Tropical Cumulus Convection and Upward Propagating Waves in Middle Atmosphere GCMs (original) (raw)
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Tropical Cumulus Convection and Upward-Propagating Waves in Middle-Atmospheric GCMs
Journal of the Atmospheric Sciences, 2003
It is recognized that the resolved tropical wave spectrum can vary considerably among general circulation models (GCMs) and that these differences can have an important impact on the simulated climate. A comprehensive comparison of low-latitude waves is presented for the December-January-February period using highfrequency data from nine GCMs participating in the GCM Reality Intercomparison Project for Stratospheric Processes and Their Role in Climate (GRIPS; SPARC). Quantitative measures of the wavenumber-frequency structure of resolved waves and their impacts on the zonal mean circulation are given. Space-time spectral analysis reveals that the wave spectrum throughout the middle atmosphere is linked to the variability of convective precipitation, which is determined by the parameterized convection. The variability of the precipitation spectrum differs by more than an order of magnitude among the models, with additional changes in the spectral distribution (especially the frequency). These differences can be explained primarily by the choice of different cumulus parameterizations: quasi-equilibrium mass-flux schemes tend to produce small variability, while the moistconvective adjustment scheme is the most active. Comparison with observational estimates of precipitation variability suggests that the model values are scattered around the observational estimates. Among the models, only those that produce the largest precipitation variability can reproduce the equatorial quasi-biennial oscillation (QBO). This implies that in the real atmosphere, the forcing from the waves, which are resolvable with the typical resolutions of present-day GCMs, is insufficient to drive the QBO. Parameterized cumulus convection also impacts the nonmigrating tides in the equatorial region. In most of the models, momentum transport by diurnal nonmigrating tides in the mesosphere is comparable to or larger than that by planetary-scale Kelvin waves, being more significant than has been thought. It is shown that the westerly accelerations in the equatorial semi-annual oscillation in the models examined are driven mainly by gravity waves with periods shorter than 3 days, with some contribution from parameterized gravity waves, and that the contribution from the wavenumber-1 Kelvin waves is negligible. These results provide a state-of-the-art assessment of the links between convective parameterizations and middle-atmospheric waves in present-day middle-atmosphere climate models.
Tropical Intraseasonal Variability in Version 3 of the GFDL Atmosphere Model
Journal of Climate, 2013
Laboratory Atmosphere Model (AM3). Compared to its predecessor AM2, AM3 uses a new treatment of deep and shallow cumulus convection and mesoscale cloud effects. The AM3 cumulus parameterization is a mass flux-based scheme but also, unlike the AM2, incorporates subgrid-scale vertical velocities that play a key role in cumulus microphysical processes. The AM3 convection scheme allows multiphase water substance produced in deep cumuli to be transported directly into mesoscale clouds, which strongly impact large-scale moisture and radiation fields.
The equatorial stratospheric semiannual oscillation and time‐mean winds in QBOi models
Quarterly Journal of the Royal Meteorological Society, 2019
We compare the response of the quasi-biennial oscillation (QBO) to a warming climate in eleven atmosphere general circulation models that performed time-slice simulations for present-day, doubled, and quadrupled CO 2 climates. No consistency was found among the models for the QBO period response, with the period decreasing by eight months in some models and lengthening by up to thirteen months in others in the doubled CO 2 simulations. In the quadruped CO 2 simulations a reduction in QBO period of 14 months was found in some models, whereas in several others the tropical oscillation no longer resembled the present day QBO, although could still be identified. In contrast, all the models projected a decrease in the QBO amplitude in a warmer climate with the largest relative decrease near 60 hPa. In simulations with doubled and quadrupled CO the multi-model mean QBO amplitudes decreased by 36% and 51%, respectively. Across the models the differences in the QBO period response were found most strongly related to how the gravity wave momentum flux entering the stratosphere and tropical vertical residual velocity responded to the increases in CO 2 amounts. Likewise it was found that the robust decrease in QBO amplitudes was correlated across the models to changes in vertical residual velocity, parameterized gravity wave momentum fluxes, and to some degree the resolved upward wave flux. We argue that uncertainty in the repre
Realistic semiannual oscillation simulated in a middle atmosphere general circulation model
Geophysical Research Letters, 2001
Although several general circulation modelling studies have presented simulations containing a tropical middle atmosphere semiannual oscillation (6 month period), none so far have been able to reproduce the observed amplitude and vertical structure extending into the mesosphere. Here we present simulations with the Canadian Middle Atmosphere Model which utilize a nonlinear broad-spectrum gravity-wave parameterization [Medvedev and Klaassen, 1995, 2000] and exhibit a semiannual oscillation in good agreement with observations. 1. Introduction Oscillations of the mean zonal wind and temperature in the tropical strato-mesosphere with a periodicity of 6 months constitute a phenomenon known as the semiannual oscillation (SAG). The observed climatological structure of the oscillation has been widely documented, e.g. [Hamilton, 1998; Garcia et al., 1997; Burrage et al, 1996; Ortland et al., 1996]. It is well-known that the middle atmosphere undergoes two distinct out-of-phase semiannual oscillations: one has maximum amplitude at the stratopause (SSAG), while the second is observed near the mesopause (MSAG). Although the SAG is clearly associated with the annual cycle of radiative forcing (the sun crosses the equator twice a year), the detailed physical mechanism driving the SAG is not completely understood. Dunkerton [1979] showed that the eastward phase of the stratospheric SAG can be maintained by large-scale Kelvin waves propagating from below. Holton and Wehrbein [1980] demonstrated that the crossequatorial transport of westward momentum by the mean meridional circulation could explain the westward phase of the SAO. Planetary Rossby waves propagating into the tropics from the winter hemisphere also supply westward momentum in the tropical stratosphere at the solstices [Hamilton, 1986; Ray et al, 1998]. Hitchman and Leovy [1988] have estimated that only 30% to 70% of the required eastward acceleration in the stratosphere is produced by large-scale Kelvin waves, and that the rest of eastward torque can be attributed to gravity waves. Dunkerton [1982] suggested that vertically propagating gravity waves (GW) are filtered by the stratopause SAG, thereby providing both the eastward and westward accelerations required to drive the mesospheric SAO. This theory also accounts for the observed 3-month phase lag between the MSAG and SSAG. Sassi and Garcia [1997] argued that Corresponding author
Journal of The Atmospheric Sciences, 1998
A 48-yr integration was performed using the Geophysical Fluid Dynamics Laboratory SKYHI tropospherestratosphere-mesosphere GCM with an imposed zonal momentum forcing designed to produce a quasi-biennial oscillation (QBO) in the tropical stratosphere. In response to this forcing, the model generates a QBO in the tropical circulation that includes some very realistic features, notably the asymmetry between the strength of the descending easterly and westerly shear zones, and the tendency for the initial westerly accelerations to appear quite narrowly confined to the equator. The extratropical circulation in the Northern Hemisphere (NH) winter stratosphere is affected by the tropical QBO in a manner similar to that observed. In particular, the polar vortex is generally weaker in winters in which there are easterlies in the tropical middle stratosphere. Roughly twothirds of the largest midwinter polar warmings occur when the equatorial 30-mb winds are easterly, again in rough agreement with observations. Despite this effect, however, the total interannual variance in the zonalmean extratropical circulation in the model apparently is slightly decreased by the inclusion of the tropical QBO. The observed QBO dependence of the winter-mean stratospheric extratropical stationary wave patterns is also quite well reproduced in the model.
Tropical variability and stratospheric equatorial waves in the IPSLCM5 model
Climate Dynamics, 2013
The atmospheric variability in the equatorial regions is analysed in the Earth System Model pre-industrial simulation done at IPSL in the framework of CMIP5. We find that the model has an interannual variability of about the right amplitude and temporal scale, when compared to the El-Niño Southern Oscillation (ENSO), but that is too confined to the western Pacific. At the intra-seasonal periods, the model variability lacks of large-scale organisation, and only produces one characteristic Madden-Julian Oscillation every 10 winters typically. At shorter timescales and in the troposphere, the model has Rossby and Kelvin Convectively Coupled Equatorial Waves (CCEWs), but underestimates the Kelvin CCEWs signal on OLR. In the model stratosphere, a composite analysis shows that the Temperature and velocities fluctuations due to the Kelvin waves are quite realistic. In the model nevertheless, the stratospheric waves are less related to the convection than in the observations, suggesting that their forcing by the midlatitudes plays a larger role. Still in the model, the Kelvin waves are not predominantly occurring during the life cycle of the tropospheric Kelvin CCEWs, a behaviour that we find to be dominant in the observations. The composite analysis is also used to illustrate how the waves modify the zonal mean-flow, and to show that the model Kelvin waves are too weak in this respect. This illustrates how a model can have a reasonable Kelvin waves signal on the velocities and temperature, but can at the same time underestimate their amplitude to modify the mean flow. We also use this very long simulation to establish that in the model, the stratospheric equatorial waves are significantly affected by ENSO, hence supporting the idea that the ENSO can have an influence on the Quasi-Biennial Oscillation.
Journal of the Meteorological Society of Japan. Ser. II
Low-frequency oscillations appearing in three GCM seasonal cycle integrations are compared with the analyses of the European Centre for Medium-Range Weather Forecasts (ECMWF). All three models have the same resolution: 4 degrees latitude by 5 degrees longitude, with 9 levels. The GLAS GCM simulates a realistic eastward propagation of the 30-60 day oscillation in the tropical upperlevel divergent flow. The eastward travelling planetary scale structure becomes more stationary over the Indonesian region and accelerates over the central Pacific, as observed. In the GLA GCM, the oscillation propagates into the higher latitudes of both hemispheres as the waves leave the convective region. The presence of the eastward propagating oscillation is not obvious in the UCLA GCM. The wavenumber-frequency spectra of the 200 mb velocity potential reveal that all the GCMs have a significantly weaker signal for eastward propagation in the 30-60 day range than the analyses. The spectrum for the GLAS GCM is dominated by 20-60 day periods, while the GLA GCM has a spectral peak around 20-30 days. There is a weak eastward propagating peak near 15 days in the UCLA GCM. The dominant phase speeds and the different vertical structures of the heating profiles in the GCMs are in general agreement with current theory involving the positive feedback between latent heating and moist static stability. The composited patterns of the observations indicate that in the tropics a Kelvin wave-type structure is dominant near the center of the oscillation. The simulated winds are fairly realistic, although the meridional component is too strong, especially in the GLA GCM. The vertical structures of the zonal wind component and moisture suggest that a mobile wave-CISK (Lau and Peng,1987) is an important mechanism in maintaining the intraseasonal oscillation in these GCMs. The vertical distribution of the moisture field further suggests that evaporation-wind feedback (Neelin, et al., 1987) may play a role in maintaining the eastward propagating tropical waves. The differences in the structure of the oscillation in the GLAS GCM and GLA GCM appear to be a consequence of the different numerical schemes used. The GCMs have preferred zones for diabatic heating, with a turn-on heating occurring when the rising branch of the intraseasonal oscillation passes over these convective regions. All three GCMs fail to capture the detailed evolution in the different stages of the development and decay of the oscillation. The results suggest that an improvement in the boundary layer moisture processes may be crucial for a better simulation of the oscillation.
Theoretical and Applied Climatology, 2014
The present manuscript deals with the spatial distribution of gravity wave activity over the tropics using ten years (2001)(2002)(2003)(2004)(2005)(2006)(2007)(2008)(2009)(2010) of CHAllenging Mini Payloads (CHAMP) and Constellation Observing System for Meteorology, Ionosphere and Climate (COSMIC) Global Positioning System (GPS) Radio Occultation (RO) data and ground-based radiosonde measurements over an equatorial station Singapore (1.36°N, 103.98°E) and four tropical stations, Guam (13.48°N, 144.80°E), Palau (7.33°N, 134.48°E) in the northern hemisphere, Darwin (12.41°S, 130.88°E) and Pago-Pago (14.33°S, 170.71°W) in the southern hemisphere from January 2001-December 2010. It also aims to quantify the difference in wave activity in the two phases of QBO, climatologically. Space-time spectra have been constructed over a latitude band of ±10°and decomposing the CHAMP/COSMIC temperature perturbations into symmetric and antisymmetric modes about the equator. Clear signature of equatorial waves with higher wavelength and a constant background of gravity waves (GW) with inertial frequency are prominent in the spectra. Strong GW and mean flow interaction can be seen in the lower stratosphere potential energy density (E P ) and momentum flux with enhanced wave activity during the westerly (eastward wind) phase of quasi-biennial oscillation (QBO) (WQBO) over the equatorial and tropical stations like Singapore and Palau/Darwin, respectively. From the latitudinal distribution of energy density, the occurrence of two-peak structure in energy density can be seen in the middle and lower latitudes with an enhancement during the WQBO phase. The E P associated with GWs are calculated at lower stratospheric (19-26 km) heights and are compared with outgoing longwave radiation (OLR) to correlate the wave events with tropical deep convection during the easterly, i.e. westward wind (EQBO) and WQBO phases of QBO. Clear coherence of convection due to Asian summer monsoon with localized enhancement of wave activity over Western Pacific, South America and African region during the WQBO phase is observed at the lower stratospheric heights. Significant enhancement is observed during Northern Hemisphere winter months and minimum during summer. The longitudinally elongated portion of E P over the equator is partially affected by Kelvin wave (KW) like disturbances with short vertical scales and also by inertia GW.
Planetary waves and the semiannual wind oscillation in the tropical upper stratosphere
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