A climatology of quasi-linear convective systems and associated synoptic-scale environments in southern Brazil

Abstract The Madden–Julian oscillation (MJO) dominates the intraseasonal variability of cloud populations of the tropical Indian and Pacific Oceans. Suppressed MJO periods consist primarily of shallow and isolated deep convection. During the transition to an active MJO, the shallow and isolated deep clouds grow upscale into the overnight hours. During active MJO periods, mesoscale convective systems occur mostly during 2–4-day bursts of rainfall activity with a statistically significant early morning peak. Yet when these rain events are separated into individual active periods, some periods do not follow the mean pattern, with the November events in particular exhibiting an afternoon peak. The radar-observed microphysical processes producing the precipitation during the major rain events of active MJO periods evolve in connection with synoptic-scale wave passages with varying influences of diurnal forcing. MJO studies that do not account for the intermittency of rainfall during active MJO phases through averaging over multiple events can lead to the misimpression that the primary rain-producing clouds of the MJO are modulated solely by the diurnal cycle.

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Different Representation of Mesoscale Convective Systems in Convection-Permitting and Convection-Parameterizing NWP Models and Its Implications for Large-Scale Forecast Evolution

Atmosphere ◽

10.3390/atmos10090503 ◽

2019 ◽

Vol 10 (9) ◽

pp. 503 ◽

Cited By ~ 4

Author(s):

Karsten Peters ◽

Cathy Hohenegger ◽

Daniel Klocke

Keyword(s):

Large Scale ◽

Weather Prediction ◽

Mesoscale Convective Systems ◽

Limited Area ◽

Synoptic Scale ◽

Mean Flow ◽

Convective Systems ◽

Western Africa ◽

Mesoscale Convective ◽

The Difference

Representing mesoscale convective systems (MCSs) and their multi-scale interaction with the large-scale atmospheric dynamics is still a major challenge in state-of-the-art global numerical weather prediction (NWP) models. This results in potentially defective forecasts of synoptic-scale dynamics in regions of high MCS activity. Here, we quantify this error by comparing simulations performed with a very large-domain, convection-permitting NWP model to two operational global NWP models relying on parameterized convection. We use one month’s worth of daily forecasts over Western Africa and focus on land regions only. The convection-permitting model matches remarkably well the statistics of westward-propagating MCSs compared to observations, while the convection-parameterizing NWP models misrepresent them. The difference in the representation of MCSs in the different models leads to measurably different synoptic-scale forecast evolution as visible in the wind fields at both 850 and 650 hPa, resulting in forecast differences compared to the operational global NWP models. This is quantified by computing the correlation between the differences and the number of MCSs: the larger the number of MCSs, the larger the difference. This fits the expectation from theory on MCS–mean flow interaction. Here, we show that this effect is strong enough to affect daily limited-area forecasts on very large domains.

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Effects of Rotation on the Multiscale Organization of Convection in a Global 2D Cloud-Resolving Model

Journal of the Atmospheric Sciences ◽

10.1175/jas-d-19-0041.1 ◽

2019 ◽

Vol 76 (11) ◽

pp. 3669-3696

Author(s):

Qiu Yang ◽

Andrew J. Majda ◽

Noah D. Brenowitz

Keyword(s):

Surface Fluxes ◽

Vertical Shear ◽

Spatiotemporal Variability ◽

Asymptotic Model ◽

Synoptic Scale ◽

Convective Systems ◽

Planetary Scale ◽

Zonal Winds ◽

Budget Analysis ◽

Effects Of Rotation

Abstract Atmospheric convection exhibits distinct spatiotemporal variability at different latitudes. A good understanding of the effects of rotation on the multiscale organization of convection from the mesoscale to synoptic scale to planetary scale is still lacking. Here cloud-resolving simulations with fixed surface fluxes and radiative cooling are implemented with constant rotation in a two-dimensional (2D) planetary domain to simulate multiscale organization of convection from the tropics to midlatitudes. All scenarios are divided into three rotation regimes (weak, order-one, and strong) to represent the idealized ITCZ region (0°–6°N), the Indian monsoon region (6°–20°N), and the midlatitude region (20°–45°N), respectively. In each rotation regime, a multiscale asymptotic model is derived systematically and used as a diagnostic framework for energy budget analysis. The results show that planetary-scale organization of convection only arises in the weak rotation regime, while synoptic-scale organization dominates (vanishes) in the order-one (strong) rotation regime. The depletion of planetary-scale organization of convection as the magnitude of rotation increases is attributed to the reduced planetary kinetic energy of zonal winds, mainly due to the decreasing acceleration effect by eddy zonal momentum transfer from mesoscale convective systems (MCSs) and the increasing deceleration effect by the Coriolis force. Similarly, the maintenance of synoptic-scale organization is related to the acceleration effect by MCSs. Such decreasing acceleration effects by MCSs on both planetary and synoptic scales are further attributed to less favorable conditions for convection provided by weaker background vertical shear of the zonal winds, resulting from the increasing magnitude of rotation.

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Characteristics of Deep Cloud Systems under Weak and Strong Synoptic Forcing during the Indian Summer Monsoon Season

Monthly Weather Review ◽

10.1175/mwr-d-18-0346.1 ◽

2019 ◽

Vol 147 (10) ◽

pp. 3741-3758 ◽

Cited By ~ 2

Author(s):

Jayesh Phadtare ◽

G. S. Bhat

Keyword(s):

Monsoon Season ◽

Propagation Speed ◽

Maximum Size ◽

Gradient Field ◽

Synoptic Scale ◽

Pressure System ◽

Cloud Systems ◽

Convective Systems ◽

The North ◽

Synoptic Forcing

Abstract Synoptic-scale weather systems are often responsible for initiating mesoscale convective systems (MCSs). Here, we explore how synoptic forcing influences MCS characteristics, such as the maximum size, lifespan, cloud-top height, propagation speed, and triggering over the Indian region. We used 30-min interval infrared (IR) data of the Indian Kalpana-1 geostationary satellite. Cloud systems (CSs) in this data are identified and tracked using an object tracking algorithm. ERA-Interim 850-hPa vorticity is taken as a proxy for the synoptic forcing. The probability of CSs being larger, longer lived, and deeper is more in the presence of a synoptic-scale vorticity field; however, the influence of synoptic forcing is not evident on the westward propagation of CSs over land. There exists a linear relationship between maximum size, lifespan, and average cloud-top height of CSs regardless of the nature of synoptic forcing. Formation of CSs peaks around 1500 LST over land, which is independent of synoptic forcing. Over the north Bay of Bengal, CSs formation is predominantly nocturnal when synoptic forcing is strong, whereas, 0300 and 1200 LST are the preferred times when synoptic forcing is weak. Long-lived CSs are preferentially triggered in the western flank of the 850-hPa vorticity gradient field of a monsoon low pressure system. Once triggered, CSs propagate westward and ahead of the synoptic system and dissipate around midnight. Formation of new CSs on the next day occurs in the afternoon hours in the wake of previous day’s CSs and where vorticity gradient is also present. Formation and westward propagations of CSs on successive days move the synoptic envelope westward.

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Models for Multiscale Interactions. Part II: Madden–Julian Oscillation, Moisture, and Convective Momentum Transport

Meteorological Monographs ◽

10.1175/amsmonographs-d-15-0005.1 ◽

2016 ◽

Vol 56 ◽

pp. 10.1-10.5 ◽

Cited By ~ 2

Author(s):

Andrew J. Majda ◽

Samuel N. Stechmann

Keyword(s):

Tropical Cyclones ◽

Momentum Transport ◽

Mesoscale Convective Systems ◽

Equatorial Waves ◽

Madden Julian Oscillation ◽

Synoptic Scale ◽

Convective Systems ◽

Convectively Coupled Equatorial Waves ◽

Mesoscale Convective ◽

Convective Momentum Transport

Abstract It is well known that the envelope of the Madden–Julian oscillation (MJO) consists of smaller-scale convective systems, including mesoscale convective systems (MCS), tropical cyclones, and synoptic-scale waves called “convectively coupled equatorial waves” (CCW). In fact, recent results suggest that the fundamental mechanisms of the MJO involve interactions between the synoptic-scale CCW and their larger-scale environment (Majda and Stechmann). In light of this, this chapter reviews recent and past work on two-way interactions between convective systems—both MCSs and CCW—and their larger-scale environment, with a particular focus given to recent work on MJO–CCW interactions.

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Study of three days duration coupling between jets in South América

Revista Ibero-Americana de Ciências Ambientais ◽

10.6008/cbpc2179-6858.2021.002.0023 ◽

2021 ◽

Vol 12 (2) ◽

pp. 240-248

Author(s):

Emily Claudia Pereira Ramos ◽

Luiz Gabriel Cassol Machado ◽

André Becker Nunes

Keyword(s):

South America ◽

Reanalysis Data ◽

Mesoscale Convective Systems ◽

Southern Brazil ◽

Vertical Movements ◽

Convective Systems ◽

Mesoscale Convective ◽

Low Level Jet ◽

Upper Level ◽

Upper Level Jet

It can be understood by coupling between jets when Upper-Level Jet (ULJ) superimposes the Low-Level Jet (L LJ). The literature shows that such couplings tend to generate or intensify surface instabilities. Thus, the objective of this study was to analyze the synoptic configuration and the coupling between the jets associated with storms during the period of October 28-30, 2019, when instabilities hit southern Brazil causing intense precipitation and several damages. This work was carried out through the analysis of meteorological fields employing ERA5 reanalysis data and GOES-16 satellite imagery. The coupling between jets was verified in the three days of study. Upward vertical movements at 500 hPa was observed in the same area of occurrence of the upper level difluent flow, as well as an intense 850 hPa northerly flow, a large amount of moisture due to the action of the Northwestern Argentinean Low, and the presence of a frontal system between Uruguay and RS, except on the first day. Storms developed east (downstream) of the area where the coupling took place. The coupling was observed before and during the development of the mesoscale convective systems, and its dissipation occurred simultaneously with the storm. However, on the 30th, the peak of coupling did not occur together with the most intense phase of the system, it occurred before.

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Derecho and MCS Development, Evolution, and Multiscale Interactions during 3–5 July 2003

Monthly Weather Review ◽

10.1175/2010mwr3218.1 ◽

2010 ◽

Vol 138 (8) ◽

pp. 3048-3070 ◽

Cited By ~ 15

Author(s):

Nicholas D. Metz ◽

Lance F. Bosart

Keyword(s):

Gravity Wave ◽

Cold Pool ◽

Jet Stream ◽

Surface Boundary ◽

Initial Surface ◽

Synoptic Scale ◽

Convective Systems ◽

Mesoscale Convective ◽

Upper Level ◽

Upper Level Jet

Abstract From 3 to 5 July 2003 during the Bow Echo and Mesoscale Convective Vortex Experiment (BAMEX), multiple mesoscale convective systems (MCSs 1 and 2) and derechos (derechos AN, AS, A, BW, and BE) progressed across a preferred upper Midwest corridor. The derechos evolved in a favorable synoptic-scale environment. However, the environmental details associated with each derecho, such as the characteristics of the initial surface boundary, the formation position relative to the upper-level jet stream, the presence of an upper-level mesoscale disturbance, and the CAPE/shear environment varied from derecho to derecho. The MCSs and derechos composed three distinct convective episodes. Multiple mesoscale interactions between the MCSs and derechos and the environment altered the character and longevity of these episodes. The first convective episode consisted of derecho A, which formed from merging derechos AN and AS (northern and southern systems, respectively). The ∼200-hPa-deep cold pool associated with derecho A decreased surface potential temperatures by 4–8 K. MCS 1 dissipated upon entering this cold pool and an inertia–gravity wave was emitted that helped to spawn MCS 2. This inertia–gravity wave connected MCSs 1 and 2 into a compound convective episode. As derecho BW (western system) approached a strong surface boundary across Iowa created by the cold pools of derecho A and MCS 1, derecho BE (eastern system) formed. The remnants of derecho BW merged with derecho BE creating another compound convective episode. The upscale effects resulting from this active convective period directly affected subsequent convective development. Upper-level diabatic heating associated with derecho A resulted in NCEP GFS 66-h negative 1000–500-hPa thickness errors of 4–8 dam (forecast too cold) and negative 200-hPa wind errors of 10–20 m s−1 (forecast too weak). The resulting stronger than forecast 200-hPa jet stream likely increased synoptic-scale forcing for the formation and evolution of derecho BW.

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Effect of Stratiform Heating on the Planetary-Scale Organization of Tropical Convection

Journal of the Atmospheric Sciences ◽

10.1175/jas-d-15-0178.1 ◽

2015 ◽

Vol 73 (1) ◽

pp. 371-392 ◽

Cited By ~ 20

Author(s):

Qiang Deng ◽

Boualem Khouider ◽

Andrew J. Majda ◽

R. S. Ajayamohan

Keyword(s):

Deep Convection ◽

Low Frequency ◽

Lower Troposphere ◽

Warm Pool ◽

Mesoscale Convective Systems ◽

Model Parameters ◽

Synoptic Scale ◽

Convective Systems ◽

Planetary Scale ◽

Mesoscale Convective

Abstract It is widely recognized that stratiform heating contributes significantly to tropical rainfall and to the dynamics of tropical convective systems by inducing a front-to-rear tilt in the heating profile. Precipitating stratiform anvils that form from deep convection play a central role in the dynamics of tropical mesoscale convective systems. The wide spreading of downdrafts that are induced by the evaporation of stratiform rain and originate from in the lower troposphere strengthens the recirculation of subsiding air in the neighborhood of the convection center and triggers cold pools and gravity currents in the boundary layer, leading to further lifting. Here, aquaplanet simulations with a warm pool–like surface forcing, based on a coarse-resolution GCM of approximately 170-km grid mesh, coupled with a stochastic multicloud parameterization, are used to demonstrate the importance of stratiform heating for the organization of convection on planetary and intraseasonal scales. When the model parameters, which control the heating fraction and decay time scale of the stratiform clouds, are set to produce higher stratiform heating, the model produces low-frequency and planetary-scale MJO-like wave disturbances, while parameters associated with lower-to-moderate stratiform heating yield mainly synoptic-scale convectively coupled Kelvin-like waves. Furthermore, it is shown that, when the effect of stratiform downdrafts is reduced in the model, the MJO-scale organization is weakened, and a transition to synoptic-scale organization appears despite the use of larger stratiform heating parameters. Rooted in the stratiform instability, it is conjectured here that the strength and extent of stratiform downdrafts are key contributors to the scale selection of convective organizations, perhaps with mechanisms that are, in essence, similar to those of mesoscale convective systems.

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