Two-dimensional Cloud-resolving Model Research Articles

Evaluating Precipitation Features and Rainfall Characteristics in a Multi-Scale Modeling Framework.

Cloud and precipitation systems are simulated with a multi‐scale modeling framework (MMF) and compared over the Tropics and Subtropics against the Tropical Rainfall Measuring Mission (TRMM) Radar‐defined Precipitation Features (RPFs) product. A methodology, in close analogy to the TRMM RPFs, is developed to produce simulated precipitation features (PFs) from the output of the embedded two‐dimensional (2D) cloud‐resolving models (CRMs) within an MMF. Despite the limitations of 2D CRMs, the simulated population distribution, horizontal and vertical structure of PFs, and the geographical location and local rainfall contribution of mesoscale convective systems (MCSs) are in good agreement with the TRMM observations. However, some model discrepancies are found and can be identified and quantified within the PF distributions. Using model biases in relative population and rainfall contributions, PFs can be characterized into four size categories: small, medium to large, very large, and extremely large. Four different major mechanisms might account for the model biases in each different category: (1) the two‐dimensionality of the CRMs, (2) a positive convection‐wind‐evaporation feedback loop, (3) an artificial dynamic constraint in a bounded CRM domain with cyclic boundaries, and (4) the limited CRM domain size. The second and fourth mechanisms tend to contribute to the excessive tropical precipitation biases commonly found in most MMFs, whereas the other mechanisms reduce rainfall contributions from small and very large PFs. MMF sensitivity experiments with various CRM domain sizes and grid spacings showed that larger domains (higher resolutions) tend to shift PF populations toward larger (smaller) sizes.

Suppression of Arctic Air Formation with Climate Warming: Investigation with a Two-Dimensional Cloud-Resolving Model

AbstractArctic climate change in winter is tightly linked to changes in the strength of surface temperature inversions, which occur frequently in the present climate as Arctic air masses form during polar night. Recent work proposed that, in a warmer climate, increasing low-cloud optical thickness of maritime air advected over high-latitude landmasses during polar night could suppress the formation of Arctic air masses, amplifying winter warming over continents and sea ice. But this mechanism was based on single-column simulations that could not assess the role of fractional cloud cover change. This paper presents two-dimensional cloud-resolving model simulations that support the single-column model results: low-cloud optical thickness and duration increase strongly with initial air temperature, slowing the surface cooling rate as the climate is warmed. The cloud-resolving model cools less at the surface than the single-column model, and the sensitivity of its cooling to warmer initial temperatures is also higher, because it produces cloudier atmospheres with stronger lower-tropospheric mixing and distributes cloud-top cooling over a deeper atmospheric layer with larger heat capacity. Resolving larger-scale cloud turbulence has the greatest impact on the microphysics schemes that best represent general observed features of mixed-phase clouds, increasing their sensitivity to climate warming. These findings support the hypothesis that increasing insulation of the high-latitude land surface by low clouds in a warmer world could act as a strong positive feedback in future climate change and suggest studying Arctic air formation in a three-dimensional climate model.

A Numerical Study of Convection in a Condensing CO2 Atmosphere under Early Mars-Like Conditions

Abstract Cloud convection of a CO2 atmosphere where the major constituent condenses is numerically investigated under a setup idealizing a possible warm atmosphere of early Mars, utilizing a two-dimensional cloud-resolving model forced by a fixed cooling profile as a substitute for a radiative process. The authors compare two cases with different critical saturation ratios as condensation criteria and also examine sensitivity to number mixing ratio of condensed particles given externally. When supersaturation is not necessary for condensation, the entire horizontal domain above the condensation level is continuously covered by clouds irrespective of number mixing ratio of condensed particles. Horizontal-mean cloud mass density decreases exponentially with height. The circulations below and above the condensation level are dominated by dry cellular convection and buoyancy waves, respectively. When 1.35 is adopted as the critical saturation ratio, clouds appear exclusively as intense, short-lived, quasi-periodic events. Clouds start just above the condensation level and develop upward, but intense updrafts exist only around the cloud top; they do not extend to the bottom of the condensation layer. The cloud layer is rapidly warmed by latent heat during the cloud events, and then the layer is slowly cooled by the specified thermal forcing, and supersaturation gradually develops leading to the next cloud event. The periodic appearance of cloud events does not occur when number mixing ratio of condensed particles is large.

Precipitation responses to radiative processes of water- and ice-clouds: an equilibrium cloud-resolving modeling study

AbstractCloud radiative processes are important in regulating weather and climate. Precipitation responses to radiative processes of water- and ice-clouds are investigated by analyzing mean equilibrium simulation data from a series of two-dimensional cloud-resolving model sensitivity experiments in this study. The model is imposed by zero vertical velocity. The exclusion of water radiative processes in the presence of ice radiative processes, as well as the removal of ice radiative processes, enhances tropospheric longwave radiative cooling and lowers air temperature and the saturation mixing ratio. The reduction in the saturation mixing ratio leads to an increase in vapor condensation and an associated release of latent heat, which increases rainfall. The elimination of water radiative processes strengthens local atmospheric warming in the upper troposphere via a reduction in longwave radiative cooling. The enhanced warming increases the rain source via an increase in the melting of graupel, which increa...

The relationship between latent heating, vertical velocity, and precipitation processes: The impact of aerosols on precipitation in organized deep convective systems.

A high-resolution, two-dimensional cloud-resolving model with spectral-bin microphysics is used to study the impact of aerosols on precipitation processes in both a tropical oceanic and a midlatitude continental squall line with regard to three processes: latent heating (LH), cold pool dynamics and ice microphysics. Evaporative cooling in the lower troposphere is found to enhance rainfall in low cloud condensation nuclei (CCN) concentration scenarios in the developing stages of a midlatitude convective precipitation system. In contrast, the tropical case produced more rainfall under high CCN concentrations. Both cold pools and low-level convergence are stronger for those configurations having enhanced rainfall. Nevertheless, latent heat release is stronger (especially after initial precipitation) in the scenarios having more rainfall in both the tropical and midlatitude environment. Sensitivity tests are performed to examine the impact of ice and evaporative cooling on the relationship between aerosols, LH and precipitation processes. The results show that evaporative cooling is important for cold pool strength and rain enhancement in both cases. However, ice microphysics play a larger role in the midlatitude case compared to the tropical. Detailed analysis of the vertical velocity governing equation shows that temperature buoyancy can enhance updrafts/downdrafts in the middle/lower troposphere in the convective core region; however, the vertical pressure gradient force (PGF) is the same order and acts in the opposite direction. Water loading is small but on the same order as the net PGF-temperature buoyancy forcing. The balance among these terms determines the intensity of convection.

Barotropic and baroclinic processes associated with convective development in the tropical deep convective regime

The budget of perturbation kinetic energy is calculated to study barotropic and baroclinic processes associated with convective development in the tropical deep convective regime. The barotropic kinetic-energy conversion is further examined through the derivation and analysis of budget of vertical transport of zonal momentum. The two-dimensional cloud-resolving model simulation data during TOGA COARE are analyzed in this budget study. The analysis of the energy budget shows that while baroclinic conversion from perturbation available potential energy is a major source for the growth of perturbation kinetic energy, barotropic conversion from the mean kinetic energy has significant modification on the variation of perturbation kinetic energy. The general evolution of barotropic conversion is associated with that of the vertical wind shear of the mean circulations, but the significant change in barotropic conversion corresponds to the change in the vertical transport of zonal momentum. Zonal transport of cloud hydrometeor is responsible for the variation of vertical transport of zonal momentum.

Effects of doubled carbon dioxide on rainfall responses to radiative processes of water clouds

The effects of doubled carbon dioxide on rainfall responses to radiative processes of water clouds are investigated in this study. Two groups of two-dimensional cloud-resolving model sensitivity experiments with regard to pre-summer heavy rainfall around the summer solstice and tropical rainfall around the winter solstice are conducted and their five-day averages over the model domain are analyzed. In the presence of radiative effects of ice clouds, doubled carbon dioxide changes pre-summer rainfall from the decrease associated with the enhanced atmospheric cooling to the increase associated with the enhanced infrared cooling as a result of the exclusion of radiative effects of water clouds. Doubled carbon dioxide leads to the reduction in tropical rainfall, caused by the removal of radiative effects of water clouds through the suppressed infrared cooling. In the absence of radiative effects of ice clouds, doubled carbon dioxide changes pre-summer rainfall from the increase associated with the strengthened atmospheric warming to the decrease associated with the weakened release of latent heat caused by the elimination of radiative effects of water clouds. The exclusion of radiative effects of water clouds increases tropical rainfall through the strengthened infrared cooling, which is insensitive to the change in carbon dioxide.

Simulations of the West African Monsoon with a Superparameterized Climate Model. Part II: African Easterly Waves

Abstract The relationship between African easterly waves and convection is examined in two coupled general circulation models: the Community Climate System Model (CCSM) and the “superparameterized” CCSM (SP-CCSM). In the CCSM, the easterly waves are much weaker than observed. In the SP-CCSM, a two-dimensional cloud-resolving model replaces the conventional cloud parameterizations of CCSM. Results show that this allows for the simulation of easterly waves with realistic horizontal and vertical structures, although the model exaggerates the intensity of easterly wave activity over West Africa. The simulated waves of SP-CCSM are generated in East Africa and propagate westward at similar (although slightly slower) phase speeds to observations. The vertical structure of the waves resembles the first baroclinic mode. The coupling of the waves with convection is realistic. Evidence is provided herein that the diabatic heating associated with deep convection provides energy to the waves simulated in SP-CCSM. In contrast, horizontal and vertical structures of the weak waves in CCSM are unrealistic, and the simulated convection is decoupled from the circulation.

Precipitation efficiency and its relationship to physical factors

The precipitation efficiency and its relationship to physical factors are examined by analyzing a two-dimensional cloud-resolving model simulation during TOGA COARE in this study. The basic physical factors include convective available potential energy, water—vapor convergence, vertical wind shear, cloud ratio, sea surface temperature, air temperature, and precipitable water. Precipitation efficiencies do not show a close relationship to air temperature nor to sea surface temperature nor to precipitable water. The precipitation efficiency increases as the water—vapor convergence rate increases and vertical wind shear weakens, whereas it decreases as the convective available potential energy dissipates and anvil clouds develop.

Effects of doubled carbon dioxide on rainfall responses to large-scale forcing: A two-dimensional cloud-resolving modeling study

Rainfall responses to doubled atmospheric carbon dioxide concentration were investigated through the analysis of two pairs of two-dimensional cloud-resolving model sensitivity experiments. One pair of experiments simulated pre-summer heavy rainfall over southern China around the summer solstice, whereas the other pair of experiments simulated tropical rainfall around the winter solstice. The analysis of the time and model domain mean heat budget revealed that the enhanced local atmospheric warming was associated with doubled carbon dioxide through the weakened infrared radiative cooling during the summer solstice. The weakened mean pre-summer rainfall corresponded to the weakened mean infrared radiative cooling. Doubled carbon dioxide increased the mean tropical atmospheric warming via the enhanced mean latent heat in correspondence with the strengthened mean infrared radiative cooling during the winter solstice. The enhanced mean tropical rainfall was associated with the increased mean latent heat.

Radiative Effects of Water Clouds on Heat, Cloud Microphysical and Surface Rainfall Budgets Associated with Pre-Summer Torrential Rainfall

This study investigates thermal, cloud microphysical and surface-rainfall responses to the radiative effects of water clouds by analyzing two pairs of two-dimensional cloud-resolving model sensitivity experiments of a pre-summer heavy rainfall event. In the presence of the radiative effects of ice clouds, exclusion of the radiative effects of water clouds reduces the model domain mean rain rate through the mean hydrometeor increase, which is associated with the decreases in the melting of graupel and cloud ice caused by enhanced local atmospheric cooling. In the absence of the radiative effects of ice clouds, removal of the radiative effects of water clouds increases model domain mean rain rate via the enhancements in the mean net condensation and the mean hydrometeor loss. The enhanced mean net condensation and increased mean latent heat are related to the strengthened mean infrared radiative cooling in the lower troposphere. The increased mean hydrometeor loss associated with the reduction in the melting of graupel is caused by the enhanced local atmospheric cooling.

Effects of water and ice clouds on cloud microphysical budget: An equilibrium modeling study

The effects of water and ice clouds on the cloud microphysical budget associated with rainfall are investigated through the analysis of grid-scale data from a series of two-dimensional cloud-resolving model equilibrium sensitivity simulations. The model is imposed without large-scale vertical velocity. In the control experiment, the contribution from rainfall (cM) associated with net evaporation and hydrometeor loss/convergence is about 29% of that from the rainfall (Cm) associated with net condensation and hydrometeor gain/divergence and about 39% of that from the rainfall (CM) associated with net condensation and hydrometeor loss/convergence. The exclusion of ice clouds enhances rainfall contribution of CM, whereas it reduces rainfall contributions of Cm and cM. The removal of radiative effects of water clouds increases rainfall contribution of CM, barely changes rainfall contribution of Cm and reduces the rainfall contribution of cM in the presence of the radiative effects of ice clouds. Elimination of the radiative effects of water clouds reduces the rainfall contributions of CM and Cm, whereas it increases the rainfall contribution of cM in the absence of the radiative effects of ice clouds.

A modified scheme that parameterizes depositional growth of ice crystal: A modeling study of pre-summer torrential rainfall case over Southern China

Depositional growth of cloud ice is estimated and its parameterization schemes are compared through the two-dimensional cloud-resolving modeling analysis of pre-summer heavy rainfall over southern China. Hsie et al. (1980) and Krueger et al. (1995) developed parameterization schemes to calculate depositional growth of cloud ice by estimating the growth timescale under the assumption that the ice crystal concentration is independent of crystal size. A new scheme is proposed by Zeng et al. (2008) under the assumption that the ice crystal concentration is proportional to the mass of ice crystal. Hsie's and Krueger's schemes produce small amount of cloud ice similar to what Zeng's scheme with low ice crystal concentration does. When ice crystal concentration is increased to a high value in Zeng's scheme, the simulation generates anomalous depositional growth of cloud ice and thus anomalous area expansion of stratiform rainfall. Zeng's scheme is modified by changing radius of base ice crystal from 0 to 40μm in the calculation of depositional growth of cloud ice. The modified scheme with high ice crystal concentration greatly reduces growth of cloud ice and thus fractional coverage of stratiform rainfall.

Torrential rainfall responses to radiation and ice clouds over Jiang-Huai Valley, China in July 2007

The effects of radiation and ice clouds on a torrential rainfall event of Jiang-Huai Valley over China are investigated through a series of two-dimensional sensitivity cloud-resolving model experiments. The model is integrated with an imposed large-scale vertical velocity and zonal wind constructed from the National Centers for Environmental Prediction (NCEP)/Global Data Assimilation System (GDAS) from 2 to 9 July 2007, while the control experiment is compared with two sensitivity experiments that exclude radiation and ice clouds, respectively. The exclusion of ice clouds decreases model domain mean surface rain rate through the weakened mean net condensation and the mean hydrometeor change from loss to gain during the life span of the rainfall event. The sensitivity of the mean rain rate to radiation occurs only in the later period of the rainfall event and is less than that to ice clouds. The reduction in the mean rain rate caused by the elimination of radiation is associated with the decreases in the mean net condensation and latent heat, which corresponds to the strengthened mean local atmospheric cooling.

Microphysical Processes Evident in Aerosol Forcing of Tropical Deep Convective Clouds

AbstractThis study investigates the effects of aerosols on tropical deep convective clouds (DCCs). A series of large-scale, two-dimensional cloud-resolving model simulations was completed, differing only in the concentration of aerosols available to act as cloud condensation nuclei (CCN). Polluted simulations contained more DCCs, wider storms, higher cloud tops, and more convective precipitation domainwide. Differences in warm cloud microphysics were largely consistent with the first and second aerosol indirect effects. The average surface precipitation produced in each DCC column decreased with increasing aerosol concentration. A detailed microphysical budget analysis showed that the reduction in collision and coalescence largely dominated the trend in average precipitation. The production of rain from ice, though it also decreased, became a more important contribution to precipitation as the aerosol concentration increased. The DCCs in polluted simulations contained more frequent extreme values of vertical velocity, but the average updraft speed decreased with increasing aerosols in DCCs above 6 km. An examination of the buoyancy term of the vertical velocity equation demonstrates that the drag associated with condensate loading is an important factor in determining the average updraft strength. The largest contributions to latent heating in DCCs were cloud nucleation and vapor deposition onto water and ice, but changes in latent heating were, on average, an order of magnitude smaller than those in the condensate loading term. The average updraft speed was largely affected by increased drag from condensate loading in more mature updrafts, while early storm updrafts experienced convective invigoration from increased latent heating.

Cloud-Resolving Model Simulations and a Simple Model of an Idealized Walker Cell

Abstract An idealized Walker cell with prescribed sea surface temperature (SST) and prescribed radiative cooling is studied using both a two-dimensional cloud-resolving model (CRM) and a simple conceptual model. In the CRM, for the same SST distribution, the width of the warm pool (area of strong precipitation) varies systematically with the magnitude of the radiative cooling, narrowing as radiative cooling is increased. The simple model is constructed to interpret these behaviors. Key aspects of the simple model include a surface wind determined from the boundary layer momentum budget, which in turn sets evaporation assuming a spatially uniform surface relative humidity, prescribed gross moist and dry stratification as a function of column water vapor and precipitation, and a gustiness enhancement on evaporation in areas of precipitation. It is found that the gustiness enhancement, likely due to mesoscale systems, creates a feedback that narrows the warm pool. This process has not been included in previous formulations of the simple model and its role is emphasized here.

Torrential rainfall responses to ice microphysical processes during pre-summer heavy rainfall over southern China

In this study, the effects of key ice microphysical processes on the pre-summer heavy rainfall over southern China during 3–8 June 2008 were investigated. A series of two-dimensional sensitivity cloud-resolving model simulations were forced with zonally uniform vertical velocity, zonal wind, horizontal temperature, and water vapor advection data from the National Centers for Environmental Prediction (NCEP)/Global Data Assimilation System (GDAS). The effects of key ice microphysical processes on the responses of rainfall to large-scale forcing were analyzed by comparing two sensitivity experiments with a control experiment. In one sensitivity experiment, ice crystal radius, associated with depositional growth of snow from cloud ice, was reduced from 100 µm in the control experiment to 50 µm, and in the other sensitivity experiment the efficiency of the growth of graupel from the accretion of snow was reduced to 50% from 100% in the control experiment. The results show that the domain-mean rainfall responses to these ice microphysical processes are stronger during the decay phase than during the onset and mature phases. During the decay phase, the increased mean rain rate resulting from the decrease in ice crystal radius is associated with the enhanced mean local atmospheric drying, the increased mean local hydrometeor loss, and the suppressed mean water vapor divergence. The increased mean rain rate caused by the reduction in accretion efficiency is related to the reduced mean water vapor divergence and the enhanced mean local hydrometeor loss.

The dependence of precipitation efficiency on rainfall type in a cloud-resolving model

[1] The rain microphysical precipitation efficiency (RMPE), the cloud microphysical precipitation efficiency (CMPE), and the large-scale precipitation efficiency (LSPE) are analyzed using 21 day two-dimensional cloud-resolving model simulation data for the period of 18 December 1992 to 9 January 1993 in the Tropical Ocean Global Atmosphere Coupled Ocean-Atmosphere Response Experiment (TOGA COARE) for eight rainfall types separated by a partitioning scheme based on the surface rainfall budget. RMPE estimated from grid-scale data is considered to be the reference precipitation efficiency. Mean versus gridded estimates of RMPE and CMPE show that they are not very sensitive to spatial scales for all rainfall types. The root-mean-square (RMS) differences between LSPE and RMPE are larger than the standard deviations of RMPE for five rainfall types, and the time-mean LSPE estimates are lower than those of RMPE for six rainfall types, indicating that LSPE is not a good estimate for RMPE. Although the RMS differences between CMPE and RMPE are smaller or marginally smaller than the standard deviations of RMPE for all rainfall types, CMPE estimates are lower than those of RMPE for four rainfall types.

A modeling study of surface rainfall processes associated with a torrential rainfall event over Hubei, China, during July 2007

The surface rainfall processes associated with the torrential rainfall event over Hubei, China, during July 2007 were investigated using a two-dimensional cloud-resolving model. The model integrated the large-scale vertical velocity and zonal wind data from National Centers for Environmental Prediction (NCEP)/Global Data Assimilation System (GDAS) for 5 days. The time and model domain mean surface rain rate was used to identify the onset, mature, and decay periods of rainfall. During the onset period, the descending motion data imposed in the lower troposphere led to a large contribution of stratiform rainfall to the model domain mean surface rainfall. The local atmospheric drying and transport of rain from convective regions mainly contributes to the stratiform rainfall. During the mature periods, the ascending motion data integrated into the model was so strong that water vapor convergence was the dominant process for both convective and stratiform rainfall. Both convective and stratiform rainfalls made important contributions to the model domain mean surface rainfall. During the decay period, descending motion data input into the model prevailed, making stratiform rainfall dominant. Stratiform rainfall was mainly caused by the water vapor convergence over raining stratiform regions.

A partitioning analysis of tropical rainfall based on cloud budget

Precipitation in the deep tropical convective regime is investigated through the partitioning analysis of grid-scale data from a two-dimensional cloud-resolving model simulation based on cloud budget. The model is forced by large-scale vertical velocity, zonal wind, horizontal advection, and sea surface temperature observed and derived from TOGA COARE. In cloud budget, rainfall is associated with cloud microphysical processes denoted by the net condensation and dynamical processes denoted by hydrometeor change/convergence. Thus, three rainfall types are separated in this scheme. Time-mean analysis shows that the rainfall type associated with hydrometeor loss/convergence only accounts for 27% of total rainfall, which corresponds to downward motions and downward water vapor and hydrometeor mass flux in the mid and lower troposphere. The two other rainfall types associated with the net condensation contribute equally to total rainfall (36%), which are generally related to upward motions and upward water vapor and hydrometeor mass flux throughout the troposphere.