The Response of a Middle-Latitude Model Atmosphere to Forcing by Topography and Stationary Heat SOURCES1,2 (original) (raw)
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Journal of the Atmospheric Sciences, 2017
The parameterization of orographic drag processes in atmospheric models remains uncertain because of a lack of observational and theoretical constraints on their formulation and free parameters. While previous studies have demonstrated that parameterized orographic drag acting near the surface has a significant impact on the atmospheric circulation, this work follows a more systematic approach to investigate its impacts on the large-scale circulation and the circulation response to climate change. A set of experiments with a comprehensive atmospheric general circulation model is used to ascertain the range of climatological circulations that may arise from parameter uncertainty. It is found that the Northern Hemisphere (NH) wintertime stationary wave field is strongly damped over the North Pacific (NP) and amplified over the North Atlantic (NA) as a result of increased low-level parameterized orographic drag, both of which are shown to be conducive to higher-latitude westerlies. A c...
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
Impact of surface waves in a Regional Climate Model
Meteorologische Zeitschrift, 2010
A coupled regional atmosphere-wave model system is developed with the purpose of investigating the impact of climate changes on the wave field, as well as feed-back effects of the wave field on the atmospheric parameters. This study focuses on the effects of introducing a two-way atmosphere-wave coupling on the atmosphere as well as on wave parameters. The model components are the regional climate model RCA, and the third generation wave model WAM. Two different methods are used for the coupling, using the roughness length and only including the effect of growing sea, and using the wave age and introducing the reduction of roughness due to decaying sea (swell). Introducing a two-way coupling results in an altered frequency distribution of wind speed and wave heights. When only including growing sea the impact of waves on the long term mean atmospheric parameters is limited, inducing a reduction of wind speed and significant wave height. When also the impact of swell is introduced, there is a shift towards higher wind speeds as well as higher significant wave heights in the four investigated areas. There is a reduction of surface heat fluxes and a decrease in near surface temperature as well as a significant increase in near surface humidity. The major conclusion is that when introducing a more realistic surface description over sea, the air-sea interaction represented by waves has a significant impact also on long term averages of parameters in the atmosphere. Waves should thus be introduced in climate models for a realistic description of processes over sea.
2003
Midwinter storm track response to zonal variations in midlatitude sea surface temperatures (SSTs) has been investigated using an atmospheric general circulation model under aquaplanet and perpetual-January conditions. Zonal wavenumber-1 SST variations with a meridionally confined structure are placed at various latitudes. Having these SST variations centered at 30ЊN leads to a zonally localized storm track, while the storm track becomes nearly zonally uniform when the same SST forcing is moved farther north at 40Њ and 50ЊN. Large (small) baroclinic energy conversion north of the warm (cold) SST anomaly near the axis of the storm track (near 40ЊN) is responsible for the large (small) storm growth. The equatorward transfer of eddy kinetic energy by the ageostrophic motion and the mechanical damping are important to diminish the storm track activity in the zonal direction. Significant stationary eddies form in the upper troposphere, with a ridge (trough) northeast of the warm (cold) SST anomaly at 30ЊN. Heat and vorticity budget analyses indicate that zonally localized condensational heating in the storm track is the major cause for these stationary eddies, which in turn exert a positive feedback to maintain the localized storm track by strengthening the vertical shear near the surface. These results indicate an active role of synoptic eddies in inducing deep, tropospheric-scale response to midlatitude SST variations. Finally, the application of the model results to the real atmosphere is discussed.
Journal of the Meteorological Society of Japan. Ser. II
The structure of stationary planetary waves in the winter stratosphere is computed by. means of a steady-state hemispheric quasi-geostrophic model with a zonal basic state and lower boundary forcing obtained from climatology. The nonlinear wave solution is found to resemble rather closely the linear one, despite the large wave amplitudes which distort considerably the westerly zonal flow. Zonal wavenumber one is the most affected by the wave-wave interactions, experiencing an increase in amplitude and a decrease in westward phase tilt in the northerly regions of the middle stratosphere. Comparison of the solutions to the corresponding climatological wave structure indicates that the inclusion of the nonlinear terms leads to an improvement of the structure of wavenumber one. An examination of the 5-year January climatological basic state reveals a distinct linear relationship between the zonal streamfunction and the tonal potential vorticity in middle and northerly latitudes. Consequently, the wave-wave interactions are to a first approximation a result of the presence of the model dissipation. Weak dissipation in this region implies only weak interactions, which explains the quasi-linear structure of the solutions.
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.
Dynamics of localized extreme heatwaves in the mid-latitude atmosphere: A conceptual examination
John Wiley & Sons, 2023
This study investigates the adjustment of large-scale localized buoyancy anomalies in mid-latitude regions and the nonlinear evolution of associated condensation patterns in both adiabatic and moist-convective environments. This investigation is carried out utilizing the two-layer idealized moist- convective thermal rotating shallow water (mcTRSW) model. Our investigation reveals that the presence of a circular positive potential temperature anomaly in the lower layer initiates an anticyclonic high-pressure rotation, accompanied by a negative buoyancy anomaly in the upper layer, resulting in an anisotropic northeast–southwest tilted circulation of heat flux. The evolution of eddy heat fluxes, such as poleward heat flux, energy, and meridional elongation of the buoyancy field, heavily depends on the perturbation's strength, size, and vertical structure. The heatwave initiates atmospheric instability, leading to precipitation systems such as rain bands and asymmetric latent heat release due to moist convection in a diabatic environment. This creates a comma cloud pattern in the upper troposphere and a comma-shaped buoyancy anomaly in the lower layer, accompanied by the emission of inertia gravity waves. The southern and eastern sectors of the buoyancy anomaly show an upward flux, generating a stronger cross-equatorial flow and inertia-gravity waves in a southward and eastward direction. Furthermore, the simulations reveal a similar asymmetric pattern of total condensed liquid water content distribution, accompanied by the intensification of moist convection as rain bands. This intensification is more pronounced in barotropic structures than in baroclinic configurations with stagnant upper layers. This study highlights the importance of considering moist convection and its effects on atmospheric and oceanic flows in mid-latitude regions, as well as the role of buoyancy anomalies in generating heatwaves and precipitation patterns.
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Geophysical Research Letters, 1995
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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.