Deep Convective Systems Research Articles

Atmospheric dynamics of extreme discharge events from 1979 to 2016 in the southern Central Andes

During the South-American Monsoon season, deep convective systems occur at the eastern flank of the Central Andes leading to heavy rainfall and flooding. We investigate the large- and meso-scale atmospheric dynamics associated with extreme discharge events (> 99.9th percentile) observed in two major river catchments meridionally stretching from humid to semi-arid conditions in the southern Central Andes. Based on daily gauge time series and ERA-Interim reanalysis, we made the following three key observations: (1) for the period 1940–2016 daily discharge exhibits more pronounced variability in the southern, semi-arid than in the northern, humid catchments. This is due to a smaller ratio of discharge magnitudes between intermediate (0.2 year return period) and rare events (20 year return period) in the semi-arid compared to the humid areas; (2) The climatological composites of the 40 largest discharge events showed characteristic atmospheric features of cold surges based on 5-day time-lagged sequences of geopotential height at different levels in the troposphere; (3) A subjective classification revealed that 80% of the 40 largest discharge events are mainly associated with the north-northeastward migration of frontal systems and 2/3 of these are cold fronts, i.e. cold surges. This work highlights the importance of cold surges and their related atmospheric processes for the generation of heavy rainfall events and floods in the southern Central Andes.

Tropical cyclones impact on tropopause and the lower stratosphere vapour based on satellite data

AbstractAs one of the deep convection systems, tropical cyclones (TCs) could significantly affect the thermodynamic structure and cloud microphysical processes near the tropopause, which may cause changes in troposphere and stratosphere water vapour activity, with important influences on global climate change. However, the effects of TCs on tropopause and the lower stratosphere water vapour remain unclear. Here, we use satellite data from year 2012 to 2016 to assess the effect of TCs on water vapour at the tropopause and in the lower stratosphere over the Northwest tropical Pacific Ocean. It is found that TCs could decrease the tropopause temperature and increase the process of water vapour deposition on cloud ice to a certain extent. This process can increase cloud ice mixing ratio and then dehydrate the tropopause. Part of the small cloud ice particles will be injected into the lower stratosphere, where they would be sublimated, hydrating the lower stratosphere. Compared to non‐TC area, the average water vapours in TC area decrease about 1.04 ppmv near the tropopause, but increase by 0.75 ppmv in the lower stratosphere.

An Observational Comparison of Level of Neutral Buoyancy and Level of Maximum Detrainment in Tropical Deep Convective Clouds

AbstractTropical deep convective clouds are important drivers of large‐scale atmospheric circulation representing the main vertical transport pathway through the depth of the troposphere for heat, momentum, water, and chemical species. The strength and depth of this transport are impacted by the convective updraft size and intensity that are driven by buoyancy, dynamical forcing, and mixing of environmental air, that is, entrainment. In this study, we identify tropical deep convective systems with well‐defined forward anvils using Atmospheric Radiation Measurement (ARM) ground‐based profiling radars, at three ARM fixed sites in the Tropical Western Pacific (TWP; i.e., Manus, Nauru, and Darwin) and three ARM Mobile Facility deployments in Niamey, Niger; Gan Island, Maldives; and Manacapuru, Brazil. We use the difference between the level of neutral buoyancy (LNB) and the level of maximum detrainment (LMD) as a proxy for the effective bulk convective entrainment (εproxy). The LNB, the theoretical height that a parcel raised above the level of free convection would reach with no mixing, is calculated based on preconvection radiosonde measurements using parcel theory. The LMD is the height of the maximum reflectivity observed in forward anvil clouds by profiling radars. Deep convective systems over the TWP show higher LNBs that extend to 16.3 km on average and larger εproxy (median value of LNB minus LMD up to 6.5 km) compared to their continental counterparts in the Amazon and West Africa. Oceanic conditions show larger convective available potential energy (CAPE) coupled with higher moisture at low levels, which favors larger εproxy. In contrast, continental cases initiate and develop, under high convective inhibition, steeper environmental lapse rate, and high wind shear conditions, which show smaller offset between LNB and LMD. Deep convective cases that promote significant cold pools at the surface experience less εproxy. Using a Random Forest regression algorithm, CAPE is associated with the highest feature importance score for predicting convective εproxy, followed by low‐level relative humidity. For continental cases, the low‐level wind shear also indicates higher importance.

Rare earth elements in the East Sea (Japan Sea): Distributions, behaviors, and applications

The distributions of rare earth elements (REEs) were measured at 14 stations from 0 to 3365 m (n = 176) in the entire East Sea for the first time. The East Sea has its own deep-convection system, which operates on a time scale that is approximately one tenth that of the global ocean; it is also a downwind region of Asian dust. The vertical distributions of REE concentrations increased with depth, similar to those in the North Atlantic Ocean. However, the concentrations of REEs were 1.3–2.7 fold higher than those in the North Atlantic Ocean, associated with relatively larger atmospheric depositions of dust from the Asian continent. In this environmental condition, the removal of Ce relative to the other REEs was about an order of magnitude more efficient than that in the North Pacific Ocean. As such, in the deep layer below 750 m, large increases of heavy-REEs (HREEs) relative to light-REEs (LREEs), opposite to the re-mineralization trend, were observed suggesting that LREEs were preferentially re-scavenged by sinking particles. In addition, the boundary scavenging of REEs was also observed for all REEs in the bottom layer of the East Sea. On the other hand, the warm surface waters of REEs, Tsushima Warm Water (TWW) showed large fractionations, relative to the other water masses. This REE pattern traced the deep physical mixing proportions of TWW up to 750 m (∼10%). Thus, the unique distribution patterns of REEs in this sea illustrate the geochemical scavenging characteristics of REEs, which are concealed in the major oceans, as well as water mass mixing patterns in the upper ocean.

Impacts of Aerosol and Environmental Conditions on Maritime and Continental Deep Convective Systems Using a Bin Microphysical Model

AbstractA series of model simulations were conducted to investigate the effects of cloud condensation nuclei (CCN) loading and convective available potential energy (CAPE) on tropical maritime and midlatitude continental deep convection. Dynamical downscaling from global aerosol reanalysis was used to represent aerosol fields for the two convective regimes. We describe a control run and multiple sensitivity experiments using a limited‐area model, employing spectral‐bin cloud microphysics. The CCN loading is perturbed between the target maritime and continental conditions, roughly 40–2,000 cm−3 at 850 hPa and 1% supersaturation. Surface precipitation rates monotonically increase with increasing CCN loading for both the maritime and continental situations, while these monotonic increases are disrupted in the simulations with reduced CAPE. The increase in precipitation is in the form of convective precipitation, at the expense of stratiform precipitation. CCN increases promote increases in supercooled cloud water, in agreement with previous modeling studies. However, in the simulations investigated herein, the changes in supercooled water have different impacts on the cloud microphysics in the maritime and continental simulations. Increased supercooled water contents lead to more hail and less graupel in the continental simulation. For the maritime simulation, enhanced supercooled cloud water contents promote an increase in graupel since little or no hail is produced. This distinction is due to the difference in relative magnitudes and peak altitudes of supercooled water and snow amounts, which is further attributable to moisture and dynamical differences in the two cases.

Regional low-temperature fluid flow indicated by quartz mineralization in Silesicum, NE Bohemian massif

The crystalline rocks of the Silesicum unit of the Bohemian Massif host two principal types of hydrothermal quartz veins. Veins associated with granitoids are primarily quartz with wollastonite and epidote and accompanied by hydrothermal alteration of host rocks whereas others are of the more complex Alpine-type. L+ V + S (liquid + vapour + solid phase) fluid inclusions within quartz crystals contain muscovite and +/- calcite (+/- haematite) as solid phases and homogenisation temperatures are between 124 and 176 degrees C. Trapped fluids are uniformly of the H2O-NaCl-CaCl2 +/- MgCl2 +/- KCl system with a salinity between 9.1 and 26.8 mass % (NaCl+CaCl2). The variable Na/Ca ratios result from different intensity of fluid-rock reactions. The ubiquitous muscovite is a product of fluid-rock interactions. Oxygen isotope evidence indicates that the fluids were a mixture of meteoric and marine waters in deep regional-scale convection systems. The studied type of fluids represents a post-Variscan hydrothermal system and shows the extent of migration pathways in the upper crust on the north-eastern edge of the Bohemian Massif.

Secondary Ice Formation in Idealised Deep Convection—Source of Primary Ice and Impact on Glaciation

Secondary ice production via rime-splintering is considered to be an important process for rapid glaciation and high ice crystal numbers observed in mixed-phase convective clouds. An open question is how rime-splintering is triggered in the relatively short time between cloud formation and observations of high ice crystal numbers. We use idealised simulations of a deep convective cloud system to investigate the thermodynamic and cloud microphysical evolution of air parcels, in which the model predicts secondary ice formation. The Lagrangian analysis suggests that the “in-situ” formation of rimers either by growth of primary ice or rain freezing does not play a major role in triggering secondary ice formation. Instead, rimers are predominantly imported into air parcels through sedimentation form higher altitudes. While ice nucleating particles (INPs) initiating heterogeneous freezing of cloud droplets at temperatures warmer than −10 °C have no discernible impact of the occurrence of secondary ice formation, in a scenario with rain freezing secondary ice production is initiated slightly earlier in the cloud evolution and at slightly different places, although with no major impact on the abundance or spatial distribution of secondary ice in the cloud as a whole. These results suggest that for interpreting and analysing observational data and model experiments regarding cloud glaciation and ice formation it is vital to consider the complex vertical coupling of cloud microphysical processes in deep convective clouds via three-dimensional transport and sedimentation.

Vertical structure and frequencies of deep convection during active periods of the West and Central African monsoon season

Deep convective systems during active periods of the West African monsoon season have not been properly investigated over better temporal and spatial resolution in West Africa. Deep convective systems are investigated over seven climatic zones of the West African sub-region, which are as follows: west-coast rainforest, dry rainforest, Nigeria-Cameroon rainforest, Nigeria savannah, Central African and South Sudan (CASS) savannah, Sudano-Sahel, and Sahel, using data from Tropical Rainfall Measurement Mission (TRMM) Precipitation Feature (PF) database. The vertical structure of the convective systems indicated by the presence of at least one 40 dBZ and reaching (attaining) at least 1 km in the atmosphere showed strong core (highest frequency (%)) of reflectivity values around 2 km which is below the freezing level (4–5 km) for all the zones. Echoes are detected above the 15-km altitude much more frequently in the rainforest and Savannah zones than the Sudano and Sahel zones during active periods in March–May (MAM), whereas during active periods in June–September (JJAS), the savannahs, Sudano, and Sahel zone convections tend to reach higher altitude more frequently than the rainforest zones. The frequencies of deep convection defined by the presence of at least one 40 dBZ, reaching (attaining) at least 1 km in the atmosphere, with sizes greater than 2000 square, and having at least one convective pixels indicate that these systems are within the range of 2.3–2.8% in frequency averaged over the 16-year period of study in both March–May (MAM) and June–September (JJAS) active periods in the rainforest and savannah zones. These were found to be less than 2% in the Sudano and Sahel zones, except for the active JJAS period in Sudano zone.

Statistical analysis of ice microphysical properties in tropical mesoscale convective systems derived from cloud radar and in situ microphysical observations

Abstract. This study presents a statistical analysis of the properties of ice hydrometeors in tropical mesoscale convective systems observed during four different aircraft campaigns. Among the instruments on board the aircraft, we focus on the synergy of a 94 GHz cloud radar and two optical array probes (OAP; measuring hydrometeor sizes from 10 µm to about 1 cm). For two campaigns, an accurate simultaneous measurement of the ice water content is available, while for the two others, ice water content is retrieved from the synergy of the radar reflectivity measurements and hydrometeor size and morphological retrievals from OAP probes. The statistics of ice hydrometeor properties are calculated as a function of radar reflectivity factor measurement percentiles and temperature. Hence, mesoscale convective systems (MCS) microphysical properties (ice water content, visible extinction, mass–size relationship coefficients, total concentrations, and second and third moments of hydrometeor size distribution) are sorted in temperature (and thus altitude) zones, and each individual campaign is subsequently analyzed with respect to median microphysical properties of the merged dataset (merging all four campaign datasets). The study demonstrates that ice water content (IWC), visible extinction, total crystal concentration, and the second and third moments of hydrometeor size distributions are similar in all four types of MCS for IWC larger than 0.1 g m−3. Finally, two parameterizations are developed for deep convective systems. The first concerns the calculation of the visible extinction as a function of temperature and ice water content. The second concerns the calculation of hydrometeor size distributions as a function of ice water content and temperature that can be used in numerical weather prediction.

Warm Rain in Southern West Africa: A Case Study at Savè

A warm-rain episode over southern West Africa is analyzed using unprecedented X-band radar observations from Savè, Benin and a Large-Eddy Simulation (LES) over a 240 × 240 km 2 domain. While warm rain contributes to 1% of the total rainfall in the LES, its spatial extent accounts for 24% of the area covered by rainfall. Almost all the warm-rain cells tracked in the observation and the LES have a size between 2 and 10 km and a lifetime varying from 5 to 60 min. During the nighttime, warm-rain cells are caused by the dissipation of large deep-convection systems while during the daytime they are formed by the boundary-layer thermals. The vertical extension of the warm-rain cells is limited by vertical wind shear at their top. In the simulation, their top is 1.6 km higher with respect to the radar observations due to the large-scale environment given by wrong initial conditions. This study shows the challenge of simulating warm rain in southern West Africa, a key phenomenon during the little dry season.

The Roles of Mineral Dust as Cloud Condensation Nuclei and Ice Nuclei During the Evolution of a Hail Storm

AbstractAerosols play important roles in the evolution of deep convective systems like hailstorms. In this study, the heterogeneous ice nucleation schemes have been improved in the Weather Research and Forecasting model coupled with a spectral bin microphysics (WRF‐SBM), which considered aerosols acting as ice nuclei (IN). A hail storm occurred around Tianshan mountains, northwestern China, was simulated with updated WRF‐SBM, and the results have been compared with satellite observations. Further, four sensitive simulation tests were conducted with different cloud condensation nuclei (CCN) and IN concentrations to investigate their respective roles during the evolution of the hailstorm. The increase in CCN concentration resulted in larger cloud droplet concentration and cloud water content, as well as enhanced condensational growth, which released more latent heat and led to stronger updraft at lower levels. The increase in IN number almost did not affect warm processes but led to larger ice crystal concentration and enhanced Bergeron process. Larger CCN concentration led to larger supercooled liquid water content, which in turn contributed to the enhanced hail growth by more efficient drop‐ice collisions and led to larger size of hail particles, while larger IN number reduced the size of graupel and suppressed the growth of hailstones. An analysis of the mobility of hail indicated increased frequency of larger hail with stronger sedimentation induced by more CCN. A further three ensemble runs with random perturbations on initial temperature and humidity were performed for each aerosol scenario, and the results suggested the robustness of simulated CCN and IN effects.

Spatial and seasonal variation of rainfall contribution by the height spectrum of precipitation systems and associated cloud bulk properties over the South Asia

AbstractThe spatial‐seasonal variation of rainfall contribution by the height spectrum of precipitation systems in terms of maximum echo top height of 20 and 40 dBZ (ETH20 dBZ (max) and ETH40 dBZ (max)) is investigated. The study is carried out during the pre‐monsoon (March–May) and monsoon (June–September) over the continental, coastal and oceanic regions of South‐Asia. For this purpose, 17 years (1998–2014) of TRMM‐PR and 4 years (2007–2010) of CloudSat‐CPR observations are considered. Over the continental (coastal/oceanic) regions, the maximum rainfall contribution is by relatively deeper ETH20 dBZ (max) during the monsoon (pre‐monsoon). During the pre‐monsoon (monsoon), the maximum rainfall contribution by Deep Convective Systems (DCSs) is over the Northern‐Myanmar‐Coast; 55% (Western‐Himalaya‐Indentation; 39%). Over the central Arabian‐Sea and central Bay‐of‐Bengal, rainfall contributions by DCSs are predominantly associated with Tropical Cyclonic Disturbances (TCDs) in the pre‐monsoon season. In terms of ETH40 dBZ (max), the maximum rainfall contribution over the continental region during the pre‐monsoon is by relatively stronger Precipitation Features (PFs) compared to the monsoon. Unlike the other regions, during the pre‐monsoon, over the Western‐Himalaya‐Indentation and Central‐Himalaya‐Foothills, rainfall contribution by Intense Convective Systems (ICSs) is significantly higher compared to DCSs. During the pre‐monsoon (monsoon), the maximum rainfall contribution by ICSs is over the Eastern‐India‐Coast; 35% (Western‐Himalaya‐Indentation; 18%). The rainfall contribution by ICSs during TCDs is insignificant. During both the seasons, the regions of high rainfall contribution by ICSs, DCSs, and shallow PFs are associated with relatively higher (≥400 mg‐m−3), moderate (300–350 mg‐m−3), and low (<150 mg‐m−3) Cloud Ice Water Content (CIWC) in the mixed‐phase region, respectively, suggesting a strong, moderate and weak mixed‐phase processes involved in the respective PFs. TCDs enhance the occurrence of DCSs with moderate CIWC in mixed‐phase region and leading to enhanced rainfall contribution by them over the active cyclone regions.

Simulating a Mesoscale Convective System Using WRF With a New Spectral Bin Microphysics: 1: Hail vs Graupel

AbstractA modified Fast Spectral Bin Microphysics scheme (FSBM‐2) embedded into the Weather Research and Forecasting (WRF) model is used to simulate a mesoscale deep convective system observed during the Midlatitude Continental Convective Clouds Experiment (MC3E). FSBM‐2 uses modified source codes as compared to the current FSBM (FSBM‐1). In contrast to FSBM‐1, FSBM‐2 can simulate hail of several centimeters in diameter and includes additional processes such as spontaneous breakup of raindrops and aerosol regeneration by drop evaporation. It is shown that allowing large hail particles of diameters exceeding about 1 cm substantially increases the agreement between the simulated and observed squall‐line structures in both the convective and stratiform regions. In contrast, if graupel particles are used to represent high‐density hydrometeors in convective areas, the ratio of convective‐to‐stratiform areas diverges from the ratio seen in observations and maximum radar reflectivities are substantially underestimated. Analysis of snow size distributions in the stratiform area shows an important link between the ice crystals formed by homogeneous freezing in the convective area to ice particle number concentration in the stratiform region. Simulated raindrop size distributions in the stratiform area below the melting level from FSBM‐2 show a good agreement with observations. The regeneration of cloud condensational nuclei (CCN) by droplet evaporation and detrainment of these CCN to the stratiform region increases the concentration of CCN there up to 8‐ to 9‐km altitude; the additional CCN penetrate clouds and produce new droplets, leading to some convection intensification.

Analysis of Thunderstorms Producing Terrestrial Gamma Ray Flashes With the Meteosat Second Generation

AbstractUp to now, only few works focused on the meteorological context leading to the production of Terrestrial Gamma ray Flashes (TGFs). In this study, we carry out, for the first time, an analysis on large scale of the meteorological scenario linked to 278 TGFs detected by RHESSI, AGILE, and Fermi, by using the Meteosat Second Generation geostationary satellites. These satellites are useful as they continuously monitor the same geographic region in time, allowing investigations on thunderstorms' development; moreover, they are endowed with channels and products that provide information about the meteorological context under analysis, such as the cloud top temperature and altitude, the cloud extension, the drop effective radius and the cloud phase. Our work confirms what previously found in other studies about the TGF‐associated thunderstorms, by using a different approach and by using for the first time the Meteosat satellites: we find TGFs mostly linked to the development phase of deep convective thunderstorm systems, exhibiting typical characteristics of tropical storms, and providing a first picture on large scale of the TGF‐associated thunderstorm systems.

Diurnal variation of high-level clouds from the synergy of AIRS and IASI space-borne infrared sounders

Abstract. By covering about 30 % of the Earth and by exerting a strong greenhouse effect, high-level clouds play an important role in the energy balance of our planet. Their warming and cooling effects within the atmosphere strongly depend on their emissivity. The combination of cloud data from two space-borne infrared sounders, the Atmospheric InfraRed Sounder, AIRS, and the Infrared Atmospheric Sounding Interferometer, IASI, which observe the Earth four times per day, allows us to investigate the diurnal variation of these high-level clouds by distinguishing between high opaque, cirrus, and thin cirrus clouds. We demonstrate that the diurnal phase and amplitude of high-level clouds can be estimated from these measurements with an uncertainty of 1.5 h and 20 %, respectively. By applying the developed methodology to AIRS and IASI cloud observations for the period of 2008–2015, we obtained monthly geographical distributions of diurnal phase and amplitude at a spatial resolution of 1∘ latitude ×1∘ longitude. In agreement with other studies, the diurnal cycle of high-level clouds is the largest over land in the tropics. At higher latitudes, their diurnal cycle is the largest during the summer. For selected continental regions we found diurnal amplitudes of cloud amount of about 7 % for high opaque clouds and for thin cirrus, and 9 % for cirrus. Over ocean, these values are 2 to 3 times smaller. The diurnal cycle of tropical thin cirrus seems to be similar over land and over ocean, with a minimum in the morning (09:00 LT) and a maximum during the night (01:00 LT). Tropical high opaque clouds have a maximum in the evening (21:00 LT over land), a few hours after the peak of convective rain. This lag can be explained by the fact that this cloud type includes not only the convective cores, but also part of the thicker anvils. Tropical cirrus show maximum coverage during the night (01:00 LT over land). This lag indicates that they are part of the deep convective cloud systems. However, the peak local times also vary regionally. We are providing a global monthly database of detected diurnal cycle amplitude and phase for each of these three high-level cloud types.

Ice Microphysical Properties near the Tops of Deep Convective Cores Implied by the GPM Dual-Frequency Radar Observations

Abstract Using three years of observations from the Dual-Frequency Precipitation Radar (DPR) aboard the Global Precipitation Measurement (GPM) Core Observatory, properties of the cores of deep convection are examined. First, deep convective systems are selected, defined as GPM precipitation features with maximum 20-dBZ echo-top heights above 10 km. The cores of deep convection are described by the profiles of Ku- and Ka-band radar reflectivity at the location of the highest echo top in each deep convective system. Then the dual-frequency ratio (DFR) profile is derived by subtracting Ka-band from Ku-band radar reflectivity. It is found that values of DFR are larger over land than over ocean in general near the top of the convection, which is consistent with larger ice particles in stronger updrafts in continental convection. The magnitude of DFR at 12 km is positively correlated with the convection intensity indicated by 20- and 30-dBZ echo tops. The microphysical properties including volume-weighted mean diameter, ice water content, and total ice particle number concentration are derived using a simple lookup table approach. Under the same particle size distribution assumption, the cores of deep convection over land have larger ice particle size, higher ice water content, and lower particle concentration than those over ocean at levels above 10 km, but with some distinct regional variations.

Main Detrainment Height of Deep Convection Systems over the Tibetan Plateau and Its Southern Slope

Deep convection systems (DCSs) can rapidly lift water vapor and other pollutants from the lower troposphere to the upper troposphere and lower stratosphere. The main detrainment height determines the level to which the air parcel is lifted. We analyzed the main detrainment height over the Tibetan Plateau and its southern slope based on the CloudSat Cloud Profiling Radar 2B_GEOPROF dataset and the Aura Microwave Limb Sounder Level 2 cloud ice product onboard the A-train constellation of Earth-observing satellites. It was found that the DCSs over the Tibetan Plateau and its southern slope have a higher main detrainment height (about 10–16 km) than other regions in the same latitude. The mean main detrainment heights are 12.9 and 13.3 km over the Tibetan Plateau and its southern slope, respectively. The cloud ice water path decreases by 16.8% after excluding the influences of DCSs, and the height with the maximum increase in cloud ice water content is located at 178 hPa (about 13 km). The main detrainment height and outflow horizontal range are higher and larger over the central and eastern Tibetan Plateau, the west of the southern slope, and the southeastern edge of the Tibetan Plateau than that over the northwestern Tibetan Plateau. The main detrainment height and outflow horizontal range are lower and broader at nighttime than during daytime.

The Relationship between the ITCZ and MJO Initiation over the Indian Ocean

Abstract Understanding convective initiation of the Madden–Julian oscillation (MJO) remains an unmet challenge. MJO initiation has been perceived as a process starting from a convectively suppressed large-scale condition with gradual growth of shallow convection to congestus and to deep convective and stratiform systems that cover a large-scale area. During the DYNAMO field campaign over the Indian Ocean, MJO initiation was observed to start from an existing intertropical convergence zone (ITCZ) south of the equator. This raises a question of what possible role the ITCZ may play in convective initiation of the MJO. This study addresses this question through analysis of satellite observations of precipitation and a global reanalysis product. By setting several criteria, MJO and ITCZ events were objectively identified and grouped according to whether MJO initiation was immediately preceded by an ITCZ. The results demonstrate that an ITCZ is neither a necessary nor sufficient condition for convective initiation of the MJO. Nonetheless, evolution of the large-scale circulation, moisture, and convective characteristics during MJO initiation can be different with and without a preexisting ITCZ. Convective growth begins gradually before and during MJO initiation when there is a preexisting ITCZ whereas it is abrupt and slightly delayed without a preexisting ITCZ. Such differences are presumably related to the existing large-scale moist condition of the ITCZ. The results from this study suggest that there are multiple mechanisms for convective initiation of the MJO, which should be considered in theoretical understanding of the MJO.

The Importance of the Ice-Phase Microphysics Parameterization for Simulating the Effects of Changes to CCN Concentrations in Deep Convection

AbstractSimulations of a well-observed squall line that occurred during the Midlatitude Continental Convective Clouds Experiment (MC3E) were conducted using a mesoscale model with a horizontal grid spacing of 1 km to examine the importance of parameterized ice-phase processes to changes in concentrations of activated cloud condensation nuclei (CCN) in a detailed two-moment bulk microphysics scheme. Numerical experiments showed that the simulated squall-line structure was sensitive to changes in activated CCN concentration not only from the direct impacts on cloud droplet sizes and autoconversion rates, but also because of changes in the growth rates and spatial distribution of ice-phase condensate. A microphysical budget analysis highlighted the importance of graupel in rain production and the sensitivity of graupel growth rates on changes to CCN concentrations. Sensitivity tests on the level of detail in the representation of graupel, specifically the treatment of its bulk density and the number of prognostic moments, indicated that changes in the reflectivity and precipitation structure of the simulated storm due to changes in CCN were sensitive to the graupel parameterization. The results suggest that the proper representation of graupel and possibly other ice-phase categories in microphysics schemes may be crucial for correctly simulating the effects of changes to CCN concentrations for continental deep convective systems.

New Insights into the Convective System Characteristics Over the Indian Summer Monsoon Region Using Space-Based Passive and Active Remote Sensing Techniques†

The characteristics of convective systems over the Indian summer monsoon (ISM) region derived by using a unique combination of four satellites METEOSAT, TRMM, COSMIC and CloudSat comprising of both active and passive remote sensing sensors are discussed for the first time. Six years of observations during ISM brought out the preferential geographical regions and time for formation of shallow, deep, and very deep convective systems. It has been observed that shallow, deep, and very deep convective clouds are abundant over ISM region and their distributions show a relatively high frequency of occurrence over the Head Bay of Bengal as compared to other regions. The mean vertical structure of clouds are also been studied using CloudSat observations over several latitudinal belts. The time of maximum occurrence of convective type cloud systems are also quantified using the present observations. The analysis suggests that the deep and very deep convective cloud systems peak during late afternoon hours over many parts of the Indian landmass with few exceptions such as over foothills of the Himalayas, where it peaks during the early morning hours. The three-dimensional structures of convective systems are also discussed. The significance of the present results lie in providing new insights into the characteristics of convective systems by investigating CAPE, clouds, and precipitation, which in turn play a vital role in understanding the internal dynamics of ISM.