Convective Regions Research Articles

Analysis of a long-lived, two-cell lightning storm on Saturn

Lightning storms in Saturn’s atmosphere can last for a few days up to several months. In this paper we analyze a lightning storm that raged for seven and a half months at a planetocentric latitude of 35° south from the end of November 2007 until mid-July 2008. Thunderstorms observed by the Cassini spacecraft before this time were characterized by a single convective storm region of ~2000 km in size, but this storm developed two distinct convective storm cells at the same latitude separated by ~25° in longitude. The second storm cell developed in March 2008, and the entire two-cell convective system was moving with a westward drift velocity of about 0.35 deg per day, which differs from the zonal wind speed. An exhaustive data analysis shows that the storm system produced ~277000 lightning events termed Saturn electrostatic discharges (SEDs) that were detected by Cassini’s Radio and Plasma Wave Science (RPWS) instrument, and they occurred in 439 storm episodes. We analyzed the SED intensity distributions, the SED polarization, the burst rates, and the burst and episode durations. During this storm Cassini made several orbits around Saturn and observed the SEDs from all local times. A comparison with optical observations shows that SEDs can be detected when the storm is still beyond the visible horizon. We qualitatively describe this so-called over-the-horizon effect which is thought to be due to a temporary trapping of SED radio waves below Saturn’s ionosphere. We also describe the first occurrence of so-called SED pre- and post-episodes, which occur in a limited frequency range around 4 MHz separated from the main episode. Pre- and post-episodes were mostly observed by Cassini located at local noon, and should be a manifestation of an extreme over-the-horizon effect. Combined radio and imaging observations suggest that some decreases in SED activity are caused by splitting of the thunderstorm into a bright cloud and a dark oval.

Simulation of Inverted Charge Structure Formation in Convective Regions of Mesoscale Convective System

Simulation of Inverted Charge Structure Formation in Convective Regions of Mesoscale Convective System

Differences in the Tropical Convective Activities at the Opposite Phases of the Quasi-Biennial Oscillation

Differences in convective activities in the tropical region (30°S–30°N, 180°E–180°W) during different phases of the quasi-biennial oscillation (QBO) are investigated over 32 years (1979–2010) using five metrics representing tropical convection: (i) precipitation and (ii) outgoing longwave radiation from observations and (iii) convective available potential energy (CAPE), (iv) deep convective heating rate, and (v) convective cloud top pressure from reanalysis data. The easterly (QBOE) and westerly (QBOW) phases of the QBO are defined using the zonal wind anomaly from the monthly climatology at 50 hPa. During the QBOE (QBOW), the convective activities are intensified (weakened) over the Maritime Continent and weakened (intensified) over the equatorial eastern and central Pacific. Therefore, the zonal mean values of the five metrics averaged over chronically convective regions show stronger convective activities during the QBOE than during the QBOW, while the opposite is true for the whole tropical region. Composite analyses are also performed during the neutral, El Nino, and La Nina periods. In the neutral period, the convective activities during QBOE are stronger than during QBOW except in the equatorial region (10°S–10°N). The convective activities over the Maritime Continent (central and eastern Pacific) are enhanced when La Nina and the QBOE (El Nino and the QBOW) occur simultaneously. All metrics show similar pattern to one another, implying that the metrics from reanalysis data represent the variations in the convective activities with respect to the QBO reasonably well. Among the five metrics, the CAPE is most sensitive to the QBO phase, likely because the virtual temperature in the upper troposphere is modulated by anomalous meridional circulations induced by different QBO phases.

Vortex-excited acoustic resonance in channel with coaxial side-branches: Vortex dynamics and aeroacoustic energy transfer

Vortex dynamics and aeroacoustic energy transfer, which play essential roles in vortex-excited acoustic resonance inside straight channels with coaxial side-branches, were investigated by phase-locked particle image velocimetry (PIV) and Howe’s acoustic analogy. In the experiments, the periodic acoustic pressure fluctuations at the endplates of the side branches were used to trigger PIV via a field-programmable gate array control system. The results revealed that the spatiotemporal evolution of vortex shedding can be classified into three regions in response to the acoustic standing-wave propagations: the formation region, the convection region, and the collapse region, along with the flapping recirculation zone and the intermittent vertical flow streaks that occur inside the side branches. Further investigation was performed in terms of phase-dependent quantities such as the shear and normal stresses; the normal stress production, which was attributed to the evolution of vortex shedding, was found to be the major contributor to the kinematics and energetics of the self-sustained flow. Finally, Howe’s acoustic analogy was used to determine the instantaneous acoustic power and the accumulated aeroacoustic energy during one acoustic resonance cycle. The aeroacoustic energy extracted from the acoustic standing-wave propagations contributed to the formation and subsequent growth of the shedding vortex, whereas the decreased turbulent kinetic energy of the shedding vortex was transferred to the acoustic standing waves to maintain the longitudinal wave propagations.

Testing Angular Momentum Transport and Wind Loss in Intermediate-mass Core-helium Burning Stars

Abstract Stars between 2 and 3 M ☉ rotate rapidly on the main sequence, and their rotation rates in the core-helium burning (secondary-clump) phase can therefore be used to test models of angular momentum loss used for gyrochronology in a new regime. Because both their core and surface rotation rates can be measured, these stars can also be used to set strong constraints on angular momentum transport inside stars. We find that they are rotating slower than angular momentum conservation and rigid rotation would predict. Our results are insensitive to the degree of core–envelope coupling because of the small moment of inertia of the radiative core. We discuss two possible mechanisms for slowing down the surfaces of these stars: (1) substantial angular momentum loss and (2) radial differential rotation in the surface convection zone. Modern angular momentum loss prescriptions used for solar-type stars predict secondary-clump surface rotation rates in much better agreement with the data than prior variants used in the literature, and we argue that such enhanced loss is required to understand the combination of core and surface rotation rates. However, we find that the assumed radial differential rotation profile in convective regions has a strong impact on the predicted surface rotation rates, and that a combination of enhanced loss and radial differential rotation in the surface convection zone is also consistent with the data. We discuss future tests that can quantify the impact of both phenomena. Current data tentatively suggest that some combination of the two processes fits the data better than either one alone.

Steric Level Fluctuations and Deep Convection in the Labrador and Irminger Seas

The paper considers steric level fluctuations in the northern Atlantic Ocean. We use a method that combines AVISO altimetry and GRACE gravity measurements to evaluate steric level fluctuations and obtain estimates of steric variations in the Labrador and Irminger seas for 2003–2015. The range of steric fluctuations in the Labrador Sea is from –11 to 10 cm, and in the Irminger Sea is from –11 to 12 cm. We estimate trends of steric fluctuations, which indicate a significant increase in the steric component of variability in the level of the North Atlantic. We propose a method for determining regions of deep convection in the Labrador and Irminger seas based on the minimum values of the steric level anomalies (the seasonal component has been excluded) from combined satellite measurement data. Possible spots of deep convection are determined in the Labrador and Irminger seas and shown on steric level fluctuation maps for different years. We demonstrate that deep convection was not manifested in the Labrador Sea in 2006, and there was a general weakening in deep convection processes in the North Atlantic after 2008.

The effect of secondary ice production parameterization on the simulation of a cold frontal rainband

Abstract. Secondary ice production via processes like rime splintering, frozen droplet shattering, and breakup upon ice hydrometeor collision have been proposed to explain discrepancies between in-cloud ice crystal and ice-nucleating particle numbers. To understand the impact of this additional ice crystal generation on surface precipitation, we present one of the first studies to implement frozen droplet shattering and ice–ice collisional breakup parameterizations in a mesoscale model. We simulate a cold frontal rainband from the Aerosol Properties, PRocesses, And InfluenceS on the Earth's Climate campaign and investigate the impact of the new parameterizations on the simulated ice crystal number concentrations (ICNC) and precipitation. Near the convective regions of the rainband, contributions to ICNC can be as large from secondary production as from primary nucleation, but ICNCs greater than 50 L−1 remain underestimated by the model. The addition of the secondary production parameterizations also clearly intensifies the differences in both accumulated precipitation and precipitation rate between the convective towers and non-convective gap regions. We suggest, then, that secondary ice production parameterizations be included in large-scale models on the basis of large hydrometeor concentration and convective activity criteria.

Modified NAM Microphysics for Forecasts of Deep Convective Storms

Abstract The Ferrier–Aligo (FA) microphysics scheme has been running operationally in the National Centers for Environmental Prediction (NCEP) North American Mesoscale Forecast System (NAM) since August 2014. It was developed to improve forecasts of deep convection in the NAM contiguous United States (CONUS) nest, and it replaces previous versions of the NAM microphysics. The FA scheme is the culmination of extensive microphysical scheme sensitivity experiments made over nearly a dozen warm- and cool-season severe weather cases, as well as an extensive real-time testing in a full, system-wide developmental version of the NAM. While the FA scheme advects each hydrometeor species separately, it was the mass-weighted rime factor (RF) that allowed rimed ice to be advected to very cold temperatures aloft and improved the vertical structure of deep convection. Rimed ice fall speeds were reduced in order to offset an increase in bias of heavy precipitation as a consequence of the mass-weighted RF advection. The FA scheme also incorporated findings from 3-km model runs using the Thompson scheme, including 1) improved closure assumptions for large precipitating ice that targeted the convective and anvil regions of storms, 2) a new diagnostic calculation of radar reflectivity from rimed ice in association with intense convection, and 3) a variable rain intercept parameter that reduced widespread spurious weak reflectivity from shallow boundary layer clouds and increased stratiform rainfall.

Penetrative Convection in Partly Stratified Rapidly Rotating Spherical Shells

Celestial objects host interfaces between convective and stable stratified interior regions. The interaction between both, e.g., the transfer of heat, mass, or angular momentum depends on whether and how flows penetrate into the stable layer. Powered from the unstable, convective regions, radial flows can pierce into the stable region depending on their inertia (overshooting). In rapidly rotating systems, the dynamics are strongly influenced by the Coriolis force and radial flows penetrate in stratified regions due to the geostrophic invariance of columnar convection even in the limit of vanishing inertia. Within this study, we numerically investigate both mechanisms and hence explore the nature of penetrative convection in rapidly rotating spherical shells. The study covers a broad range of system parameters, such as the strength of the stratification relative to the Coriolis force or the inertia. Guided by the application to Saturn, we model a sandwiched stable stratified layer (SSL) surrounded by two convective zones. A comprehensive analysis of the damping behavior of convective flows at the edges of the SSL showed that the mean penetration depth is controlled by the ratio of stratified and unstratified buoyancy gradients and is hence independent of rotation. A scaling law is derived and suggests that the penetration depth decreases with the square root of the ratio of unstabilizing and stabilizing entropy gradients. The influence of the Coriolis force, however, is evident by a modulation of the penetration depth along latitude, since convective columns are elongated vertically and hence pierce predominantly into the SSL around mid-latitudes and outside the tangent cylinder. Our result also show that the penetration depth decreases linearly with the flow length scale (low pass filter), confirming predictions from the linear theory of rotating partially stratified convection.

Southern Hemisphere forcing of South Asian monsoon precipitation over the past ~1 million years

The orbital-scale timing of South Asian monsoon (SAM) precipitation is poorly understood. Here we present new SST and seawater δ18O (δ18Osw) records from the Bay of Bengal, the core convective region of the South Asian monsoon, over the past 1 million years. Our records reveal that SAM precipitation peaked in the precession band ~9 kyrs after Northern Hemisphere summer insolation maxima, in phase with records of SAM winds in the Arabian Sea and eastern Indian Ocean. Precession-band variance, however, accounts for ~30% of the total variance of SAM precipitation while it was either absent or dominant in records of the East Asian monsoon (EAM). This and the observation that SAM precipitation was phase locked with obliquity minima and was sensitive to Southern Hemisphere warming provides clear evidence that SAM and EAM precipitation responded differently to orbital forcing and highlights the importance of internal processes forcing monsoon variability.

Towards understanding the global and regional climatic impacts of Modoki magnitude

Abstract Earlier studies suggest that the frequency and the strength of ENSO Modoki events increased considerably since last few decades (1970–2010) resulting in a need to revisit the climatic impacts of varying magnitude of ENSO Modoki. Hence, to better understand the impact of ENSO Modoki amplitude over the tropical and extra-tropical regions, especially in the Pacific, Asian and African regions, we conducted ENSO Modoki sensitivity experiments using ICTP-AGCM (SPEEDY). One of the main interests of this study is to see how ICTP-AGCM can reproduce teleconnections induced by ENSO Modoki events and how sensitive is the global and regional climate to ENSO Modoki strength. SPEEDY model qualitatively reproduces the impact of ENSO Modoki over the Pacific, Atlantic, North and South America and African regions very well. However, it underestimates ENSO Modoki-induced teleconnection patterns and associated changes in South Asia, particularly in the Indian region. This study suggests a nonlinear climatic response to increased magnitude of ENSO Modoki. Our results reveal that like conventional ENSO, ENSO Modoki also induces considerable impact over North Pacific (Atlantic) region and initiates strong PNA (NAO) like response. ENSO Modoki-induced negative/positive NAO-like response and associated changes in Southern Europe and North Africa region get significantly strong following increased intensity of El Nino/La Nina Modoki in the boreal winter. We further find that ENSO Modoki magnitude significantly impacts tropical and high latitude circulation cells. The positive phase of ENSO (El Nino) overall strengthens Hadley Cell and a reverse is true for La Nina phase. ENSO Modoki-induced strengthening and weakening of Hadley Cell induce significant impact over South Asian and African ITCZ convective regions through modification of ITCZ/monsoon circulation system.

Dissolved gas analysis of thermal faults in transformer liquids simulated using immersed heating method

Dissolved gas analysis (DGA) is considered as one of the most effective techniques to diagnose transformer incipient faults. With the increasing applications of alternative liquids in transformers, fault gas generation characteristics of the liquids were investigated in laboratory under simulated faults. Immersed heating method has been widely used to simulate thermal faults, of which the complete fault temperature range was hardly achieved. In this paper, pool boiling theory was introduced to explain the underlying reason that why the complete fault temperature range cannot be achieved using immersed heating method. There are three stages in pool boiling including natural convection region, nucleate boiling region and film boiling region. The Nukiyama temperature of a mineral oil was found to be 330°C. This is the highest stable temperature achieved by the immersed heating method in T2 region, beyond which the heating element temperature is unstable and hence temperature measurement is unreliable. The Nukiyama temperature of an ester liquid was found to be 390°C, which is higher than that of the mineral oil due to its higher boiling point. The findings imply that the previously published T2 DGA results obtained using immersed heating method at reported temperatures beyond the Nukiyama temperature have likely been in T3 region near to the melting temperature of the heating element (normally higher than 1000°C).

Magnetotail Configuration During a Steady Convection Event as Observed by Low‐Altitude and Magnetospheric Spacecraft

AbstractMotivated by ongoing discussion regarding the magnetic configuration in the near‐Earth and midtail regions and its role in populating the inner magnetosphere during long‐duration steady magnetospheric convection (SMC) events, we analyze a rich collection of observations during ~10 hr of strong, steady solar wind driving. Auroral boundaries and regions of stretched and dipolarized magnetic field in the plasma sheet were monitored using solar electron loss cone anisotropy observed by low‐altitude spacecraft. Following a southward turning of the interplanetary magnetic field and a subsequent 3‐ to 4‐hr period of large‐scale substorm‐related reconfigurations and plasma injections, the near‐Earth magnetic configuration evolved into a nonstandard type, which lasted until the end of this SMC event (5 hr). During that time a dipolarized region with complicated Bz landscape persisted in the midtail, while the configuration was very stretched in the near tail. This was manifested as a highly depressed magnetic Bz component at geostationary orbit and as persistent nonadiabatic electron scattering at the periphery of the outer radiation belt. In addition, in situ observations suggest that a thin current sheet extended longitudinally toward the dawn terminator. In the return convection region near the terminator, observations of this azimuthal current sheet were sporadically interrupted/modulated by earthward convecting plasma structures, either remnants of reconnection‐produced plasma bubbles or flapping waves. The hybrid magnetotail configuration (dipolar in the midtail and stretched in the near tail) observed during this long‐duration SMC event poses a challenge for empirical magnetospheric modeling.

Characteristics of a mesoscale convective system in a warm zone that created an extreme, short rainstorm in Southern China in May 2016

An extremely intense rainstorm occurred on 20 May 2016. It had been the heaviest rainfall event recorded in Xinyi, Guangdong Province, China, since 1960. The black body temperature (TBB) output of FY-2B satellite images, weather radar, and NCEP/NCAR reanalysis data were used to investigate the characteristics of a mesoscale convective system (MCS) associated with the rainstorm. Heavy rainstorms formed under the influence of an eastward migration of a low trough at 500 hPa, typical of warm area heavy rain. In front of the 500 hPa trough, there was strong coupling of high and low jet streams, high convective effective bit energy, strong middle and low water vapor convergence and high ground dew point temperature, creating a favorable environment for the formation and development of MCSs. The convective region of the linear convective cloud developed into an organized MCS, and Xinyi’s heavy rainfall occurred during the MCS lifecycle from formation to maturity to extinction. Five MCSs, with radar echo intensities of 40-55 dBz, created a ‘train effect’, which was the main driver of the heavy rainstorm. The strong echoes observed above 40 dBz were below 0°C, and the strongest echo center was near 2 km. The convective precipitation system had a low mass center and high precipitation efficiency. The echo top (with intensity ≥ 10 dBz) was as high as 19 km, and there was a deep ice phase growth belt above -10°C, which contributed to the heavy rainfall, which peaked at 132.8 mm in one hour.

Long-Range Transport of Water Channelized through the Southern Subtropical Jet

In this study, an air mass (containing a cirrus cloud) was detected by light detection and ranging (lidar) above São Paulo (Brazil) in June 2007 and tracked around the globe, thanks to Lagrangian calculations as well as ground-based and satellite observations. Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) data were also used to provide locations of occurrence of cirrus around the globe and extract their respective macro physical parameters (altitude and temperature). An analysis of the air mass history based on Lagrangian trajectories reveals that water coming from the Equator is channelized through the southern subtropical jet for weeks. In this case, the back-trajectories showed that the cirrus cloud detected at São Paulo was a mixture of air masses from two different locations: (1) the active convective area located around the Equator, with transport into the upper troposphere that promotes cirrus cloud formation; and (2) the South Pacific Ocean, with transport that follows the subtropical jet stream (STJ). Air masses coming from equatorial convective regions are trapped by the jet, which contributes to maintaining the lifetime of the cirrus cloud for a few days. The cloud disappears near the African continent, due to a southern excursion and warmer temperatures, then reappears and is detected again by the lidar system in São Paulo after 12 days. The observed cloud is located at a similar altitude, revealing that sedimentation is small or compensated by radiative uplift.

Computational modeling of batch stirred tank reactor based on spherical catalyst particles

This paper presents a model of a batch stirred tank reactor with spherical catalyst particles as microreactors. The model involves three regions: an array of porous enzyme-loaded microreactors where enzyme reaction as well as mass transfer by diffusion take place, a diffusion limiting region surrounding the particles and a convective region where the substrate is of uniform concentration. The microbioreactors are mathematically modeled by a two-compartment model based on reaction–diffusion equations containing a nonlinear term related to the Michaelis–Menten enzyme kinetics. The influence of the physical and kinetic parameters of the microbioreactors on the transient effectiveness of the bioreactor system is numerically investigated in a wide range of model parameters. The numerical simulation was carried out using the finite difference technique. The simulation results show non-monotonic effect of the initial substrate concentration and nonlinear effects of the internal and external diffusion limitations as well as adsorption capacity of the microreactors on the transient effectiveness.

Radar-Observed Bulk Microphysics of Midlatitude Leading-Line Trailing-Stratiform Mesoscale Convective Systems

AbstractIn 2013, all NEXRAD WSR-88D units in the United States were upgraded to dual polarization. Dual polarization allows for the identification of precipitation particle shape, size, orientation, and concentration. In this study, dual-polarization NEXRAD observations from 34 recent events are used to identify the bulk microphysical characteristics of a specific subset of mesoscale convective systems (MCSs), the leading-line trailing-stratiform (LLTS) MCS. NEXRAD observations are used to examine hydrometeor distributions in relative altitude to the 0°C level and as a function of storm life cycle, precipitation source (convective or stratiform), and storm environment. The analysis reveals that graupel particles are the most frequently classified hydrometeor class in a layer extending from the 0°C-level altitude to approximately 5 km above within the convective region. Below the 0°C level, rain is the most frequently classified hydrometeor, with small hail and graupel concentrations present throughout the LLTS system’s life cycle. The stratiform precipitation region contains small graupel concentrations in a shallow layer above the 0°C level, with pristine ice crystals being classified as the most frequently observed hydrometeor at higher altitudes and snow aggregates being classified as the most frequently observed hydrometeor at lower altitudes above the environmental 0°C level. Variations in most unstable convective available potential energy (MUCAPE) have the largest impact on the vertical distribution of hydrometeors, because more-unstable environments are characterized by a greater production of rimed ice.

The evolution and intensification of Cyclone Pam (2015) and resulting strong winds over the southern Pacific islands

Cyclone Pam (2015), a category-5 storm, in the southern Pacific in March 2015 caused severe damages over the islands states in the southern Pacific. Cyclone Pam was originated from an active convective region in the Madden-Julian oscillation (MJO) and evolved from convective clouds into a tropical cyclone. This study numerically investigated the effects of convective processes on the evolution and intensification of Cyclone Pam and the resulting strong winds with the use of the Weather Research and Forecasting (WRF) model through examining sensitivities to cloud microphysics schemes. The numerical simulations successfully reproduced the eastward propagation of the MJO and the aggregation of convective clouds that transformed to a tropical cyclone. It was found that microphysics processes affect the diabatic heating and therefore the evolution of tropical convection into a tropical cyclone. The potential instability for convective development and the actual development of deep cumulus clouds both contribute to the sensitivity of the simulated TC to the choice of the cloud microphysics schemes. Even with such sensitivity, this study successfully reproduced the cyclone track accurately in all the experiments and therefore is able to generate consistent hazard information from the cyclone.

Continuous Assimilation of Lightning Data Using Time‐Lagged Ensembles for a Convection‐Allowing Numerical Weather Prediction Model

AbstractIn this study, a lightning data assimilation method based on the time‐lagged ensembles for predicting severe convection is presented. With the lightning data assimilation scheme, the background error covariances are computed using time‐lagged ensembles, which consist of deterministic forecasts from eight forecast cycles initialized every 3 hr. Pseudo‐observations of graupel mixing ratio (qg) are retrieved from total lightning rates by utilizing empirical vertical profiles obtained from the simulation results of the previous forecast cycles, and the corresponding observation errors are estimated according to the uncertainties in the lightning observations and the empirical vertical profiles of qg. The increments of the model state variables are computed with the Kalman gain matrices and are continuously ingested into the Weather Research and Forecasting model via nudging terms acting on the prognostic equations over each time step during model integration. The effect of the lightning data assimilation scheme on convection analysis and forecast was assessed through a case study of a severe convective event, which took place in the Guangdong of China. Assimilating lightning data recovered many of the observed convective cells, suppressed the spurious convection, and corrected the displacement errors of the convective systems. Quantitative verifications indicate that forecast skills were improved mainly in the convective regions with the impact of assimilating lightning data on stratiform regions being overall less effective.

The energy flux spectrum of internal waves generated by turbulent convection

We present three-dimensional direct numerical simulations of internal waves excited by turbulent convection in a self-consistent, Boussinesq and Cartesian model of mixed convective and stably stratified fluids. We demonstrate that in the limit of large Rayleigh number ($Ra\in [4\times 10^{7},10^{9}]$) and large stratification (Brunt–Väisälä frequencies$f_{N}\gg f_{c}$, where$f_{c}$is the convective frequency), simulations are in good agreement with a theory that assumes waves are generated by Reynolds stresses due to eddies in the turbulent region as described in Lecoanet & Quataert (Mon. Not. R. Astron. Soc., vol. 430 (3), 2013, pp. 2363–2376). Specifically, we demonstrate that the wave energy flux spectrum scales like$k_{\bot }^{4}\,f^{-13/2}$for weakly damped waves (with$k_{\bot }$and$f$the waves’ horizontal wavenumbers and frequencies, respectively), and that the total wave energy flux decays with$z$, the distance from the convective region, like$z^{-13/8}$.