Convective Cloud Clusters and Squall Lines along coastal Amazon

2021 ◽  
Author(s):  
A. C. Sousa ◽  
L. A. Candido ◽  
P. Satyamurty
Keyword(s):  
Mixed Layer ◽  
Convective Cloud ◽  
La Niña ◽  
Squall Line ◽  
Mixing Ratio ◽  
Squall Lines ◽  
La Nina ◽  
Coastal Belt ◽  
Cloud Clusters ◽  
The Mean

AbstractMesoscale convective cloud clusters develop and organize in the form of squall lines along the coastal Amazon in the afternoon hours and propagate inland during the evening hours. The frequency, location, organization into lines and movement of the convective systems are determined by analyzing the “precipitation features” obtained from the TRMM satellite for the period 1998-2014. The convective clusters and their alignments into Amazon coastal squall lines are more frequent from December through July and they mostly stay within 170 km from the coast line. Their development and movement in the afternoon and evening hours of about 14 m s-1 are helped by the sea breeze. Negative phase of Atlantic Dipole and La Niña combined increase the frequency of convective clusters over coastal Amazon. Composite environmental conditions of 13 large Amazon coastal squall line cases in April show that conditional instability increases from 09 LT to 12 LT and the wind profiles show a jet like structure in low levels. The differences in the vertical profiles of temperature and humidity between the large squall line composites and no-squall line composites are weak. However, appreciable increase in the mean value of CAPE from 09 LT to 15 LT is found in large squall line composite. The mean mixing ratio of mixed layer at 09 LT in La Niña situations is significantly larger in the large squall line composite. Thus, CAPE and mixed layer mixing ratio are considered promising indicators of the convective activity over the coastal belt of the Amazon Basin.

scholarly journals La Niña–like Mean-State Response to Global Warming and Potential Oceanic Roles

2017 ◽  
Vol 30 (11) ◽  
pp. 4207-4225 ◽  
Author(s):  
Tsubasa Kohyama ◽  
Dennis L. Hartmann ◽  
David S. Battisti
Keyword(s):  
Global Warming ◽  
Southern Oscillation ◽  
Forced Response ◽  
Equatorial Pacific ◽  
La Niña ◽  
The West ◽  
La Nina ◽  
The Mean ◽  
Mean State ◽  
Sst Gradient

Abstract The majority of the models that participated in phase 5 of the Coupled Model Intercomparison Project global warming experiments warm faster in the eastern equatorial Pacific Ocean than in the west. GFDL-ESM2M is an exception among the state-of-the-art global climate models in that the equatorial Pacific sea surface temperature (SST) in the west warms faster than in the east, and the Walker circulation strengthens in response to warming. This study shows that this “La Niña–like” trend simulated by GFDL-ESM2M could be a physically consistent response to warming, and that the forced response could have been detectable since the late twentieth century. Two additional models are examined: GFDL-ESM2G, which differs from GFDL-ESM2M only in the oceanic components, warms without a clear zonal SST gradient; and HadGEM2-CC exhibits a warming pattern that resembles the multimodel mean. A fundamental observed constraint between the amplitude of El Niño–Southern Oscillation (ENSO) and the mean-state zonal SST gradient is reproduced well by GFDL-ESM2M but not by the other two models, which display substantially weaker ENSO nonlinearity than is observed. Under this constraint, the weakening nonlinear ENSO amplitude in GFDL-ESM2M rectifies the mean state to be La Niña–like. GFDL-ESM2M exhibits more realistic equatorial thermal stratification than GFDL-ESM2G, which appears to be the most important difference for the ENSO nonlinearity. On longer time scales, the weaker polar amplification in GFDL-ESM2M may also explain the origin of the colder equatorial upwelling water, which could in turn weaken the ENSO amplitude.

scholarly journals Dipole Structure of Mixed Layer Salinity in Response to El Niño‐La Niña Asymmetry in the Tropical Pacific

2019 ◽  
Vol 46 (21) ◽  
pp. 12165-12172 ◽  
Author(s):  
Cong Guan ◽  
Shijian Hu ◽  
Michael J. McPhaden ◽  
Fan Wang ◽  
Shan Gao ◽  
...  
Keyword(s):  
Mixed Layer ◽  
El Niño ◽  
El Nino ◽  
Tropical Pacific ◽  
La Niña ◽  
Dipole Structure ◽  
La Nina ◽  
The Tropical Pacific

A Wave-Relative Framework Analysis of AEW–MCS Interactions Leading to Tropical Cyclogenesis

2020 ◽  
Vol 148 (11) ◽  
pp. 4657-4671
Author(s):  
Kelly M. Núñez Ocasio ◽  
Jenni L. Evans ◽  
George S. Young
Keyword(s):  
West Africa ◽  
Thermodynamic Characteristics ◽  
Convective Cloud ◽  
Type Systems ◽  
Squall Line ◽  
Tropical Cyclogenesis ◽  
Framework Analysis ◽  
Convective Systems ◽  
Cloud Clusters ◽  
Mesoscale Convective

AbstractAn African easterly wave (AEW) and associated mesoscale convective systems (MCSs) dataset has been created and used to evaluate the propagation of MCSs, AEWs, and, especially, the propagation of MCSs relative to the AEW with which they are associated (i.e., wave-relative framework). The thermodynamic characteristics of AEW–MCS systems are also analyzed. The analysis is done for both AEW–MCS systems that develop into tropical cyclones and those that do not to quantify significant differences. It is shown that developing AEWs over West Africa are associated with a larger number of convective cloud clusters (CCCs; squall-line-type systems) than nondeveloping AEWs. The MCSs of developing AEWs propagate at the same speed of the AEW trough in addition to being in phase with the trough, whereas convection associated with nondeveloping AEWs over West Africa moves faster than the trough and is positioned south of it. These differences become important for the intensification of the AEW vortex as this slower-moving convection (i.e., moving at the same speed of the AEW trough) spends more time supplying moisture and latent heat to the AEW vortex, supporting its further intensification. An analysis of the rainfall rate (MCS intensity), MCS area, and latent heating rate contribution reveals that there are statistically significant differences between developing AEWs and nondeveloping AEWs, especially over West Africa where the fraction of extremely large MCS areas associated with developing AEWs is larger than for nondeveloping AEWs.

Impact of Environmental Variations on Simulated Squall Lines Interacting with Terrain

2011 ◽  
Vol 139 (10) ◽  
pp. 3163-3183 ◽  
Author(s):  
Casey E. Letkewicz ◽  
Matthew D. Parker
Keyword(s):  
Wind Profile ◽  
Windward Side ◽  
Squall Line ◽  
Cold Pool ◽  
Convective System ◽  
Low Level ◽  
Convective Systems ◽  
Squall Lines ◽  
Strong Convection ◽  
The Mean

Abstract The complex evolution of convective systems crossing (or attempting to cross) mountainous terrain represents a substantial forecasting challenge. This study examines the processes associated with environments of “crossing” squall lines (which were able to redevelop strong convection in the lee of a mountain barrier) and “noncrossing” squall lines (which were not able to redevelop strong convection downstream of the barrier). In particular, numerical simulations of mature convective systems crossing idealized terrain roughly approximating the Appalachian Mountains were used to test the first-order impact of variations in the vertical wind profile upon system maintenance. By itself, the wind profile showed no ability to uniquely discriminate between simulated crossing and noncrossing squall lines; each test revealed a similar pattern of orographic enhancement, suppression, and lee reinvigoration in which a hydraulic jump deepened the system’s cold pool and renewed the low-level lifting. Increasing the mean wind led to greater enhancement of vertical velocities on the windward side of the barrier and greater suppression on the lee side. Variations in the low-level shear influenced the temperature and depth of the outflow, which in turn altered the lifting along the system’s gust front. However, in all of the wind profile tests, convection redeveloped in the lee. Additional simulations explored more marginal environments in which idealized low-level cooling or drying stabilized the downstream environment. In most such tests, the systems weakened but the presence of CAPE aloft still enabled the systems to survive in the lee. However, the combination of a stronger mean wind with diminished CAPE and increased convective inhibition (CIN) was ultimately found to eliminate downstream redevelopment and produce a noncrossing mesoscale convective system (MCS). Within these experiments, the ability of a squall line to cross a barrier similar to the Appalachians is primarily tied to the characteristics of the downstream thermodynamic environment; however, as the lee thermodynamic environment becomes less favorable, the mean wind exerts a greater influence on system intensity and redevelopment.

Causes of the El Niño and La Niña Amplitude Asymmetry in the Equatorial Eastern Pacific

2010 ◽  
Vol 23 (3) ◽  
pp. 605-617 ◽  
Author(s):  
Jingzhi Su ◽  
Renhe Zhang ◽  
Tim Li ◽  
Xinyao Rong ◽  
J-S. Kug ◽  
...  
Keyword(s):  
Data Assimilation ◽  
Mixed Layer ◽  
El Niño ◽  
El Nino ◽  
La Niña ◽  
Eastern Pacific ◽  
Ocean Data Assimilation ◽  
La Nina ◽  
Nonlinear Advection ◽  
Far Eastern

Abstract The amplitude asymmetry between El Niño and La Niña is investigated by diagnosing the mixed-layer heat budget during the ENSO developing phase by using the three ocean assimilation products: Simple Ocean Data Assimilation (SODA) 2.0.2, SODA 1.4.2, and the Global Ocean Data Assimilation System (GODAS). It is found that the nonlinear zonal and meridional ocean temperature advections are essential to cause the asymmetry in the far eastern Pacific, whereas the vertical nonlinear advection has the opposite effect. The zonal current anomaly is dominated by the geostrophic current in association with the thermocline depth variation. The meridional current anomaly is primarily attributed to the Ekman current driven by wind stress forcing. The resulting induced anomalous horizontal currents lead to warm nonlinear advection during both El Niño and La Niña episodes and thus strengthen (weaken) the El Niño (La Niña) amplitude. The convergence (divergence) of the anomalous geostrophic mixed-layer currents during El Niño (La Niña) results in anomalous downwelling (upwelling) in the far eastern equatorial Pacific, which leads to a cold nonlinear vertical advection in both warm and cold episodes.

The NCEP GODAS Ocean Analysis of the Tropical Pacific Mixed Layer Heat Budget on Seasonal to Interannual Time Scales

2010 ◽  
Vol 23 (18) ◽  
pp. 4901-4925 ◽  
Author(s):  
Boyin Huang ◽  
Yan Xue ◽  
Dongxiao Zhang ◽  
Arun Kumar ◽  
Michael J. McPhaden
Keyword(s):  
Data Assimilation ◽  
Mixed Layer ◽  
Heat Budget ◽  
La Niña ◽  
Sst Variability ◽  
Ocean Data Assimilation ◽  
La Nina ◽  
Mixed Layer Heat Budget ◽  
Data Assimilation System ◽  
The Tropical Pacific

Abstract The mixed layer heat budget in the tropical Pacific is diagnosed using pentad (5 day) averaged outputs from the Global Ocean Data Assimilation System (GODAS), which is operational at the National Centers for Environmental Prediction (NCEP). The GODAS is currently used by the NCEP Climate Prediction Center (CPC) to monitor and to understand El Niño and La Niña in near real time. The purpose of this study is to assess the feasibility of using an operational ocean data assimilation system to understand SST variability. The climatological mean and seasonal cycle of mixed layer heat budgets derived from GODAS agree reasonably well with previous observational and model-based estimates. However, significant differences and biases were noticed. Large biases were found in GODAS zonal and meridional currents, which contributed to biases in the annual cycle of zonal and meridional advective heat fluxes. The warming due to tropical instability waves in boreal fall is severely underestimated owing to use of a 4-week data assimilation window. On interannual time scales, the GODAS heat budget closure is good for weak-to-moderate El Niños. A composite for weak-to-moderate El Niños suggests that zonal and meridional temperature advection and vertical entrainment/diffusion all contributed to the onset of the event and that zonal advection played the dominant role during decay of the event and the transition to La Niña. The net surface heat flux acts as a damping during the development stage, but plays a critical role in the decay of El Niño and the transition to the following La Niña. The GODAS heat budget closure is generally poor for strong La Niñas. Despite the biases, the GODAS heat budget analysis tool is useful in monitoring and understanding the physical processes controlling SST variability associated with ENSO. Therefore, it has been implemented operationally at CPC in support of NOAA’s ENSO forecasting.

scholarly journals Characteristics of Wind Structure and Nowcasting of Gust Associated with Subtropical Squall Lines over Hong Kong and Shenzhen, China

Atmosphere ◽  
2020 ◽  
Vol 11 (3) ◽  
pp. 270 ◽  
Author(s):  
Junyi He ◽  
P.W. Chan ◽  
Qiusheng Li ◽  
Lei Li ◽  
Chao Lu ◽  
...  
Keyword(s):  
Surface Wind ◽  
Wind Engineering ◽  
Inertial Subrange ◽  
Squall Line ◽  
High Temporal Resolution ◽  
Wind Gust ◽  
Wind Structure ◽  
Squall Lines ◽  
The Mean ◽  
The Pearl River

A number of squall line passages over the Pearl River Delta (PRD) are documented for the first time based on the wind data collected by a 356 m-high meteorological tower with high temporal resolution (10 Hz). The mean wind and turbulence characteristics are studied based on a sample of six cases. The mean wind profile is consistent with the international standard for wind engineering, but the turbulence intensity profile has quite significant deviations. Moreover, the energy spectrum of the fluctuating wind is found to be consistent with the −5/3 law, as expected in the inertial subrange. For the purpose of wind gust nowcasting, the performance of a nowcasting algorithm based on upper air wind and thermodynamic profiles is examined using the limited dataset. The results highlight that the mean wind gust estimates are sufficient to nowcast the surface wind gust as measured by the anemometers, but for an extreme squall line case, the maximum wind gust estimates would be useful. The information in this paper is believed to be useful for wind engineering applications and weather nowcasting for wind gusts associated with subtropical squall lines, and, to the best of the authors’ knowledge, this is the first study of its kind for the PRD region.

scholarly journals Climate Variability and the Shape of Daily Precipitation: A Case Study of ENSO and the American West

2011 ◽  
Vol 24 (10) ◽  
pp. 2483-2499 ◽  
Author(s):  
Nicole Feldl ◽  
Gerard H. Roe
Keyword(s):  
American West ◽  
Large Scale ◽  
Daily Precipitation ◽  
La Niña ◽  
Precipitation Distribution ◽  
Enso Variability ◽  
La Nina ◽  
The Mean ◽  
Change In Mean

Abstract Characterizing the relationship between large-scale atmospheric circulation patterns and the shape of the daily precipitation distribution is fundamental to understanding how dynamical changes are manifest in the hydrological cycle, and it is also relevant to issues such as natural hazard mitigation and reservoir management. This relationship is pursued using ENSO variability and the American West as a case study. When considering the full range of wintertime precipitation and consistent with conventional wisdom, mean precipitation intensity is enhanced during El Niño relative to La Niña in the Southwest and vice versa in the Northwest. This change in mean is attributed to a shift in the distribution of daily precipitation toward more intense daily rainfall rates. In addition, fundamental changes in the shape of the precipitation distributions are observed, independent of shifts in the mean. Surprisingly, for intense precipitation, La Niña winters actually demonstrate a significant increase in intensity (but not frequency) across the Southwest. A main lesson from this analysis is that, in response to ENSO variability, changes in extreme events can be significantly different from changes in the mean. In some instances, even the sign of the change is reversed. This result suggests that patterns of large-scale variability have an effect on the precipitation distribution that is nuanced, and they cannot be regarded as simply causing a shift in climatic zones. It also raises interesting questions concerning how best to establish confidence in climate predictions.

scholarly journals Detecting spatial patterns with the cumulant function – Part 2: An application to El Niño

2008 ◽  
Vol 15 (1) ◽  
pp. 169-177 ◽  
Author(s):  
A. Bernacchia ◽  
P. Naveau ◽  
M. Vrac ◽  
P. Yiou
Keyword(s):  
Spatial Patterns ◽  
Principal Components ◽  
El Niño ◽  
El Nino ◽  
Value Theory ◽  
La Niña ◽  
Sample Mean ◽  
Cumulant Function ◽  
La Nina ◽  
The Mean

Abstract. The spatial coherence of a measured variable (e.g. temperature or pressure) is often studied to determine the regions of high variability or to find teleconnections, i.e. correlations between specific regions. While usual methods to find spatial patterns, such as Principal Components Analysis (PCA), are constrained by linear symmetries, the dependence of variables such as temperature or pressure at different locations is generally nonlinear. In particular, large deviations from the sample mean are expected to be strongly affected by such nonlinearities. Here we apply a newly developed nonlinear technique (Maxima of Cumulant Function, MCF) for detection of typical spatial patterns that largely deviate from the mean. In order to test the technique and to introduce the methodology, we focus on the El Niño/Southern Oscillation and its spatial patterns. We find nonsymmetric temperature patterns corresponding to El Niño and La Niña, and we compare the results of MCF with other techniques, such as the symmetric solutions of PCA, and the nonsymmetric solutions of Nonlinear PCA (NLPCA). We found that MCF solutions are more reliable than the NLPCA fits, and can capture mixtures of principal components. Finally, we apply Extreme Value Theory on the temporal variations extracted from our methodology. We find that the tails of the distribution of extreme temperatures during La Niña episodes is bounded, while the tail during El Niños is less likely to be bounded. This implies that the mean spatial patterns of the two phases are asymmetric, as well as the behaviour of their extremes.

scholarly journals Impacts of ENSO on Philippine Tropical Cyclone Activity

2016 ◽  
Vol 29 (5) ◽  
pp. 1877-1897 ◽  
Author(s):  
Irenea L. Corporal-Lodangco ◽  
Lance M. Leslie ◽  
Peter J. Lamb
Keyword(s):  
Tropical Cyclone ◽  
El Niño ◽  
El Nino ◽  
Southern Oscillation ◽  
The Philippines ◽  
Life Cycles ◽  
La Niña ◽  
Cyclone Activity ◽  
La Nina ◽  
The Mean

Abstract This study investigates the El Niño–Southern Oscillation (ENSO) contribution to Philippine tropical cyclone (TC) variability, for a range of quarterly TC metrics. Philippine TC activity is found to depend on both ENSO quarter and phase. TC counts during El Niño phases differ significantly from neutral phases in all quarters, whereas neutral and La Niña phases differ only in January–March and July–September. Differences in landfalls between neutral and El Niño phases are significant in January–March and October–December and in January–March for neutral and La Niña phases. El Niño and La Niña landfalls are significantly different in April–June and October–December. Philippine neutral and El Niño TC genesis cover broader longitude–latitude ranges with similar long tracks, originating farther east in the western North Pacific. In El Niño phases, the mean eastward displacement of genesis locations and more recurving TCs reduce Philippine TC frequencies. Proximity of La Niña TC genesis to the Philippines and straight-moving tracks in April–June and October–December increase TC frequencies and landfalls. Neutral and El Niño accumulated cyclone energy (ACE) values are above average, except in April–June of El Niño phases. Above-average quarterly ACE in neutral years is due to increased TC frequencies, days, and intensities, whereas above-average El Niño ACE in July–September is due to increased TC days and intensities. Below-average La Niña ACE results from fewer TCs and shorter life cycles. Longer TC durations produce slightly above-average TC days in July–September El Niño phases. Fewer TCs than neutral years, as well as shorter TC durations, imply less TC days in La Niña phases. However, above-average TC days occur in October–December as a result of higher TC frequencies.

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