Observations and Predictability of a Nondeveloping Tropical Disturbance over the Eastern Atlantic

2018 ◽  
Vol 146 (9) ◽  
pp. 3079-3096 ◽  
Author(s):  
Alan Brammer ◽  
Chris D. Thorncroft ◽  
Jason P. Dunion
Keyword(s):  
Deep Convection ◽  
Recirculation Region ◽  
Tropical Cyclogenesis ◽  
Short Term ◽  
Ensemble Forecasts ◽  
Dry Air ◽  
Easterly Wave ◽  
Closed Circulation ◽  
West African Coast ◽  
Favorable Conditions

Abstract A strong African easterly wave (AEW) left the West African coast in early September 2014 and operational global numerical forecasts suggested a potential for rapid tropical cyclogenesis of this disturbance in the eastern Atlantic, despite the presence of a large region of dry air northwest of the disturbance. Analysis and in situ observations show that after leaving the coast, the closed circulation associated with the AEW trough was not well aligned vertically, and therefore, low-level or midlevel dry air was advected below or above, respectively, areas of closed circulation. GPS dropwindsonde observations highlight the dry air undercutting the midlevel recirculation region in the southwestern quadrant. This advection of dry air constrains the spatial extent of deep convection within the AEW trough, leading to the vortex decaying. As the column continues to be displaced horizontally, losing vertical alignment, this enables increased horizontal advection of dry air into the system further limiting convective activity. Ensemble forecasts indicate that short-term errors in precipitation rate and vorticity generation can lead to an over intensified and well-aligned vortex, which then interacts less with the unfavorable environment, allowing for further convection and intensification. The stronger vortex provides more favorable conditions for precipitation through a more vertically coherent closed circulation and thus a positive feedback loop is initiated. The short-term forecasts of precipitation were shown to be sensitive to lower-tropospheric moisture anomalies around the AEW trough through ensemble sensitivity analysis from Global Ensemble Forecast System real-time forecasts.

scholarly journals Coarse, intermediate and high resolution numerical simulations of the transition of a tropical wave critical layer to a tropical storm

2010 ◽  
Vol 10 (22) ◽  
pp. 10803-10827 ◽  
Author(s):  
M. T. Montgomery ◽  
Z. Wang ◽  
T. J. Dunkerton
Keyword(s):  
High Resolution ◽  
Deep Convection ◽  
Potential Temperature ◽  
Wrf Model ◽  
Tropical Storm ◽  
Critical Layer ◽  
Tropical Cyclogenesis ◽  
Cyclonic Vorticity ◽  
Easterly Wave ◽  
Tropical Depressions

Abstract. Recent work has hypothesized that tropical cyclones in the deep Atlantic and eastern Pacific basins develop from within the cyclonic Kelvin cat's eye of a tropical easterly wave critical layer located equatorward of the easterly jet axis. The cyclonic critical layer is thought to be important to tropical cyclogenesis because its cat's eye provides (i) a region of cyclonic vorticity and weak deformation by the resolved flow, (ii) containment of moisture entrained by the developing flow and/or lofted by deep convection therein, (iii) confinement of mesoscale vortex aggregation, (iv) a predominantly convective type of heating profile, and (v) maintenance or enhancement of the parent wave until the developing proto-vortex becomes a self-sustaining entity and emerges from the wave as a tropical depression. This genesis sequence and the overarching framework for describing how such hybrid wave-vortex structures become tropical depressions/storms is likened to the development of a marsupial infant in its mother's pouch, and for this reason has been dubbed the "marsupial paradigm". Here we conduct the first multi-scale test of the marsupial paradigm in an idealized setting by revisiting the Kurihara and Tuleya problem examining the transformation of an easterly wave-like disturbance into a tropical storm vortex using the WRF model. An analysis of the evolving winds, equivalent potential temperature, and relative vertical vorticity is presented from coarse (28 km), intermediate (9 km) and high resolution (3.1 km) simulations. The results are found to support key elements of the marsupial paradigm by demonstrating the existence of a rotationally dominant region with minimal strain/shear deformation near the center of the critical layer pouch that contains strong cyclonic vorticity and high saturation fraction. This localized region within the pouch serves as the "attractor" for an upscale "bottom up" development process while the wave pouch and proto-vortex move together. Implications of these findings are discussed in relation to an upcoming field experiment for the most active period of the Atlantic hurricane season in 2010 that is to be conducted collaboratively between the National Oceanic and Atmospheric Administration (NOAA), the National Science Foundation (NSF), and the National Aeronautics and Space Adminstration (NASA).

Analysis of African Easterly Wave Structures and Their Role in Influencing Tropical Cyclogenesis

2010 ◽  
Vol 138 (4) ◽  
pp. 1399-1419 ◽  
Author(s):  
Susanna B. Hopsch ◽  
Chris D. Thorncroft ◽  
Kevin R. Tyle
Keyword(s):  
West African ◽  
Core Structure ◽  
Tropical Cyclogenesis ◽  
Convective Activity ◽  
The West ◽  
Dry Air ◽  
West African Coast ◽  
African Coast ◽  
Low Levels ◽  
Upper Level

Abstract Composite structures of African easterly waves (AEWs) that develop into named tropical cyclones in the Atlantic are compared and contrasted with nondeveloping AEWs using the 40-yr ECMWF Re-Analysis (ERA-40) data and satellite brightness temperature between 1979 and 2001. Developing AEWs are characterized by a more distinctive cold-core structure two days before reaching the West African coast. As they move westward, the convective activity increases further in the vicinity of the Guinea Highlands region. At the same time the AEW trough increases its vorticity at low levels consistent with a transformation toward a more warm-core structure before it reaches the ocean. As the AEW moves over the ocean convection is maintained in the trough, consistent with the observed tropical cyclogenesis. The nondeveloping AEW has a similar evolution before reaching the coast except that the amplitudes are weaker and there is less convective activity in the Guinea Highlands region. The nondeveloping AEW composite has a more prominent dry signal just ahead of the AEW trough at mid- to upper levels. It is argued that the weaker west coast development (i.e., reduced convective activity and reduced spinup at low levels) combined with the closer proximity of the trough to mid- to upper-level dry air aloft are consistent with the nondevelopment. The most intense nondeveloping AEWs were characterized by more intense convection and stronger mid- and low-level synoptic circulations at the West African coast than the developing AEWs. The analysis strongly suggests that the lack of development was due to the presence of dry mid- to upper-level air just ahead of the AEW trough that may have been enhanced because of equatorward advection of dry air by the AEW itself.

Genesis of Hurricane Julia (2010) within an African Easterly Wave: Sensitivity Analyses of WRF-LETKF Ensemble Forecasts

2014 ◽  
Vol 71 (9) ◽  
pp. 3180-3201 ◽  
Author(s):  
Stefan F. Cecelski ◽  
Da-Lin Zhang
Keyword(s):  
Deep Convection ◽  
Empirical Orthogonal Functions ◽  
Sensitivity Analyses ◽  
Tropical Cyclogenesis ◽  
Tropospheric Temperature ◽  
Orthogonal Functions ◽  
Ensemble Forecasts ◽  
Low Level ◽  
Modes Of Variability ◽  
Two Parameters

Abstract In this study, the predictability of tropical cyclogenesis (TCG) is explored by conducting ensemble sensitivity analyses on the TCG of Hurricane Julia (2010). Using empirical orthogonal functions (EOFs), the dominant patterns of ensemble disagreements are revealed for various meteorological parameters such as mean sea level pressure (MSLP) and upper-tropospheric temperature. Using the principal components of the EOF patterns, ensemble sensitivities are generated to elucidate which mechanisms drive the parametric ensemble differences. The dominant pattern of MSLP ensemble spread is associated with the intensity of the pre–tropical depression (pre-TD), explaining nearly half of the total variance at each respective time. Similar modes of variance are found for the low-level absolute vorticity, though the patterns explain substantially less variance. Additionally, the largest modes of variability associated with upper-level temperature anomalies closely resemble the patterns of MSLP variance, suggesting interconnectedness between the two parameters. Sensitivity analyses at both the pre-TD and TCG stages reveal that the MSLP disturbance is strongly correlated to upper-tropospheric temperature and, to a lesser degree, surface latent heat flux anomalies. Further sensitivity analyses uncover a statistically significant correlation between upper-tropospheric temperature and convective anomalies, consistent with the notion that deep convection is important for augmenting the upper-tropospheric warmth during TCG. Overall, the ensemble forecast differences for the TCG of Julia are strongly related to the processes responsible for MSLP falls and low-level cyclonic vorticity growth, including the growth of upper-tropospheric warming and persistent deep convection.

scholarly journals Atlantic Tropical Cyclogenesis: A Three-Way Interaction between an African Easterly Wave, Diurnally Varying Convection, and a Convectively Coupled Atmospheric Kelvin Wave

2012 ◽  
Vol 140 (4) ◽  
pp. 1108-1124 ◽  
Author(s):  
Michael J. Ventrice ◽  
Christopher D. Thorncroft ◽  
Matthew A. Janiga
Keyword(s):  
West Africa ◽  
Diurnal Cycle ◽  
Deep Convection ◽  
Active Phase ◽  
Boreal Summer ◽  
Tropical Cyclogenesis ◽  
Tropical Atlantic ◽  
Kelvin Wave ◽  
African Easterly Wave ◽  
Easterly Wave

This paper explores a three-way interaction between an African easterly wave (AEW), the diurnal cycle of convection over the Guinea Highlands (GHs), and a convectively coupled atmospheric equatorial Kelvin wave (CCKW). These interactions resulted in the genesis of Tropical Storm Debby over the eastern tropical Atlantic during late August 2006. The diurnal cycle of convection downstream of the GHs during the month of August is explored. Convection associated with the coherent diurnal cycle is observed off the coast of West Africa during the morning. Later, convection initiates over and downstream of the GHs during the afternoon. These convective features were pronounced during the passage of the pre-Debby AEW. The superposition between the convectively active phase of a strong CCKW and the pre-Debby AEW occurred shortly after merging with the diurnally varying convection downstream of the GHs. The CCKW–AEW interaction preceded tropical cyclogenesis by 18 h. The CCKW provided a favorable environment for deep convection. An analysis of high-amplitude CCKWs over the tropical Atlantic and West Africa during the Northern Hemisphere boreal summer (1979–2009) highlights a robust relationship between CCKWs and the frequency of tropical cyclogenesis. Tropical cyclogenesis is found to be less frequent immediately prior to the passage of the convectively active phase of the CCKW, more frequent during the passage, and most frequent just after the passage.

scholarly journals Intermediate and high resolution numerical simulations of the transition of a tropical wave critical layer to a tropical storm

2009 ◽  
Vol 9 (6) ◽  
pp. 26143-26197 ◽  
Author(s):  
M. T. Montgomery ◽  
Z. Wang ◽  
T. J. Dunkerton
Keyword(s):  
High Resolution ◽  
Deep Convection ◽  
Potential Temperature ◽  
Wrf Model ◽  
Tropical Storm ◽  
Critical Layer ◽  
Trade Wind ◽  
Tropical Cyclogenesis ◽  
Cyclonic Vorticity ◽  
Easterly Wave

Abstract. Recent work has hypothesized that tropical cyclones in the deep Atlantic and eastern Pacific basins develop from the cyclonic Kelvin cat's eye of a tropical easterly wave critical layer located equatorward of the easterly jet axis that typifies the trade wind belt. The cyclonic critical layer is thought to be important to tropical cyclogenesis because its cat's eye provides (i) a region of cyclonic vorticity and weak deformation by the resolved flow, (ii) containment of moisture entrained by the developing flow and/or lofted by deep convection therein, (iii) confinement of mesoscale vortex aggregation, (iv) a predominantly convective type of heating profile, and (v) maintenance or enhancement of the parent wave until the developing proto-vortex becomes a self-sustaining entity and emerges from the wave as a tropical depression. This genesis sequence and the overarching framework for describing how such hybrid wave-vortex structures become tropical depressions/storms is likened to the development of a marsupial infant in its mother's pouch, and for this reason has been dubbed the "marsupial paradigm". Here we conduct the first multi-scale test of the marsupial paradigm in an idealized setting by revisiting the problem of the transformation of an easterly wave-like disturbance into a tropical storm vortex using the WRF model. An analysis of the evolving winds, equivalent potential temperature, and relative vertical vorticity is presented from coarse (28 km) and high resolution (3.1 km) simulations. The results are found to support key elements of the marsupial paradigm by demonstrating the existence of a vorticity dominant region with minimal strain/shear deformation within the critical layer pouch that contains strong cyclonic vorticity and high saturation fraction. This localized region within the pouch serves as the "attractor" for an upscale "bottom up" development process while the wave pouch and proto-vortex move together. Implications of these findings are discussed in relation to an upcoming field experiment for the most active period of the Atlantic hurricane season in 2010 that is to be conducted collaboratively between the National Oceanic and Atmospheric Administration (NOAA), the National Science Foundation (NSF), and the National Aeronautics and Space Adminstration (NASA).

A First Look at the Structure of the Wave Pouch during the 2009 PREDICT–GRIP Dry Runs over the Atlantic

2012 ◽  
Vol 140 (4) ◽  
pp. 1144-1163 ◽  
Author(s):  
Zhuo Wang ◽  
Michael T. Montgomery ◽  
Cody Fritz
Keyword(s):  
Tropical Cyclone ◽  
Large Scale ◽  
Deep Convection ◽  
Vertical Shear ◽  
Recirculation Region ◽  
Necessary Condition ◽  
Tropical Cyclogenesis ◽  
Sufficient Condition ◽  
Easterly Waves ◽  
Tropical Cyclone Formation

In support of the National Science Foundation Pre-Depression Investigation of Cloud-systems in the tropics (NSF PREDICT) and National Aeronautics and Space Administration Genesis and Rapid Intensification Processes (NASA GRIP) dry run exercises and National Oceanic and Atmospheric Administration Hurricane Intensity Forecast Experiment (NOAA IFEX) during the 2009 hurricane season, a real-time wave-tracking algorithm and corresponding diagnostic analyses based on a recently proposed tropical cyclogenesis model were applied to tropical easterly waves over the Atlantic. The model emphasizes the importance of a Lagrangian recirculation region within a tropical-wave critical layer (the so-called pouch), where persistent deep convection and vorticity aggregation as well as column moistening are favored for tropical cyclogenesis. Distinct scenarios of hybrid wave–vortex evolution are highlighted. It was found that easterly waves without a pouch or with a shallow pouch did not develop. Although not all waves with a deep pouch developed into a tropical storm, a deep wave pouch had formed prior to genesis for all 16 named storms originating from monochromatic easterly waves during the 2008 and 2009 seasons. On the other hand, the diagnosis of two nondeveloping waves with a deep pouch suggests that strong vertical shear or dry air intrusion at the middle–upper levels (where a wave pouch was absent) can disrupt deep convection and suppress storm development. To sum up, this study suggests that a deep wave pouch extending from the midtroposphere (~600–700 hPa) down to the boundary layer is a necessary condition for tropical cyclone formation within an easterly wave. It is hypothesized also that a deep wave pouch together with other large-scale favorable conditions provides a sufficient condition for sustained convection and tropical cyclone formation. This hypothesized sufficient condition requires further testing and will be pursued in future work.

scholarly journals On the potential causes of the recent Pelagic Sargassum blooms events in the tropical North Atlantic Ocean

2017 ◽  
Author(s):  
Sandrine Djakouré ◽  
Moacyr Araujo ◽  
Aubains Hounsou-Gbo ◽  
Carlos Noriega ◽  
Bernard Bourlès
Keyword(s):  
Atlantic Ocean ◽  
North Atlantic Ocean ◽  
Large Mass ◽  
Recirculation Region ◽  
Brown Macroalgae ◽  
The West ◽  
The North ◽  
Tropical North Atlantic ◽  
Equatorial Upwelling ◽  
Favorable Conditions

Abstract. Since 2011, unprecedented and repetitive blooms and large mass strandings of the floating brown macroalgæ, Sargassum natans and Sargassum fluitans have been reported along the West Indies, the Caribbean, the Brazilian and the West Africa coasts. Recent studies have highlighted a new tank of Sargassum: the North Equatorial Recirculation Region of the Atlantic Ocean. This region is located off the northeast of Brazil, approximately between the equator and 10° N and from 50° W to 25° W. The potential causes of these recent blooms and mass strandings are still poorly understood. Observational datasets and modelling outputs involving hydrological parameters and climate events are examined focusing on their potential feedback on the observed blooms and mass strandings. The results show that combined conditions have been in favor of these recent changes. High anomalously unprecedented positive sea surface temperature observed in the tropical Atlantic in 2010–2011 could have induced favorable temperature conditions for Sargassum blooms. These favorable conditions were then fed by additional continental nutrients inputs, principally from the Amazon River. These continental nutrients load are the consequences of deforestation, agroindustrial and urban activities in the Amazonian forest. The results also suggest that subsurface intake of nutrients from the equatorial upwelling could also contribute to the blooms of the Sargassum seaweed in the Atlantic Ocean but further studies are needed to confirm these additional inputs.

scholarly journals Mesoscale Processes during the Genesis of Hurricane Karl (2010)

2019 ◽  
Vol 76 (8) ◽  
pp. 2235-2255 ◽  
Author(s):  
Michael M. Bell ◽  
Michael T. Montgomery
Keyword(s):  
Structural Changes ◽  
Doppler Radar ◽  
Deep Convection ◽  
Tropical Cyclogenesis ◽  
Primary Role ◽  
Low Level ◽  
Stratiform Precipitation ◽  
Environmental Air ◽  
Mesoscale Processes

Abstract Observations from the Pre-Depression Investigation of Cloud Systems in the Tropics (PREDICT), Genesis and Rapid Intensification Processes (GRIP), and Intensity Forecast Experiment (IFEX) field campaigns are analyzed to investigate the mesoscale processes leading to the tropical cyclogenesis of Hurricane Karl (2010). Research aircraft missions provided Doppler radar, in situ flight level, and dropsonde data documenting the structural changes of the predepression disturbance. Following the pre-Karl wave pouch, variational analyses at the meso-β and meso-α scales suggest that the convective cycle in Karl alternately built the low- and midlevel circulations leading to genesis episodically rather than through a sustained lowering of the convective mass flux from increased stabilization. Convective bursts that erupt in the vorticity-rich environment of the recirculating pouch region enhance the low-level meso-β- and meso-α-scale circulation through vortex stretching. As the convection wanes, the resulting stratiform precipitation strengthens the midlevel circulation through convergence associated with ice microphysical processes, protecting the disturbance from the intrusion of dry environmental air. Once the column saturation fraction returns to a critical value, a subsequent convective burst below the midlevel circulation further enhances the low-level circulation, and the convective cycle repeats. The analyses suggest that the onset of deep convection and associated low-level spinup were closely related to the coupling of the vorticity and moisture fields at low and midlevels. Our interpretation of the observational analysis presented in this study reaffirms a primary role of deep convection in the genesis process and provides a hypothesis for the supporting role of stratiform precipitation and the midlevel vortex.

Four Years of Tropical ERA-40 Vorticity Maxima Tracks. Part II: Differences between Developing and Nondeveloping Disturbances

2009 ◽  
Vol 137 (8) ◽  
pp. 2576-2591 ◽  
Author(s):  
Brandon Kerns ◽  
Edward Zipser
Keyword(s):  
Large Scale ◽  
Deep Convection ◽  
Vertical Wind Shear ◽  
Skill Score ◽  
Relative Vorticity ◽  
Prediction Skill ◽  
Tropical Cyclogenesis ◽  
Linear Discriminant ◽  
Predictive Skill ◽  
Heidke Skill Score

Abstract Using a subset of the relative vorticity maxima (VM) tracks described in Part I, large-scale environmental fields, cold cloud area, and rainfall area are used to discriminate between developing and nondeveloping tropical disturbances in the eastern North Pacific (EPAC) and Atlantic Oceans. By using a minimum cold cloud coverage requirement, the nondeveloping VM are limited to disturbances with enhanced low-level relative vorticity and widespread deep convection. Linear discriminant analysis is used to determine the overall discrimination and the relative importance of each predictor for each basin separately. It is important to distinguish the two basins because, for many predictors, the differences between the basins are greater than the differences between developing and nondeveloping VM in each basin. Using the parametric forecast method, there is greater discrimination and prediction skill in the EPAC than in the Atlantic. There are also significant differences between the two basins in terms of the degree of discrimination provided by each of the predictors. Surprisingly, the mean vertical wind shear magnitude is greater for EPAC developing VM than for EPAC nondeveloping VM. Incorporating the satellite-derived predictors marginally improves the potential forecast skill in the EPAC but not in the Atlantic. The prediction skill (Heidke skill score) of tropical cyclogenesis in the Atlantic is similar to what has been obtained in previous studies using cloud cluster tracks. There is greater predictive skill in the EPAC.

Global Survey of Precipitation Properties Observed during Tropical Cyclogenesis and Their Differences Compared to Nondeveloping Disturbances

2020 ◽  
Vol 148 (4) ◽  
pp. 1585-1606
Author(s):  
Jonathan Zawislak
Keyword(s):  
Brightness Temperature ◽  
North Pacific ◽  
Deep Convection ◽  
Passive Microwave ◽  
Tropical Cyclogenesis ◽  
Global Database ◽  
Areal Coverage ◽  
The Pacific ◽  
Rain Rates ◽  
The Western Pacific

Abstract This study evaluates precipitation properties involved in tropical cyclogenesis by analyzing a multiyear, global database of passive microwave overpasses of the pregenesis stage of developing disturbances and nondeveloping disturbances. Precipitation statistics are quantified using brightness temperature proxies from the 85–91-GHz channels of multiple spaceborne sensors, as well as retrieved rain rates. Proxies focus on the overall raining area, areal coverage of deep convection, and the proximity of precipitation to the disturbance center. Of interest are the differences in those proxies for developing versus nondeveloping disturbances, how the properties evolve during the pregenesis stage, and how they differ globally. The results indicate that, of all of the proxies examined, the total raining area and rain volume near the circulation center are the most useful precipitation-related predictors for genesis. The areal coverage of deep convection also differentiates developing from nondeveloping disturbances and, similar to the total raining area, generally also increases during the pregenesis stage, particularly within a day of genesis. As the threshold convective intensity is increased, pregenesis cases are less distinguishable from nondeveloping disturbances. Relative to the western Pacific and Indian Oceans, the Atlantic and eastern North Pacific Oceans have less precipitation and deep convection observed during genesis and the smallest differences between developing and nondeveloping disturbances. This suggests that the total raining area and areal coverage of deep convection associated with tropical disturbances are better predictors of tropical cyclogenesis fate in the Pacific and Indian Oceans than in the Atlantic and eastern North Pacific.

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