Moist Convective Adjustment Scheme Research Articles

Characteristics of the Meiyu and Baiu Frontal Precipitation Zone in the CMIP3 20th Century Simulation and 21st Century Projection

Features of the Meiyu frontal rain zone (MFZ, in 110–125°E) and Baiu frontal rain zone (BFZ, in 125–140°E) in the 20th Century simulation (20C3M) and 21st Century projection (Special Report on Emission Scenarios (SRES) A1B), obtained by 22 climate models contributed to the World Climate Research Programme's Coupled Model Intercomparison Project phase 3 (CMIP3) are studied. Characteristics of the MFZ and BFZ in the 20th century climate simulation are compared with two sets of observed precipitation data. The latitude of the zone and the precipitation in the zone (“precipitation in FZ”) reproduced by the models are examined, using the 20-year (1980–1999) averaged values for May, June and July. The “precipitation in FZ” in each month obtained from the multi-model ensemble average (MEA) coincides approximately with the observation data. However, MEA reproduces MFZ and BFZ to the north of their observed latitude in May. MEA reasonably reproduces the latitude BFZ for June and July, and the latitude of MFZ only for June. The standard deviation (STD) of the “precipitation in FZ” does not change widely from zone to zone, and from month to month. However, STD of the latitude of zones varies widely. The STD of latitude of MFZ and BFZ became larger in July. The STD of the latitude of MFZ is larger than that of BFZ. The low and medium horizontal resolution models with moist convective adjustment scheme tend to reproduce MFZ and BFZ in the northern latitude, while the models with mass flux scheme and models with Arakawa-Schubert scheme tend to reproduce MFZ and BFZ in the southern latitude. Features of the MFZ and BFZ in the 21st century climate projection by the models are examined using the 20-year (2080–2099) averaged values for May, June and July. The characteristics of the models in regard to MFZ and BFZ in 20th century simulation are commonly found in the 21st century projection. The MFZ and BFZ shift slightly northward, and the “precipitation in FZ” decreased slightly from the 20th century to 21st century. However, these changes are very small, as compared with the respective STD. The change of MFZ and BFZ under the climate changes in the 21st century can not be definitely concluded by models of CMIP3.

The MJO in an AGCM with three different cumulus parameterization schemes

The Madden–Julian oscillation (MJO) is simulated using an AGCM with three different cumulus parameterization schemes: a moist convective adjustment (MCA) scheme, the Zhang–McFarlane (ZM) mass-flux scheme, and the Tiedtke scheme. Results show that the simulated MJO is highly dependent on the cumulus parameterization used. Among the three cumulus parameterizations, only the MCA scheme produces MJO features similar to observations, including the reasonable spatial distribution, intraseasonal time scales and eastward propagation. Meanwhile, the amplitude is too large and the eastward propagation speed too fast than observations and the relationship between precipitation and low-level wind anomaly is unrealistic with enhanced convection occurring within easterly anomalies instead of westerly anomalies as in observations. The over-dependence of precipitation on boundary convergence produced by the MCA scheme is presumably responsible for this unrealistic phase relation in the simulation. The other two schemes produce very poor simulations of the MJO: spectral power of westward propagation is larger than that of eastward propagation in zonal wind and precipitation, indicating a westward propagation of the intraseasonal variability. The mean state and vertical profile of diabatic heating are perhaps responsible for the differences in these simulations. The MCA scheme produces relatively realistic climate background. When either ZM or Tiedtke scheme is used, the observed extension of westerly winds from the western Pacific to the dateline is missing and precipitation over the equatorial region and SPCZ is dramatically underestimated. In addition, diabatic heating produced by both ZM and Tiedtke schemes are very weak and nearly uniform with height. The heating profile produced by the MCA scheme has a middle-heavy structure with much larger magnitude than those produced by the other two schemes. In addition, a very unrealistic boundary layer heating maximum produced by the MCA scheme induces too strong surface convergence, which perhaps contributes to the too strong intraseasonal variability in the simulation.

The Impacts of Convective Parameterization and Moisture Triggering on AGCM-Simulated Convectively Coupled Equatorial Waves

Abstract This study examines the impacts of convective parameterization and moisture convective trigger on convectively coupled equatorial waves simulated by the Seoul National University (SNU) atmospheric general circulation model (AGCM). Three different convection schemes are used, including the simplified Arakawa–Schubert (SAS) scheme, the Kuo (1974) scheme, and the moist convective adjustment (MCA) scheme, and a moisture convective trigger with variable strength is added to each scheme. The authors also conduct a “no convection” experiment with deep convection schemes turned off. Space–time spectral analysis is used to obtain the variance and phase speed of dominant convectively coupled equatorial waves, including the Madden–Julian oscillation (MJO), Kelvin, equatorial Rossby (ER), mixed Rossby–gravity (MRG), and eastward inertio-gravity (EIG) and westward inertio-gravity (WIG) waves. The results show that both convective parameterization and the moisture convective trigger have significant impacts on AGCM-simulated, convectively coupled equatorial waves. The MCA scheme generally produces larger variances of convectively coupled equatorial waves including the MJO, more coherent eastward propagation of the MJO, and a more prominent MJO spectral peak than the Kuo and SAS schemes. Increasing the strength of the moisture trigger significantly enhances the variances and slows down the phase speeds of all wave modes except the MJO, and usually improves the eastward propagation of the MJO for the Kuo and SAS schemes, but the effect for the MCA scheme is small. The no convection experiment always produces one of the best signals of convectively coupled equatorial waves and the MJO.

The Variability of Modeled Tropical Precipitation

This paper investigates the temporal properties of tropical precipitation in the Canadian Centre for Climate Modelling and Analysis (CCCma) third-generation atmospheric general circulation model (AGCM3). AGCM3 employs the penetrative mass-flux (PMF) scheme of Zhang and McFarlane (ZM) for the parameterization of deep cumulus convection. It is found that the temporal variability of the ZM scheme is sensitive to a number of its internal parameters as well to the use of a prognostic, rather than diagnostic, closure condition for the cloud-base mass flux. Sensitivity experiments suggest that the ZM scheme can produce realistic amounts of variability when compared to direct radar observations of deep cumulus convection in the Tropics. A central finding of this study is that the resolved large-scale stratiform precipitation (LSP) in the model can participate in the modeling of deep latent heating and so compete with the ZM scheme in the Tropics. In modeling deep latent heating the LSP is found to mimic the behavior of a moist-convective adjustment scheme. In AGCM3 it is found that typical parameter settings of the ZM scheme place it in a regime in which the temporal variability of tropical precipitation is dominated by this behavior of the LSP, while the temporal mean is dominated by the ZM scheme. In such circumstances it is the LSP, and not the ZM scheme, that provides the primary source of resolved tropical Kelvin and mixed Rossby‐gravity waves in the GCM. Such competition between LSP and the parameterization of deep convection appears to be active in other modeling studies. Consequently, it has the potential to complicate efforts to understand the nature of resolved tropical waves in GCMs and their role in the forcing of the quasi-biennial and semiannual oscillations.

Tropical Cumulus Convection and Upward-Propagating Waves in Middle-Atmospheric GCMs

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 high-frequency 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 moist-convective 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.

Simulation of the Baiu Front in a High Resolution AGCM.

The features of the Baiu front are examined using an AGCM with high resolution (T106L60). The Arakawa-Schubert scheme is used for the cumulus parameterization. The model clearly simulates the eastward-moving band of rain concentrated south of Japan during June, which can be considered the Baiu front. The characteristics of the Baiu front, such as the low level jet stream, a moist neutral stratification and a strong meridional gradient of moisture, are well simulated. Meso-α-scale disturbances appearing in the model are also comparable to the observational results. Experiments with varying horizontal resolutions keeping the same vertical resolution are also performed to examine resolution dependences. The distribution of precipitation is improved using higher resolution. In the T106L20 experiment, the Baiu-like frontal zone is formed by using a strong gravity wave drag parameterization. Thee strength of the subtropical jet is important over the Baiu front for a good simulation. Experiments with two different cumulus parameterizations (the moist convective adjustment, and Kuo schemes) are performed at T106L20 resolution to examine the effect of the parameterization on the simulation of the Baiu front. The moist convective adjustment scheme leads to high rainfall in the extra tropical region. The patterns of the Indian monsoon, and the North Pacific subtropical anticyclone, are quite different from those favorable to the formation of the Baiu front. On the other hand, a better distribution of precipitation is found using the Kuo scheme. However, an earlier disappearance of the Baiu front is seen in each cumulus parameterization experiment.

Comparison of seasonal and intraseasonal variation of tropical climate in NCAR CCM2 GCM with two different cumulus schemes

The seasonal and intraseasonal variation of tropical climate in National Center for Atmospheric Research (NCAR) Community Climate Model Version 2 (CCM2) General Circulation Model (GCM) has been examined using two different cumulus parameterization schemes, the moist convective adjustment scheme of Manabe et al. (1965) and the mass-flux scheme of Hack (1994). Ten-year simulations have been undertaken with each of these schemes with SST prescribed according to the monthly mean climatology. The seasonal mean rainfall in the tropics simulated by the moist convective adjustment scheme (MCA) scheme was found to be more realistic than the mass-flux (Hack) scheme. The more realistic simulation by the MCA scheme was found to be on account of the fact that the mean moist static energy of the lower troposphere in the MCA scheme was closer to the observations than in the Hack scheme. In both the schemes, the precipitation in the tropics increases montonically with precipitable water vapour when the precipitable water vapour is above 40 mm. This is consistent with relationship between precipitation and precipitable water in the observations. The Hack scheme tends to simulate lower precipitation (for a given amount of precipitable water) when compared to observations. The MCA scheme simulates the eastward migration of convective systems along the equator quite well, although the speed of propagation is somewhat low. The poleward migration of convective systems in the Indian region is more realistically simulated by the MCA scheme than the Hack scheme. This is because the latitudinal gradient of the mean moist static energy in the MCA scheme is more realistic than in the Hack scheme. Over most of the tropics, simulation by the MCA scheme is more realistic on both seasonal and intraseasonal timescales.

台風Flo(1990)の強度予報に対する積雲対流のパラメタリゼーションの影響

The impact of two cumulus parameterizations on intensity of a simulated tropical cyclone was examined using a limited area numerical model. A moist convective adjustment scheme and Arakawa-Schubert scheme were used as a cumulus parameterization for each numerical experiment. Typhoon Flo (1990) was selected as a case study.Difference in the cumulus parameterizations had drastic influence on a difference in sea-level pressure averaged over the core region (within the 250km radius from the storm center) of the simulated storm from an early stage of the integration. That is, compared with the experiment with moist convective adjustment, the averaged sea-level pressure with Arakawa-Schubert scheme was relatively low. Central pressure in the latter was also lower than the former, which showed that the latter produced a more intense tropical cyclone consistent with the observation. The difference in central pressure, however, appeared at a later stage, compared with the difference in the averaged sea-level pressure.Temperature tendency due to a moist convective adjustment was considerably negative in the lower troposphere and so was actual temperature tendency, which resulted in a rapid drop of temperature. Consequently, the total air mass within the inner-core region became greater in the experiment with moist convective adjustment than with the Arakawa-Schubert scheme. On the other hand, the difference in the low-level temperature field between the two experiments results in the difference in radial profile of rainfall, and the radius where the eyewall is located. The difference probably brings about the delayed appearance of the central pressure difference.Another numerical experiment was conducted with modified moist convective adjustment not to be cool in the lower troposphere. The result confirmed that cooling in the lower troposphere within the inner core-region greatly inhibits intensification of the simulated storm.

The GFDL Hurricane Prediction System and Its Performance in the 1995 Hurricane Season

Abstract The Geophysical Fluid Dynamics Laboratory (GFDL) Hurricane Prediction System was adopted by the U.S. National Weather Service as an operational hurricane prediction model in the 1995 hurricane season. The framework of the prediction model is described with emphasis on its unique features. The model uses a multiply nested movable mesh system to depict the interior structure of tropical cyclones. For cumulus parameterization, a soft moist convective adjustment scheme is used. The model initial condition is defined through a method of vortex replacement. It involves generation of a realistic hurricane vortex by a scheme of controlled spinup. Time integration of the model is carried out by a two-step iterative method that has a characteristic of frequency-selective damping. The outline of the prediction system is presented and the system performance in the 1995 hurricane season is briefly summarized. Both in the Atlantic and the eastern Pacific, the average track forecast errors are substantially red...

1993年8月1日に南九州で発生したライン状豪雨の数値シミュレーション

The stationary band-shaped torrential rain, observed over southern Kyushu on 1 August 1993, was successfully simulated by a three-dimensional anelastic non-hydrostatic model. A warm rain scheme that explicitly predicts cloud water and rainwater is employed in the model. The Japan Spectral Model (JSM) had previously failed to reproduce the stationary feature of the rainband and had predicted the precipitation area to move eastward.The failure of JSM was examined by using a 10km-resolution model. The failure was traced to the use of the moist convective adjustment scheme. Neither coarse-resolution nor absence of hydrostatic water loading led to the failure. By the moist convective adjustment scheme stabilizing unstable layers, the air was cooled and dried in the lower adjusted levels. This drying reduced the near-surface moisture and cut off the supply of highly humid air required for the maintenance of the rainband.The causes of the organization and maintenance of the simulated stationary rainband were then investigated using 2km- and 5km-resolution models. Highly humid air in the lower layer was advected by southwesterlies, condensing over the region with a large temperature contrast in the north-south direction corresponding to the Baiu frontal zone. At first, shallow convective rolls formed parallel to the low-level westerlies. As they developed, individual cells grouped into severe convective systems, resulting in a marked depression of pressure in the lower layer by more than 1.5hPa over the region of (100km)2 for two hours after the condensation. This low pressure induced the convergence of winds. An inflow of intensified cold northwesterlies increased meridional temperature contrast, forming a strong convergence line and an organized rainband. The maintenance system of this rainband had the characteristic features of the back-building type, with significant vertical wind shear in the repeated generation of convective cells upstream of the rainband. For this torrential rain event, evaporation was not very important in the enhancement and maintenance of the rainband, while the relatively large meridional temperature contrast over the Baiu frontal zone played a significant role.The maintenance and enhancement processes of the low-level jet (LLJ) in the torrential rain event were also investigated by calculating the mean horizontal momentum budget and examining the trajectories of air parcels. The pressure gradient force produced by the convection-induced low pressure accelerated the mean horizontal velocity in the LLJ core, while horizontal advection acted to decelerate the jet. It was found that the acceleration and deceleration of the jet canceled each other to maintain the jet over southern Kyushu. The inflow of southwesterly winds into the lower layer below 1km was also accelerated, by 6ms-1 over 150min after the start of integration, by the pressure gradient force. These accelerated horizontal winds were transported upward by convection, a portion of which intensified the LLJ to the north of the rainband.

United Mechanisms for the Generation of Low- and High-Frequency Tropical Waves. Part I: Control Experiments with Moist Convective Adjustment

To examine several mechanisms for the generation of low- and high-frequency tropical waves, numerical experiments are conducted using an idealized nine-level R21 spectral model with the original scheme of moist convective adjustment (MCA). The model prescribes globally uniform, time-independent distributions of sea surface temperatures and insolation, thereby excluding stationary waves and extratropical baroclinic waves. The idealized model, however, still produces tropical intraseasonal oscillations, superclusters, Kelvin waves, and mixed Rossby‐gravity waves. When eliminating the wind fluctuations in the parameterized surface fluxes of latent and sensible heat, the intraseasonal oscillations are profoundly weakened while other waves are not substantially weakened. Subsequently, the MCA scheme is modified to neutralize any conditionally unstable stratification that would otherwise develop during periods of nonsaturation. This modification suppresses the part of the MCA process that neutralizes, upon saturation, any preexisting unstable stratification. In spite of the presence of moisture convergence, all tropical transient waves then disappear, in contrast to the wave-CISK (conditional instability of the second kind) mechanism. The above results are consistent with the united mechanisms proposed as follows. Intraseasonal oscillations are maintained primarily through the evaporation‐wind feedback mechanism. Other waves are maintained primarily through the ‘‘saturation-triggering mechanism’’ and/or the lateral-triggering mechanism. The saturationtriggering mechanism hypothesizes that transient waves can be triggered by the intermittent onset of nonequilibrium moist convection, upon saturation, to neutralize any preexisting unstable stratification.

準2年振動を再現した大循環モデルにおける赤道波について

The quasi-biennial oscillation (QBO) in the equatorial lower stratosphere has been well simulated in the first version of the Center for Climate System Research / National Institute for Environmental Studies (CCSR/NIES) atmospheric general circulation model (Takahashi, 1996). The model horizontal resolution is T21 with 60 layers of vertical resolution of about 500 m in the upper troposphere and lower stratosphere, with a moist convective adjustment scheme. The period of the simulated oscillation is 1.5 years, which is slightly shorter than that observed. The equatorial wave behavior in the model is examined using the spectral analysis method. The westerly acceleration phase of the QBO-like oscillation in the model is due to Kelvin waves and eastward-propagating gravity waves. The easterly acceleration phase of the oscillation is due to westward-propagating n = 1 equatorial gravity waves, random westward-propagating gravity waves and Rossby waves propagating from the mid-latitudes in the northern winter hemisphere to the equatorial region. The Rossby-gravity wave has a role in the easterly acceleration of the QBO-like oscillation in the present general circulation model.

Improvement of Spin-up of Precipitation Calculation with Use of Observed Rainfall in the Initialization Scheme

ABSTRACT A new approach is proposed for the initialization of the primitive equations model (hereafter, NWP model), taking account of the effect of non-adiabatic heating due to release of the latent heat into humidity and divergent wind fields. In the present paper, an operational model of the Japan Meteorological Agency (JMA), the Japanese Spectral Model (JSM) is taken up as the NWP model. For the purpose of the present study, rainfall amount is estimated, with a resolution of about 5-km mesh, from radar-AMeDAS composite precipitation data (hereafter, R-A data), where AMeDAS stands for the Automated Meteorological Data Acquisition System which is a network of automated surface observation stations including raingauge measurements. The observed rainfall is considered to represent the mesoscale precipitation field. The total heating rate calculated from the R-A data is assumed to be partitioned parabolically in the vertical between the lifting condensation level (LCL) and the cloud top (TBB CT) estimated by the GMS (Japanese geostationary meteorological satellite), where TBB means the black body temperature. The diabatic initialization method proposed here consists of two steps. The first step is a physical initialization of the water vapour field, where the relative humidity field is changed to be more moist over the area of rainfall (>1 mm hr−1). The moistening is performed in such a way that, if the air is not moist enough over the area of rainfall, the lapse rate at each model layer from the LCL to the TBB CT becomes a critical value presumably required by the moist convective adjustment scheme used in the NWP model concerned. On the other hand, over the area where no rainfall is observed, the relative humidity is left untouched. The second step is a nonlinear normal mode initialization (NNMI) which includes the non-adiabatic heating rate obtained from the R-A data among other factors. The first four vertical modes with the period less than 6 hours are adopted. It should be remarked that the divergent component of wind field which is necessary to continuously maintain the condensation due to mesoscale rainfall is also initialized at the stage of NNMI with the large-scale condensation and the moist convective adjustment. Improvement in the operational performance of the NWP model with the use of the proposed diabatic initialization method is demonstrated for a case study and statistical verification. The forecasts with and without the present diabatic initialization are compared. Particularly, amelioration of the spin-up of mesoscale precipitation calculation for the first 6 hours after the start is clearly seen. The initialization method shows stable execution and seems to be useful for the prediction of the mesoscale phenomena, particularly in the tropics where meteoroligical data are relatively sparse.

低・高周波熱帯波動の生成のための連合機構第II部

It is assumed that low- and high-frequency tropical waves are generated by the united mechanisms consisting of the evaporation-wind feedback (EWF), saturation-triggering (ST), and lateral-triggering mechanisms. Through the EWF mechanism, some waves become unstable owing to evaporation-wind feedback. Through the ST mechanism, other waves are triggered by the intermittent onset of moist convection, upon saturation, to neutralize any pre-existing conditionally unstable stratification. These mechanisms are theoretically interpreted by partitioning moist convective adjustment into two consecutive processes of diagnostic and prognostic adjustments. The two processes respectively restore and maintain convective equilibrium, and are crucial to the ST and EWF mechanisms.As a step to toward a unified theory, EWF instability is examined by the use of a theoretical Kelvin-wave model, which incorporates only the prognostic-adjustment process in the linearized perturbation equations, thereby excluding the ST mechanism. The solutions indicate that wave instability results from the EWF mechanism and not from the wave-CISK mechanism. For a plausible choice of adjustable parameters, one strongly unstable mode corresponds to the observed 40-50-day oscillation, while two weakly unstable modes correspond to the observed 25-30-day and 10-20-day oscillations.These results are compared with those from the numerical experiments conducted in Part I, using a nonlinear model incorporating the original moist convective adjustment scheme. It is then speculated that the 40-50- and 25-30-day modes can strongly grow through the linear and nonlinear EWF mechanisms respectively, while the 10-20-day mode can strongly amplify through the ST mechanism.

The Madden-Julian oscillation in the Canadian Climate Centre general circulation model

The Madden-Julian oscillation (MJO) simulated by the Canadian Climate Centre general circulation model (CCC GCM) is identified by a principal oscillation pattern (POP) analysis and compared with that observed in the real atmosphere. The results are based upon two integrations of the CCC GCM, one with a parameterization of penetrative cumulus convection (EXP1) and the other with a moist convective adjustment scheme (EXP2). The signal of MJO can be detected in both integrations as the first POP of the 200 hPa velocity potential along the equator. The disturbances show a distinctive wave number one structure with the strongest local amplitude found in the longitudes corresponding to the region of the Asian monsoon. The phase speed of the eastward wave propagation is higher in the eastern Pacific and lower in the monsoon region where the convective activities are strongest. These features are in good agreement with the observations. The energy spectrum of the velocity potential peaks at the frequency corresponding to a period of about 38 days for EXP1, which is somewhat shorter compared to the observed periods of 40–50 days. On the other hand, two spectral peaks can be clearly identified for EXP2, one with a period of 24 days and the other with a much longer period, somewhere near 112 days. Both peaks appear statistically significant at 95% level. Long term data of the observed atmosphere show little indication of such spectral separation. The horizontal patterns identified by the POP analysis resemble to some extent the baroclinic response of tropical flow to a heat source travelling with the speed of MJO. At the upper level, Rossby wave energy propagates westward with winds generally following the height contours, whereas Kelvin wave energy propagates to the east from the heat source with strong cross-contour flow near the equator. At the lower level, the patterns are essentially reversed. The model-generated precipitation and diabatic heating are examined by compositing against the moving MJO. It is found in EXP2 that the composite heating distribution is coherent with the flow pattern only in a certain sector of the equator, depending on whether the fast or slow mode is used to determine the reference point. The composite vertical heating profile of a slower mode tends to have a maximum found at a lower level. The sensitivity of simulated MJO to the cumulus convection scheme in the model is discussed.

Validation of a limited area model in cases of mediterranean cyclogenesis: Surface fields and precipitation scores

We have examined the performance of a limited-area mesoscale model of our design in cases of winter cyclones over the southern European-central Mediterranean area with particular ticular attention to standard precipitation statistical scores (bias, threat score, false alarms), obtaining results which compare favourably with other state-of-the-art LAM's documented in the literature. The model, which we briefly describe here, uses standard procedures and includes Geleyn's radiation package and Emanuel's moist convective adjustment scheme. We also discuss the model's performance from the viewpoint of individual surface fields, which display a variety of-mesoscale features correctly reproducing, in most cases, those of the observed fields. The latter have been re-analyzed for this purpose using conventional SYNOP data and ECMWF analyses as first guess.

Maintenance of the Intertropical Convergence Zones and the Large-Scale Tropical Circulation on a Water-covered Earth

Abstract How the surface boundary heating (sea surface temperature) and cumulus adjustment process affect the location, structure, energetics, and dynamics of the intertropical convergence zones (ITCZs) is investigated. A series of experiments is performed with a general circulation model where the lower boundary is specified to be water at a fixed sea surface temperature (SST), an aqua planet. All experiments are run using equinoctal insolation with no longitudinal variation in SST. Two different convective parameterization schemes (Kuo and moist convective adjustment) and several different zonally symmetric SST distributions are used in these experiments. The location of the ITCZ is found to be sensitive to the convective parameterization scheme and the SST distribution. The model with the moist convective adjustment scheme produces an ITCZ over the tropical SST maximum, even under conditions where the SST gradient is weak. By contrast, the model with the Kuo convective parameterization is not as sensit...

大気大循環モデルに現れた混合ロスビー・重力波と対流との関係

We investigate the relationship between mixed Rossby-gravity waves (MRGWs) and convection in a general circulation model. The experiments described are performed in a general circulation model with the lower boundary set to that of an ocean surface everywhere. Several experiments are run varying the convective parameterization scheme (using either a modified Kuo scheme or a moist convective adjustment scheme) and varying the tropical sea surface temperatures (specified to be zonally symmetric in all cases), thereby changing the location of the modeled intertropical convergence zones (ITCZs). The appearance of a robust MRGW occurs when the sea surface temperature is such that two ITCZs straddle the equator. The particular sea surface temperature distribution used and the parameterization scheme for convection also affect the structure and strength of the modeled MRGW. The vertical structure of MRGWs is analyzed in the experiment in which this wave mode is the most energetic. We show that MRGWs of several different zonal length scales exist in the troposphere in association with convection; however, it is only the longer length scales which can be discerned in the upper troposphere and lower stratosphere.

Response of a GCM with a Hybrid Convection Scheme to a Tropical Pacific Sea Surface Temperature Anomaly

Abstract A tropical Pacific sea surface temperature (SST) anomaly is included in a global general circulation model with a hybrid convection scheme. Results are compared with a similar version of the same model that includes a simple moist convective adjustment scheme and with observed data analyses from the European Centre for Medium-Range Weather Forecasts. The model with the hybrid scheme transports proportionately more heat and moisture higher into the troposphere in the vicinity of the SST anomaly, and the extratropical sensitivity to the SST anomaly is greater (larger-amplitude anomalies). Improvements in the simulated mean climate with the hybrid scheme result in an altered and more realistic extratropical response to the tropical SST anomaly. This is especially true for the Southern Hemisphere where the improvements to the mean climate simulation are most dramatic. This result is consistent with linear models that show a great dependence on the mean extratropical climate simulation for the type of...

「水惑星」モデルにおける熱帯域の積雲活動のふるまいと循環構造

As an extension of the 'aqua planet' experiments by Hayashi and Sumi (1986, HS86), GCM experiments with various SST distributions and cumulus parameterizations are performed in order to examine the structure of cumulus activity and its parameter dependence. In part I, the results of the analyses concentrating on the super clusters are described.The super clusters, which were named by HS86, are expressed in the model as continuously moving precipitating areas of the grid scale. The associated circulation structure can be explained in terms of the wave-CISK dynamics of Kelvin waves. The super clusters become more active when a warmer SST is used. On the other hand, the 30 day oscillation, that is an eastward-moving planetary scale structure, is more distinct when a colder SST is used.The behavior of cumulus activity is highly sensitive to the choice of cumulus parameterization. When the moist convective adjustment scheme is used instead of Kuo's (1974) scheme, the eastward-moving structure of the grid scale is no longer observed at the equator. However, the planetary scale structure still clearly appears in the experiment with the moist convective adjustment scheme.