Convective Adjustment Research Articles

Modeling of the three-dimensional structure of hydrological fields in a karst cavity under the action of submarine discharge of underground waters

We discuss the results of numerical simulation of the process of submarine discharge in a karst cavity. It is assumed that fresh water percolates through cracks in the lateral boundaries of the cavity, and its temperature is, generally speaking, not equal to the temperature of water in the grotto. The calculations are carried out within the framework of a three-dimensional model on the σ -coordinates in the hydrostatic approximation by using the procedure of “convective adjustment” guaranteeing the existence of stable density stratification with conservation of the amounts of heat and salt. The comparison with the data of observations shows that the model gives a qualitatively correct description of the structure of the fields of temperature, salinity, and current velocities formed as a result of submarine discharge in the cavity. The possibilities of subsequent improvement of the model are discussed.

Impacts of Convective Lifetime on Moist Geostrophic Adjustment

Abstract This paper presents a theoretical study of the effects of moist convection on geostrophic adjustment in an infinite channel. The governing equations correspond to a linearized shallow water system of equations for the atmosphere first vertical baroclinic mode, which is coupled to a vertically averaged moisture equation. The coupling is through a parameterization that represents precipitation. The transient behavior and final state of the flow initially at rest with active precipitation limited to half of the channel is investigated, both numerically and analytically. It is shown that an initial imbalance resulting from precipitation induces a circulation that dries out the nonprecipitating region and further enhances precipitation. This interaction between precipitation and dynamics leads to a sharper temperature gradient and stronger jet in the final state, when compared to the dry adjustment. Unlike in the dry case, the moist geostrophic adjustment cannot be entirely determined from the initial unbalanced flow, since it depends on the time scale for convection. Analytic approximations are derived in limits of both fast and slow convective adjustment time.

An Analysis of Convectively Coupled Kelvin Waves in 20 WCRP CMIP3 Global Coupled Climate Models

Abstract Output from 20 coupled global climate models is analyzed to determine whether convectively coupled Kelvin waves exist in the models, and, if so, how their horizontal and vertical structures compare to observations. Model data are obtained from the World Climate Research Program’s (WCRP’s) Coupled Model Intercomparison Project phase 3 (CMIP3) multimodel dataset. Ten of the 20 models contain spectral peaks in precipitation in the Kelvin wave band, and, of these 10, only 5 contain wave activity distributions and three-dimensional wave structures that resemble the observations. Thus, the majority (75%) of the global climate models surveyed do not accurately represent convectively coupled Kelvin waves, one of the primary sources of submonthly zonally propagating variability in the tropics. The primary feature common to the five successful models is the convective parameterization. Three of the five models use the Tiedtke–Nordeng convective scheme, while the other two utilize the Pan and Randall scheme. The 15 models with less success at generating Kelvin waves predominantly contain convective schemes that are based on the concept of convective adjustment, although it appears that those schemes can be improved by the addition of convective “trigger” functions. Three-dimensional Kelvin wave structures in the five successful models resemble observations to a large degree, with vertically tilted temperature, specific humidity, and zonal wind anomalies. However, no model completely captures the observed signal, with most of the models being deficient in lower-tropospheric temperature and humidity signals near the location of maximum precipitation. These results suggest the need for improvements in the representations of shallow convection and convective downdrafts in global models.

Fluctuation of Mass Flux in a Cloud-Resolving Simulation with Interactive Radiation

AbstractIt was shown by Craig and Cohen that fluctuations of cumulus clouds under homogeneous large-scale forcing satisfy the Gibbs canonical ensemble in a strict radiative–convective equilibrium (RCE). In the limit of random noninteracting convective cells, an analytical expression for the distribution function of total mass flux over a region of given size was derived.The authors examine the consistency of the Gibbs canonical ensemble as a representation for the mass flux fluctuations when the large-scale forcing is time dependent. A cloud-resolving simulation (CRM) with interactive radiation, fixed imposed surface temperature, and diurnally varying solar forcing to mimic the diurnal cycle over the tropical ocean is used.As a necessary condition for the existence of a state of quasi-equilibrium, the time-scale separation between convective processes and forcing is studied. Detailed evaluation of time scales of convective adjustment and memory in a three-month run confirms the hypothesis of time-scale separation.The Craig and Cohen theory, in a varying range of heights between the cloud base up to the level of neutral buoyancy (LNB), is tested. It is shown that, although the theory is capable of reproducing the qualitative features of the variability, systematic deviations are detected. By quantifying the spatial distribution of the clouds, the authors suggest that deviations are associated with clustering effects.

Convective adjustment in box models

A convective adjustment (CA) algorithm is thought to be responsible for grid-scale oceanic-state sustained oscillations seen in oceanic general circulation models (OGCM), an effect that is most evident in simulations with coarse spatio-temporal scales. The CA algorithm is thought to inadvertently create a salt oscillator. Several studies have confirmed that a flip-flop type salt oscillator, which is reminiscent in some respects of simple CA schemes, can develop sustained oscillations. Subsequently, several researchers were able to show how coupled salt oscillators, reacting in a particular temporal sequence, are capable of producing large-scale oscillations not unlike those found in the OGCM simulations. However, the proxy models used to study how CA can create these oscillations in large-scale simulations were never directly related to OGCM results. Here we couple hydrodynamics to the CA and look at zonally-driven flows in the low-frequency, large-scale limit. Adding flow is a step in the direction of developing an analytically tractable model with which to understand the basics of OGCMs. We analytically determine whether, and under what circumstances, the CA scheme is responsible for sustained oscillations. We carry out this program for four basic box-model configurations, each inspired by the general shape of the eigenfunctions and constraints of the large-scale zonally-averaged forced flow over a hemisphere. Furthermore, in order to make our results relevant to the Meridional Ocean Circulation, we also investigate the effect of replacing the usual assumption of a linear relation between thermohaline flow rate and horizontal density gradient with a nonlinear hydraulic relationship. We find that a salt oscillator does not occur in the most common box-model configurations. In one of our models, however, we find wide parameter ranges in which all steady states calculated for the model fail to satisfy the CA scheme, the situation which is expected to result in CA-induced oscillations. The model in question corresponds to a hemispheric shallow thermohaline flow over a deep reservoir. However, we find that oscillations occur in these parameter ranges only if the density threshold for convection is negative, i.e., if the CA scheme turns on convection between vertically adjacent boxes when the density stratification between them is still slightly stable. In this situation, the amplitude and period of the oscillations depend strongly on the size of the density threshold, both vanishing as the threshold is taken toward zero. We also show that the same is true in the Welander flip-flop model of a single salt oscillator. For positive values of the threshold, that is, when the CA scheme is allowed to ignore small unstable stratification changes, oscillations do not occur in the limit of integration time step going to zero, but can still be seen when the time step is finite, even if small. Moreover, the system evolves toward a new steady state, one in which the stratification in one box is exactly the threshold value itself. We show how to calculate these new steady states, and explain why they give way to oscillations when the density threshold is negative.

The Energetics of Global Thermohaline Circulation and Its Wind Enhancement

Abstract The energy budget of global thermohaline circulation (THC) is numerically investigated using an ocean general circulation model (OGCM) under a realistic configuration. Earlier studies just discuss a globally integrated energy budget. This study intends to draw a comprehensive picture of the global THC by separately calculating the energy budgets for three basins (the Atlantic, Indo-Pacific, and Southern Ocean). The largest mechanical energy source is a kinetic energy (KE) input to the general circulation by wind. Of that, 0.3 TW is converted to gravitational potential energy (GPE), and 80% of the energy conversion occurs in the Southern Ocean. Almost the same quantity of GPE is supplied by vertical mixing. Injected GPE is almost equally dissipated by convective adjustment and the effect of cabbeling, and a large part of that is consumed in the Southern Ocean. A dominant role of the Southern Ocean in the energy balance of THC and importance of the interbasin transport of GPE are found. Then, the enhancement of the meridional overturning circulation in the Atlantic induced by wind in the Southern Ocean is examined. Calculating the energy budget anomaly enables the authors to identify its mechanism as a component of THC.

A gravity wave analysis near to the Andes Range from GPS radio occultation data and mesoscale numerical simulations: Two case studies

Global maps of potential wave energy per unit mass, recently performed with the Global Positioning System (GPS) Radio Occultation (RO) technique and different satellite missions (CHAMP and SAC-C since 2001, GRACE and COSMIC since 2006) revealed in Argentina, at the eastern side of the highest Andes Mountains, a considerable wave activity (WA) in comparison with other extra-tropical regions. The main gravity wave (GW) sources in this natural laboratory are deep convection (mainly during late Spring and Summer), topographic forcing and geostrophic adjustment. The mesoscale numerical WRF (Weather Research and Forecasting) 2.1.2 model was used to simulate the atmospheric parameters during two representative RO events showing apparent intense WA in this region. The significance of the relative position of the RO lines of sight, the line of tangent points and GW phase surfaces during each event is discussed in relation with the apparent WA detected. The GPS RO technique may not be by itself reliable enough to quantify and locate WA of single events. Nevertheless, it should be considered a useful tool to observe the global WA from statistical studies. We also discuss the relative contribution of high and medium intrinsic frequency mountain waves regularly observed, coexisting with inertio gravity waves, their origin and propagation characteristics.

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.

A new model to simulate the Martian mesoscale and microscale atmospheric circulation: Validation and first results

The Laboratoire de Météorologie Dynamique (LMD) Mesoscale Model is a new versatile simulator of the Martian atmosphere and environment at horizontal scales ranging from hundreds of kilometers to tens of meters. The model combines the National Centers for Environmental Prediction(NCEP)‐National Center for Atmospheric Research (NCAR) fully compressible nonhydrostatic Advanced Research Weather Research and Forecasting (ARW‐WRF) dynamical core, adapted to Mars, with the LMD‐general circulation model (GCM) comprehensive set of physical parameterizations for the Martian dust, CO2, water, and photochemistry cycles. Since LMD‐GCM large‐scale simulations are also used to drive the mesoscale model at the boundaries of the chosen domain of interest, a high level of downscaling consistency is reached. To define the initial state and the atmosphere at the domain boundaries, a specific “hybrid” vertical interpolation from the coarse‐resolution GCM fields to the high‐resolution mesoscale domain is used to ensure the stability and the physical relevancy of the simulations. Used in synoptic‐scale mode with a cyclic domain wrapped around the planet, the mesoscale model correctly replicates the main large‐scale thermal structure and the zonally propagating waves. The model diagnostics of the near‐surface pressure, wind, and temperature daily cycles in Chryse Planitia are in accordance with the Viking and Pathfinder measurements. Afternoon gustiness at the respective landing sites is adequately accounted for on the condition that convective adjustment is turned off in the mesoscale simulations. On the rims of Valles Marineris, intense daytime anabatic (∼30 m s−1) and nighttime katabatic (∼40 m s−1) winds are predicted. Within the canyon corridors, topographical channeling can amplify the wind a few kilometers above the ground, especially during the night. Through large‐eddy simulations in Gusev Crater, the model describes the mixing layer growth during the afternoon, and the associated dynamics: convective motions, overlying gravity waves, and dust devil–like vortices. Modeled temperature profiles are in satisfactory agreement with the Miniature Thermal Emission Spectrometer (Mini‐TES) measurements. The ability of the model to transport tracers at regional scales is exemplified by the model's prediction for the altitude of the Tharsis topographical water ice clouds in the afternoon. Finally, a nighttime “warm ring” at the base of Olympus Mons is identified in the simulations, resulting from adiabatic warming by the intense downslope winds along the flanks of the volcano. The surface temperature enhancement reaches +20 K throughout the night. Such a phenomenon may have adversely affected the thermal inertia derivations in the region.

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 role of quasi-stationary flaw polynya in formation of dense shelf waters in the wintertime and their subsequent slope cascading (the Laptev Sea case study)

Shelf areas in the region of the Severnaya Zemlya Archipelago in the Laptev Sea are characterized by existing quasi-stationary flaw polynya that periodically opens throughout the entire wintertime under the action of strong offshore winds, which occur during the passage of cyclones. In periods of the open water surface, a near-surface turbulent layer or forced convection layer is formed in the flaw polynya; the water in the layer formed undergoes intense salinization and its dense increases due to active volumetric frazil ice production. As a result of the gravity force action, intense three-dimensional convective circulation develops in the underlying layers. It leads to a fast convective adjustment of the entire water column, especially, in the late winter, when residual stratification in the area of polynya is weakened with the total action of salinization due to the background static ΣMsback and periodical local frazil ice formation ΣMsf. On the whole for the entire winter period ΣMsf is 3.4 times greater than ΣMsback, although, during one month, probable lifetime of polynya with open water surface is several days. However, in these periods, salt fluxes with frazil ice production exceed background salt fluxes in the congelation polynya and background salt fluxes under heavy ice (limiting the polynya) 10–80 times. Spreading outside the polynia, dense shelf waters form in the area of polynya mesoscale baroclinic circulation, first generating intense shelf cascading, then intense slope cascading, which is of a local and random character. Some estimates of elements of baroclinic circulation of a convective origin in the area of polynia were obtained from the laboratory modeling results and are confirmed by field observation data.

On the sensitivity of cloud‐to‐ground lightning activity to surface air temperature changes at different timescales in São Paulo, Brazil

This paper presents a study about the sensitivity of cloud‐to‐ground (CG) lightning activity to changes in surface air temperature at daily, monthly, yearly, and decadal timescales in the city of São Paulo (Brazil). Lightning data collected in the city by the Brazilian Lightning Detection Network (BrasilDat) from 1999 to 2006 and thunderstorm day data obtained from 1951 to 2006 were analyzed and compared with surface air temperature data. The lightning activity increases significantly with increasing temperature, with a sensitivity of approximately 40% per 1°C for daily and monthly timescales and approximately 30% per 1°C for decadal timescale. For the yearly timescale, the increase is not statistically significant. The lower sensitivity for the decadal timescale suggests that the lightning sensitivity to changes in surface air temperature decreases for larger timescales, in agreement with what is expected on the basis of convective adjustment. The decadal lightning sensitivity found in this study is in reasonable agreement with the increase in the global lightning activity estimated by most climate models. The study is the first to investigate in detail this relationship in a large urban area inside the tropics and should contribute to the effort to understand the impact of the global warming on lightning activity.

Mediterranean Overflow Water (MOW) simulation using a coupled multiple‐grid Mediterranean Sea/North Atlantic Ocean model

A z‐level, 4th‐order‐accurate ocean model is applied in six two‐way‐coupled grids spanning the Mediterranean Sea and North Atlantic Ocean (MEDiNA). Resolutions vary from 1/4° in central North Atlantic to 1/24° in Strait of Gibraltar region. This allows the MEDiNA model to efficiently resolve small features (e.g., Strait of Gibraltar) in a multibasin, multiscale model. Such small features affect all scales because of nonlinearity and low dissipation. The grid coupling using one coarse grid overlap is nearly seamless without intergrid sponge layers. No instant convective adjustment or other highly diffusive process is used. The deep water in the 1/8° Mediterranean Sea grid is formed by the resolved flows that emulate subgrid‐scale processes directly. Downslope migration of Mediterranean Overflow Water (MOW) water involves dense water flowing away from the bottom laterally over bottom stairsteps in the z‐level model, thus flowing over less dense underlying water. Without excessively water mass‐diluting process, the advection dominates the downslope migration of thin, dense MOW in the simulation. The model results show realistic MOW migration to the observed equilibrium depth, followed by lateral spreading near that depth. The results are also consistent with the climatology along 43°N, where the MOW hugs a steep shelfslope centered at ∼1 km depth and then spreads westward, with the salinity core (S > 35.7) reaching 18°W. This study clearly restores z‐level models to a competitive status doing density current simulations.

Advances in simulating atmospheric variability with the ECMWF model: From synoptic to decadal time‐scales

AbstractAdvances in simulating atmospheric variability with the ECMWF model are presented that stem from revisions of the convection and diffusion parametrizations. The revisions concern in particular the introduction of a variable convective adjustment time‐scale, a convective entrainment rate proportional to the environmental relative humidity, as well as free tropospheric diffusion coefficients for heat and momentum based on Monin–Obukhov functional dependencies.The forecasting system is evaluated against analyses and observations using high‐resolution medium‐range deterministic and ensemble forecasts, monthly and seasonal integrations, and decadal integrations with coupled atmosphere‐ocean models. The results show a significantly higher and more realistic level of model activity in terms of the amplitude of tropical and extratropical mesoscale, synoptic and planetary perturbations. Importantly, with the higher variability and reduced bias not only the probabilistic scores are improved, but also the midlatitude deterministic scores in the short and medium ranges. Furthermore, for the first time the model is able to represent a realistic spectrum of convectively coupled equatorial Kelvin and Rossby waves, and maintains a realistic amplitude of the Madden–Julian oscillation (MJO) during monthly forecasts. However, the propagation speed of the MJO is slower than observed. The higher tropical tropospheric wave activity also results in better stratospheric temperatures and winds through the deposition of momentum.The partitioning between convective and resolved precipitation is unaffected by the model changes with roughly 62% of the total global precipitation being of the convective type. Finally, the changes in convection and diffusion parametrizations resulted in a larger spread of the ensemble forecasts, which allowed the amplitude of the initial perturbations in the ensemble prediction system to decrease by 30%. Copyright © 2008 Royal Meteorological Society

Precipitation fronts and the reflection and transmission of tropical disturbances

AbstractThis paper investigates the interactions between precipitation and the circulation in an idealized model for the tropical atmosphere where convection is represented by a quasi‐equilibrium closure. When studying large‐scale circulation in the Tropics, the governing equations can be further simplified by making the strict quasi‐equilibrium assumption which considers that convection acts to instantaneously adjust the atmospheric temperature profile to a moist adiabatic lapse rate. It is shown here that, under this assumption, the interface between the precipitating and non‐precipitating regions exhibits a discontinuity in the precipitation rate and vertical velocity. Furthermore, this interface, referred to as a precipitation front, moves at a velocity distinct from the propagation speed of dry and moist disturbances. The theory predicts the existence of three types of precipitation fronts: drying fronts, slow moistening fronts and fast moistening fronts. In previous studies, numerical simulations have demonstrated the existence of the three types of front, and have also confirmed that the precipitation front theory offers a good approximation for the behaviour of the interface between dry and moist regions for finite value of the convective adjustment time.A new framework is proposed here in which the encounter of an atmospheric disturbance and the edge of a precipitation region is recast as a Riemann problem. It is shown that, for any precipitation fronts, there are three incident characteristics but only two outgoing characteristics. This makes it possible to solve simultaneously for the intensity of the outgoing invariants and the propagation speed of the front. This also implies that atmospheric disturbances will be partially transmitted and partially reflected when encountering a precipitation front. For small perturbations, linear reflection and transmission coefficients can be computed analytically. These results also indicate that that disturbances can be amplified through over‐transmission or over‐reflection. These theoretical results are confirmed in numerical simulations of an idealized Walker circulation. A solution that includes a stationary precipitation front is perturbed by adding a small‐amplitude gravity wave, convectively coupled gravity wave, or moisture disturbance. All numerical simulations exhibit reflection and transmission of the incoming signal, and one simulation shows a case of over‐transmission of the incoming disturbance. Copyright © 2008 Royal Meteorological Society

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.

Diurnal cycle of precipitation in the NASA Seasonal to Interannual Prediction Project atmospheric general circulation model

The global statistics of the diurnal cycle of warm‐season precipitation simulated by NASA's Seasonal to Interannual Prediction Project (NASA/NSIPP) atmospheric general circulation model (AGCM) were evaluated using the Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI) rain retrievals. The model has notable biases in the phase of the diurnal cycle over land, where it produces rainfall maxima in the early afternoon, several hours earlier than the observed evening maxima. The model also produces too little precipitation in the nighttime over land. Similar biases in the phase were found over ocean, although the statistics are less robust. An analysis of the convective and stratiform contributions to the precipitation indicates that the incorrect representation of the diurnal cycle is primarily tied to deficiencies in the deep convection scheme. A set of sensitivity experiments shows that the phase of the maximum diurnal precipitation is quite sensitive to the change of the convection starting (parcel origination) level and the increase of the convection adjustment (relaxation) timescale in the Relaxed Arakawa‐Schubert scheme. Both modifications act to delay the timing for the maximum development of CAPE, which lead to improvements in the diurnal cycle of precipitation, especially in correcting the phase errors over land, although the modifications tend to reduce the diurnal amplitudes substantially. The study suggests that improvements to the diurnal cycle in current models require improvements to the parameterized deep convection schemes, including the coupling with the boundary layer, the characteristic timescale of convection adjustment, and the triggering process for nocturnal precipitation.

Using Betts-Miller-Janjic convective parameterization scheme in H14-31 model to forecast heavy rainfall in Vietnam

According to Krishnamurti, improvements of physical parameterizations will mainly affect simulations for the tropics [10]. The study of William A. Gallus Jr. showed that the higher the model resolution and more detailed convective parameterizations, the better the skill in quantitative precipitation forecast (QPF) in general [16]. The quality of precipitation forecast is so sensitive to convective parameterization scheme (CPS) used in the model as well as model resolution. The fact shows that for high resolution regional model like H14-31 CPS based on low-level moisture convergence as Tiedtke did not give good heavy rainfall forecast in Vietnam. In this paper we used the scheme of Betts-Miller-Janjic (BMJ) based on the convective adjustment toward tropical observationally structures in reality instead of Tiedtke in Hl4-31. Statistical verification results and verification using CRA method of Hl4-31 of two CPSs for seperated cases and for three rain seasons (2003-2005) shows that heavy rainfall forecast of Hl4-31/BMJ is better than one of H14-31/TK for Vietnam-South China Sea. CRA verification also shows that it is possible to say that heavy rainfall forecast skill of l-I14-31/BMJ in tropics is nearly similar to the skill of LAPS of Australia.

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.

Statistical Parameterization of Heterogeneous Oceanic Convection

Abstract A statistical convective adjustment scheme is proposed that attempts to account for the effects of mesoscale and submesoscale variability of temperature and salinity typically observed in the oceanic convective regions. Temperature and salinity in each model grid box are defined in terms of their mean, variance, and mutual correlations. Subgrid-scale instabilities lead to partial mixing between different layers in the water column. This allows for a smooth transition between the only two states (convection on and convection off) allowed in standard convective adjustment schemes. The advantage of the statistical parameterization is that possible instabilities associated with the sharp transition between the two states, which are known to occasionally affect the large-scale model solution, are eliminated. The procedure also predicts the generation of correlations between temperature and salinity and the presence of convectively induced upgradient fluxes that have been obtained in numerical simulations of heterogeneous convection and that cannot be represented by standard convective adjustment schemes.