Autoconversion Of Cloud Water Research Articles

Subgrid-scale horizontal and vertical variation of cloud water in stratocumulus clouds: a case study based on LES and comparisons with in situ observations

Abstract. Stratocumulus clouds in the marine boundary layer cover a large fraction of ocean surface and play an important role in the radiative energy balance of the Earth system. Simulating these clouds in Earth system models (ESMs) has proven to be extremely challenging, in part because cloud microphysical processes such as the autoconversion of cloud water into precipitation occur at scales much smaller than typical ESM grid sizes. An accurate autoconversion parameterization needs to account for not only the local microphysical process (e.g., the dependence on cloud water content qc and cloud droplet number concentration Nc) but also the subgrid-scale variability of the cloud properties that determine the process rate. Accounting for subgrid-scale variability is often achieved by the introduction of a so-called enhancement factor E. Previous studies of E for autoconversion have focused more on its dependence on cloud regime and ESM grid size, but they have largely overlooked the vertical dependence of E within the cloud. In this study, we use a large-eddy simulation (LES) model, initialized and constrained with in situ and surface-based measurements from a recent airborne field campaign, to characterize the vertical dependence of the horizontal variation of qc in stratocumulus clouds and the implications for E. Similar to our recent observational study (Zhang et al., 2021), we found that the inverse relative variance of qc, an index of horizontal homogeneity, generally increases from cloud base upward through the lower two-thirds of the cloud and then decreases in the uppermost one-third of the cloud. As a result, E decreases from cloud base upward and then increases towards the cloud top. We apply a decomposition analysis to the LES cloud water field to understand the relative roles of the mean and variances of qc in determining the vertical dependence of E. Our analysis reveals that the vertical dependence of the horizontal qc variability and enhancement factor E is a combined result of condensational growth throughout the lower portion of the cloud and entrainment mixing at cloud top. The findings of this study indicate that a vertically dependent E should be used in ESM autoconversion parameterizations.

Characteristics of the Indian Summer Monsoon Rainfall Simulated by the Regional Climate Model (RegCM4)

The regional climate model (version RegCM4) is used in this study to simulate the Indian summer monsoon rainfall. Model integrations were carried out for 16 monsoon seasons from 1985 to 2000. The model-simulated rainfall at a daily scale as well the seasonal mean were compared with observed rainfall. It was observed that the model underestimates the rainfall amount, especially over the eastern and central parts of India. Frequency distributions of daily rainfall showed that there were more days with light rainfall and fewer days with moderate rainfall in the model as compared to the observation over most parts of India. The ability of the RegCM4 to simulate interannual variability in rainfall over two sub-regions of India (one in western parts and the other in eastern India) was examined for the months of July and August. Composite analyses of rainfall, winds, moisture and evaporation for excess and deficient rainfall years for these two regions were carried out and compared with similar analysis using ERA-Interim data. When the eastern parts of India experienced more rainfall, the cyclonic circulation over Odisha was more intense, with stronger easterlies over the Gangetic plains, enhancing moisture transport to the Indian land mass from the adjoining seas. A cyclonic circulation difference between excess and deficient years over this region and an anticyclonic circulation difference over peninsular India led to the development of a strong convergence zone over parts of Maharashtra and Gujarat in the excess years. Several sensitivity experiments were also carried out using various schemes, domain sizes and diffusion parameters to study the role of these parameters in defining the model bias. The key parameters that emerged from these experiments as affecting the moisture convergence and rainfall bias over the Indian region were the horizontal diffusion and auto-conversion of cloud water into precipitation. However, these parameters were not able to improve the simulation of interannual variability over most parts of India.

Global well-posedness for the primitive equations coupled to nonlinear moisture dynamics with phase changes

In this work we study the global solvability of the primitive equations for the atmosphere coupled to moisture dynamics with phase changes for warm clouds, where water is present in the form of water vapor and in the liquid state as cloud water and rain water. This moisture model contains closures for the phase changes condensation and evaporation, as well as the processes of auto conversion of cloud water into rainwater and the collection of cloud water by the falling rain droplets. It has been used by Klein and Majda in [] and corresponds to a basic form of the bulk microphysics closure in the spirit of Kessler [] and Grabowski and Smolarkiewicz []. The moisture balances are strongly coupled to the thermodynamic equation via the latent heat associated to the phase changes. In [] we assumed the velocity field to be given and proved rigorously the global existence and uniqueness of uniformly bounded solutions of the moisture balances coupled to the thermodynamic equation. In this paper we present the solvability of a full moist atmospheric flow model, where the moisture model is coupled to the primitive equations of atmospherical dynamics governing the velocity field. For the derivation of a priori estimates for the velocity field we thereby use the ideas of Cao and Titi [], who succeeded in proving the global solvability of the primitive equations.

Sea-Salt Aerosol Effects on the Simulated Microphysics and Precipitation in a Tropical Cyclone

We investigate the effects of sea-salt aerosol (SSA) activated as cloud condensation nuclei on the microphysical processes, precipitation, and thermodynamics of a tropical cyclone (TC). The Weather Research and Forecasting model coupled with Chemistry (WRF-Chem) was used together with a parameterization of SSA production. Three simulations, with different levels of SSA emission (CTL, LOW, HIGH), were conducted. The simulation results show that SSA contributes to the processes of autoconversion of cloud water and accretion of cloud water by rain, thereby promoting rain formation. The latent heat release increases with SSA emission, slightly increasing horizontal wind speeds of the TC. The presence of SSA also regulates the thermodynamic structure and precipitation of the TC. In the HIGH simulation, higher latent heat release gives rise to stronger updrafts in the TC eyewall area, leading to enhanced precipitation. In the LOW simulation, due to decreased latent heat release, the temperature in the TC eye is lower, enhancing the downdrafts in the region; and because of conservation of mass, updrafts in the eyewall also strengthen slightly; as a result, precipitation in the LOW experiment is a little higher than that in the CTL experiment. Overall, the relationship between the precipitation rate and SSA emission is nonlinear.

Modeling stable orographic precipitation at small scales: The impact of the autoconversion scheme

This study presents idealized numerical simulations of moist airflow over a narrow isolated mountain in order to investigate the impact of the autoconversion scheme on simulated precipitation. The default setup generates an isolated water cloud over the mountain, implying that autoconversion of cloud water into rain is the only process capable of initiating precipitation. For comparison, a set of sensitivity experiments considers the classical seeder-feeder configuration, which means that ambient precipitation generated by large-scale lifting is intensified within the orographic cloud. Most simulations have been performed with the nonhydrostatic COSMO model developed at the German Weather Service (DWD), comparing three different autoconversion schemes of varying sophistication. For reference, a subset of experiments has also been performed with a spectral (bin) microphysics model. While precipitation enhancement via the seeder-feeder mechanism turns out to be relatively insensitive against the autoconversion scheme because accretion is the leading process in this case, simulated precipitation amounts can vary by 1-2 orders of magnitude for purely orographic precipitation. By comparison to the reference experiments conducted with the bin model, the Seifert-Beheng autoconversion scheme (which is the default in the COSMO model) and the Berry-Reinhardt scheme are found to represent the nonlinear behaviour of orographic precipitation reasonably well, whereas the linear approach of the Kessler scheme appears to be less adequate.

The 13–14 December 2001 IMPROVE-2 Event. Part III: Simulated Microphysical Budgets and Sensitivity Studies

Abstract This paper investigates the microphysical pathways and sensitivities within the Reisner-2 bulk microphysical parameterization (BMP) of the fifth-generation Pennsylvania State University–National Center for Atmospheric Research (PSU–NCAR) Mesoscale Model (MM5) for the Improvement of Microphysical Parameterization through Observational Verification Experiment (IMPROVE)-2 field experiment on 13–14 December 2001. A microphysical budget over the windward slope at 1.33-km horizontal grid spacing was calculated, in which the importance of each microphysical process was quantified relative to the water vapor loss (WVL) rate. Over the windward Cascades, the largest water vapor loss was associated with condensation (73% of WVL) and snow deposition (24%), and the windward surface precipitation resulted primarily from accretion of cloud water by rain (27% of WVL), graupel fallout and melt (19%), and snowmelt (6%). Two-thirds of the snow generated aloft spilled over into the lee in an area of model overprediction, resulting in windward precipitation efficiency of only 50%. Even with the large amount of precipitation spillover, the windward precipitation was still overpredicted in many locations. A series of experiments were completed using different snowfall speeds, cloud water autoconversion, threshold riming values for snow to graupel autoconversion, and slope intercepts for snow. The surface precipitation was most sensitive to those parameters associated with the snow size distribution and fall speed, while decreasing the riming threshold for snow to graupel conversion had the greatest positive impact on the precipitation forecast. All simulations overpredicted cloud water over the lower windward slopes, had too little cloud water over the crest, and had too much ice at moderate-to-large sizes aloft. Riming processes were important, since without supercooled water there were bull’s-eyes of spurious snow spillover over the lee slopes.

Bulk Microphysical Sensitivities within the MM5 for Orographic Precipitation. Part II: Impact of Barrier Width and Freezing Level

This paper investigates the impact of barrier width and freezing level on the microphysical processes and pathways within the Reisner2 bulk microphysical parameterization (BMP) using a two-dimensional version of the fifth-generation Pennsylvania State University‐National Center for Atmospheric Research (PSU‐NCAR) Mesoscale Model (MM5). As the barrier half-width is decreased incrementally from 50 km (relatively wide mountain) to 10 km (narrow mountain) for a deep orographic cloud and a 750-mb freezing level, the percentage of water vapor loss (WVL) rate over the windward slope leading to snow deposition decreases from 23% to 7%, while condensation increases from 74% to 93% of WVL rate. A narrow (10 km) barrier has less snow aloft, twice as much cloud water over the windward slope, and a shallow region of intense riming over the crest that results in twice as much graupel as the wide (50 km) barrier. It is found that a relatively wide barrier ( $30 km half-width) allows more time for snow growth aloft; therefore, it is more sensitive to snow parameters over the windward slope such as the slope intercept for number concentration and fall speeds. In contrast, a narrower barrier is more sensitive to rain and graupel processes, such as the cloud water autoconversion and graupel fall speeds. The wide barrier has a larger sensitivity to cloud water processes when the freezing level is elevated to 500 mb, while the narrow barrier is more sensitive to snow processes when the freezing level is lowered to 1000 mb. For a 1000-mb freezing level, the lack of riming and accretion reduces the rapid increase in maximum precipitation that is shown to occur when the barrier halfwidth is reduced from 20 and 10 km for a higher freezing level.

Liquid and Ice Cloud Microphysics in the CSU General Circulation Model. Part 1: Model Description and Simulated Microphysical Processes

Abstract Microphysical processes responsible for the formation and dissipation of water and ice clouds have been incorporated into the Colorado State University General Circulation Model in order to 1) yield a more physically based representation of the components of the atmospheric moisture budget, 2) link the distribution and optical properties of the model-generated clouds to the predicted cloud water and ice amounts, and 3) produce more realistic simulations of cloudiness and the earth's radiation budget. The bulk cloud microphysics scheme encompasses five prognostic variables for the mass of water vapor, cloud water, cloud ice, rain, and snow. Graupel and hail are neglected. Cloud water and cloud ice are predicted to form through large-scale condensation and deposition processes and also through detrainment at the tops of cumulus towers. The production of rain and snow occur through autoconversion of cloud water and cloud ice. Rain drops falling through clouds can grow by collecting cloud water, and ...

Numerical Modeling of a Line of Towering Cumulus on Day 226 of GATE

Abstract A three-dimensional numerical model with warm rain bulk cloud physics is used to investigate the shallow convection observed on day 226 of GATE. This convection had cloud tops at 3.0 km, cloud bases at 0.4 km and approximately 0.1 cm of rain at the surface. The simulated convection shows a strong sensitivity to the criterion for the onset of autoconversion of cloud water into rain water. The strongest convection occurs for the case in which no rain water forms. This case, however, does not conform to the observed convection, lacking the downdraft below cloud base and the observed strong surface outflow. The primary simulation produces a “finger” of convection propagating to the northeast, perpendicular to the northwest–southeast orientation of the larger-scale line of convection. The orientation and propagation speed of the calculated convection are in excellent agreement with observed radar data. This simulation also has a well-defined leading edge and strong surface outflow as observed. In poor...