APE Workshop 20 - 22 April 2005 at Reading ------------------------------------------- Presentations registered 13.04.2005 ----------------------------------- H. Borth, L. Goering, V. Wirth and D. Majewski, University of Mainz Tropical precipitation and Hadley circulation on the DWD-Aqua-Planet for the APE sea surface temperature distributions We will give a summary of the results reached so far for the APE runs carried through with the DWD Aqua Planet. The DWD Aqua Planet is based on the German operational global weather forecast model GME, which is a hydrostatic icosahedral gridpoint model. Two items will be treated in more detail: First the characteristic precipitation regimes in the tropics for the different sea-surface-temperature distributions and second the influence of the different tropical precipitation dynamics (one ore two ITCZs) on the location, structure and intensity of the Hadley-Circulation and subtropical jets. ------------------------------------------- Yoshi-Yuki Hayashi AGU for APE / Hokkaido University, Japan On the varieties of forced and spontaneously generated tropical precipitation patterns: some expectations and comparison. Tropical rainfall patterns appearing in aqua planet GCMs show significant diversities. One example is the difference of equatorial disturbances between Hayashi and Sumi (1986) and Lee et al (2001). Another example is the difference in the signatures of rainfall responses to the east of an equatorial warm SST anomaly in Hosaka et al (1998) and Hoskins et al (1999) both of which are roughly similar in their setups. Facing with even more diverging results of APE, we are going to summarize our experiences including the above mentioned classical results from the viewpoint of rather simple equatorial linear dynamics modified with the moist effect, and expectations from them, hopefully, to help our comparison. As for the equatorial disturbances with zonally symmetric SST, low resolution models seem to present both or one of eastward propagating features with a structure of more or less Kelvin wave-CISK like and westward features advected by the background easterly wind. As for the remote response of rainfall to the SST 'anomaly', it is demonstrated that the differences are caused not only by the model used but also by the 'basic state' SST distributions. This suggests the important roles of the basic states of 'moist' variables such as moisture and precipitation, which depend both on the basic state SST and properties of the GCM used. ------------------------------------------- Myong-In Lee Global Modeling and Assimilation Office, NASA/GSFC Diurnal and semi-diurnal cycles of convection in an aqua-planet GCM The mechanism for the observed diurnal cycle of convection and nocturnal rainfall over the tropical ocean has not been clearly understood. With an aqua-planet version of the AGCM, the influence of the atmospheric tide on the diurnal (24-hour) and semi-diurnal (12-hour) cycles of oceanic convection was investigated by comparing two idealized experiments-with and without diurnal ozone forcing. The magnitude and phase of the diurnal variation of surface pressure in the aqua-planet model are in good agreement with the observation. Simulated precipitation is dominated by diurnal component with a maximum phase at night (around 1-5 LST), rather than semi-diurnal component. The nocturnal precipitation is accompanied with meridional convergence pattern along the equator, which responds to the off-equatorial anticyclones at 15^(o) N and S. Once the diurnal variation of ozone forcing was removed in the model, the amplitude of diurnal variation of precipitation decreased by 20 %, but with no change in the phase. The off-equatorial divergences are also weakened and further examinations are needed to understand the role of off-equatorial divergence in regulating the equatorial nocturnal convection. ------------------------------------------- Xiaoyun Liang, Chinese Academy of Sciences The mid-latitude wave activity of the LASG model A series of 8 numerical experiments have been run under controlled conditions identified in the aqua-planet inter-comparison project by using the LASG spectral climate model SAMIL R42L9. The mid-latitude wave activity is analyzed with the multiple level zonal mean fluxes. Results show that northward transfer of momentum (Figure 1) is symmetrical about the equator except for the stationary eddy fluxes. The transient eddy fluxes predominate in the upper troposphere. The contribution of stationary mean meridional circulation to the total northward transfer of momentum is less than the contribution of the transient eddy besides the surface boundary. The above results are consistent with observations. Moreover, the location of maximum transfer in upper troposphere is sensitive to latitudinal profile of SST. The larger the curvature of SST profile is, the nearer the maximum of transfer to equator. The sensitivity of northward transfer of heat flux (Figure 2) to the SST profile is less than that of northward transfer of momentum flux. ------------------------------------------- John McGregor and Martin Dix CSIRO Atmospheric Research Aqua-planet simulations using CCAM We have performed the set of aqua-planet experiments using the CSIRO conformal-cubic atmospheric model (CCAM). The model uses a recent CSIRO mass-flux convective parameterization. Shallow convection is modelled only by enhanced vertical mixing within the cloudy boundary layer. The precipitation patterns of the various CCAM simulations are examined, comparing resolved and convective components, as well as diurnal aspects. Tropical circulation patterns for the simulations are also compared. Aspects of the transient behaviour are also studied, including the propagation of tropical disturbances. ------------------------------------------- Andrea Molod Department of Earth, Atmospheric and Planetary Sciences, MIT Aquaplanet Simulations using Different Vertical Grids for Dynamics and Physics in the MIT GCM ------------------------------------------- Wataru Ohfuchi, Earth Simulator Center Sensitivity of intensities of subtropical jet and Hadley circulation on cumulus parameterizations and resolutions AFES, the atmospheric general circulation model maintained at the Earth Simulator Center, has been run with various cumulus parameterizations and resolutions. A strong sensitivity is found in the intensity of the Hadley circulation on horizontal resolutions. When the resolution is high with T79 or T159 (low with T39), the Hadley circulation is strong (weak), both total and convective precipitations are strong (weak), and precipitation system is organized (noisy). A little more detailed analysis will be reported (hopefully!). ------------------------------------------- Robin Smith Max-Planck Institute for Meteorology, Hamburg A low-resolution coupled OAGCM Aquaplanet simulation The low resolution version of the FORTE OAGCM is integrated in an Aquaplanet configuration. A warm, humid climate results, with increased cloud cover and a strong greenhouse effect. A much reduced equator-pole surface temperature gradient is found, maintained more by changes in local radiative forcing than by changes in heat transport within the system. Complete inhibition of the ocean heat transport however does have a significant effect, producing a large drop in global average temperatures. ------------------------------------------- Rachel Stratton Met Office Sensitivity of the Met office Aqua-planet results to convection The tropical performance of an aqua-planet simulation depends strongly on convection. The convective parametrisation scheme in the Met Office model is a mass flux scheme based on Gregory and Rowntree (G-R)(1990) with various extensions. These include; a change to force convection triggered from the boundary layer to trigger from the lifting condensation level, diagnosis of deep and shallow convection, different entrainment rates and closures for deep and shallow convection, deep convection has a CAPE closure and the convective momentum parametrisation is based on an eddy viscosity model. This study investigates various changes to the convection scheme. The first is the difference between the original G-R scheme as used in HadAM3 and the extended version used here. The extended version of the G-R scheme tends have more intermittant deep convection. The second is changing the deep closure to be dependent on moist static energy flux at the surface. This gives more continous deep convection. ------------------------------------------- Hirofumi Tomita, Frontier Research Center for Global Change A cloud-resolving aqua planet experiment We have challenged to perform the control case of APE by using a global cloud-resolving model NICAM with super-high resolution. The horizontal resolution increases from 15km up to 3.5km over the globe. For the sake of such high resolution, our approach can treat explicitly individual cloud motions to avoid the ambiguity of cumulus parameterization. Although the simulated term in this run is shorter than the other runs, we hope that our result is regarded as one reference solution in comparison with parameterization models. In this talk, we focus on the tropical features, i.e., intra-seasonal oscillation and hierarchal structure of cloud from individual cloud to super cloud cluster: each of cloud cluster reasonably moves westward and the envelope of these cloud cluster ( super cloud cluster ) moves eastward with period of 25-40 days. We also show the climatology such as precipitation rate: we found that ITCZ region obtained by the cloud resolving simulation tends to be narrower than those of the parameterization models. ------------------------------------------- Zaizhi Wang, Guangzhou Institute of Tropical & Marine Meteorology Wavenumber-frequency characteristics of the LASG model The experiments required by APE are completed with the spectral climate model SAMIL of LASG/IAP. The tropical wavenumber-frequency characteristics are analyzed based on the convective precipitation. Following Wheeler and Kiladis (1999), we split the wave into components that are symmetric and antisymmetric about the equator and normalize each by a background spectrum. The results for Control and Control5n experiments in which the only differences are the SST distributions, show that the wave behaviours are in agreement with those of the theoretical waves. As the SST is not symmetric about the equator, the symmetric component in Control5n is much weaker than that in Control, while the eastward waves in the antisymmetric component in Control5n is stronger. Since the SSTs in Control5n are similar with the real SST distributions in summer, the results imply that the antisymmetric eastward waves are more active in summer, while the symmetric low frequency waves may be suppressed. ------------------------------------------- Masahiro Watanabe Hokkaido University On the extratropical low-frequency variability in aqua-planet simulations A multi-model comparison among the APE simulations may be the most relevant to the tropical mean state and disturbances, we would like to propose the APE to be useful to study the atmospheric low-frequency variability in middle latitudes as well. In particular, the dominant low-frequency variability in aqua-planet models may be viewed as a prototype to the observed annular mode, so that the dynamics of such a 'pure' annular mode in different model will be worth to examine. Preliminary results are obtained from extended integrations of CCSR/NIES/FRCGC model. Analyses to low-frequency anomaly fields in the control experiment reveal that the annular variability indeed dominates and, furthermore, a set of zonal wavenumber-5, quasi-stationary perturbations is weakly interrelated to the model annular mode. The role of this wave-zonal flow interaction and the dominant wavenumber selection mechanism in different experiments and models are interested in the model intercomparison. ------------------------------------------- David Williamson NCAR Low-frequency mid-latitude variability in the APE simulations We have run the APE series of simulations with the Community Atmosphere Model Version 3 (CAM3) at NCAR. We will present analyses of an extended run of the CONTROL experiment which indicates low-frequency mid-latitude variability in longitudinal wavenumber five. We will present wavenumber-frequency diagrams to characterize the variability. Similar calculations from the three-year standard experiment and from a series of one-year sub-samples of the extended run (the length requested for the high frequency samples in the APE standard data sets) will be compared. These indicate that the presence of the variability can be established, but the short samples cannot establish the details. To the extent possible, we will compare analyses from all the models submitted to APE. ------------------------------------------- Steve Woolnough CGAM, University of Reading The diurnal cycle of convection and atmospheric tides in an aquaplanet GCM The diurnal cycle of tropical convection and its relationship to the atmospheric tides is investigated using an aquaplanet GCM. The diurnal and semidiurnal harmonics of precipitation are both found to contribute significantly to the total diurnal variability of precipitation in the model, which is broadly consistent with observations of the diurnal cycle of convection over the open ocean. The semidiurnal tide is found to be the dominant forcing for the semidiurnal harmonic of precipitation. In contrast the diurnal tide plays only a small role in forcing the diurnal harmonic of precipitation, which is dominated by the variations in shortwave and longwave heating. In both the diurnal and semidiurnal harmonics, the feedback onto the convection by the humidity tendencies due to the convection is found to be important in determining the phase of the harmonics. Further experiments show that the diurnal cycle of precipitation is sensitive to the choice of closure in the convection scheme. While the surface pressure signal of the simulated atmospheric tides in the model agree well with both theory and observations in their magnitude and phase, sensitivity experiments suggest that the role of the stratospheric ozone in forcing the semidiurnal tide is much reduced compared to theoretical predictions. Furthermore, the influence of the cloud radiative effects seems small. It is suggested that the radiative heating profile in the troposphere, associated primarily with the water vapor distribution, is more important than previously thought for driving the semidiurnal tide. However, this result may be sensitive to the vertical resolution and extent of the model. ------------------------------------------- Yukiko Yamada Hokkaido University A resolution dependence of equatorial precipitation activities represented in a general circulation model Dependence of the representation of equatorial precipitation activities on the horizontal and vertical resolutions of a model is investigated. The numerical model utilized is AFES, an AGCM based on CCSR/NIES AGCM5 ver 5.4.02 optimized for the Earth Simulator. Cumulus parameterization is not employed for the sake of simplicity. When the horizontal resolution is increased from T39 to T319, a hierarchical structure appears. With the resolution of T319, westward and eastward moving precipitation structures may be regarded as isolated features with different scales of time or space. The westward moving features which have the scale of several grids seem to be advected by the background easterly wind. The eastward moving features, within which the westward moving features are embedded, are now subjected to composite analysis to identify its structure. -------------------------------------------