Cumulonimbus Research Articles

Improvements to the parametrization of snow in AROME in the context of ice crystal icing

AbstractIce crystal icing (ICI) poses a threat nowadays for airplane pilots crossing the anvils of tropical mesoscale convective systems (MCSs). The use of fine‐scale operational numerical weather predictions as provided by the French limited‐area model AROME could help to better understand this phenomenon and to help its anticipation. To enable AROME to simulate ICI‐prone conditions, modifications of its single‐moment microphysical scheme Intercity‐Express 3 (ICE3) are tested. Using a temperature‐dependent snow particle distribution deeply impacts the organization and the ice phase of the simulated MCS. Notably, while the size of convective regions decreases, the size of anvil clouds increases and the low stratiform rain increases as well. As a result, by increasing the quantity of snow and decreasing the quantity of graupel, the simulation of ICI‐prone conditions in the anvils of convective systems is enabled. Using this parametrization, further modifications fine‐tune the representation of snow and further increase the size of the anvil cloud. The Marshall–Palmer snow distribution is replaced by a generalized gamma and the terminal fall velocities of snow hydrometeors are parametrized so that they are in closer agreement with observations.

A semi-Lagrangian method for detecting and tracking deep convective clouds in geostationary satellite observations

Abstract. Automated methods for the detection and tracking of deep convective clouds in geostationary satellite imagery have a vital role in both the forecasting of severe storms and research into their behaviour. Studying the interactions and feedbacks between multiple deep convective clouds (DCC), however, poses a challenge for existing algorithms due to the necessary compromise between false detection and missed detection errors. We utilise an optical flow method to determine the motion of deep convective clouds in GOES-16 ABI imagery in order to construct a semi-Lagrangian framework for the motion of the cloud field, independently of the detection and tracking of cloud objects. The semi-Lagrangian framework allows severe storms to be simultaneously detected and tracked in both spatial and temporal dimensions. For the purpose of this framework we have developed a novel Lagrangian convolution method and a number of novel implementations of morphological image operations that account for the motion of observed objects. These novel methods allow the accurate extension of computer vision techniques to the temporal domain for moving objects such as DCCs. By combining this framework with existing methods for detecting DCCs (including detection of growing cores through cloud top cooling and detection of anvil clouds using brightness temperature), we show that the novel framework enables reductions in errors due to both false and missed detections compared to any of the individual methods, reducing the need to compromise when compared with existing frameworks. The novel framework enables the continuous tracking of anvil clouds associated with detected deep convection after convective activity has stopped, enabling the study of the entire life cycle of DCCs and their associated anvils. Furthermore, we expect this framework to be applicable to a wide range of cases including the detection and tracking of low-level clouds and other atmospheric phenomena. In addition, this framework may be used to combine observations from multiple sources, including satellite observations, weather radar and reanalysis model data.

The Response of the Large‐Scale Tropical Circulation to Warming

AbstractPrevious work has found that as the surface warms the large‐scale tropical circulations weaken, convective anvil cloud fraction decreases, and atmospheric static stability increases. Circulation changes inevitably lead to changes in the humidity and cloud fields which influence the surface energetics. The exchange of mass between the boundary layer (BL) and the midtroposphere has also been shown to weaken in global climate models. What has remained less clear is how robust these changes in the circulation are to different representations of convection, clouds, and microphysics in numerical models. We use simulations from the Radiative‐Convective Equilibrium Model Intercomparison Project to investigate the interaction between overturning circulations, surface temperature, and atmospheric moisture. We analyze the underlying mechanisms of these relationships using a 21‐member model ensemble that includes both General Circulation Models and Cloud‐system Resolving Models. We find a large spread in the change of intensity of the overturning circulation. Both the range of the circulation intensity, and its change with warming can be explained by the range of the mean upward vertical velocity. There is also a consistent decrease in the exchange of mass between the BL and the midtroposphere. However, the magnitude of the decrease varies substantially due to the range of responses in both mean precipitation and mean precipitable water. We hypothesize based on these results that despite well understood thermodynamic constraints, there is still a considerable ability for the cloud fields and the precipitation efficiency to drive a substantial range of tropical convective responses to warming.

Application of the Pseudo‐Global Warming Approach in a Kilometer‐Resolution Climate Simulation of the Tropics

AbstractClouds over tropical oceans are an important factor in the Earth's response to increased greenhouse gas concentrations, but their representation in climate models is challenging due to the small‐scale nature of the involved convective processes. We perform two 4‐year‐long simulations at kilometer‐resolution (3.3 km horizontal grid spacing) with the limited‐area model COSMO over the tropical Atlantic on a 9,000 × 7,000 km2 domain: A control simulation under current climate conditions driven by the ERA5 reanalysis, and a climate change scenario simulation using the Pseudo‐Global Warming approach. We compare these results to the changes projected in the CMIP6 scenario ensemble. Validation shows a good representation of the annual cycle of albedo, in particular for trade‐wind clouds, even compared to the ERA5 reanalysis. Also, the vertical structure and annual cycle of the intertropical convergence zone (ITCZ) is accurately simulated, and the simulation does not suffer from the double ITCZ problem commonly present in global climate models (GCMs). The response to global warming differs between the COSMO simulation and the analyzed GCMs. While both exhibit an overall weakening of the Hadley circulation, the narrowing of the ITCZ (known as the deep‐tropics squeeze) is not so pronounced in the kilometer‐resolution simulation, likely due to the absence of a double ITCZ bias. Also, there is a more pronounced intensification of the ITCZ at the equator in the kilometer‐resolution COSMO simulation, and a stronger associated increase in the anvil cloud fraction.

Long‐term variability of the low‐level cloud base height in Poland

AbstractThe aim of the study is to characterize the long‐term variability of the low‐level cloud base heights (CBH) in Poland in the years 1971–2020. Data were used from six weather stations belonging to the Institute of Meteorology and Water Management – the National Research Institute (IMWM‐NRI), where CBH was first measured using the IWO device and, since the 1990s, using a ceilometer. The analysis allowed for data from eight observation periods (0000, 0300, 0600, 0900, 1200, 1500, 1800, 2100 UTC), which referred to several height ranges (between the ground surface and 2500 m a.s.l.). In the analysed multiyear period, in the cool half of the year, the height of the Cumulonimbus cloud bases increased significantly, that is, the number of cases increased at a height of >1000 m and, in the warm half of the year, at >1500 m. In recent years, the number of cases of the Cumulus cloud with a base at a height between 300 and 999 m has clearly decreased, while it has increased at a height of 1000–1499 m (in October–March) and 1500–1999 m (April–September). A similar increase in CBH was found in the case of the Stratocumulus cloud (their greater share between 1500 and 2499 m). It was only in the case of the Stratus cloud that a lowering of the cloud base was noticed, possibly owing to the imperfect measurement of the fractus species. The positive upward trend in the base height of most low‐level clouds can be explained by a statistically significant increase in temperature and a decrease in relative air humidity in the lower troposphere.

Radiation, Clouds, and Self‐Aggregation in RCEMIP Simulations

AbstractThe responses of tropical anvil cloud and low‐level cloud to a warming climate are among the largest sources of uncertainty in our estimates of climate sensitivity. However, most research on cloud feedbacks relies on either global climate models with parameterized convection, which do not explicitly represent small‐scale convective processes, or small‐domain models, which cannot directly simulate large‐scale circulations. We investigate how self‐aggregation, the spontaneous clumping of convection in idealized numerical models, depends on cloud‐radiative interactions with different cloud types, sea surface temperatures (SSTs), and stages of aggregation in simulations that form part of RCEMIP (the Radiative‐Convective Equilibrium Model Intercomparison Project). Analysis shows that the presence of anvil cloud, which tends to enhance aggregation when collocated with anomalously moist environments, is reduced in nearly all models when SSTs are increased, leading to a corresponding reduction in the aggregating influence of cloud‐longwave interactions. We also find that cloud‐longwave radiation interactions are stronger in the majority of General Circulation Models (GCMs), typically resulting in faster aggregation compared to Cloud‐system Resolving Models (CRMs). GCMs that have stronger cloud‐longwave interactions tend to aggregate faster, whereas the influence of circulations is the main factor affecting the aggregation rate in CRMs.

Increasing Precipitation Efficiency Amplifies Climate Sensitivity by Enhancing Tropical Circulation Slowdown and Eastern Pacific Warming Pattern

AbstractThe role of precipitation efficiency (PE)—the fraction of column‐integrated condensate that reaches the surface as rain—in the global temperature response to CO2 rise is yet to be quantified. Here we employ 36 limited‐domain cloud resolving models (CRMs) from the Radiative‐Convective Equilibrium Model Intercomparison Project and find that they strongly imply higher PE at warmer temperatures. We then analyze 35 general circulation models (GCMs) from the Coupled Model Intercomparison Project Phase 6 and find that increasing PE is associated with tropical circulation slowdown and greater eastern equatorial Pacific warming. These changes trigger pan‐tropical positive cloud feedback through stratiform anvil cloud reduction and stratocumulus suppression, resulting in higher Effective Climate Sensitivity (ECS). We find that in 24 of 35 GCMs matching the CRMs in simulating increasing PE with greenhouse warming, mean ECS is 1 K higher than in PE‐decreasing GCMs. Thus, further constraining PE sensitivity to temperature could reduce uncertainty over future climate projections.

Observed Concentric Eyewalls of Supertyphoon Hinnamnor (2022)

Ground-based radar observations of the concentric eyewalls (CEs) of supertyphoon Hinnamnor (2022) showed that CEs evolved as follows: (Stage 0) development of a distinct stationary band complex (SBC) downshear in moderate south-southeastward vertical wind shear (VWS), (Stage 1) maturity of the secondary eyewall (SE), (Stage 2) contraction of the SE, and (Stage 3) decay of the CE structures. From Stage 0 to Stage 1, the SBCs transitioned into an axisymmetric SE, and moats appeared a few hours after SE formation. Outer rainbands formed successively in the down-to-left shear quadrants and developed wide stratiform regions with a vast anvil cloud extending outward as they moved upshear. When the anvil cloud covered the right-shear quadrants, the stratiform regions started contracting. Evolutions of the doppler velocity fields were detected under the anvil and stratiform regions of each rainband. An intense convection developed at the inner edge of the contracting SE as regions with relatively high doppler-velocity merged. When the core region was surrounded by multiple well-developed rainbands, the inner eyewall weakened rapidly. The radar observations revealed the importance of moderate-to-weak VWS and development of a SE as a rainband complex during evolution of CEs.

Analisis Citra Satelit Himawari 8/9 Terkait Kejadian Hujan Es Di Wilayah Klaten Tanggal 21 Oktober 2021

One of the extreme phenomena that often occurs is hail, hail can occur because when water vapor condenses it turns into ice particles due to the low air temperature factor. Hail includes local rain which covers an area of 5-10 km, and lasts a maximum of 10 minutes. On October 21, 2021, there was hail in Jogonalan Klaten. The incident occurred at around 14.18 WIB. By using Himawari-8 satellite data reference which was analyzed using the SATAID application and UV wind data obtained from the Copernicus web and processed with OpenGrADS software. Then as other supporting data used cloud top temperature data with band 13 himawari 8 which was processed using time series and also air lability data from the web weather.uwyo.edu. From the four data, it will be obtained that the formation of cumulonimbus clouds in the area around Klaten and Jogja as an indication of hail is strengthened by the convergence pattern in the northern region of the island of Java, precisely Central Java which causes a slowdown in the air mass that forms convective clouds which is the cause of hail in Klaten Regency. . This is reinforced by the results of the analysis of the cloud top temperature which reached -18°c.

Temperatures of Anvil Clouds and Radiative Tropopause in a Wide Array of Cloud-Resolving Simulations

Abstract We present 123 cloud-resolving simulations to study how temperatures of anvil clouds and radiative tropopause (RT) change with surface warming. Our simulation results show that the RT warms at approximately the same rate as anvil clouds. This relationship persists across a variety of modeling choices, including surface temperature, greenhouse gas concentration, and the representation of radiative transfer. We further show that the shifting ozone profile associated with climate warming may give rise to a fixed RT temperature as well as a fixed anvil temperature. This result points to the importance of faithful treatment of ozone in simulating clouds and climate change; the robust anvil–RT relationship may also provide alternative ways to understand what controls anvil temperature.

Characteristics of a Jumping Cirrus Retrieved From Rapid Scans by Himawari‐8

AbstractA jumping cirrus (JC) is a cloud that forms a few kilometers above an anvil cloud downwind of an overshooting top (OT). A JC transports water vapor from the troposphere into the stratosphere, where water vapor is an important greenhouse gas. However, the microphysics of JC has not been explored thus far. Herein, 2.5‐min rapid scan images obtained from the geostationary weather satellite Himawari‐8 were analyzed for JC observations. A dark plume was observed to appear suddenly above an anvil cloud downwind of OTs and moved 1 km upwind within 2.5 min. The brightness temperature of the plume showed that it overlaid the anvil cloud. These characteristics matched those of a JC. The reflectivity of the assumed JC at a wavelength of 0.64 μm was 0.62, whereas that of the anvil cloud on the sunny side of the JC was 1.02. The measured reflectivities were compared to those simulated using ray tracing to retrieve the microphysics of the JC. The results indicated that dark reflectivity occurred because the JC was isolated above the anvil cloud, and bright reflectivity occurred because the anvil was illuminated by the sun and scattered light from the JC. The retrieved effective radius and ice water content of the JC were 23.4–28.0 μm and 0.12–0.15 g m−3, respectively. The JC appeared to transport 10 × 103 t of ice water above deep convection within 10 min. We applied the results to analyzing the best satellite geometry for detecting and measuring JC.

Cloud Radiative Effects on MJO Development in DYNAMO

Abstract Observed Madden–Julian oscillation (MJO) events are examined with the aid of regional model simulations to understand the role of cloud radiative effects in the MJO development. The importance of this role is demonstrated by the absence of the MJO in the model simulations that contain no cloud radiative effects. Comparisons of model simulations with and without the cloud radiative effects and observation help identify the major processes arising from those effects. Those processes develop essentially from heating in the upper troposphere due to shortwave absorption within anvil clouds in the upper troposphere and the convergence of longwave radiation in the middle to upper troposphere, with a peak at 300 hPa, during deep convection. First, that heating adds extra buoyancy and accelerates the rising motion in the upper troposphere in deep convection. The vertical acceleration in the upper troposphere creates a vacuum effect and demands for more deep convection to develop. Second, in response to that demand and required by mass balance arises the large-scale horizontal and vertical mass, moisture, and energy convergence. It strengthens deep convection and, with the feedback from continuing cloud radiative effect, creates conditions that can perpetuate deep convection and MJO development. That perpetuation does not occur however because those processes arising from the cloud radiative heating in the upper troposphere stabilize the troposphere until it supports no further deep convection. Weakening deep convection reduces cloud radiative effects. The subsequent reduction of the vacuum effect in the upper troposphere diminishes deep convection completing an MJO cycle. These results advance our understanding of the development of the MJO in the radiative–convective system over warm waters in the tropics. They show that while the embryo of intraseasonal oscillation may exist in the system its growth/development is largely dependent on cloud radiative effects and feedbacks.

Three Rules for the Decrease of Tropical Convection With Global Warming

AbstractTropical convection is expected to decrease with warming, in a variety of ways. Specific incarnations of this idea include the “stability‐iris” hypothesis of decreasing anvil cloud coverage, as well as the decrease of both tropospheric and cloud‐base mass fluxes with warming. This paper seeks to encapsulate these phenomena into three “rules,” and to explore their interrelationships and robustness, using both analytical reasoning as well as cloud‐resolving and global climate simulations. We find that each of these rules can be derived analytically from the usual expression for clear‐sky subsidence, so they all embody the same essential physics. But, these rules do not all provide the same degree of constraint: the stability‐iris effect is not entirely robust due to unconstrained microphysical degrees of freedom, and the decrease in cloud‐base mass flux is not entirely robust due to unconstrained effects of entrainment and detrainment. Tropospheric mass fluxes on the other hand are shown to be well‐constrained theoretically, and when evaluated in temperature coordinates they exhibit a monotonic decrease with warming at all vertical levels and across a hierarchy of models.

Clear-sky control of anvils in response to increased CO2 or surface warming or volcanic eruptions

Anvil clouds produced by deep convection cover extensive areas of the tropics, and their response to external perturbations matters for the Earth’s climate sensitivity. It has been suggested that variations in the height and spatial extent of these clouds can be understood from basic physical arguments related to the conservation of mass and energy in the clear-sky areas of the tropics. Based on satellite observations, meteorological reanalyses, and climate model simulations, we show that these arguments can be used to interpret the response of anvil cloud fraction to a range of perturbations in the current climate and under climate change. This includes the response to interannual and long-term surface temperature changes, to the direct effect of carbon dioxide, and the decrease of anvil cloud fraction after explosive volcanic eruptions. Therefore, the control of tropical anvils by clear-sky radiative cooling and static stability in the upper troposphere can explain a large diversity of the responses of anvil cloud fraction to natural and anthropogenic perturbations. These findings should also be considered when assessing the impacts of geo-engineering techniques.

Convective Cloud Classification Model for Reconstruction of Heavy Rain That Triggers the Flood and Landslide in Parapat, North Sumatera

Rainfall in Parapat as a part of Lake Toba, North Sumatera and a super-priority tourist destination in Indonesia on May, 13 2021 causes flood, it exceeds Meteorologi, Climatologi and Geophysics Agency’s threshold about intensity of extreme condition. It is related to dynamic of weather’s parameter, especially with convection process and clouds. The landslide material was very closed the road so that the SiantarParapat access was temporarily closed down. Heavy rains that have occurred since noon are said to be the cause of the floods and landslides that occurred at night. This study aims to describe the distribution of rain in Parapat and surrounding areas during floods and landslides. The data used are measured rainfall at the nearest AWS (Automatic Weather Station), and rainfall estimation data from GPM IMERG (Global Precipitation Measurement – The Integrated Multi-SatellitE Retrievals). The results of the study show that convective cloud classification model can be identify the reconstruction of heavy rain that trigger the flood and landslide in parapet. There has been rain with moderate to heavy intensity since 12 May 2021. Heavy rains that have occurred in the last few days have caused soil conditions to become saturated, resulting in landslides. Cloud classification shows the presence of Cumulonimbus cloud clusters that grow and develop around the Lake Toba area before and during heavy rains.

A study on the anomalous TOA net radiative warming by clouds in a sub-region within the Indian summer monsoon region

Indian summer monsoon clouds generally exert a net radiative cooling at top of atmosphere. In contrast, clouds over a sub-region comprising parts of South peninsular India, Sri Lanka and adjoining Bay of Bengal (SISB) inside the Indian monsoon region exert a net radiative warming. In this work, an attempt is made to understand the reasons behind the anomalous radiative warming found over SISB. Ten-year (2000–2009) top of atmosphere radiative flux data from Clouds and the Earth’s Radiant Energy System payloads onboard Aqua and Terra satellites and cloud data from International Satellite Cloud Climatology Project during the peak Indian summer monsoon season of July–August are analyzed to understand the underlying causes. Top of Atmosphere net radiative forcing is positive and as high as ∼15 Wm−2 over SISB during the peak Indian summer monsoon season.Cloud cover amounts over SISB with net radiative warming and north Bay of Bengal with net radiative cooling are compared. Cloud cover amounts of high-level cirrostratus and deep convective cumulonimbus clouds are less by 65% and 87% respectively over SISB when compared to north Bay of Bengal. SISB is located on the leeward side of the Western Ghats mountain chain with descending motion. Adiabatic compression and associated warming of descending air leads to dehydration of air parcel. The upper tropospheric tropical easterly jet sweeps the deep convective cloud tops of the Indian monsoon and advect them to large distance as upper level thin cirrus family of clouds. When these clouds reach SISB with descending motion, they thin out by melting and evaporation or sublimation as the air mass is compressed and heated adiabatically. Over SISB, pressure vertical velocity at 500 hPa is moderately inversely correlated to cloud cover amount of cirrostratus and cumulonimbus clouds. Cloud cover amount of these two clouds is significantly correlated to shortwave cloud radiative forcing and longwave cloud radiative forcing over SISB. When cloud cover amount of cirrostratus and cumulonimbus clouds are low (<∼15% and <∼5% respectively), magnitude of longwave cloud radiative forcing is mostly higher than the magnitude of shortwave cloud radiative forcing which results in net radiative warming. Reduction in cloud cover amount of cirrostratus and cumulonimbus clouds by subsidence appears to be the cause behind the observed warming over SISB. When the magnitude of shortwave cloud radiative forcing is below (above) ∼55 Wm−2, longwave cloud radiative forcing is more (less) than the magnitude of shortwave cloud radiative forcing which results in net warming (cooling). Net radiative warming is found in 65% of grid points over SISB.

APPLICATION OF SOFTWARE, REPRESENTING CUMULONIMBUS CLOUDS, IN AIR TRAFFIC CONTROL

In this paper we attempt to answer the question of whether the display of cumulonimbus clouds on air traffic controller radar screens has a significant impact on air traffic control. The developed cumulonimbus cloud mapping simulator allowed the study to be performed by performing simulations with a flight control simulator. This allowed an assessment of how the air traffic controller makes decisions when cumulonimbus is displayed and when it is not. The benefits of cumulonimbus imaging for aircraft trajectories and flight time were also assessed. The research was conducted in the space of Vilnius terminal manoeuvring area.

Microphysical Analysis of Precipitation in the Central and Eastern Margins of the Tibetan Plateau

Through the observation and study of the raindrop spectrum, we can not only explore the evolution law of precipitation but also understand the microphysical characteristics of different types of precipitation clouds. This paper uses the raindrop spectrum data observed at Naqu Station, Yushu Station, Linzhi Station, and Emei Mount Station, as well as cloud radar data in the Yushu region in the Tibetan Plateau. Raindrop spectral characteristics are studied, and the raindrop size distribution (DSD) characteristics of the four stations are analyzed. The results are as follows: (1) The overall raindrop spectral concentration of the four stations decreases with the increase in particle size after the peak value. The downtrend is most gentle at the peak. (2) All rain intensity levels show the characteristics of multiple vertices. At Linzhi Station, Naqu Station and Yushu Station, the particles with a diameter of &lt;1 mm contributed the most to the precipitation rate, while the particles with a diameter of &lt;1 mm at Emei Mount Station contributed the least to the precipitation rate. (3) The precipitation in the central and eastern margins of the Tibetan Plateau is dominated by small and medium-sized particles, accounting for 95–99% of the precipitation particle number concentration. (4) The raindrop spectra of the three types of precipitation clouds at the four sites are all cumulonimbus precipitation Cumulonimbus in the uppermost layer, stratiform cloud precipitation stratiform cloud in the lowermost layer, and mixed cloud precipitation stratiform cloud in between. (5) Cloud radar analysis of precipitation structure shows that cumulonimbus and mixed clouds develop vigorously, and the highest cloud height can reach 13 km, but the average precipitation duration of mixed clouds is shorter than that of cumulonimbus; stratiform clouds develop relatively smoothly, with cloud heights of 6–7 km, the average precipitation duration is the shortest.

The Global Nature of Early‐Afternoon and Late‐Night Convection Through the Eyes of the A‐Train

AbstractCharacterizing macrophysical properties of deep convective systems on a global scale is a precursor to understanding their influence on Earth's Energy Budget and Water Cycle. This study documents the properties of global convective objects (COs) in the early afternoon (1:30 p.m. local time; LT) and overnight (1:30 a.m. LT) measurements from the A‐Train satellite constellation. CloudSat measurements are used to identify convective cores and establish their intensity, while other A‐Train data sets define cloud structure, storm spatial extent, rainfall yield, and radiative effects of each CO. Global distributions of storm characteristics are consistent with previous studies in which the most intense convection is located over tropical land, particularly over the Amazon and Congo Basin, while the largest COs occur over the Maritime Continent. Despite their limited twice‐daily sampling, A‐Train measurements capture that early afternoon convection over tropical land is both more intense and produces heavier rainfall than nighttime land‐based convection, while the day‐night differences are minimal over the tropical ocean. High‐resolution estimates of updraft cores reveal that CO size increases as the number of distinct cores in a CO increases owing to an increase in nonconvective rain and anvil cloud area. Convective objects generally cool the environment, which is strongest over the Northern Hemisphere midlatitudes, but cooling weakens as the nonconvective cloud fraction increases such that 30% of COs actually exert a net warming effect. These results suggest that A‐Train measurements may capture bulk connections between deep convective cloud features, precipitation, and radiative effects despite a lack of complete diurnal sampling.

Criticality in the Shallow-to-Deep Transition of Simulated Tropical Marine Convection

Abstract Idealized simulations of tropical, marine convection depict shallow, nonprecipitating cumuli located beneath the 0°C level transitioning into cumulonimbi that reach up to 12 km and higher. The timing of the transition was only weakly related to environmental stability, and 13 of the 15 simulations run with 5 different lapse-rate profiles had rain develop at nearly the same time after model start. The key quantity that apparently controlled deep convective formation was vertical acceleration inside cloudy updrafts between cloud base and the 0°C level. Below a critical value of updraft vertical acceleration, little rainfall occurred. Just as the domain-mean updraft acceleration reached the critical value, the first convection quickly grew to past 12 km altitude. Then, as acceleration increased above the critical value, rain rate averaged in the model domain increased quickly over about a 3-h-long period. The specific value of the critical updraft acceleration depended on how updrafts were defined and in what layer the acceleration was averaged; however, regardless of how criticality was defined, a robust relationship between domain-mean updraft vertical acceleration and rain rate occurred. Positive acceleration of updrafts below the 0°C level was present below 2.75 km and was largest in the 500 m above cloud base. However, the maximum difference between updraft and environmental temperatures occurred between 2 and 3 km. The domain-mean Archimedean buoyancy of updrafts relative to some reference state was a poor predictor for domain-mean rain rate. The exact value of the critical updraft acceleration likely depends on numerous other factors that were not investigated. Significance Statement A numerical model is utilized to investigate potential thermodynamic and dynamic quantities related to the growth of cumulus clouds into cumulonimbus clouds over tropical oceans when the atmosphere is sufficiently moist to support rainfall. Archimedean buoyancy alone cannot be used to predict rain rate reliably. Instead the total buoyancy not relative to an arbitrary reference state must be considered. The simulated relationship between total vertical acceleration in updrafts and rain rate was robust. While the processes that control the vertical acceleration remain unclear, our results highlight the importance of observing processes that occur on spatial scales of tens of meters and temporal scales of a few minutes.