1. Introduction
Tropical cyclogenesis, the series of processes by which a loosely organized tropical convective system1 transitions into a tropical cyclone (TC), is still poorly understood. The primary challenge when studying tropical cyclogenesis has been the remote, data-sparse locations in which it occurs. There are several common methods for overcoming this challenge. One solution is to composite data gathered from a large number of individual cases to determine the “typical” structure of a tropical system (e.g., McBride and Zehr 1981). Another solution is to examine a small number of cases in which, either by chance or design, the density of observations is significantly higher than normal (e.g., Davis and Ahijevych 2012). Using such an approach allows for greater detail in the analysis although the number of available cases is limited. Alternatively, many studies forgo direct observations entirely and instead use numerical model simulations (e.g., Nolan 2007). Finally, a fourth approach, which will not be discussed here in the interest of brevity, is to use statistical techniques to identify large-scale predictors of tropical cyclogenesis (e.g., Hennon and Hobgood 2003; Kerns and Zipser 2009).
In their study on developing and nondeveloping systems, McBride and Zehr (1981) examined composited rawinsonde data, described in McBride (1981), from the western Atlantic and west Pacific basins. The study found a number of differences between developing and nondeveloping tropical convective systems in terms of temperature, vorticity, divergence, and wind shear. Developing systems were found to be associated with a more pronounced upper-level warm anomaly (although the authors point out that the composite magnitude is too small to be noticed on an individual basis), stronger large-scale low-level cyclonic vorticity (by a factor of 2 over nondeveloping systems), zero vertical wind shear near the system center, and, in the case of Atlantic systems, strong upper-level divergence. Additionally, the authors determined that both developing and nondeveloping systems had little to no difference in moisture anomaly and static stability.
In a similar study, Lee (1989a) examined composited rawinsonde data from the west Pacific basin, including 660 nondeveloping and 341 developing cloud clusters. The study found that the developing cloud clusters were, on average, located within environments with slightly lower surface pressures (
Using dropsonde data from a number of recent field campaigns as well as reconnaissance flights, Zawislak and Zipser (2014a) studied the thermodynamic differences between developing and nondeveloping tropical convective systems in the Atlantic basin between 2005 and 2010. The dataset used by the study includes data from 12 developing systems (667 pregenesis dropsondes and 1537 postgenesis dropsondes) and four nondeveloping systems (245 dropsondes). Their study found that developing systems tend to have more moisture above 800 hPa than nondeveloping systems and tended to have an increase in tropospheric stability as genesis nears. Additionally, nondeveloping systems were found to become drier at midlevels with time and more convectively unstable at low levels.
Komaromi (2013) used dropsonde data collected during the 2010 Pre-Depression Investigation of Cloud-Systems in the Tropics (PREDICT; Montgomery et al. 2012) field experiment to examine a variety of fields. The study found that in developing systems an upper-level positive temperature anomaly builds gradually while nondeveloping systems tended to have temperatures colder than the PREDICT mean sounding at upper levels. Additionally, nondeveloping systems were found to have significantly drier midlevels and greater convective available potential energy (suggesting greater potential instability) than developing systems. In developing systems a broad vorticity maximum between 900 and 600 hPa was found at 48–72 and 24–48 h prior to genesis. The study also found low-level divergence appearing simultaneously with this vorticity maximum during the 48–72-h pregenesis period and low-level negative temperature anomalies appearing during the 24–72-h period. Finally, the study found that an 800-hPa vorticity maximum forms during the final 24-h period prior to tropical cyclogenesis.
Using cloud-resolving model simulations, Nolan (2007) performed a sensitivity study on the triggers of tropical cyclogenesis. A number of simulations were performed with varying depths of initial vorticity maxima and moisture. The study found that, under realistic conditions, 1–3 days were required for the model TC to form. Furthermore, the formation coincided with the very rapid generation of a small-scale vorticity maximum at low levels by a single, long-lived updraft located beneath the midlevel vortex. Nolan (2007) also noted that repeated bursts of deep convection in the region of the vortex acted to steadily moisten the atmosphere. Evidence of the formation of a midlevel vortex appeared around the time the relative humidity between the surface and 10 km reached 80%. In the simulations initialized with a midlevel vortex, intensification did not occur until high values of midlevel moisture were present. These results, although model based, reaffirm the importance of the mid- and upper-tropospheric moisture and vortex spinup to the genesis process.
Raymond et al. (2011) examined five tropical convective systems in the northwest Pacific that were sampled during the THORPEX Pacific Asian Regional Campaign (T-PARC)/Tropical Cyclone Structure-2008 (TCS-08) field campaign (Elsberry and Harr 2008; Parsons et al. 2008). In the case of pre-Typhoon Nuri (2008), their analysis indicated that the maximum vertical mass flux was initially at a height of approximately 10 km and resulted in a region of maximum convergence at midlevels. This convergence acted as the primary mode of vorticity generation, creating a strong midlevel vorticity maximum (thus a low-level cold core). The authors suggested that this enhanced cold core changed the vertical mass flux profile such that it is maximized at approximately 5 km, as analyzed using a subsequent mission one day later. As a result of this downward shift, the maximum convergence would also shift downward and act to generate a low-level vorticity maximum (and thus lead to the formation of a warm core). The study also located similar features and evolution in a nondeveloping system resulting in weaker vorticity generation (in particular, vorticity was observed to decrease at the surface). The authors suggested the difference was due to disparities in strength and organization between the two systems.
Using observational data gathered during the PREDICT field experiment, Davis and Ahijevych (2012) performed case studies of three systems located in the western Atlantic: two developing systems (namely, the pre-Karl and pre-Matthew systems) and one system that failed to reintensify after having weakened from a tropical storm (Gaston). For their study, the authors defined the center of circulation as the location at which the area-averaged tangential winds are maximized within a 3° radius. Profiles of a number of variables were then generated by compositing the dropsondes within this 3° radius. Their findings indicated that the genesis of the two developing systems was linked to the vertical alignment of the vorticity column combined with the presence of quasi-persistent deep moist convection. The authors pointed out that these two factors were not present in the weakening system and suggest that the lack of vertical alignment generates strong system-relative flow resulting in greater susceptibility to a dry environment. The results also indicated that, in the two developing systems, the midlevel vorticity spins up earlier than does the low-level vorticity, which did not undergo significant strengthening until the time of genesis. Additionally, the two genesis cases were associated with moist midlevels and warming in the mid- to upper troposphere. Finally, the analyses suggested there is only a short period within the diurnal cycle of convection during which vorticity generation was large enough to build the system and, due to dissipative effects (e.g., friction), the system tended to weaken between these peaks. As such the brief peak in vorticity generation must be great enough to account for the prior weakening period before system intensification can proceed.
The current theories of tropical cyclogenesis have focused on the formation of the deep warm core structure associated with a TC. Simpson et al. (1997) and Ritchie and Holland (1997) suggest a mechanism by which the merger of midlevel mesoscale convective vortices (MCVs) lead to genesis. When the potential vorticity (PV) anomalies associated with the MCVs merge, the combined PV anomaly is larger than the two individual components whereas their corresponding thermal anomalies have not grown. Outside of diabatic heat sources, the only method by which the system can attain thermal wind balance is by forcing ascent below and descent above the merged PV anomaly. This ascent forces surface convergence, which acts to concentrate and stretch the background planetary vorticity, leading to a warm core system. Bister and Emanuel (1997) have suggested an alternative theory in which a midlevel MCV can lead to the formation of a warm core cyclone. In their theory, an initial midlevel MCV forms in a stratiform heating region and a low-level anticyclone associated with sinking air below this generates warming and drying. The sinking air brings the MCV circulation down to the boundary layer where the surface fluxes and evaporation of precipitation have created a cool, moist atmosphere. Once the MCV circulation reaches the boundary layer, new downdraft-free convection (made possible by the near saturation of the midlevels) is triggered in which background vorticity is concentrated and stretched, forming a warm core cyclone. More recently, Montgomery et al. (2006) proposed a genesis pathway in which a series of mergers of vortical hot towers acts to rapidly build low-level vorticity, which then axisymmetrizes with a midlevel vorticity maximum to form a complete warm core associated with a mature TC.
Finally, although they do not attempt to explain the process of tropical cyclogenesis directly, Dunkerton et al. (2009) have proposed a theory explaining the formation and maintenance of a region of favorable low-level conditions within an easterly wave by a system-scale gyre, termed a “pouch,” present in the wave-relative flow. Within this pouch the flow becomes separated from the environment outside of the pouch, impeding the entrainment of dry air at low levels. As a result, the pouch is a favored location for the quasi-persistent convective activity associated with the genesis process. A more detailed discussion of these theories can be found in Tory and Frank (2010).
Collectively, these studies highlight a number of key differences between developing and nondeveloping tropical convective systems. Throughout the studies, developing systems tended to have stronger upper-level warm anomalies, stronger low- and midlevel cyclonic vorticity, and stronger low-level convergence and upper-level divergence. Most studies found moisture, especially in the middle troposphere, to be greater in developing systems than nondeveloping systems. Additionally, many studies found that developing systems tended to have higher stability than nondeveloping ones. A notable exception to these moisture and stability findings is the McBride and Zehr (1981) study, which found little difference in moisture and stability between developing and nondeveloping systems. Tropical cyclogenesis theories tend to focus on the importance of persistent moist convection for generating the vortical structures necessary to form a TC.
While there have been numerous studies examining the composite differences between developing and nondeveloping systems, there have been relatively fewer observational studies examining the differences in structure and evolution between individual developing and nondeveloping cases, especially in a three-dimensional sense. The present study aims to perform a detailed analysis of the structural evolution of kinematic and thermodynamic fields (viz., vorticity, divergence, temperature anomaly, and relative humidity) with the goal of highlighting differences in developing and nondeveloping tropical convective systems. Furthermore, by examining the evolution of these fields in three dimensions on a case-study level, this study acts to build on the findings of the previous examinations of the PREDICT and the Genesis and Rapid Intensification Processes (GRIP) dropsonde datasets (e.g., Davis and Ahijevych 2012; Komaromi 2013; Smith and Montgomery 2012; Zawislak and Zipser 2014a). To this end, five cases from the 2010 Atlantic hurricane season have been selected for detailed examination: two nondeveloping systems, two developing systems, and one dissipating system. As only five cases are examined, the applicability of the conclusions from this study are dependent on the representativeness of the cases chosen.
A brief overview of the PREDICT and GRIP datasets as well as the case selection process and methodology is given in the next section. The results of the analyses are detailed in section 3 followed by a discussion of these results in section 4. Conclusions and a summary of the study are presented in section 5.
2. Data and methodology
The analyses presented here use dropsonde measurements from two field campaigns: the PREDICT (Montgomery et al. 2012) experiment and the GRIP (Braun et al. 2013) experiment. Data support for both experiments is provided by the Earth Observing Laboratory (EOL) at the National Center for Atmospheric Research (NCAR) under sponsorship of the National Science Foundation (NSF).
The dropsondes used in the experiments are developed by NCAR and use global positioning system (GPS) signals for tracking. For project-specific documentation of the dropsonde quality control procedures and issues, see Young et al. (2011a) for the GRIP campaign and Young et al. (2011b) for the PREDICT campaign. The Airborne Vertical Atmospheric Profiling System (AVAPS) is capable of collecting data simultaneously from multiple dropsondes at any given time. The instrument reports values of temperature, relative humidity, and pressure every 0.5 s. Wind speed and direction are calculated using the GPS-derived location (0.25-s intervals) while the altitude is calculated using the GPS position as well as using the hydrostatic equation. The time-differentiated hydrostatic equation is used to calculate the dropsonde descent rate that, given the mass, parachute area, and a drag coefficient of the dropsonde, can be used to estimate vertical velocities. Wang et al. (2009) provide details on the vertical velocity calculation method with examples using a different dropsonde system. Unfortunately, the wind tunnel data required to calculate the drag coefficient are unavailable (J. Wang 2012, personal communication) and there is some uncertainty as to whether the GPS position has sufficient accuracy to capture small fluctuations in vertical motion necessary for the type of analyses employed here (T. Hock 2013, personal communication). As such, dropsonde-derived vertical velocity measurements are not used in the present study.
a. PREDICT
The PREDICT field experiment (Montgomery et al. 2012) was designed with the goal of documenting the evolution of an easterly wave into a TC through the collection of data in and around the low-level, wave-relative gyre described by Dunkerton et al. (2009). The campaign took place during August and September 2010 and flew a total of 26 missions into 8 systems, providing 558 dropsonde soundings after quality control. Data collection was conducted using the NCAR/NSF Gulfstream-V aircraft usually from a flight level of approximately 12–13 km. In addition to the typical atmospheric variables routinely measured by aircraft, the aircraft also carried instruments to take detailed measurements of cloud ice microphysics as well as equipment designed to generate atmospheric profiles using GPS radio occultation [for more information on this technique, see Xie et al. (2008)].
b. GRIP
The goal of the GRIP field experiment (Braun et al. 2013) was to document the processes responsible for the rapid intensification of a mature TC as well as the processes involved in tropical cyclogenesis. The experiment took place concurrently with the PREDICT campaign, lasting from August to September 2010, flying into 5 tropical systems over the course of 16 missions and generating 328 quality-controlled dropsonde soundings. NASA’s DC-8 was the primary data collection platform used in the campaign and typically flew at approximately 10–11 km, somewhat lower than the PREDICT aircraft. Additionally, an unmanned NASA Global Hawk flew over storms at an altitude of approximately 20 km. In addition to the DC-8 dropsonde system, GRIP also flew instruments to gather data related to cloud microphysics, aerosol concentrations, lightning detection, and precipitation. Although not employed by the present study, data from these other instruments may be the focus of a future publication.
c. Data limitations and biases
One notable bias inherent in most dropsonde datasets is that the dropsonde profiles are almost always located outside of deep convective cores. The bias is a result of flight safety restrictions requiring that minimum horizontal and vertical distances be maintained from the cores of deep convection, particularly in the presence of lightning. Although this may be an issue for studies aiming to explore smaller-scale in-cloud dynamics, the strong bias nicely serves the goals of the current study. By removing the direct influence of deep convective cores, the analyses presented herein become representative of a combination of the preexisting background field and the upscale impacts of any deep convection. As the end result of the mesoscale convective evolution and organization occurring within a developing system is a synoptic-scale TC, the upscale convective impact is precisely the topic of interest. This is, of course, not to say that the observations do not fall through stratiform clouds and precipitation, where sensor wetting can be an issue.
The data for an individual mission is limited in that it is collected over the span of a 4–8-h flight. Since the observation times are not simultaneous, a time correction is applied to each observation based on the system center position. These center positions are taken from model-derived consensus pouch positions2 and linearly interpolated to the observation times. For the analysis presented below, the observation locations are displayed relative to the average time of the observations during that flight. This correction assumes that any changes in the system over the duration of the flight are negligible. While this is not typically a problematic assumption on the spatial scale of these observations, instances where observations are spatially close but temporally distant can result in significant errors, especially in the calculated divergence fields.
A particularly troublesome limitation of any localized in situ observational dataset is the problem of areal coverage. As the goal of the PREDICT field experiment was to collect data in and around the low-level circulations, the sampling of the low-level structures present in the nondeveloping systems and in the early stages of the developing systems are well documented. Unfortunately, the systems are often not vertically aligned during the early stages of organization and, as a result, some mid- and upper-level structure may have been outside the observation domain. While the goal of the GRIP campaign was much less targeted at a specific feature, the pregenesis GRIP mission tended to occur after the systems were nearing vertical alignment and, as such, sampling tended to adequately cover both the low- and midtropospheric structures (Braun et al. 2013). A key issue experienced by both research teams was the restrictions imposed on airspace use. The most notable of these restrictions was imposed on the Venezuelan flight information region, which extends northward from the coast to the central eastern Caribbean Sea. The impact of these restrictions was partially mitigated by sampling the eastern portion of the system before flying around the flight information region and sampling the western half as it moved out of the restricted airspace.
d. Case selection
A combined total of 11 individual systems were flown by the GRIP and PREDICT field campaigns. Of these, two reached hurricane strength (Earl and Karl), four peaked as a tropical storm (Fiona, Gaston, Matthew, and Nicole), one was a tropical depression (Tropical Depression 5), and another system was part of a test flight into a nontropical surface trough. Additionally, one tropical system (PGI51L)3 is difficult to categorize as either developing or nondeveloping. The remaining two systems (PGI27L and PGI30L) were both of tropical origin but failed to develop. Of the six developing systems, arguably only three of them (Karl, Matthew, and Nicole) were sampled prior to genesis. A fourth system (Fiona) was declared a tropical storm during the first mission into the system and provides little information about the pregenesis structure. Since the present study is examining differences between developing and nondeveloping systems, it becomes important to have sufficient pregenesis sampling to accurately document the structural changes associated with development. Only one mission was flown into Nicole prior to genesis, thus preventing examination of the structural evolution (this is also the case for the uncategorized PGI51L). As such, the cases available for analysis are naturally reduced to the two nondeveloping and two developing systems with sufficient pregenesis data, all of which are included in the analysis presented here. An additional clarification to the selection method is needed for Gaston, which was sampled during and after a period of weakening to a tropical wave before its eventual dissipation. Examination of the system suggests the structural evolution is not that of a typical nondeveloping system for much of the observation period due to the preexisting structures associated with a TC. Despite this, the ex-Gaston system is included in this analysis in order to document the dissipation stage of a moderately organized system, which presumably shares similarities to the dissipation of the more organized of nondeveloping systems. Furthermore, as the ex-Gaston system has been featured in a number of other publications (e.g., Davis and Ahijevych 2012; Komaromi 2013; Smith and Montgomery 2012; Zawislak and Zipser 2014a), its inclusion provides a more complete comparison to previous findings. A breakdown of the missions flown into each of the systems analyzed in this study is included in Table 1.
Breakdown of the systems analyzed in the present study. Note that PGI[No.][No.]L is the system identifier used during the PREDICT, GRIP, and Intensity Forecast Experiment (IFEX) campaigns (Montgomery et al. 2012).
e. Methodology
For each mission flown into a tropical convective system, the vorticity, divergence, relative humidity, and temperature anomaly on isobaric surfaces are calculated every 25 hPa from 1000 to 200 hPa. To obtain values at each of these levels, the nearest observation above and the nearest observation below the analysis level are linearly interpolated to that level by the logarithm of pressure. To ensure that no two values depend on the same observations, the nearest observations are only considered if they are within 12.5 hPa of the analysis level. If one of the pair is not available within these bounds, the value is reported as missing and has no further impact on the resulting analyses. To better grasp the structural evolution of the systems, cross sections have been generated using these pressure-level fields. When possible, these cross sections have been subjectively positioned such that the important vorticity features are captured.
The values of vorticity and divergence are calculated by evaluating a line integral around a closed region as per Green’s theorem. The benefit of using a Green’s theorem approach over the more traditional method of interpolating data to a grid is that it better captures the gradients present in the wind field (Spencer and Doswell 2001). As both vorticity and divergence are calculated using gradients in the wind field, a Green’s theorem approach will provide more accurate vorticity and divergence fields. A further reduction in errors is achieved by performing the line integration around higher-order polygonal regions instead of more typical triangular regions. Helms and Hart (2013) explain the method and provide a detailed analysis of the accuracy improvements resulting from the use of higher-order polygons instead of triangles.
The temperature anomaly is defined here as the observed departure from the Dunion (2011) moist tropical mean sounding. The choice of using the Dunion (2011) moist tropical mean sounding is based on the predominantly moist nature of tropical convective systems and the similar locations of the dropsonde observations employed here and the rawinsonde observations used in Dunion (2011). Since the reference sounding, as provided in Dunion (2011), is available every 50–100 hPa, reference temperatures are linearly interpolated every 25 hPa.
3. Results
Sections 3a–c present detailed analyses of the structure and evolution of the five selected tropical convective systems. For ease of comparison, the cases have been split between three subsections based on their development (or lack thereof).
For the purposes of these results, “low level,” “midlevel,” and “upper level” refer to the layers between the surface and 650 hPa, between 650 and 350 hPa, and altitudes above 350 hPa, respectively. It is also important to note that, contrary to the World Meteorological Organization definition, relative humidity is defined here with respect to ice where temperatures are below freezing and with respect to water everywhere else. This choice of definition gives a better view of the extent of favorable conditions for cloud formation or erosion and the resultant changes in latent heating, although it should be noted that this definition does not account for the possibility of supercooled water droplets.
a. Nondeveloping cases
1) Nondeveloping PGI27L
PGI27L was a nondeveloping system associated with an easterly wave that moved off of Africa on 9 August 2010. It moved into the Caribbean Sea on 16 August and into the Gulf of Mexico on 20 August. The PREDICT campaign flew two missions into PGI27L on 17 and 18 August while the system was over the central Caribbean Sea. The first of these missions took place just after the diurnal maximum. This diurnal maximum was the second peak in convective activity to have large convective features, suggestive of a mesoscale convective system (MCS), observed in the infrared satellite imagery since shortly after the system exited Africa. The second mission took place just after the diurnal maximum of the third such peak in convective activity.
Observations taken during the mission into PGI27L on 17 August reveal the presence of an easterly wave with an associated column of weak cyclonic vorticity between 900 and 600 hPa (eastern dashed lines in Fig. 1a). This vorticity column is located in a region of active deep convection and increased surface convergence (Fig. 1d) but is lacking a closed circulation at any level (not shown).
Cross sections of (a) relative vorticity, (b) relative humidity, (c) temperature anomaly, and (d) horizontal divergence from the nondeveloping PGI27L system at 1300 UTC 17 Aug 2010. The thick solid red or black line in the three smaller plots at the top of each panel indicates the location of the cross-sectional data for each variable. The labels “A” and “B” at the end of these solid lines indicate the location of the corresponding labels in the cross sections. The dashed lines in the cross sections indicate the approximate axis of maximum vorticity. The vertical spacing between the data points in the cross sections is 25 hPa and the plots are contoured in intervals of (a) 2 × 10−5 s−1, (b) 10%, (c) 1.0 K, and (d) 2 × 10−5 s−1. Note that relative humidity is defined with respect to ice where temperatures are below freezing. The thick contour in the cross sections corresponds to (a) 0 s−1, (b) 70%, (c) 0 K, and (d) 0 s−1.
Citation: Monthly Weather Review 143, 8; 10.1175/MWR-D-14-00242.1
Cross sections of (a) relative vorticity, (b) relative humidity, (c) temperature anomaly, and (d) horizontal divergence from the nondeveloping PGI27L system at 1300 UTC 17 Aug 2010. The thick solid red or black line in the three smaller plots at the top of each panel indicates the location of the cross-sectional data for each variable. The labels “A” and “B” at the end of these solid lines indicate the location of the corresponding labels in the cross sections. The dashed lines in the cross sections indicate the approximate axis of maximum vorticity. The vertical spacing between the data points in the cross sections is 25 hPa and the plots are contoured in intervals of (a) 2 × 10−5 s−1, (b) 10%, (c) 1.0 K, and (d) 2 × 10−5 s−1. Note that relative humidity is defined with respect to ice where temperatures are below freezing. The thick contour in the cross sections corresponds to (a) 0 s−1, (b) 70%, (c) 0 K, and (d) 0 s−1.
Citation: Monthly Weather Review 143, 8; 10.1175/MWR-D-14-00242.1
Cross sections of (a) relative vorticity, (b) relative humidity, (c) temperature anomaly, and (d) horizontal divergence from the nondeveloping PGI27L system at 1300 UTC 17 Aug 2010. The thick solid red or black line in the three smaller plots at the top of each panel indicates the location of the cross-sectional data for each variable. The labels “A” and “B” at the end of these solid lines indicate the location of the corresponding labels in the cross sections. The dashed lines in the cross sections indicate the approximate axis of maximum vorticity. The vertical spacing between the data points in the cross sections is 25 hPa and the plots are contoured in intervals of (a) 2 × 10−5 s−1, (b) 10%, (c) 1.0 K, and (d) 2 × 10−5 s−1. Note that relative humidity is defined with respect to ice where temperatures are below freezing. The thick contour in the cross sections corresponds to (a) 0 s−1, (b) 70%, (c) 0 K, and (d) 0 s−1.
Citation: Monthly Weather Review 143, 8; 10.1175/MWR-D-14-00242.1
A separate region of midlevel cyclonic vorticity, maximized near 500 hPa, is located approximately 200 km to the west of the eastern vorticity column. The presence of strong midlevel convergence and a layer of increased stability coinciding with the midlevel vorticity maximum suggest vorticity is being generated as a consequence of concentration of absolute vorticity. This convergence is likely associated with a hydrostatic adjustment in response to warming by a heat source4 such as the one responsible for generating the positive temperature anomaly above 500 hPa in the vicinity of the midlevel vorticity maximum (Fig. 1c).
The temperature profile in the vicinity of the midlevel vorticity maximum closely resembles that of a stratiform heating profile with a warm anomaly above 500 hPa and a cold anomaly around 600 hPa. Based on the satellite presentation and the relative humidity cross section (Fig. 1b), the extent of the convectively generated stratiform cloud deck is likely being limited by a region of drier air above 400 hPa just west of the midlevel vorticity maximum and the very dry midlevel air just to the east.
Analysis of the second mission into the system, which occurred on 18 August, indicates that the vorticity column has become stronger and considerably more upright (Fig. 2a). Despite this stronger, vertically aligned vorticity column, the system has still not developed a closed circulation. Above 350 hPa, the vorticity maximum is associated with an upper-level trough and ridge pattern and corresponds to a region of strong easterly vertical wind shear located west of 77°W (not shown). Between 275 and 200 hPa the flow in this region shifts from being around 5–10 m s−1 from the northeast at 275 hPa to around 10–15 m s−1 from the northwest. For comparison, between 700 and 350 hPa the wind in the same region shifts from being around 5–10 m s−1 from the east at 700 hPa to around 5–10 m s−1 from the east-northeast at 350 hPa. Because of the significant changes PGI27L experienced between the first and second mission, it is difficult to determine what direct impacts this top-heavy shear profile had on the system.
As in Fig. 1, but the plots are valid for the nondeveloping PGI27L system at 1400 UTC 18 Aug 2010.
Citation: Monthly Weather Review 143, 8; 10.1175/MWR-D-14-00242.1
As in Fig. 1, but the plots are valid for the nondeveloping PGI27L system at 1400 UTC 18 Aug 2010.
Citation: Monthly Weather Review 143, 8; 10.1175/MWR-D-14-00242.1
As in Fig. 1, but the plots are valid for the nondeveloping PGI27L system at 1400 UTC 18 Aug 2010.
Citation: Monthly Weather Review 143, 8; 10.1175/MWR-D-14-00242.1
In addition to being associated with a relatively shallow layer of strong wind shear, the upper-level trough is collocated with a maximum in horizontal convergence (Fig. 2d) located above a layer of divergent flow. This divergence profile is indicative of upper-level subsidence. This subsidence is likely responsible for the dry upper-level air (Fig. 2b) collocated with the upper-level vorticity maximum. Although mostly associated with a single dropsonde sounding, there is also a warm anomaly (Fig. 2c) that may be due, in part, to subsidence warming generated by this descending air.
In contrast to the previous day, the second mission indicates that the stratiform heating profile region of the system is positioned to the northeast of the midlevel vorticity maximum. This offset is likely due to the previously mentioned upper-level dry air preventing condensation and subsequent stratiform heating in the vicinity of the midlevel vorticity maximum. This offset pattern would result in a difference in the location of maximum vorticity and maximum vorticity generation at midlevels. As a result, the strengthening of the system would likely halt until either a new vorticity maximum forms or the stratiform heating is reestablished over the location of maximum midlevel vorticity. Evidence of such a shift in the midlevel vorticity maximum is shown in Zawislak and Zipser (2014b), which points out that the PGI27L 600-hPa vorticity maximum, as present in the National Centers for Environmental Prediction (NCEP) model analysis, is displaced northeast of the low-level vorticity maximum by 0000 UTC 19 August.
2) Nondeveloping PGI30L
The PGI30L system was associated with an African easterly wave that moved off of Africa on 16 August 2010. Although the wave did not produce deep convective activity for the majority of its life, the shallow, low-level cloud formations associated with the wave are seen to pass north of the Cape Verde Islands late in the day on 17 August. A total of two missions were flown by the PREDICT research team into PGI30L: one on 21 August and another on 23 August. With the exception of a small region of clouds that lasted from 1500 UTC 20 August to 2000 UTC 21 August, the first organized initiation of isolated deep convection did not occur until 22 August and was located along the wave axis.
While PGI27L possessed deep vortical structures, PGI30L lacks any notable cyclonic vorticity maxima above 600 hPa (Figs. 3a,b). It is likely noncoincidental that the system is almost devoid of moisture above this level (Figs. 3c,d), especially on 21 August. The low-level region of cyclonic vorticity on 21 August appears to be associated with the axis of the wave trough and is unlikely to have had any influence from convective activity, which has yet to occur on any noteworthy scale at this point.
As in Fig. 1, but the plots are valid for the nondeveloping PGI30L system at 1300 UTC 21 Aug 2010 for (a) relative vorticity and (c) relative humidity and 1100 UTC 23 Aug 2010 for (b) relative vorticity and (d) relative humidity. Note that the circle indicates the region of cyclonic low-level vorticity.
Citation: Monthly Weather Review 143, 8; 10.1175/MWR-D-14-00242.1
As in Fig. 1, but the plots are valid for the nondeveloping PGI30L system at 1300 UTC 21 Aug 2010 for (a) relative vorticity and (c) relative humidity and 1100 UTC 23 Aug 2010 for (b) relative vorticity and (d) relative humidity. Note that the circle indicates the region of cyclonic low-level vorticity.
Citation: Monthly Weather Review 143, 8; 10.1175/MWR-D-14-00242.1
As in Fig. 1, but the plots are valid for the nondeveloping PGI30L system at 1300 UTC 21 Aug 2010 for (a) relative vorticity and (c) relative humidity and 1100 UTC 23 Aug 2010 for (b) relative vorticity and (d) relative humidity. Note that the circle indicates the region of cyclonic low-level vorticity.
Citation: Monthly Weather Review 143, 8; 10.1175/MWR-D-14-00242.1
Although the structural development continues to be confined to the lower atmosphere two days later (23 August; Fig. 3b) a weak cyclonic vorticity maximum has formed near 700 hPa on the western edge of the observation domain. While this vorticity feature has some similarities with an MCV in terms of temperature (not shown) and divergence (not shown) fields, the relative humidity field (Fig. 3d) indicates that this feature is located in dry air, which inhibits cloud formation, suggesting that the feature is not a result of stratiform cloud processes or convective activity. Additionally, the satellite representation indicates that a line of convection has initiated along the eastern edge of the wave axis, whose location corresponds approximately to the maximum of vorticity located on the eastern edge of the domain between 900 and 600 hPa. There is, not surprisingly, also a region of weak, widespread convergence (not shown) at low levels in the region of the convective line.
b. Developing cases
1) Developing PGI44L (Karl)
The initial PGI44L disturbance formed on 8 September 2010 in association with the interaction of a trough over South America and an easterly wave (Stewart 2011). The system was labeled as a tropical depression at 1200 UTC 14 September and became Tropical Storm Karl at 1800 UTC 14 September. In total, 11 missions were flown into the systems from 10 September to 17 September by both PREDICT and GRIP.
As was the case with PGI27L, the first mission into PGI44L, which took place around 1200 UTC 10 September, occurred just after the diurnal convective maximum. By this time, the low-level circulation was already very well defined below 800 hPa near 13°N, 60.5°W (Fig. 4a) with a closed circulation appearing in the observations as high as 600 hPa (not shown). Between 600 and 800 hPa, the circulation tilts to the south and is located on the southern edge of the domain, near 12°N, 60.5°W, at 600 hPa. Examination of the convective evolution in the satellite imagery (not shown) indicates a large region of deep convection in the vicinity of the midlevel circulation apparent on 9 September. Given that the convective elements near the midlevel circulation on 10 September have limited spatial coverage and the atmosphere above 400 hPa in this region is relatively dry (Fig. 4b), this midlevel vorticity feature may be a remnant from the previous convective cycle.
As in Fig. 1, but the plots are valid for the developing pre-Karl PGI44L system at 1200 UTC 10 Sep 2010.
Citation: Monthly Weather Review 143, 8; 10.1175/MWR-D-14-00242.1
As in Fig. 1, but the plots are valid for the developing pre-Karl PGI44L system at 1200 UTC 10 Sep 2010.
Citation: Monthly Weather Review 143, 8; 10.1175/MWR-D-14-00242.1
As in Fig. 1, but the plots are valid for the developing pre-Karl PGI44L system at 1200 UTC 10 Sep 2010.
Citation: Monthly Weather Review 143, 8; 10.1175/MWR-D-14-00242.1
Despite the presence of a closed circulation between the surface and 600 hPa, the divergence field does not reveal any coherent convergent structures above 700 hPa at 1200 UTC 10 September (Fig. 4d), although the flow is generally convergent below this level. This is in sharp contrast to the region of strong midlevel convergence present in PGI27L (e.g., Fig. 1d). Approximately six hours later, during the second flight into the system, a strong midlevel convergence maximum (not shown) does appear along the southern edge of the observation domain, although there is little change in the vorticity field (not shown) during this time interval.
The midlevel vorticity maximum present in the 11 September analysis (Fig. 5a) is associated with a closed circulation between 650 and 500 hPa. There are a number of features that identify this midlevel circulation as an MCV. Most notable is the upper-level warm anomaly (Fig. 5c) whose base (approximately between 400 and 600 hPa) is collocated with a maximum of vorticity and convergence (Fig. 5d). Furthermore, the anomaly is located in a region of moist air (Fig. 5b) that would be capable of supporting the stratiform cloud deck associated with MCV production. At low levels, there is a closed circulation between the surface and 850 hPa that is collocated with a region of weak convergent flow.
As in Fig. 1, but the plots are valid for the developing pre-Karl PGI44L system at 1800 UTC 11 Sep 2010.
Citation: Monthly Weather Review 143, 8; 10.1175/MWR-D-14-00242.1
As in Fig. 1, but the plots are valid for the developing pre-Karl PGI44L system at 1800 UTC 11 Sep 2010.
Citation: Monthly Weather Review 143, 8; 10.1175/MWR-D-14-00242.1
As in Fig. 1, but the plots are valid for the developing pre-Karl PGI44L system at 1800 UTC 11 Sep 2010.
Citation: Monthly Weather Review 143, 8; 10.1175/MWR-D-14-00242.1
The following day, around 1300 UTC 12 September, the observed vorticity field (Fig. 6a) suggests that the midlevel vorticity feature still exists although the system no longer has a closed circulation at any level within the observation domain. Meanwhile, the temperature anomaly (Fig. 6c) continues to become warmer with time (the rate is between 0.5 and 1 K day−1). The flow between 300 and 400 hPa appears to be advecting the dry air located near 15.5°N, 69°W (Fig. 6b) from the south (not shown), possibly eroding the existing cloud mass. Additionally, the mid- and low-level vorticity maxima no longer appear as two distinct entities and, instead, appear as an uninterrupted vorticity column. Observations also suggest that the horizontal distance between the low- and midlevel vorticity maxima has decreased, although it is difficult to determine by how much because of the limitations imposed by the observation domain. As evidenced by the divergence cross section and the 850-hPa divergence field (Fig. 5d), the low-level convergence is limited to the northwest edge of the domain. In the vicinity of the vorticity axis, the flow is nearly nondivergent. This is in sharp contrast to the generally convergent low-level flow observed the following day (Fig. 6d). As a number of previous studies have pointed out (e.g., Komaromi 2013; Zawislak and Zipser 2014a), establishing convergent low-level flow is a necessary step in the genesis process.
As in Fig. 1, but the plots are valid for the developing pre-Karl PGI44L system at 1300 UTC 12 Sep 2010.
Citation: Monthly Weather Review 143, 8; 10.1175/MWR-D-14-00242.1
As in Fig. 1, but the plots are valid for the developing pre-Karl PGI44L system at 1300 UTC 12 Sep 2010.
Citation: Monthly Weather Review 143, 8; 10.1175/MWR-D-14-00242.1
As in Fig. 1, but the plots are valid for the developing pre-Karl PGI44L system at 1300 UTC 12 Sep 2010.
Citation: Monthly Weather Review 143, 8; 10.1175/MWR-D-14-00242.1
The flight on 13 September found the vorticity column (Fig. 7a) to have become nearly vertically upright with a prominent warm anomaly at upper levels (Fig. 7c). At this time, the vorticity column is encased in a column of high relative humidity (Fig. 7b) surrounded by dry air above 500 hPa. This setup is reminiscent of the pouch described by Dunkerton et al. (2009), although it is uncertain if a closed streamline exists in the comoving reference frame. Also of note, the divergence profile (Fig. 7d) in the vicinity of the vorticity column indicates strong convergence in the boundary layer and strong divergence at upper levels. Between these two extremes are layers of weakly convergent and divergent flow between 850 and 350 hPa. Within the 850–350-hPa layer, the flow on the upstream (eastern) side of the vorticity column has a fairly constant speed and direction while the downstream (western) flow experiences changes in speed that result in the corresponding layers of convergence and divergence.
As in Fig. 1, but the plots are valid for the developing pre-Karl PGI44L system at 1300 UTC 13 Sep 2010.
Citation: Monthly Weather Review 143, 8; 10.1175/MWR-D-14-00242.1
As in Fig. 1, but the plots are valid for the developing pre-Karl PGI44L system at 1300 UTC 13 Sep 2010.
Citation: Monthly Weather Review 143, 8; 10.1175/MWR-D-14-00242.1
As in Fig. 1, but the plots are valid for the developing pre-Karl PGI44L system at 1300 UTC 13 Sep 2010.
Citation: Monthly Weather Review 143, 8; 10.1175/MWR-D-14-00242.1
While the vorticity column has become vertically aligned by 13 September, the observed wind field does not indicate a closed circulation. One possibility is that a closed circulation is located to the south of the observation domain, placing it just outside of the deep convection. Alternatively, the National Hurricane Center (NHC) TC report (TCR; Stewart 2011) suggests that a temporary interruption in the genesis process occurred and resulted in the temporary loss of a closed circulation. As there is no signature of a circulation evident in the cloud patterns, it is likely that the circulation has weakened temporarily. Additionally, the observed winds on 12 September do not show a closed midlevel circulation and show a weakened low-level circulation, further supporting the loss of a closed circulation on 13 September. Despite this weakening and the dry midlevel air surrounding the system, the system quickly spins up and is named a tropical storm the next day.
2) Developing PGI46L (Matthew)
In contrast to the pre-Karl disturbance, the pre-Matthew system originated in a more typical fashion: from an easterly wave. The progenitor wave moved off the coast of western Africa on 11 September 2010 and the northern portion of the wave spawned Tropical Storm Julia on 21 September (Brennan 2010). Very little convective activity of a significant scale was associated with the wave after the genesis of Julia until 18 September, two days prior to the first mission into the pre-Matthew system. After the formation of this convection, convective activity persisted uninterrupted, with the exception of a moderate weakening associated with the diurnal variability typical of tropical convective systems, until the time of genesis (1200 UTC 23 September). The pre-Matthew system was located within the Caribbean Sea during every mission flown into the system. After developing into a tropical storm on 23 September at approximately 1800 UTC, the system eventually made landfall in Belize.
While most of the previous flights occurred shortly after the diurnal peak in convection, the timing of the flights into the pre-Matthew disturbance occur approximately three hours later in terms of local solar time. At the time of the first mission, 1500 UTC 20 September, persistent convection has been present for the past two days. The analysis on 20 September (Fig. 8) reveals several structures similar to those seen during the first flight into the nondeveloping PGI27L (Fig. 1). While the low-level flow in the early pre-Matthew system has noticeably more curvature than that of PGI27L, neither system has a closed circulation. The vorticity maxima (Fig. 8a) in both systems do not form a connected column but are instead broken into three parts, although the inclusion of these breaks is somewhat subjective. The moisture fields (Fig. 8b) indicate both systems are located in zonally wide regions of moist air with dry midlevel air located to the northeast. Both systems also have a warm anomaly (Fig. 8c) present at mid- to upper levels, although the anomaly in the pre-Matthew system is warmer and more extensive than the anomaly in the PGI27L system. Additionally, both systems have strong convergence at midlevels with strong divergence aloft.
As in Fig. 1, but the plots are valid for the developing pre-Matthew PGI46L system at 1500 UTC 20 Sep 2010.
Citation: Monthly Weather Review 143, 8; 10.1175/MWR-D-14-00242.1
As in Fig. 1, but the plots are valid for the developing pre-Matthew PGI46L system at 1500 UTC 20 Sep 2010.
Citation: Monthly Weather Review 143, 8; 10.1175/MWR-D-14-00242.1
As in Fig. 1, but the plots are valid for the developing pre-Matthew PGI46L system at 1500 UTC 20 Sep 2010.
Citation: Monthly Weather Review 143, 8; 10.1175/MWR-D-14-00242.1
Twenty-four hours later, around 1500 UTC 21 September, the midlevel vorticity maximum (Fig. 9a) has strengthened and become a single entity rather than two disjoint pieces. Reminiscent of the temperature anomaly and divergence patterns seen during the second flight into the nondeveloping PGI27L (Figs. 2c,d), the maximum vertical temperature gradient (Fig. 9c) and peak convergence (Fig. 9d), as well as the associated implied vorticity generation, are located to the east of the midlevel vorticity maximum. Unlike PGI27L, these features are located above the low-level vorticity maximum and may be acting to bring the low- and midlevel maxima into vertical alignment. Although the low-level vorticity maximum is approximately 200 km east of the midlevel maximum, the region of elevated low-level cyclonic vorticity is fairly broad and overlaps with the location of the midlevel maximum.
As in Fig. 1, but the plots are valid for the developing pre-Matthew PGI46L system at 1500 UTC 21 Sep 2010.
Citation: Monthly Weather Review 143, 8; 10.1175/MWR-D-14-00242.1
As in Fig. 1, but the plots are valid for the developing pre-Matthew PGI46L system at 1500 UTC 21 Sep 2010.
Citation: Monthly Weather Review 143, 8; 10.1175/MWR-D-14-00242.1
As in Fig. 1, but the plots are valid for the developing pre-Matthew PGI46L system at 1500 UTC 21 Sep 2010.
Citation: Monthly Weather Review 143, 8; 10.1175/MWR-D-14-00242.1
The presence of a pocket of a slightly drier air below 700 hPa and maximized to the south of the cross section on 21 September (Fig. 9b) may be causing enhanced downdraft formation [as per James and Markowski (2010)]. Regardless of the cause, the outflow boundaries generated by these enhanced downdrafts are evidenced by the collocated regions of strong near-surface divergence (Fig. 9d) and cold surface temperature anomalies (Fig. 9c). Although not of direct consequence to the present study, it is of interest to note that the gust fronts of these outflow boundaries have very little impact on convective initiation due to the almost nonexistent near-surface vertical wind shear (not shown) as per the mechanisms proposed by Rotunno et al. (1988). It is unknown whether these prominent outflow boundaries are rare in tropical convective systems or if these features are simply missed by the observations because of limitations in either spatial or temporal resolution. Weak, isolated, near-surface cool anomalies do appear in several other analyses included in this study, but any regions of surface divergence, if present, are not captured in the observations.
Analysis of the 1600 UTC 22 September mission suggests the formation of a potentially closed circulation (not shown) between 500 and 850 hPa (possibly extending farther downward). Additionally, the low- and midlevel vorticity maxima (Fig. 10a) have become more vertically aligned and now resemble a vorticity column rather than two distinct vorticity maxima. Three hours later (1900 UTC 22 September; Fig. 11), the analysis reveals a potentially closed circulation extending from the surface up to at least 550 hPa (not shown). The latter flight also indicates that the vorticity column has become deeper during this period (Fig. 11a). The system was operationally declared a tropical depression the following day, with the poor satellite representation likely responsible for the delayed declaration in comparison to the presence of a low-level circulation analyzed here.
As in Fig. 1, but the plots are valid for the developing pre-Matthew PGI46L system at 1600 UTC 22 Sep 2010.
Citation: Monthly Weather Review 143, 8; 10.1175/MWR-D-14-00242.1
As in Fig. 1, but the plots are valid for the developing pre-Matthew PGI46L system at 1600 UTC 22 Sep 2010.
Citation: Monthly Weather Review 143, 8; 10.1175/MWR-D-14-00242.1
As in Fig. 1, but the plots are valid for the developing pre-Matthew PGI46L system at 1600 UTC 22 Sep 2010.
Citation: Monthly Weather Review 143, 8; 10.1175/MWR-D-14-00242.1
As in Fig. 1, but the plots are valid for the developing pre-Matthew PGI46L system at 1900 UTC 22 Sep 2010.
Citation: Monthly Weather Review 143, 8; 10.1175/MWR-D-14-00242.1
As in Fig. 1, but the plots are valid for the developing pre-Matthew PGI46L system at 1900 UTC 22 Sep 2010.
Citation: Monthly Weather Review 143, 8; 10.1175/MWR-D-14-00242.1
As in Fig. 1, but the plots are valid for the developing pre-Matthew PGI46L system at 1900 UTC 22 Sep 2010.
Citation: Monthly Weather Review 143, 8; 10.1175/MWR-D-14-00242.1
Both flights into the system on 22 September indicate the system is in a widespread region of moist air (Figs. 10b and 11b), although the latter mission shows an increase in the relative humidity with large regions of saturated air captured in the observations. There is also a pocket of dry air evident above 350 hPa just west of the vorticity column at both times and the beginnings of this pocket are captured in the moisture and cloud fields of the previous day (Fig. 9b). Although the exact cause of this feature is difficult to discern given the horizontal resolution of the data, it is hypothetically possible that localized subsidence is being forced by the nearby deep, sustained convection. While support for a link between this dry pocket and localized subsidence can be found in the presence of an isolated region of convergence above 300 hPa during the earlier flight on 22 September (Fig. 10d) collocated with the dry air, there is no corresponding feature in the divergence analysis from the previous day (Fig. 9d), when the dry air pocket first appeared in the observations.
In addition to the similarities in the moisture fields, the 22 September flights both depict similar structures in the temperature anomaly and divergence fields. The temperature anomaly analyses on 22 September indicate a continued warming of the mid- and upper-level atmosphere in comparison to the previous day. The divergence fields show strong convergence at midlevels, although observations during the second mission of the day indicate the convergent region is shifted west of the midlevel center. As with the displacement between the midlevel convergence and vorticity maxima the previous day, the convergence maximum observed during the 1900 UTC 22 September flight is located on the same side of the midlevel vorticity maximum as the low-level vorticity maximum. Presumably, such an orientation would encourage the formation of a midlevel vorticity maximum over the low-level maximum, resulting in a vertically aligned vorticity column.
c. Dissipating PGI38L (ex-Gaston)
Tropical storm Gaston was first designated as a tropical depression at 0600 UTC 1 September 2010 and as a tropical storm at 1200 UTC 1 September. The system was downgraded to a tropical depression at 0000 UTC 2 September and a remnant low at 1800 UTC 2 September. The NHC TCR for Gaston (Blake 2010) indicates that the system was close to being redesignated as a tropical depression on 4 September but remained a remnant low due to a lack of convective organization. The report also states that the system dissipated into an open trough on 8 September. In total, PREDICT flew five missions into the system. The midpoint of the first mission occurred around 1700 UTC 2 September, shortly before the system was downgraded to a remnant low. The final mission occurred on 7 September, the day before the system devolved into an open trough.
During the first mission into the system (not shown), the column of elevated cyclonic vorticity extended from the surface to approximately 200 hPa. Within this column, the midlevel vorticity maximum was displaced approximately 175 km to the northwest. Despite this displacement, the observations show a closed circulation from the surface to at least 300 hPa. As was the case with the other systems, the midlevel vorticity maximum is located below a maximum in temperature anomaly. While the atmosphere is moist in the direct vicinity of the vorticity maxima (>80% relative humidity), observations indicate very dry air (<40% relative humidity) just west of the system as low as 825 hPa.
By the time of the second mission, around 1700 UTC 3 September, the vorticity column (Fig. 12a) has become notably weaker and no longer has an upper-level presence. The observations suggest that the closed circulation has become open above 400 hPa. The midlevel dry air, evident on the southeast end of the cross section (Fig. 12b), is being wrapped around the south side of the midlevel circulation. Although there is a strong warm anomaly above the midlevel vorticity maximum (Fig. 12c), there is not a strong corresponding convergent structure evident in the divergence field (Fig. 12d).
As in Fig. 1, but the plots are valid for the ex-Gaston PGI38L system at 1700 UTC 3 Sep 2010.
Citation: Monthly Weather Review 143, 8; 10.1175/MWR-D-14-00242.1
As in Fig. 1, but the plots are valid for the ex-Gaston PGI38L system at 1700 UTC 3 Sep 2010.
Citation: Monthly Weather Review 143, 8; 10.1175/MWR-D-14-00242.1
As in Fig. 1, but the plots are valid for the ex-Gaston PGI38L system at 1700 UTC 3 Sep 2010.
Citation: Monthly Weather Review 143, 8; 10.1175/MWR-D-14-00242.1
The third mission into the system occurred two days later, around 1700 UTC 5 September. At this time the midlevel vorticity maximum is now located southwest of the low-level center (Fig. 13a). Whether the change in position was due to a new vorticity maximum being generated or the old maximum being advected around the system is difficult to determine given the temporal resolution of this data, although strong midlevel convergence (Fig. 13d) suggests the vorticity may have been locally generated. The vertical extent of the closed circulation continues to shrink with evidence only appearing up to 650 hPa at this time. The system continues to struggle because of dry air, which can be seen wrapping around the south and east sides of the low-level circulation at 850 hPa (Fig. 13b). While the system has a midlevel convergence maximum, the extent of the upper-level divergence is limited, which may also be hampering the system. Additionally, the low-level temperature anomaly continues to increase in magnitude (Fig. 13c). This low-level warm anomaly is the strongest of any of the cases examined in this study.
As in Fig. 1, but the plots are valid for the ex-Gaston PGI38L system at 1700 UTC 5 Sep 2010.
Citation: Monthly Weather Review 143, 8; 10.1175/MWR-D-14-00242.1
As in Fig. 1, but the plots are valid for the ex-Gaston PGI38L system at 1700 UTC 5 Sep 2010.
Citation: Monthly Weather Review 143, 8; 10.1175/MWR-D-14-00242.1
As in Fig. 1, but the plots are valid for the ex-Gaston PGI38L system at 1700 UTC 5 Sep 2010.
Citation: Monthly Weather Review 143, 8; 10.1175/MWR-D-14-00242.1
Observations during the fourth mission into the ex-Gaston system, which took place around 1400 UTC 6 September, indicate that the midlevel vorticity maximum has all but dissipated (Fig. 14a) with generally anticyclonic flow above 500 hPa. Furthermore, the previously closed circulation has become open as low as 850 hPa, although the dropsonde pattern makes it difficult to determine the exact level at which the circulation becomes open. Above 600 hPa, the atmosphere is very dry (Fig. 14b) with the exception of isolated pockets of moist air, primarily associated with the convective activity on the western edge of the observation domain. The vertical gradient in temperature anomaly has largely disappeared (Fig. 14c). The divergence field (Fig. 14d) indicates the presence of weak convergence at low levels and weak divergence at upper levels with almost no noteworthy structure at midlevels.
As in Fig. 1, but the plots are valid for the ex-Gaston PGI38L system at 1400 UTC 6 Sep 2010.
Citation: Monthly Weather Review 143, 8; 10.1175/MWR-D-14-00242.1
As in Fig. 1, but the plots are valid for the ex-Gaston PGI38L system at 1400 UTC 6 Sep 2010.
Citation: Monthly Weather Review 143, 8; 10.1175/MWR-D-14-00242.1
As in Fig. 1, but the plots are valid for the ex-Gaston PGI38L system at 1400 UTC 6 Sep 2010.
Citation: Monthly Weather Review 143, 8; 10.1175/MWR-D-14-00242.1
The final mission into the system (not shown) occurred around 1500 UTC 7 September. As was the case the previous day, the circulation is open above 850 hPa except that the region of cyclonic vorticity extends only as high as 700 hPa. Above this level the flow shows no evidence of cyclonic curvature. Recent convective activity has resulted in isolated pockets of increased moisture at mid- and upper levels. The warm anomaly continues to be maximized at low levels, although a minimum has appeared around 500 hPa, likely in connection with the convective activity.
4. Discussion
A particularly interesting (and somewhat unexpected) finding of these analyses is the formation of a near-vertical vorticity column in the nondeveloping PGI27L case. As only two strictly nondeveloping cases are included in the analysis and only one of these possesses any midlevel cyclonic vorticity structures, the present study is unable to determine how common a vertically aligned cyclonic vorticity column is in a nondeveloping system. An expansion of this analysis to past and future field experiment data will hopefully shed light on this topic.
In terms of vorticity, there is very little difference between the developing systems and the nondeveloping PGI27L system during the period of vertical alignment. This would suggest that vorticity structure was not a limiting factor in preventing PGI27L from developing. When present, differences are manifest in the magnitude of the vorticity. Comparing the individual cases, the pre-Matthew system contains much greater vorticity magnitude than either the pre-Karl system or PGI27L, likely due to the deep closed circulation present only in the pre-Matthew system at the time of the observed vertical alignment. In the cases of the pre-Karl system and PGI27L, where the closed circulation has either opened or has not yet formed, the vorticity magnitudes are similar. While this might suggests that differences in the vorticity magnitude are insufficient to differentiate between developing and nondeveloping systems, composite studies have found that developing systems typically have a higher magnitude of relative vorticity (e.g., McBride and Zehr 1981; Kerns and Chen 2013). This discrepancy suggests that PGI27L was an unusually strong nondeveloping system and had above-average relative vorticity magnitude. The lack of distinction in vertical alignment also suggests that the mechanisms by which the maxima form and subsequently align are present even under less than favorable development conditions. Without the benefit of similarly high-density repeated sampling of additional Atlantic tropical convective systems (especially nondeveloping systems), it is difficult to make any observationally based conclusions regarding the mechanism by which these systems become vertically aligned.
A possible related source of the bifurcation between development and nondevelopment is in the extent of curvature present in the flow. Although the pre-Karl system did not have a closed circulation at the time of vertical alignment, it did have highly curved flow at this time. In both nondeveloping systems, the flow curvature was much less throughout the observing periods. A caveat to this is highlighted by the presence of a persistent closed circulation in the dissipating ex-Gaston system: although flow curvature may play a part in the bifurcation between development and nondevelopment, it is not able to distinguish between the two paths.
Comparison of the divergence and vorticity fields suggests that the primary mode of vorticity generation is one of absolute vorticity convergence and stretching. This agrees with the importance placed on this mechanism in genesis theory (e.g., Bister and Emanuel 1997; Ritchie and Holland 1997; Simpson et al. 1997; Montgomery et al. 2006). Although a detailed vorticity tendency analysis (e.g., Creighton et al. 2013) might be able to confirm or refute this conclusion, the available data are not capable of supporting such an analysis under the methodology framework employed here.5 Examination of the temperature anomaly fields further suggests that the midlevel convergence is thermodynamically driven by stratiform heating, which is observed to be collocated with the midlevel convergence and vorticity maxima during most stages of system evolution. While the presence of strong convergence at midlevels agrees with the findings of Raymond et al. (2011), it is difficult to determine if the level of maximum convergence is shifting downward with time due to the noisiness of the divergence field. It is also worth noting that the region of strong convergence is present in the nondeveloping PGI27L. This suggests that the presence of strong midlevel convergence does not indicate future development (although it is plausible that a lack of convergence suggests nondevelopment). In contrast to the midlevels, the low-level convergence does not correspond to a temperature anomaly at the scales resolved by these observations and may be forced either synoptically (in the cases presented here, by the easterly wave in which the system is embedded) or by the convective inflow or a combination of the two.
Of particular interest to understanding the bifurcation between these developing and nondeveloping systems is the relative humidity, particularly at mid- and upper levels. The convectively active cases have mid- to upper-level moisture plumes of varying extent that are generated by persistent, widespread convective activity. In the mid- and upper troposphere, these plumes are often associated with stratiform cloud decks, which are responsible for generating an upper-level warm anomaly. Warming by this upper-level heat source produces a maximum in convergence at midlevels, which generates vorticity via absolute vorticity convergence. It is this process that builds the midlevel vorticity maximum.
In the nondeveloping PGI27L case, the moisture plume, and the associated upper-level stratiform latent heat source, is collocated with the midlevel vorticity maximum during the first flight into the system. This configuration results in the midlevel vorticity maximum being reinforced by the convergence of absolute vorticity. During the second flight, however, the vorticity column extends into dry mid- and upper-level air. This dry air prevents the stratiform cloud deck from extending over the vorticity column; instead, the stratiform cloud deck, and the convergence maximum, is displaced to the northeast of the near vertically aligned vorticity column. Although there were no additional flights to sample the impacts of this displacement, Zawislak and Zipser (2014b) indicate the model-analyzed 600-hPa vorticity maximum has shifted to the northeast of the low-level center over the following 12 hours. It is plausible that the nondevelopment of the PGI27L system is a result of the loss of vertical alignment of the vorticity column due to the disruptive presence of mid- and upper-level dry air over the low-level vorticity maximum. Additionally, the presence of a shallow shear layer at upper levels acts to further complicate the analysis and more work is needed to better understand the impacts of such a top-heavy shear profile on system evolution.
The displacement of the midlevel convergence maximum from the midlevel vorticity maximum is also observed in the developing pre-Matthew system. Unlike in the PGI27L case, the displaced midlevel convergence in the pre-Matthew system is located above the low-level vorticity maximum. As such, it is plausible that this displacement in convergence (and implied absolute vorticity convergence) is responsible, in part, for the increased vertical alignment observed the following day.
While this “displaced warm anomaly” pattern is observed in two of the cases examined here, it is not observed in the developing pre-Karl, dissipating ex-Gaston, or convectively suppressed nondeveloping PGI30L. Additional research is needed to determine how frequently this pattern occurs and how great an influence it has on the genesis process. Although the importance of midlevel moisture to the genesis process is not a novel finding (e.g., Nolan 2007; Davis and Ahijevych 2012; Komaromi 2013; Zawislak and Zipser 2014a), the present study has demonstrated its importance through a detailed observational analysis employing an alternative methodology.
As pointed out in prior studies (e.g., Raymond and Sessions 2007; Davis and Ahijevych 2012; Zawislak and Zipser 2014a), the warm anomaly in the mid- and upper troposphere acts to convectively stabilize the atmosphere within the system. As a stable atmosphere dissipates vorticity at a slower rate than an unstable atmosphere, systems with a well-developed mid- to upper-level warm anomaly would require less work to regenerate the vorticity dissipated during the convective lull, leading to a faster rate of development. On the other hand, if the midlevel atmosphere is too stable, it will inhibit the vertical mass flux necessary for vorticity generation. If the preserving effects of stability on the midlevel vorticity are an important factor in the genesis process, the presence of dry midlevels may act to destabilize the atmosphere both directly (by modifying the local buoyancy) and indirectly (via evaporative cooling and preventing latent heat release aloft), thus encouraging the dissipation of system-scale vorticity. This interaction may help explain the nondevelopment of PGI27L and the dissipation of ex-Gaston, although a large number of additional cases in similar conditions are necessary to observationally verify this possibility.
There are a number of properties associated with the presence of a midlevel vorticity maximum that may be beneficial to tropical cyclogenesis. In Zhang and Zhu (2012), it is suggested that the upper-level warm anomaly can produce a drop in sea level pressure that, in turn, would result in enhanced low-level convergence. If this process is critical to tropical cyclogenesis, the presence of dry air over the low-level vorticity maximum would certainly act to inhibit development. Even if the temperature anomaly is the key feature, the midlevel vorticity is not necessarily irrelevant to the process as elevated vorticity can act to slow the ingestion of dry air into a system (Dunkerton et al. 2009). Additionally, vorticity factors into the inertial stability of the system. Stronger cyclonic vorticity increases the inertial stability, which, in turn, increases the efficiency of latent heat release in warming the local atmosphere.
In addition to the role of moisture in the development process, the analysis of PGI27L suggests an additional possible discriminating feature. Above 250 hPa, PGI27L contains a region of strong convergence, associated with an upper-level trough, while both developing systems are analyzed to have strong divergence at upper levels, typical of systems with persistent deep convection. The impact of this convergence on the net horizontal mass flux may be somewhat mitigated by a layer of strong divergence just below the feature. Although the lack of strong upper-level divergence above the vorticity column in a nondeveloping system agrees with the findings of previous studies (e.g., McBride and Zehr 1981), which have identified the presence of strong upper-level divergence as a feature typical of developing systems, it is unknown if there is a direct relationship in terms of mass flux between the convergent upper-level feature in PGI27L and its subsequent nondevelopment.
If the developing cases examined here are indicative of the general population of developing systems, by the time the system becomes vertically aligned, the midlevel vorticity maximum and upper-level warm anomaly are already well established and only the low-level circulation need be spun up.6 This agrees with prior studies (e.g., Nolan 2007; Davis and Ahijevych 2012), which have found that the low-level circulation tends to spin up relatively rapidly just prior to the time of genesis. Further work is needed to determine if a TC can form in such a way that the low-level vortex is spun up before an established midlevel vorticity maximum is in place.
5. Conclusions
Five Atlantic tropical convective systems were analyzed with the goal of identifying structural and evolutionary differences that may impart greater insight into both the processes involved in, and the forecasting of, tropical cyclogenesis. Of these systems, two were classified as nondeveloping, two as developing, and one as a dissipating system. The results of this study, as discussed in the previous section, corroborate the findings of a number of prior studies of developing and nondeveloping systems (e.g., McBride and Zehr 1981; Nolan 2007; Davis and Ahijevych 2012; Komaromi 2013; Zawislak and Zipser 2014a).
The analyses suggest that a vertically aligned vorticity column, a moisture-rich troposphere (especially above the low-level vorticity maximum), low-level convergence (generated synoptically or otherwise), midlevel convergence (at least in the early to middle stages of development), upper-level divergence, and an upper-level warm anomaly are each individually a necessary, but not sufficient, condition of tropical cyclogenesis. The upper-level warm temperature anomaly, produced by stratiform latent heat release, is responsible for the generation of a midlevel MCV. The presence of subsaturated air in the mid- and upper troposphere can act to prevent this stratiform latent heating and may potentially erode existing warm anomalies via evaporative cooling.
Concerning identifying differences in structural evolution that may be indicative of future development, the cases analyzed suggest that there are no marked differences in vorticity or lower-tropospheric divergence. In particular, vertical alignment of the low- and midlevel vorticity maxima occurred in three systems including one that did not develop. The analyses suggest that both developing and nondeveloping systems generate vorticity via the same processes (i.e., concentration and stretching) and the vorticity columns of both can become increasingly upright with time. The results also suggest that differences in moisture can play a large part in future development, but a moist environment does not guarantee future system survival or development. Differences observed in the upper-level warm temperature anomaly and midlevel convergence structures may also play a role depending on their location relative to the vorticity structures. Because of its relation to midlevel vorticity generation, a misalignment between the midlevel convergence and the midlevel vorticity maximum may act to either shift the midlevel vorticity toward or away from the low-level vorticity maximum. In one nondeveloping case, the upper-level warm anomaly and associated midlevel convergence was displaced in such a way so as to shift the midlevel vorticity maximum out of vertical alignment with the low-level vorticity maximum. In contrast, the upper-level warm anomaly in one developing system was positioned such that the associated convergence would act to move the midlevel vorticity maximum closer to the low-level vorticity maximum. Additionally, one nondeveloping case contained a region of strong convergence at upper levels suggesting the importance of upper-level divergence. Again, as the present study has only examined five cases located in the western Atlantic, a significant number of additional case studies are required to determine if these conclusions are representative of other tropical convective systems and other basins.
Acknowledgments
The authors thank Mark Bourassa and Vasubandhu Misra of the Florida State University for their feedback on the thesis on which this work was based. Additional thanks are owed to Bob Dattore for his efforts in providing data from a number of field experiments. This paper has also benefited from helpful discussions with Jack Beven and Chris Davis that have greatly aided the analysis process. The authors would also like to thank Patrick Duran, Jon Zawislak, and two anonymous reviewers for their helpful insights during the preparation of this manuscript. Data support for the PREDICT and GRIP field experiments is provided by NCAR/EOL under sponsorship of the National Science Foundation. This work was funded through the NASA Genesis and Rapid Intensification Processes (GRIP) field experiment (Grant NNX09AC43G).
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A tropical convective system is defined here as the collection of convective elements that make up the meso-
The consensus pouch positions are produced in real time by M. Boothe using the method described in Dunkerton et al. (2009) with a number of operational forecast model analyses. These pouch positions are available online at http://www.met.nps.edu mtmontgo/storms2010.html.
PGI denotes PREDICT–GRIP–IFEX, the three experiments that took place during 2010.
Specifically the response to warming that produces a drop (rise) in geopotential heights on an isobaric surface below (above) the source thereby inducing convergence below (divergence above) in order to remove the geopotential anomaly.
While the vorticity tendency equation was successfully expressed in line integral form using Green’s theorem, errors arose when kinematically calculating vertical velocity despite using the O’Brien (1970) corrections. It would appear that errors in divergence, caused by the high sensitivity of the divergence calculations to the errors introduced by the time–space conversion (see section 2e), were magnified by the vertical velocity calculations such that any useful signal was overwhelmed.
This is, of course, after any low-level divergence is replaced by convergence as per Komaromi (2013) and Zawislak and Zipser (2014a).
