Downdraft Convective Available Potential Energy Research Articles

A case study of a derecho storm in dry, high-shear environment

The present study examines the origin and environmental conditions of the severe convective windstorm on September 17, 2017, which affected several countries in the central and southeastern Europe, above all Serbia and Romania. The large area of the damage swath (at least 500 km long) and high wind gusts (up to 40 m/s) would classify this event as a derecho or at least as a storm very similar to derechos (with respect to newer definition proposals). Small-scale bow echoes were found in areas with highest reported wind gusts, and some thunderstorms within the storm-producing convective system were probably supercells. The existence of high wind shear and storm rotation could be also related to the significant rightward deflection of the system with respect to the mean wind and propagation of other thunderstorms and systems observed on that day. In contrary to many other known derecho events, this storm propagated toward a very dry airmass exhibiting only low or moderate convective available potential energy (CAPE) values. This is shown by soundings, ECMWF model outputs, and vertical profiles from the IASI L2 satellite sounder. Several convective parameters (e.g. CAPE, downdraft CAPE, derecho composite parameter, 0-3-km relative humidity, 0-6-km shear) were evaluated and compared with proximity soundings of other described European derechos or with the available climatology. The possibility of a balance between the cold pool-generated horizontal vorticity and the environmental shear is also discussed. It is concluded that identification of low-level humidity sources (with aid of storm-relative wind vectors or streamlines) can be important in forecasting of thunderstorm systems moving toward an airmass, which is seemingly too dry for development and maintenance of deep convection. It is also shown that due to low CAPE values, some composite parameters would not indicate favourable conditions for a long-lived convective system. The lack of radiosonde observations can be partially supplemented by data from the IASI L2 sounder, which profiles can be largely different from model forecasts, showing much drier air in the mid- and upper troposphere in this case. It is concluded that due to the absence of strong synoptic forcing and larger pressure gradient at surface, convective processes played major role in the windstorm development. The presence of high temperature lapse rates at low- and mid-levels, high wind shear and unusually dry pre-storm airmass could be considered as the most important signatures related to the storm severity.

Analysis of Severe Elevated Thunderstorms over Frontal Surfaces Using DCIN and DCAPE

A 10-year study of elevated severe thunderstorms was performed using The National Centers for Environmental Information Storm Events Database. A total of 80 elevated thunderstorm cases were identified, verified, and divided into “Prolific” and “Marginal” classes. These severe cases occurred at least 80 km away from, and on the cold side of, a surface boundary. The downdraft convective available potential energy (DCAPE), downdraft convective inhibition (DCIN), and their ratio are tools to help estimate the potential for a downdraft to penetrate through the depth of a stable surface layer. The hypothesis is that as the DCIN/DCAPE ratio decreases, there exists enhanced possibility of severe surface winds. Using the initial fields from the Rapid Refresh numerical weather prediction model, datasets of DCIN, DCAPE, and their ratio were created. Mann-Whitney U tests on the Prolific versus Marginal case sets were undertaken to determine if the DCAPE and DCIN values come from different populations for the two different case sets. Results show that the Prolific cases have values of DCIN closer to zero, suggesting the downdraft is able to penetrate to the surface causing severe winds. Thus, comparing DCIN and DCAPE is a viable tool in determining if downdrafts will reach the surface from elevated thunderstorms.

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

This study presents a 9‐year climatology and composite analysis of quasi‐linear convective systems (QLCSs) in southern Brazil (SB). QLCSs are identified using radar imagery and defined as a 100‐km long, 40‐dBZ convective line that lasts for at least 1 hr. QLCS cases associated with at least three severe wind reports are classified as severe. Composites of the synoptic‐scale environments and severe convective weather parameters are constructed for severe and non‐severe cases of three synoptic patterns found as the most recurrent in QLCS cases. QLCSs are more frequent and more likely to be severe during spring, while very few cases occur during winter. Most cases occur during late night and morning, corroborating previous research of mesoscale convective systems in this area, but the percentages of severe cases are higher in late afternoon and early night. Faster QLCSs, particularly those with velocities greater than 50 km/hr, have higher probability of being severe in comparison to slower systems. The three synoptic patterns more often related to QLCS occurrence in SB are characterized by a mid‐level trough upstream of SB (type 1), a mid‐level trough west of the Andes (type 2) and predominance of zonal flow over SB (type 3). In general, there is a low‐pressure system over northwestern Argentina extending a trough to SB, where low‐level moisture flux convergence and warm advection occur. In severe cases, the sea‐level pressure is lower in northwestern Argentina and there is greater low‐level northwesterly flow over SB. The type 1 is the most frequent pattern when QLCSs are observed in SB and is also the type with higher percentage of severe cases. Convective available potential energy (CAPE) and downdraft CAPE are good discriminators between severe and non‐severe cases in type 1 and 2 environments, while the 1,000–500‐hPa bulk wind difference is a better predictor in type 3 cases.

Dynamic Ensemble Analysis of Frontal Placement Impacts in the Presence of Elevated Thunderstorms during PRECIP Events

The Program for Research on Elevated Convection with Intense Precipitation (PRECIP) field campaign sampled 10 cases of elevated convection during 2014 and 2015. These intense observing periods (IOP) mostly featured well-defined stationary or warm frontal zones, over whose inversion elevated convection would form. However, not all frontal zones translated as expected, with some poleward motions being arrested and even returning equatorward. Prior analyses of the observed data highlighted the downdrafts in these events, especially diagnostics for their behavior: the downdraft convective available potential energy (DCAPE) and the downdraft convective inhibition (DCIN). With the current study, the DCAPE and DCIN are examined for four cases: two where frontal motion proceeded poleward, as expected, and two where the frontal motions were slowed significantly or stalled altogether. Using the Weather Research and Forecasting (WRF) model, a multi-model ensemble was created for each of the four cases, and the best performing members were selected for additional deterministic examination. Analyses of frontal motions and surface cold pools are explored in the context of DCAPE and DCIN. These analyses further establish the DCAPE and DCIN, not only as a means to classify elevated convection, but also to aid in explaining frontal motions in the presence of elevated convection.

The Role of Mesoscale-Convective Processes in Explaining the 21 November 2016 Epidemic Thunderstorm Asthma Event in Melbourne, Australia

AbstractA major thunderstorm asthma epidemic struck Melbourne and surrounding Victoria, Australia, on 21 November 2016, which led to multiple deaths, a flood of residents seeking medical attention for respiratory problems, and an overwhelmed emergency management system. This case day had all the classic ingredients for an epidemic, including high rye grass pollen concentrations, a strong multicellular thunderstorm system moving across the region, and a large population of several million people in the vicinity of Melbourne. A particular characteristic of this event was the strong, gusty winds that likely spread the pollen grains and/or allergenic contents widely across the region to increase population exposure. This exploratory case study is the first to examine the usefulness of low-to-middle-atmospheric thermodynamic information for anticipating epidemic thunderstorm asthma outbreaks by allowing the forecast of strong downdraft winds. The authors investigated the utility of several mesoscale products derived from atmospheric soundings such as downdraft convective available potential energy (DCAPE) and indices for predicting surface wind gusts such as microburst wind speed potential index (MWPI) and a wind gust index (GUSTEX). These results indicate that DCAPE levels reached “high” to “very high” thresholds for strong downdraft winds in the lead-up to the thunderstorm, and the MWPI and GUSTEX indices accurately predicted the high maximum surface wind observations. This information may be useful for diagnostic and prognostic assessment of epidemic thunderstorm asthma and in providing an early warning to health practitioners, emergency management officials, and residents in affected areas.

Evaluating elevated convection with the downdraft convective inhibition

AbstractA method for evaluating the penetration of a stable layer by an elevated convective downdraft is discussed. Some controversy exists on the community's ability to define truly elevated convection from surface‐based convection. By comparing the downdraft convective inhibition (DCIN) to the downdraft convective available potential energy (DCAPE), we determine that downdraft penetration potential is progressively enabled as the DCIN is progressively smaller than the DCAPE; inversely as DCIN increases over DCAPE, so does the likelihood of purely elevated convection. Serial vertical soundings and accompanying analyses are provided to support this finding.

Environments of Northeast U.S. Severe Thunderstorm Events from 1999 to 2009

Abstract An investigation of the environments and climatology of severe thunderstorms from 1999 through 2009 across the northeastern United States is presented. A total of 742 severe weather events producing over 12 000 reports were examined. Given the challenges that severe weather forecasting can present in the Northeast, this study is an effort to distinguish between the more prolific severe-weather-producing events and those that produce only isolated severe weather. The meteorological summer months (June–August) are found to coincide with the peak severe season. During this time, 850–500- and 700–500-hPa lapse rates, mixed layer convective inhibition (MLCIN), and downdraft convective available potential energy (DCAPE) are found to be statistically significant in discriminating events with a large number of reports from those producing fewer reports, based on observed soundings. Composite synoptic pattern analyses are also presented to spatially characterize the distribution of key meteorological variables associated with severe weather events of differing magnitudes. The presence of a midlevel trough and particular characteristics of its tilt, along with an accompanying zone of enhanced flow, are found in association with the higher-report severe weather events, along with cooler midlevel temperatures overlaying warmer low-level temperatures (i.e., contributing to the steeper lapse rates). During the meteorological fall and winter months (September–February), large-scale ascent is often bolstered by the presence of a coupled upper-level jet structure.

Convective Squalls over the Eastern Equatorial Atlantic

Abstract The Congo Basin and the adjacent equatorial eastern Atlantic are among the most active regions of the world in terms of intense deep moist convection, leading to frequent lightning and severe squalls. Studying the dynamics and climatology of this convection is difficult due to a very sparse operational network of ground-based observations. Here, a detailed analysis of recently available high temporal resolution meteorological observations from three oil platforms off the coast of Angola spanning the three wet seasons from 2006/07 to 2008/09 is presented. The annual cycle of squall days as identified from wind data closely follows that of convective available potential energy (CAPE) and therefore mirrors the cycle of wet and dry seasons. The diurnal cycle of squall occurrence varies from station to station, most likely related to local features such as coastlines and orography, which control the initiation of storms. An attempt to classify squalls based on the time evolution of the station meteorology and satellite imagery suggests that microbursts account for at least one-third of the strong gusts, while mesoscale squall lines appear to be quite rare. On a daily basis the probability of squall occurrence increases with increasing values of CAPE, downdraft CAPE, and 925–700-hPa wind shear, and decreases for high convective inhibition, all calculated from vertical profiles of temperature and humidity at the nearest grid point in the NCEP–NCAR and ECMWF reanalysis datasets. Both the climatological results and the stability indices can be used for local forecasting to avoid squalls impacting on operations on the offshore platforms.

Environment and Early Evolution of the 8 May 2009 Derecho-Producing Convective System

This study documents the complex environment and early evolution of the remarkable derecho that traversed portions of the central United States on 8 May 2009. Central to this study is the comparison of the 8 May 2009 derecho environment to that of other mesoscale convective systems (MCSs) that occurred in the central United States during a similar time of year. Synoptic-scale forcing was weak and thermodynamic instability was limited during the development of the initial convection, but several mesoscale features of the environment appeared to contribute to initiation and upscale growth, including a mountain wave, a midlevel jet streak, a weak midlevel vorticity maximum, a “Denver cyclone,” and a region of upper-tropospheric inertial instability. The subsequent MCS developed in an environment with an unusually strong and deep low-level jet (LLJ), which transported exceptionally high amounts of low-level moisture northward very rapidly, destabilized the lower troposphere, and enhanced frontogenetical circulations that appeared to aid convective development. The thermodynamic environment ahead of the developing MCS contained unusually high precipitable water (PW) and very large midtropospheric lapse rates, compared to other central plains MCSs. Values of downdraft convective available potential energy (DCAPE), mean winds, and 0–6-km vertical wind shear were not as anomalously large as the PW, lapse rates, and LLJ. In fact, the DCAPE values were lower than the mean values in the comparison dataset. These results suggest that the factors contributing to updraft strength over a relatively confined area played a significant role in generating the strong outflow winds at the surface, by providing a large volume of hydrometeors to drive the downdrafts.

A statistical analysis of sounding derived indices and parameters for extreme and non-extreme thunderstorm events over Cyprus

Abstract. The main purpose of this study is to provide a simple statistical analysis of several stability indices and parameters for extreme and non-extreme thunderstorm events during the period 1997 to 2001 in Cyprus. For this study, radiosonde data from Athalassa station (35°1´ N, 33°4´ E) were analyzed during the aforementioned period. The stability indices and parameters set under study are the K index, the Total Totals (TT) index, the Convective Available Potential Energy related parameters such as Convective Available Potential Energy (CAPE), Downdraft CAPE (DCAPE) and the Convective Inhibition (CIN), the Vorticity Generator Parameter (VGP), the Bulk Richardson Number (BRN), the BRN Shear and the Storm Relative Helicity (SRH). An event is categorized as extreme, if primarily, CAPE was non zero and secondary, if values of both the K and the TotalTotals (TT) indices exceeded 26.9 and 50, respectively. The cases with positive CAPE but lower values of the other indices, were identified as non-extreme. By calculating the median, the lower and upper limits, as well as the lower and upper quartiles of the values of these indices, the main characteristics of their distribution were determined.

Uplift of Saharan dust south of the intertropical discontinuity

In situ observations from a flight made during the Geostationary Earth Radiation Budget Intercomparison of Longwave and Shortwave Radiation (GERBILS) field campaign (June 2007) show significant dust uplift into the monsoon flow immediately south of the intertropical discontinuity in the western Sahara. Dust loadings were highest in the moist monsoon air and the observations are consistent with dust uplift by the nocturnal monsoon winds. There is some evidence that cold pools within the monsoon flow contributed to the dust uplift: regions of elevated dust, water vapor, and ozone within the monsoon air are consistent with precipitation cooling and moistening air from upper levels and the resultant dusty cold pools propagating northward. However, only southward propagating cold pool outflows could be observed in satellite imagery. Using European Centre for Medium‐Range Weather Forecasts analyses and satellite data, it is shown that the asymmetry in the seasonal dust cycle is closely related to the downdraft convective available potential energy (DCAPE) from convective storms. There is both more dust and more DCAPE during monsoon onset than during retreat. The larger DCAPE values during monsoon onset, as well as the stronger nocturnal monsoon flow and the stronger heat trough circulation, are expected to contribute to the higher dust loadings at this time. Both the monsoon flow and cold pool outflows within it result in dust uplift in the western Sahara during the monsoon onset, which is when the maximum dust uplift occurs. For dust modeling, this shows the importance of accurately modeling not only the monsoon flow itself but also deep convection and cold pools.

The Influence of Midtropospheric Dryness on Supercell Morphology and Evolution

Abstract This work studies the relationship between midtropospheric dryness and supercell thunderstorm morphology and evolution using a three-dimensional, nonhydrostatic cloud model. Environments that differ only in midtropospheric dryness are found to produce supercells having different low-level outflow and rotational characteristics. Thunderstorms forming in environments with moderate vertical wind shear, large instability, and very dry midtropospheric air produce strong low-level outflow. When this low-level outflow propagates faster than the midlevel mesocyclone, the storm updraft and low-level mesocyclone weaken. However, in environments with larger vertical wind shear or with higher-altitude dry midtropospheric air, the low-level outflow is not as detrimental to the supercell. This provides a possible explanation for why some environments that appear favorable for the development of strong low-level mesocyclones in supercells fail to do so. Downdraft convective available potential energy (DCAPE) is...