atmosphere by various weather phenomena. They propagate upward, carry energy and momentum to higher altitudes, and appreciably influence the general circulation upon depositing them in the middle and upper atmosphere. We use a three-dimensional first-principle general circulation model (GCM) with implemented nonlinear whole atmosphere GW parameterization to study the global climatology of wave activity and produced effects at altitudes up to the upper thermosphere. The numerical experiments were guided by the GW momentum fluxes and temperature variances as measured in 2010 by the SABER (Sounding of the Atmosphere using Broadband Emission Radiometry) instrument onboard NASA's TIMED (Thermosphere Ionosphere Mesosphere Energetics Dynamics) satellite. This includes the latitudinal dependence and magnitude of GW activity in the lower stratosphere for the boreal summer season. The modeling results were compared to the SABER temperature and total absolute momentum flux and Upper Atmosphere Research Satellite (UARS) data in the mesosphere and lower thermosphere. Simulations suggest that, in order to reproduce the observed circulation and wave activity in the middle atmosphere, GW fluxes that are smaller than observed fluxes have to be used at the source level in the lower atmosphere. This is because observations contain a broader spectrum of GWs, while parameterizations capture only a portion relevant to the middle and upper atmosphere dynamics. Accounting for the latitudinal variations of the source appreciably improves simulations.
Journal of the Atmospheric Sciences, 2019
In this work, a new parameterization scheme is developed to account for the directional absorption of orographic gravity waves (OGWs) using elliptical mountain wave theory. The vertical momentum transport of OGWs is addressed separately for waves with different orientations through decomposition of the total wave momentum flux (WMF) into individual wave components. With the new scheme implemented in the Weather Research and Forecasting (WRF) model, the impact of directional absorption of OGWs on the general circulation in boreal winter is studied for the first time. The results show that directional absorption can change the vertical distribution of OGW forcing, while maintaining the total column-integrated forcing. In general, directional absorption inhibits wave breaking in the lower troposphere, producing weaker orographic gravity wave drag (OGWD) there and transporting more WMF upwards. This is because directional absorption can stabilize OGWs by reducing the local wave amplitude. Owing to the increased WMF from below, the OGWD in the upper troposphere at midlatitudes is enhanced. However, in the stratosphere of mid-to-high latitudes, the OGWD is still weakened due to greater directional absorption occurring there. Changes in the distribution of midlatitude OGW forcing are found to weaken the tropospheric jet locally and enhance the stratospheric polar night jet remotely. The latter occurs as the adiabatic warming (associated with the OGW-induced residual circulation) is increased at midlatitudes and suppressed at high latitudes, giving rise to stronger thermal contrast. Resolved waves are likely to contribute to the enhancement of polar stratospheric winds as well, because their upward propagation into the high-latitude stratosphere is suppressed.
Geophysical Research Letters, 2005
We extend the Chun-Baik parameterization of convectively forced stationary gravity-wave drag by adding a set of discrete nonzero phase speeds to incorporate the effects of nonstationary gravity waves. The extended scheme is computationally very efficient in comparison with full spectral parameterizations and eliminates the need to specify the wave source information at the interface level. We validate the extended parameterization against an explicit simulation of convection over a tropical ocean. The distribution of the cloud-top momentum flux for a typical range of phase speeds is roughly similar to those from more refined studies. It is shown that nonstationary waves should be included to reproduce the vertical variation of explicitly simulated momentum flux.
Comprehensive meteorological modelling of the middle atmosphere: a tutorial review
Journal of Atmospheric and Terrestrial Physics, 1996
This paper reviews the current state of comprehensive, three-dimensional, time-dependent modelling of l:he circulation in the middle and upper atmosphere from a meteorologist's perspective. The paper begins with a consideration of the various components of a comprehensive model (or general circulation model, GCM), including treatments of processes that can be explicitly resolved and those that occur on scales too small to resolve (and that must be parameterized). The typical performance of GCMs in simulating the tropospheric climate is discussed. Then some important background on current ideas concerning the general circulation of the stratosphere and mesosphere is presented. In particular, the transformed-Eulerian mean flow formalism, the role of vertically-propagating internal gravity waves in driving the large-scale circulation, and the notion of a stratospheric surf zone are all briefly reviewed. Using this background as a guide, some middle atmospheric GCM results are discussed, with a focus on simulations made recently with the GFDL 'SKYHI' troposphere-stratosphere-mesosphere GCM. The presentation attempts to emphasize the interaction between theory and comprehensive modelling. Many theoretical notions cannot be confirmed in detail from observations of the real atmosphere due to the various limitations in the observational methods, but can be very completely examined in GCMs in which every atmospheric variable is known perfectly (within the limits of the numerical methods). It will be shown that our understanding of both the role of gravity waves in the general circulation and the nature of the stratospheric surf zone has benefited from analysis of GCM results.
Lower-Tropospheric Enhancement of Gravity Wave Drag in a Global Spectral Atmospheric Forecast Model
Weather and Forecasting, 2008
The impacts of enhanced lower-tropospheric gravity wave drag induced by subgrid-scale orography on short-and medium-range forecasts as well as seasonal simulations are examined. This study reports on the enhanced performance of the scheme proposed by Kim and Arakawa, which has been used in the National Centers for Environmental Prediction (NCEP) Global Spectral Model since 1997. The performance is evaluated against a traditional upper-level drag scheme that is also available in the model. The experiment results reveal that the Kim-Arakawa scheme improves the movement and intensity of an extratropical cyclone and a continental high pressure system that was accompanied by heavy snowfall over Korea on 14-15 February 2001. The monthly verification for medium-range forecasts in December 2006, which are initialized by the NCEP operational analysis, demonstrates overall improvements in the forecasts of largescale fields in the Northern Hemisphere. Moderate improvements are also found in the seasonal simulation of December-February for the years 1996/97, 1997/98, and 1999/2000. This study concludes that the enhanced lower-level drag should be properly parameterized in global atmospheric models for numerical weather prediction and seasonal prediction.