CGD's Dr. Jadwiga Richter

Abstract

Some parameterizations of gravity wave mean flow forcing in global circulation models (GCMs) add realism by describing wave generation by tropospheric convection. Because the convection in GCMs is itself a parameterized process, these convectively generated wave parameterizations necessarily use many simplifying assumptions. In this work, the authors use a realistic simulation of wave generation by convection described in previous work, which was validated by observations from the Darwin Area Wave Experiment (DAWEX), to test these assumptions and to suggest some possible improvements to the parameterizations. In particular, the authors find that wave trapping in the troposphere significantly modifies the spectrum of vertically propagating waves entering the stratosphere above convective wave sources, and offer a linear method for computing wave transmission and reflection effects on the spectrum suitable for inclusion in the parameterizations. The wave fluxes originate from both a time-varying heating mechanism and an obstacle effect mechanism acting in the simulation. Methods for including both mechanisms in the parameterizations are described. Waves emanating from the obstacle effect remain very sensitive to the depth of penetration of latent heating cells into an overlying shear zone, which will continue to make it difficult to accurately parameterize in a GCM where the convective cells are not resolved.

Figure caption: Wave momentum flux vs propagation direction and phase speed for three different sets of parameterization settings: (top) case W uses the Beres et al. (2005) settings that were applied in WACCM, (middle) case D uses settings modified to include the known properties of the resolved convection in the radar domain and wave reflection effects, and (bottom) cases D plus S adds the obstacle effect wave generation (case S) to case D. Flux units are 10^-6 kg m^-2 s^-1.

Support: This work was supported by the National Science Foundation Physical Meteorology Program Grant Number 0234230. The National Center for Atmospheric Research is operated by the University Corporation for Atmospheric Research, under sponsorship of the National Science Foundation.

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Chang, L., S. Palo, M. Hagan, J. H. Richter, R. R. Garcia, D. Riggin, and D. Fritts, 2007: Structure of the Migrating Diurnal Tide in the Whole Atmosphere Community Climate Model, Adv. Space Research, In Press.

Abstract

As part of an ongoing effort to understand the migrating diurnal tide generated by the NCAR Whole Atmosphere Community Climate Model, version 3 (WACCM3), we compare the WACCM3 migrating diurnal tide in the horizontal wind and temperature fields to similar results from the Global Scale Wave Model (GSWM). The WACCM3 diurnal tidal wind fields are also compared to tropical radar measurements at Kauai (22E). The large-scale features of the WACCM3 results, such as the global spatial structure and the semiannual amplitude variation are in general agreement with past tidal studies; however, several differences do exist. WACCM3 exhibits a much higher degree of hemispheric asymmetry, lower overall amplitudes around the equinoxes, and peaks which are more confined in latitude when compared with the GSWM. Factors which may contribute to such differences between WACCM3 and GSWM are the solar heating profiles from ozone and water vapor, dissipation, and the zonal mean zonal winds. We find that the internally generated heating in WACCM3 and eddy dissipation values are both smaller than the values specified in the GSWM; the eddy dissipation fields and zonal mean zonal winds of the two models also display measurable differences in spatial structure. Comparisons with radar data show several differences in spatial and seasonal structure. In particular, the diurnal tide zonal winds in WACCM3 above Kauai are considerably larger in amplitude than those observed in the radar data, due to contributions from nonmigrating tidal components including wave numbers eastward 1 through 3, westward 2, and stationary components, which interfere constructively with the migrating component around equinox in WACCM3.

Figure caption: Migrating diurnal tidal amplitudes for January from WACCM3 (left column) and GSWM (right column) for the zonal wind (top), the meridional wind (center), and temperature (bottom). Wind contours of 5 m/s, temperature contours of 5 K.

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Richter, J. H. and P. J. Rasch, 2007: Effects of convective momentum transport on the atmospheric circulation in the Community Atmosphere Model, version 3 (CAM3). Journal of Climate, in press.

Abstract

Transport of momentum by convection is an important process affecting global circulation. Due to the lack of global observations, the quantification of the impact of this process on the tropospheric climate is difficult. Here, we present an implementation of two convective momentum transport parameterizations, Schneider and Lindzen (1976) and Gregory et al. (1997), in the Community Atmosphere Model, version3 (CAM3), and we examine in detail how they affect global climate. An analysis of the tropospheric zonal momentum budget reveals that convective momentum transport affects tropospheric climate mainly through changes to the Coriolis torque. These changes result in improvement of the representation of the Hadley circulation: in DJF, the upward branch of the circulation is weakened in the Northern hemisphere and strengthened in the Southern hemisphere; the lower Northerly branch is weakened. In JJA, similar improvements are noted. The inclusion of convective momentum transport in CAM3 reduces many of the model's biases in the representation of surface winds, as well as in the representation of tropical convection. In an annual mean, the Tropical easterly bias, subtropical westerly bias, and the bias in the 60S jet are improved. Representation of convection is improved along the Equatorial belt, with decreased precipitation in the Indian Ocean and increased precipitation in the Western Pacific. The improvements of representation of tropospheric climate are greater with the implementation of the Schneider and Lindzen (1976) parameterization.

Figure caption: Annually averaged near surface winds in m s-1 : a) QSCAT Observations, b) Control - Observations, c) Schneider and Lindzen (1976) - Control, d) Gregory et al (1997) - Control. Color shading represents wind speed (panel a) and wind speed difference (panels b - d). The vectors depict wind direction (panel a) and vector wind difference (panels b - d).

The National Center for Atmospheric Research is operated by the University Corporation for Atmospheric Research, under sponsorship of the National Science Foundation.