Convection Scheme Research Articles

Effects of convective schemes and geometric reconstruction scheme on interface of multiple melt flows

Multi-phase polymer melt processing techniques, e.g., multi-layer co-extrusion and multi-melt multi-injection molding (M3IM), are typical methods for preparing multilayer composites. Numerical simulations are now broadly acknowledged to be an advanced and effective method to investigate the evolution of the interface between the melts during these processing techniques. However, how to describe the sharp interface between the melts precisely is still a big challenge. Herein, five different interpolation schemes were used to investigate the melt penetration in M3IM, paying close attention to the effects of different schemes on interface diffusion, wrinkle, and shape. In addition, the morphologies of the interface were characterized experimentally. By comparing the numerical results and experimental results, it can be concluded that the compressive interface capturing scheme for arbitrary meshes (CICSAM) is the most accurate method. Our work is instructive for expounding the variation patterns of interfacial morphologies during the multi-phase polymer melt processing.

Model development in practice: a comprehensive update to the boundary layer schemes in HARMONIE-AROME cycle 40

Abstract. The parameterised description of subgrid-scale processes in the clear and cloudy boundary layer has a strong impact on the performance skill in any numerical weather prediction (NWP) or climate model and is still a prime source of uncertainty. Yet, improvement of this parameterised description is hard because operational models are highly optimised and contain numerous compensating errors. Therefore, improvement of a single parameterised aspect of the boundary layer often results in an overall deterioration of the model as a whole. In this paper, we will describe a comprehensive integral revision of three parameterisation schemes in the High Resolution Local Area Modelling – Aire Limitée Adaptation dynamique Développement InterNational (HIRLAM-ALADIN) Research on Mesoscale Operational NWP In Europe – Applications of Research to Operations at Mesoscale (HARMONIE-AROME) model that together parameterise the boundary layer processes: the cloud scheme, the turbulence scheme, and the shallow cumulus convection scheme. One of the major motivations for this revision is the poor representation of low clouds in the current model cycle. The newly revised parametric descriptions provide an improved prediction not only of low clouds but also of precipitation. Both improvements can be related to a stronger accumulation of moisture under the atmospheric inversion. The three improved parameterisation schemes are included in a recent update of the HARMONIE-AROME configuration, but its description and the insights in the underlying physical processes are of more general interest as the schemes are based on commonly applied frameworks. Moreover, this work offers an interesting look behind the scenes of how parameterisation development requires an integral approach and a delicate balance between physical realism and pragmatism.

The Flexible Modelling Framework for the Met Office Unified Model (Flex-UM, using UM 12.0 release)

Abstract. The Met Office Unified Model (UM) is a world-leading atmospheric weather and climate model. In addition to comprehensive simulations of the atmosphere, the UM is capable of running idealised simulations, such as the dry physics Held–Suarez test case, radiative convective equilibrium and simulating planetary atmospheres other than Earth. However, there is a disconnect between the simplicity of the idealised UM model configurations and the full complexity of the UM. This gap inhibits the broad use of climate model hierarchy approaches within the UM. To fill this gap, we have developed the Flexible Modelling Framework for the UM – Flex-UM – which broadens the climate model hierarchy capabilities within the UM. Flex-UM was designed to replicate the atmospheric physics of the Geophysical Fluid Dynamics Laboratory (GFDL) idealised moist physics aqua-planet model. New parameterisations have been implemented in Flex-UM, including simplified schemes for convection, large-scale precipitation, radiation, boundary layer and sea surface temperature (SST) boundary conditions. These idealised parameterisations have been implemented in a modular way, so that each scheme is available for use in any model configuration. This has the advantage that we can incrementally add or remove complexity within the model hierarchy. We compare Flex-UM to ERA5 and aqua-planet simulations using the Isca climate modelling framework (based on the GFDL moist physics aqua-planet model) and comprehensive simulations of the UM (using the GA7.0 configuration). We also use two SST boundary conditions to compare the models (fixed SST and a slab ocean). We find the Flex-UM climatologies are similar to both Isca and GA7.0 (though Flex-UM is generally a little cooler, with higher relative humidity and a less pronounced storm track). Flex-UM has a single Intertropical Convergence Zone (ITCZ) in the slab-ocean simulation but a double-ITCZ in the fixed-SST simulation. Further work is needed to ensure that the atmospheric energy budget closes to within 1–2 W m−2, as the current configuration of Flex-UM gains 9–11 W m−2 (the range covers the two SST boundary conditions). Flex-UM greatly extends the modelling hierarchy capabilities of the UM and offers a simplified framework for developing, testing and evaluating parameterisations within the UM.

An improved regional coupled modeling system for Arctic sea ice simulation and prediction: a case study for 2018

Abstract. The improved and updated Coupled Arctic Prediction System (CAPS) is evaluated using a set of Pan-Arctic prediction experiments for the year 2018. CAPS is built on the Weather Research and Forecasting model (WRF), the Regional Ocean Modeling System (ROMS), the Community Ice CodE (CICE), and a data assimilation based on the local error subspace transform Kalman filter. We analyze physical processes linking improved and changed physical parameterizations in WRF, ROMS, and CICE to changes in the simulated Arctic sea ice state. Our results show that the improved convection and boundary layer schemes in WRF result in an improved simulation of downward radiative fluxes and near-surface air temperature, which influences the predicted ice thickness. The changed tracer advection and vertical mixing schemes in ROMS reduce the bias in sea surface temperature and change ocean temperature and salinity structure in the surface layer, leading to improved evolution of the predicted ice extent (particularly correcting the late ice recovery issue in the previous CAPS). The improved sea ice thermodynamics in CICE have noticeable influences on the predicted ice thickness. The updated CAPS can better predict the evolution of Arctic sea ice during the melting season compared with its predecessor, though the prediction still has some biases at the regional scale. We further show that the updated CAPS can remain skillful beyond the melting season, which may have a potential value for stakeholders to make decisions for socioeconomic activities in the Arctic.

Resolution dependence of tropical cyclones simulated by a spectral cumulus parameterization

The resolution dependence of tropical cyclones (TCs) simulated by a spectral cumulus parameterization (spectral scheme) was investigated using an atmospheric general circulation model with three ensemble members and atmosphere-only experiments. The focus of the analysis was the cause of resolution dependence of TCs which has not been clarified in previous studies. The results showed that underestimation of the TC track density and TC intensity was improved by an increase in the horizontal resolution as reported in previous studies. A mean state analysis revealed that the mean states were qualitatively similar regardless of model resolution although TC statistics were different; this implies that local differences in the atmospheric condition may have a strong impact on the TC statistics. Analyses of the horizontal structure indicated that the local TC structure better approximated observations as the resolution increased, but this was not due to enhanced atmospheric instability because the TC center location coincided with the highly unstable region only in the early stage of TC development. Influential differences for TC statistics were found in the vertical TC structure. A temperature anomaly with a large radial gradient induced by diabatic heating within the eyewall of a TC first impacted the vertical–radial circulation and then caused strong boundary layer inflow toward the eyewall. If a low resolution was adopted, the radial gradient of the temperature anomaly with strong low-level inflow was not well simulated, resulting in a weaker TC intensity and lower TC genesis frequency. Some TC property changes associated with resolution in the current spectral scheme were found to be similar to those of existing convection scheme, but there were some improvements that were specific to this spectral scheme. The cause of the improvements introduced by the present spectral scheme is briefly discussed.

Numerical simulation of merging of two rising bubbles with different densities and diameters using an enhanced Volume-Of-Fluid (VOF) model

In this study, the transient evolution of two rising bubbles with different densities is investigated numerically using an enhanced version of the VOF model, aiming to establish an state-of-the-art benchmark solutions and up-to-date data set for CFD validations. The simulations are performed on the staggered grid system where a novel third‐order accurate monotone convection scheme is applied for the discretization of the convection terms in Navier-Stokes and volume fraction equations while the semi-iterative PISOC algorithm (a combined version of the classical PISO and Chorin's model) are used to solve the pressure-velocity coupling. To reduce the false diffusion errors and mitigate smearing of interface thickness in the regions of physical discontinuities, the interface compression technique is also incorporated into the transport equation. To further enhance the accuracy of the numerical solutions, the idea of Piecewise Linear Interface Calculation (PLIC) based on the ELVIRA technique (Efficient Least-square Volume-of-fluid Interface Reconstruction Algorithm) is also utilized for the interface reconstruction and accurate implementation of surface tension force. The validity and accuracy of the enhanced VOF model is further demonstrated against a series of challenging benchmark cases including draining of liquids from the storage tank (tank draining), single rising bubble, three-phase Rayleigh-Taylor Instability and dam-break flows over dry and wet beds. The comparison of the obtained results with previously published data vividly demonstrates the superiority of the proposed method over the standard VOF/Level-Set models in handling multiphase/multi-fluid flow problems with large topological changes. In the last stage, the morphology and hydrodynamic characteristics of merging of two rising bubbles with different densities and diameters are examined and analyzed in details. The results show that, the initial/final deformations and the subsequent steady-state rise of two bubbles are remarkably influenced by the diameters of leading (upper) and trailing (lower) bubbles.

Idealized Simulations of the Tropical Climate and Variability in the Single Column Atmosphere Model (SCAM): Radiative‐Convective Equilibrium

AbstractTo explore the interactions among column processes in the Community Atmosphere Model (CAM), the single‐column version of CAM (SCAM) is integrated for 1000 days in radiative‐convective equilibrium (RCE) with tropical values of boundary conditions, spanning a parameter or configuration space of model physics versions (v5 vs. v6), vertical resolution (standard and 60 levels), sea surface temperature (SST), and some interpretation‐driven experiments. The simulated time‐mean climate is reasonable, near observations and RCE of a cyclic cloud‐resolving model. Updraft detrainment in the deep convection scheme produces distinctive grid‐scale structures in humidity and cloud, which also interact with radiative transfer processes. These grid artifacts average out in multi‐column RCE results reported elsewhere, illustrating the nuts‐and‐bolts interpretability that SCAM adds to the hierarchy of model configurations. Multi‐day oscillations of precipitation arise from descent of warm convection‐capping layers starting near the tropopause, eventually reset by a burst of convective deepening. Experiments reveal how these oscillations depend critically on an internal parameter that controls the number of neutral buoyancy levels allowed for determining cloud top and computing dilute convective available potential energy in the deep convection scheme, and merely modified a little by disabling cloud‐base radiation (heating of cloud base). This strong dependence of transient behavior in 1D on this parameter will be tested in the second part of this work, in which SCAM is coupled to a parameterized dynamics of two‐dimensional, linearized gravity wave, and in the 3D simulations in future study.

Evaluating and Improving Scale‐Awareness of a Convective Parameterization Closure Using Cloud‐Resolving Model Simulations of Convection

AbstractThis study aims to evaluate a revised closure of the Zhang‐McFarlane (ZM) scheme for deep convection and to further improve its scale awareness. Output from two cloud‐resolving model (CRM) simulations of both midlatitude organized (MC3E) and tropical unorganized (TWP‐ICE) convection are used for the evaluation. By averaging the CRM output over different subdomain sizes from 64 to 4 km, convection‐associated fields are obtained to represent the fields at different horizontal resolutions of global climate models (GCMs). The convection parameterization closure is then tested using these fields. Results show that the cloud base mass flux Mb determined from the closure has a relatively good relationship with CRM‐simulated convection at coarse resolutions for both MC3E and TWP‐ICE, but their correlation degrades at higher resolutions, especially when approaching the gray zone scale under 10 km. The fluctuation of CAPE consumption rate per unit cloud base mass flux for different convective events is found to be an important factor affecting the correlation. When averaged over the base domain, the cloud base mass flux determined by the closure is found to increase with decreasing subdomain size, implying that the closure is not scale‐aware. To improve its scale‐awareness, a modified closure is designed to alleviate the overprediction of convection at high GCM resolutions. Results show that the modified closure has the scale‐aware ability to suppress the overprediction problem at high GCM resolutions for both MC3E and TWP‐ICE.

Implementation and Evaluation of a Double-Plume Convective Parameterization in NCAR CAM5

AbstractPerformance of global climate models (GCMs) is strongly affected by the cumulus parameterization (CP) used. Similar to the approach in GFDL AM4, a double-plume CP, which unifies the deep and shallow convection in one framework, is implemented and tested in the NCAR Community Atmospheric Model version 5 (CAM5). Based on the University of Washington (UW) shallow convection scheme, an additional plume was added to represent the deep convection. The shallow and deep plumes share the same cloud model, but use different triggers, fractional mixing rates, and closures. The scheme was tested in single-column, short-term hindcast, and AMIP simulations. Compared with the default combination of the Zhang–McFarlane scheme and UW scheme in CAM5, the new scheme tends to produce a top-heavy mass flux profile during the active monsoon period in the single-column simulations. The scheme increases the intensity of tropical precipitation, closer to TRMM observations. The new scheme increased subtropical marine boundary layer clouds and high clouds over the deep tropics, both in better agreement with observations. Sensitivity tests indicate that regime-dependent fractional entrainment rates of the deep plume are desired to improve tropical precipitation distribution and upper troposphere temperature. This study suggests that a double-plume approach is a promising way to combine shallow and deep convections in a unified framework.

WRF Sensitivity for Seasonal Climate Simulations of Precipitation Fields on the CORDEX South America Domain

Dynamic numerical models of the atmosphere are the main tools used for weather and climate forecasting as well as climate projections. Thus, this work evaluated the systematic errors and areas with large uncertainties in precipitation over the South American continent (SAC) based on regional climate simulations with the weather research and forecasting (WRF) model. Ten simulations using different convective, radiation, and microphysical schemes, and an ensemble mean among them, were performed with a resolution of 50 km, covering the CORDEX-South America domain. First, the seasonal precipitation variability and its differences were discussed. Then, its annual cycle was investigated through nine sub-domains on the SAC (AMZN, AMZS, NEBN, NEBS, SE, SURU, CHAC, PEQU, and TOTL). The Taylor Diagrams were used to assess the sensitivity of the model to different parameterizations and its ability to reproduce the simulated precipitation patterns. The results showed that the WRF simulations were better than the ERA-interim (ERAI) reanalysis when compared to the TRMM, showing the added value of dynamic downscaling. For all sub-domains the best result was obtained with the ensemble compared to the satellite TRMM. The largest errors were observed in the SURU and CHAC regions, and with the greatest dispersion of members during the rainy season. On the other hand, the best results were found in the AMZS, NEBS, and TOTL regions.

Identification of airborne pollutant sources within a slot ventilated porous enclosure depending on backward model and downwind scheme

Identification of pollutant source, depending on the inverse computational fluid dynamics (CFD) methodology, within a slot-ventilated porous enclosure has been proposed in this paper. The whole fluid flow system is assumed to be two-dimensional, laminar and porous-saturated. As the inverse CFD modeling was ill-posed, a downwind scheme is employed to discretize the convection term to improve the accuracy and the corresponding time step range is derived to achieve numerical stability. Afterward, the downwind scheme simplifies the discretization process and its direct inversion algorithm. The effects of the Reynolds number (), the Darcy number (), the Schmidt number () and locations of pollutant source on the backward time CFD simulations are investigated, respectively. Results show that the temporal size/step is a critical factor of computational stability. In addition, the thickness of velocity or concentration boundary layer directly impacts the sensitivity of the inverse CFD simulation. The present research could aid in the safety of gas-supplying pipe network and relevant pollutant leakage accidents. Highlights Identification of prior-unknown sources was proposed regarding gas-supplying network. Original transport equations were maintained to enhance the accuracy and generality. The downwind convection scheme was implemented to enhance backward CFD procedures. Diverse influencing factors were numerically tested to recover pollutant sources. The proposed new algorithm was validated thoroughly by experiments and benchmark solutions.

Discrete maximum principle of a high order finite difference scheme for a generalized Allen–Cahn equation

We consider solving a generalized Allen-Cahn equation coupled with a passive convection for a given incompressible velocity field. The numerical scheme consists of the first order accurate stabilized implicit explicit time discretization and a fourth order accurate finite difference scheme, which is obtained from the finite difference formulation of the $Q^2$ spectral element method. We prove that the discrete maximum principle holds under suitable mesh size and time step constraints. The same result also applies to construct a bound-preserving scheme for any passive convection with an incompressible velocity field.

A Parameterization of Slantwise Convection in the WRF Model

Abstract In this study, we introduce a parameterization scheme for slantwise convection (SC) to be considered for models that are too coarse to resolve slantwise convection explicitly (with a horizontal grid spacing coarser than 15 km or less). This SC scheme operates in a locally defined 2D cross section perpendicular to the deep-layer-averaged thermal wind. It applies momentum tendency to adjust the environment toward slantwise neutrality with a prescribed adjustment time scale. Condensational heating and the associated moisture loss are also considered. To evaluate the added value of the SC scheme, we implement it in the Weather Research and Forecasting (WRF) Model to supplement the existing cumulus parameterization schemes for upright convection and test for two different numerical setups: a 2D idealized, unforced release of conditional symmetric instability (CSI) in an initially conditionally stable environment, and a 3D real-data precipitation event containing both CSI and conditional instability along the cold front of a cyclonic storm near the United Kingdom. Both test cases show significant improvements for the coarse-gridded (40-km) simulations when parameterizing slantwise convection. Compared to the 40-km simulations with only the upright convection scheme, the counterparts with the additional SC scheme exhibit a larger extent of CSI neutralization, generate a stronger grid-resolved slantwise circulation, and produce greater amounts of precipitation, all in better agreement with the corresponding fine-gridded reference simulations. Given the importance of slantwise convection in midlatitude weather systems, our results suggest that there exist potential benefits of parameterizing slantwise convection in general circulation models.

Simulation of ambient parameters using Grell convective scheme to study rainfall heterogeneity over western India

Regional climate variability is considered to be the major factor causing the advancement in global warming. Its effect influences precipitation patterns, which are more sensitive for monsoon dominated region like India. High-resolution (0.5°x0.5°) climate models play a significant role to understand variation in rainfall amount through simulations of various types of ambient parameters such as an atmospheric parameter-relative humidity (RH) (at 850 hPa), a cloud macrophysical parameter-cloud fraction (CF) (total column), a radiance parameter-outgoing longwave radiation (OLR) (total column) and a precipitation parameter-precipitation rate (PR) (total column). These parameters are studied over two different regions-an inland region which has more heterogeneity (central India-M.P.) as well as over coastal region which has less heterogeneity (western India-Gujarat) to understand spatial heterogeneity. The present study show daily averaged data of monsoon (June-September) from 2003 to 2017 for selected parameters over these regions with the help of linear correlation method with their uncertainties, which uses satellite (AIRS and TRMM) and model (RegCM 4.4) data. In this study, simulations are carried out by implementing Grell convective scheme during different rainfall scenarios. Results show that the correlation values are ∼ 0.35, ∼ 0.68, ∼ 0.58 and ∼ 0.18 for RH, CF, OLR and PR respectively, which signify good correlation coefficients during deficit years over coastal region. Also, these correlation values are almost same but their uncertainties are minimum for deficit as compared to normal and excess rainfall years. It shows that deficit rainfall scenarios are captured well over coastal regions as compared to excess rainfall scenarios, which is may be due to the rainfall departure as deficit rainfall years are less extreme (∼−22.34%) than excess rainfall years (∼42.12%). Further study continues to understand regional heterogeneity with respect to rainfall departure during excess and normal rainfall years. Further statistics can be improved with the simulation of more number of atmospheric/cloud parameters. Studies may be useful to determine heterogeneity and can be utilized in climate models, which further can help to improve weather predictability.

Evolution of Tropical Cyclone Properties Across the Development Cycle of the GISS-E3 Global Climate Model.

The next‐generation global climate model from the NASA Goddard Institute for Space Studies, GISS‐E3, contains many improvements to resolution and physics that allow for improved representation of tropical cyclones (TCs) in the model. This study examines the properties of TCs in two different versions of E3 at different points in its development cycle, run for 20 years at 0.5° resolution, and compares these TCs with observations, the previous generation GISS model, E2, and other climate models. E3 shares many TC biases common to global climate models, such as having too few tropical cyclones, but is much improved from E2. E3 produces strong enough TCs that observation‐based wind speed thresholds can now be used to detect and track them, and some storms now reach hurricane intensity; neither of these was true of E2. Model development between the first and second versions of E3 further increased the number and intensity of TCs and reduced TC count biases globally and in most regions. One‐year sensitivity tests to changes in various microphysical and dynamical tuning parameters are also examined. Increasing the entrainment rate for the more strongly entraining plume in the convection scheme increases the number of TCs (though also affecting other climate variables, and in some cases increasing biases). Variations in divergence damping did not have a strong effect on simulated TC properties, contrary to expectations based on previous studies. Overall, the improvements in E3 make it more credible for studies of TC activity and its relationship to climate.

Two‐fluid single‐column modelling of Rayleigh–Bénard convection as a step towards multi‐fluid modelling of atmospheric convection

AbstractMulti‐fluid models have recently been proposed as an approach to improving the representation of convection in weather and climate models. This is an attractive framework as it is fundamentally dynamical, removing some of the assumptions of mass‐flux convection schemes which are invalid at current model resolutions. However, it is still not understood how best to close the multi‐fluid equations for atmospheric convection. In this paper we develop a simple two‐fluid, single‐column model with one rising and one falling fluid. No further modelling of sub‐filter variability is included. We then apply this model to Rayleigh–Bénard convection, showing that, with minimal closures, the correct scaling of the heat flux () is predicted over six orders of magnitude of buoyancy forcing (). This suggests that even a very simple two‐fluid model can accurately capture the dominant coherent overturning structures of convection.

Lightning forecasting in Bangladesh based on the lightning potential index and the electric potential

This work documents the performances of diagnostic and explicit lightning forecasts for selected high impact weather events over Bangladesh. The lightning flashes are calculated by three methods: (1) a diagnostic lightning parameterization that bases its algorithm on the level of neutral buoyancy from a convective parameterization scheme, (2) lightning potential index (LPI) using local graupel and ice contents, and (3) explicit prediction of electric fields and lightning initiation from WRF-ELEC. Five lightning events that occurred on 02 April 2019, 17 May 2019, 20 April 2020, 26 May 2020, and 20 May 2021 were examined to evaluate the forecasting capability of each scheme for the lightning over a 24-h forecast period. Prior to diagnosing lightning activity, the composite radar reflectivity and the neighborhood score metrics (ETS, FSS) for hourly rainfall were analyzed to determine if the simulated precipitation structure is consistent with the observations. Analyses based on performance diagrams were also included for a better illustration of how simulations perform on predicting precipitation. The spatial distribution of LPI and lightning flashes is compared against the NASA Lightning Imaging Sensor (LIS) dataset. The qualitative results show that in most of the studied cases, there is a good agreement between the observations and the WRF-ELEC-based simulations in terms of detecting the primary regions of lightning activity. The LPI based predictions perform also considerably well. The results from this endeavor constitute an essential initiative towards the implementation of an effective operational lightning warning system to mitigate lightning-related hazards in Bangladesh and northeast India.

Long‐term single‐column model intercomparison of diurnal cycle of precipitation over midlatitude and tropical land

AbstractGeneral Circulation Models (GCMs) have for decades exhibited difficulties in modelling the diurnal cycle of precipitation (DCP). This issue can be related to inappropriate representation of the processes controlling sub‐diurnal phenomena like convection. In this study, 11 single‐column versions of GCMs are used to investigate the interactions between convection and environmental conditions, processes that control nocturnal convections, and the transition from shallow to deep convection on a diurnal time‐scale. Long‐term simulations are performed over two continental land sites: the Southern Great Plains (SGP) in the USA for 12 summer months from 2004 to 2015 and the Manacapuru site at the central Amazon (MAO) in Brazil for two full years from 2014 to 2015. The analysis is done on two regimes: afternoon convective regime and nocturnal precipitation regime. Most models produce afternoon precipitation too early, likely due to the missing transition of shallow‐to‐deep convection in these models. At SGP, the unified convection schemes better simulate the onset time of precipitation. At MAO, models produce the heating peak in a much lower level compared with observation, indicating too shallow convection in the models. For nocturnal precipitation, models that produce most of nocturnal precipitation all allow convection to be triggered above the boundary layer. This indicates the importance of model capability to detect elevated convection for simulating nocturnal precipitation. Sensitivity studies indicate that (a) nudging environmental variables towards observations has a minor impact on DCP, (b) unified treatment of shallow and deep convection and the capability to capture mid‐level convection can help models better capture DCP, and (c) the interactions of the atmosphere with other components in the climate system (e.g. land) are also important for DCP simulations in coupled models. These results provide long‐term statistical insights on which physical processes are essential in climate models to simulate DCP.

Simulation of Rainfall over Bangladesh Using Regional Climate Model (RegCM4.7)

Regional climate model is a scientific tool to monitor present climate change and to provide reliable estimation of future climate projection. In this study, the Regional Climate Model version 4.7 (RegCM4.7) developed by International Centre for Theoretical Physics (ICTP) has been adopted to simulate rainfall scenario of Bangladesh. The study examines model performance of rainfall simulation through the period of 1991-2018 with ERA-Interim75 data of 75 km horizontal resolution as lateral boundaries, downscaled at 25km resolution using the mixed convective precipitation scheme; MIT-Emanuel scheme over land and Grell scheme with Fritsch-Chappell closure over ocean. The simulated rainfall has been compared both at spatial and temporal scales (monthly, seasonal and annual) with observed data collected from Bangladesh Meteorological Department (BMD) and Climate Research Unit (CRU). Simulated annual rainfall showed that the model overestimated in most of the years. Overestimation has been observed in the monsoon and underestimation in pre-monsoon and post-monsoon seasons. Spatial distribution of simulated rainfall depicts overestimation in the southeast coastal region and underestimation in the northwest and northeast border regions of Bangladesh. Better estimation of rainfall has been found in the central and eastern parts of the country. The simulated annual rainfall has been validated through the Linear Scaling bias correction method for the years of 2016, 2017, and 2018 considering the rainfall of 1991-2015 as reference. The bias correction with linear scaling method gives fairly satisfactory results and it can be considered in the future projection of rainfall over Bangladesh.

Convection Modeling of Pure-steam Atmospheres

Abstract Condensable species are crucial to shaping planetary climate. A wide range of planetary climate systems involve understanding nondilute condensable substances and their influence on climate dynamics. There has been progress on large-scale dynamical effects and on 1D convection parameterization, but resolved 3D moist convection remains unexplored in nondilute conditions, though it can have a profound impact on temperature/humidity profiles and cloud structure. In this work, we tackle this problem for pure-steam atmospheres using three-dimensional, high-resolution numerical simulations of convection in postrunaway atmospheres. We show that the atmosphere is composed of two characteristic regions, an upper condensing region dominated by gravity waves and a lower noncondensing region characterized by convective overturning cells. Velocities in the condensing region are much smaller than those in the lower, noncondensing region, and the horizontal temperature variation is small. Condensation in the thermal photosphere is largely driven by radiative cooling and tends to be statistically homogeneous. Some condensation also happens deeper, near the boundary of the condensing region, due to triggering by gravity waves and convective penetrations and exhibits random patchiness. This qualitative structure is insensitive to varying model parameters, but quantitative details may differ. Our results confirm theoretical expectations that atmospheres close to the pure-steam limit do not have organized deep convective plumes in the condensing region. The generalized convective parameterization scheme discussed in Ding & Pierrehumbert is appropriate for handling the basic structure of atmospheres near the pure-steam limit but cannot capture gravity waves and their mixing which appear in 3D convection-resolving models.