Moist Convective Adjustment Research Articles

Nonuniqueness in a Single-Column Model for Moist Convection

Abstract We investigate a moist atmospheric column convection model by considering the atmosphere as a single vertical column of air parcels, each of which contains water vapor. The moist convective adjustment of both air and water mass in the column is considered from an (unstable) initial state to a statically stable final configuration of parcels. Two variations of an algorithm based upon swapping neighboring parcels are compared: after swapping, no parcels remain supersaturated. The results of these algorithms are compared directly with those of the adjustment algorithm of Cheng et al., which adjusts an atmospheric column to achieve the global maximum of a relevant cost functional. Two examples are considered: in the first, the algorithms adjust to similar arrangements, showing that the global maximum of the functional is the dynamically preferred state, while in the second, the algorithms adjust to significantly different states. Thus, we identify a nonuniqueness to the solution to the adjustment problem in terms of local and global cost functional maximizers. We then discuss the relevance of this nonuniqueness to numerical prediction in weather and climate models.

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.

Effects of multicumulus convective ensemble on East Asian summer monsoon rainfall simulation

Simulating the Asian summer monsoon is one of the most challenging tasks in climate modeling. In particular, monsoon simulations strongly depend on the cumulus parameterization scheme being used, as attested by many modeling studies. In this paper, the performances of a “unified multicumulus convective ensemble scheme” (or unified scheme), Kuo, simplified Arakawa‐Schubert, Zhang and McFarlane convective parameterization schemes and moist convective adjustment in simulating the East Asian summer monsoon in a coupled General Circulation Model (GCM) are assessed. In the unified scheme, the latter four individual parameterizations are run in parallel without any modification. Simple average of outputs from each parameterization is then incorporated to the atmospheric model at every time step. Simulations based on these convection schemes during the boreal summer season are compared with observations, focusing on the formation and seasonal evolution of the Meiyu‐Changma‐Baiu rainband. It is found that the East Asian summer monsoon evolution in the GCM is sensitive to the convective parameterization being used. While the Kuo scheme is able to capture a reasonably strong subtropical front, its mean onset time is about two to three pentads earlier than the observed. Simulations based on the simplified Arakawa‐Schubert scheme give a more realistic onset date. However, the model subtropical rainband tends to be confined over land near the coastal region of East Asia. The unified scheme is able to reproduce a prominent East Asian subtropical rainband. Moreover, the seasonal evolution of the rainband is also well captured in the unified scheme simulations. Further diagnostics are carried out to elucidate the role of various circulation elements in the Meiyu‐Changma‐Baiu front formation. In the western part of the front over land regions, there is robust precursor signal in incoming shortwave radiation before monsoon onset in observations as well as model simulations. Comparison between performances of different convective parameterizations suggests that sensitivity to low‐level vorticity can be crucial for the northward migration of the subtropical front and its extension into the open ocean east of southern Japan.

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 Dynamics of Idealized Convection Schemes and Their Effect on the Zonally Averaged Tropical Circulation

In this paper, the effect of a simple convection scheme on the zonally averaged tropical general circulation is examined within an idealized moist GCM to obtain broad classifications of the influence of convection on the Tropics. This is accomplished with a simplified convection scheme in the style of Betts and Miller. The scheme is utilized in a moist GCM with simplified physical parameterizations (gray radiation, with zonally symmetric, slab mixed layer ocean boundary conditions). Comparisons are made with simulations without a convection scheme [i.e., with large-scale condensation (LSC) only], with the moist convective adjustment (MCA) parameterization, and with various formulations and parameter sets with a simplified Betts–Miller (SBM) scheme. With the control run using the SBM scheme, the Tropics become quieter and less dependent on horizontal resolution as compared with the LSC or MCA simulations. The Hadley circulation mass transport is significantly reduced with the SBM scheme, as is the ITCZ precipitation. An important factor determining this behavior is the parameterization of shallow convection: without shallow convection, the convection scheme is largely ineffective at preventing convection from occurring at the grid scale. The sensitivities to convection scheme parameters are also examined. The simulations are remarkably insensitive to the convective relaxation time, and only mildly sensitive to the relative humidity of the reference profile, provided significant large-scale condensation is not allowed to occur. The changes in the zonally averaged tropical circulation that occur in all the simulations are understood based on the convective criteria of the schemes and the gross moist stability of the atmosphere.

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.

The effects of precipitation schemes and horizontal resolution on the major rainband in typhoon Flo (1990) predicted by the MRI mesoscale nonhydrostatic model

¶This paper describes a numerical study of the major spiral rainband in typhoon Flo (1990) using the Meteorological Research Institute Mesoscale Nonhydrostatic Model (MRI-NHM). The effects of precipitation schemes and horizontal resolution on the representation of the simulated rainband are discussed. Dynamic and thermodynamic structures of the simulated major rainband to the north of the storm center are well represented in the model with a 5 km horizontal resolution. The structures are consistent with observational results reported for other tropical cyclones. Among the realistic features are: a cold pool and convergence on the inner side of the band; convergence above low-level inflow layers; and the outward slope of the updraft with height. The band is caused by the motion of the storm through its surroundings where horizontal wind has vertical shear. The simulation of the structure and precipitation pattern associated with the major rainband depends on the precipitation scheme rather than the horizontal resolution. The band appears more realistic when using explicit cloud microphysics as a precipitation scheme, rather than moist convective adjustment. This result is attributable to the difference in scheme triggering. In the simulation with moist convective adjustment, the elimination of vertical instability in low-level atmosphere is excessive, suppressing band formation. The overall structure of the band is also more realistic in the simulation using explicit cloud microphysics, because a cold pool exists in the lower layers and the vertical axis of upward flow tilts outward. This result suggests that prediction will partly depend on variables associated with cloud microphysics, such as the mixing ratio of cloud water. The horizontal grid distance, which varied between 5 and 20 km, quantitatively influenced the rainfall amount, although the large-scale band structure remained unchanged. The rainfall amount increased as the grid interval was reduced from 20 to 10-km but decreased as the interval was further reduced from 10 to 5 km.

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.

Impacts of Cumulus Convection Parameterization on Aqua-planet AGCM Simulations of Tropical Intraseasonal Variability

Using an aqua-planet version of an atmospheric general circulation model (AGCM), the dependence of the tropical intraseasonal oscillation (ISO) simulation on the cumulus parameterization was examined with three different cumulus schemes—simplified Arakawa-Schubert, Kuo, and moist convective adjustment. The simulated intensity and propagation characteristics of the ISO depend significantly on the choice of cumulus scheme, in which more constrained convection scheme produces stronger intraseasonal variability in tropics. Mean thermodynamic state and the Intertropical Convergence Zone (ITCZ) structure also vary among the simulations, demonstrating that the ISO variability and the mean states are mutually dependent. Following Tokioka et al. (1988), the simplified Arakawa-Schubert scheme was modified by posing a minimum entrainment rate constraint for cumuli, and the relationship between tropical intraseasonal variability and zonal mean rainfall structure was examined. More constraining deep convections, the tropical ISO variability becomes stronger with narrower ITCZ structures. Vertical and horizontal structures of eastward propagating waves appeared in the aqua-planet experiments were further investigated. The vertical structures of propagating waves are consistent with observations of the Madden-Julian Oscillation, but the vertical profile of ISO-modulated heating exhibits a middle-heavy structure and the simulated waves show relatively faster propagations compared with the observed. The horizontal composite structures show the boundary-layer moisture frictional convergence to the east, and divergence to the west of the convective region, and this suggests that a frictional Kelvin wave-CISK mechanism is important to these eastward propagating waves.

A Simple Model of a Convectively Coupled Walker Circulation Using the Weak Temperature Gradient Approximation

An idealized model of a Walker circulation based on the weak temperature gradient (WTG) approximation and a single baroclinic vertical mode for all fields is analyzed. The circulation is forced by a sinusoidal variation of sea surface temperature (SST). A simple feedback of deep convective cloud radiative forcing on tropospheric radiative cooling is included, and a moist convective adjustment is used to interactively specify the location and intensity of deep convection. A goal is to understand how the fraction of the domain undergoing deep convection depends on the SST difference across the domain. The WTG approximation greatly simplifies the calculation of the circulation. For small SST differences, convection occurs everywhere and a fully analytic solution is possible; for larger SST differences, a simple nonlinear algorithm is used to determine the edge of the convective region and the tropospheric temperature. The solution is invariant to changes of domain size. The divergent circulation is independent of the Coriolis parameter as long as the domain is sufficiently narrow so WTG remains accurate. The convective region narrows and intensifies considerably when cloud‐radiation feedbacks are considered. As the SST difference increases, the convective region shrinks and the troposphere warms; simple approximate formulas for these trends in terms of the control parameters are derived. For large SST differences, the convection over the warmest water becomes susceptible to a radiative‐convective instability and there is no steady-state solution.

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.

Sensitivity of tropical storms simulated by a general circulation model to changes in cumulus parametrization

AbstractA number of recent studies have examined the statistics of tropical storms simulated by general circulation models (GCMs) forced by observed sea surface temperatures. Many GCMs have demonstrated an ability to simulate some aspects of the observed interannual variability of tropical storms, in particular, variability in storm frequency. This has led to nascent attempts to use GCMs as part of programs to produce operational seasonal forecasts of tropical‐storm numbers.In this study, the sensitivity of the statistics of GCM‐simulated tropical storms to changes in the model's physical parametrizations is examined. After preliminary results indicated that these statistics were most sensitive to details of the convective parametrization, GCM simulations with identical dynamical cores but different convective parametrizations were created. The parametrizations examined included moist convective adjustment, two variants of the Arakawa‐Schubert scheme, and several variants of the relaxed Arakawa‐Schubert (RAS) scheme; the impact of including a shallow‐convection parametrization was also examined.The simulated tropical‐storm frequency, intensity, structure, and interannual variability were all found to exhibit significant sensitivities to changes in convective parametrization. A particularly large sensitivity was found when the RAS and Arakawa‐Schubert parametrizations were modified to place restrictions on the production of deep convection.Climatologies of the GCM tropical atmosphere and composites of tropical storms were examined to address the question of whether the tropical‐storm statistics were directly impacted on by changes in convection associated with tropical storms, or if they were indirectly affected by parametrization‐induced changes in the tropical mean atmosphere. A number of results point to the latter being the primary cause. A regional hurricane model, initialized with mean states from the GCM simulation climatologies, is used to further investigate this point. Particularly compelling is the fact that versions of the RAS scheme that produce significantly less realistic simulations of tropical storms nevertheless produce a much more realistic interannual variability of storms, apparently due to an improved tropical mean climate.A careful analysis of the background convective available potential energy (CAPE) is used to suggest that this quantity is particularly relevant to the occurrence of tropical storms in the low‐resolution GCMs, although this may not be the case with observations. If the tropical CAPE is too low, tropical storms in the low‐resolution GCMs cannot form with realistic frequency.

Maintenance of Tropical Intraseasonal Variability: Impact of Evaporation–Wind Feedback and Midlatitude Storms

An intraseasonal tropical oscillation with a period of 20–80 days is simulated in the Neelin–Zeng Quasi-Equilibrium Tropical Circulation Model. This model is an intermediate-level atmospheric model that includes primitive equation nonlinearity, radiative– convective feedbacks, a simple land model with soil moisture, and a Betts–Miller-type moist convective adjustment parameterization. Vertical temperature and moisture structures in the model are based on quasi-equilibrium profiles taken from deep convective regions. The tropical intraseasonal variability is reasonably broadband. The eastward propagating 20–80-day variability is dominated by zonal wavenumber 1, shows features similar to an irregular Madden–Julian oscillation (MJO), and exhibits amplitude and phase speeds that vary both seasonally and between events. At higher wavenumbers, the model has a distinction between the low-frequency MJO-like band and the moist Kelvin wave band, similar to that found in observations. In the model, it is conjectured that this arises by interaction of the wavenumber-1 moist Kelvin wave with the zonally asymmetric basic state. Experiments using climatological sea surface temperature forcing are conducted using this model to examine the effects of evaporation–wind feedback and extratropical excitation on the maintenance of intraseasonal variability, with particular attention paid to the low wavenumber mode in the 20–80-day band. These experiments indicate that evaporation–wind feedback partially organizes this intraseasonal variability by reducing damping, but it is not by itself sufficient to sustain this oscillation for the most realistic parameters. Excitation by extratropical variability is a major source of energy for the intraseasonal variability in this model. When midlatitude storms are suppressed, tropical intraseasonal variability is nearly eliminated. However, the eastward propagating intraseasonal signal appears most clearly when midlatitude excitation is aided by the evaporation–wind feedback.

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 Dependence on Convection Parameterization of the Tropical Intraseasonal Oscillation Simulated by the UIUC 11-Layer Atmospheric GCM

Abstract Numerical experiments are conducted using the University of Illinois, Urbana–Champaign (UIUC), 11-layer atmospheric general circulation model (GCM) to investigate the dependence of the simulated tropical intraseasonal oscillation (TIO) on convection parameterization. Three convection parameterizations have been tested: 1) the UIUC GCM’s original cumulus–convection parameterization, which includes a modified version of the Arakawa–Schubert penetrative–convection parameterization and a middle-level convection parameterization, 2) the parameterization of Kuo, and (3) the moist convective adjustment parameterization of Manabe et al For each parameterization a relative humidity criterion (RHc) for convection or convective heating to occur is used, as in many GCMs. Perpetual-March simulations with these convection parameterizations have been performed for different values of RHc. It is found that the simulated TIO is highly dependent on RHc. As RHc increases, the oscillation in the simulations becomes ...

Simulation of the Quasi‐Biennial Oscillation in a general circulation model

Simulation of the equatorial stratospheric quasi‐biennial oscillation (QBO) is accomplished, using a T42‐horizontal resolution, 60‐layer general circulation model (GCM), with a scheme of moist convective adjustment. In a previous paper, we succeeded in simulating a QBO in a T21 GCM using a damping time of 20 days for the maximum wavenumber in the fourth‐order horizontal diffusion. It was noted that the damping time is a factor of 10 larger than that employed in standard GCMs. In this paper, we explore whether it is possible to use a smaller damping term by going to a finer horizontal resolution. We successfully simulated a QBO using a damping time of 4 days. The period of the simulated QBO is 800 days, being almost the same as that observed. The amplitude of the zonal‐wind oscillation is 20 ms−1. However, this is achieved at the expense of much noiser precipitation and the damping time is still a factor of 7 larger than the standard values.

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...

Linear stability and single-column analyses of several cumulus parametrization categories in a shallow-water model

Using a mass-flux based approach, the thermodynamic cumulus parametrization problem is reformulated in a simple atmospheric model, which is an analogue of the shallow-water equations. The objective is to investigate basic effects of elementary representations of several parametrization categories. In particular, a linear stability analysis and a single-column experiment are performed to infer the characteristics of each parametrization as regards its ability to simulate the large-scale organization or coherence of tropical convection. The moisture-convergence closure (MC) scheme, which assumes that the ensemble of cumulus convection is controlled by the low-level moisture convergence as in Kuo-type scheme, predicts the largest growth at the smallest scale. Hence, although it ensures the generation of a coherent propagating structure, its scale always corresponds to the grid size. Furthermore, the MC tends to produce a catastrophic positive feedback of moist convection to the large-scale convergence. In contrast, the statistical equilibrium scheme, which assumes an instantaneous adjustment of the large-scale environment to a quasi-equilibrium state, such as Arakawa-Schubert and moist convective adjustment schemes, asymptotes to a constant growth rate at small scales. Hence, this type of parametrization tends to generate a field like white noise with no large-scale coherence. The lagged-adjustment (LA) schemes, which have a short time-lag for the cumulus growth, as in the Betts–Miller scheme, feature a finite scale selection in the linear growth rate. This ensures a smooth large-scale coherence that is independent of the grid size, and is consistent with the scale-separation principle. A new type of parametrization is also tested. This convective life-cycle (CLC) scheme represnets the life cycle of a common type of convective system made up of deep precipitating convection and a subsequent mesoscale response. It uses a buoyancy-based closure. The growth-rate curve is similar to the other LA schemes, but the behaviour in the zero-dimensional (single-column) version of the model is qualitatively different. Although the CLC scheme does not automatically saisfy the scale-separation principle, its grid-size dependence can be treated by a re-normalization principle. The result are used to interpret some reported general-circulation-model results regarding the impact of different parametrization schemes on the tropical atmosphere at large scales.