Characteristic Analysis of the Downburst in Greely, Colorado on 30 July 2017 Using WPEA Method and X-Band Radar Observations

atmosphere

Article Characteristic Analysis of the Downburst in Greely, Colorado on 30 July 2017 Using WPEA Method and X-Band Radar Observations

Hao Wang 1,*, Venkatachalam Chandrasekar 2, Jianxin He 3, Zhao Shi 1 and Lijuan Wang 1 1 College of Atmospheric Sounding, Chengdu University of Information Technology, Chengdu 610225, China; [email protected] (Z.S.); [email protected] (L.W.) 2 College of Engineering, Colorado State University, Fort Collins, CO 80521, USA; [email protected] 3 Key Open Laboratory of Atmospheric Sounding, Chengdu University of Information and Technology, Chengdu 610225, China; [email protected] * Correspondence: [email protected]; Tel.: +86-028-859-67291

Received: 10 June 2018; Accepted: 4 September 2018; Published: 6 September 2018

Abstract: As a manifestation of low-altitude wind shear, a downburst is a localized, strong downdraft that can lead to disastrous wind on the ground surface. For effective pre-warning and forecasting of downbursts, it is particularly critical to understand relevant weather features that occur before and during a downburst process. It is important to identify the macroscopic features associated with the downburst weather process before considering fine-scale observations because this would greatly increase the accuracy and timeliness of forecasts. Therefore, we applied the wind-vector potential-temperature energy analysis (WPEA) method and CSU-CHILL X-band dual-polarization radar to explore the features of the downburst process. Here it was found that prior to the occurrence of the downburst of interest, the specific areas that should be monitored in future events could be determined by studying the atmospherically unstable areas using the WPEA method. Combining the WPEA method with dual-polarization radar observations, we can better distinguish the phase distribution of the hydrometeor in the process and greatly enhance the judgment of the possibility of the downburst. From exploration of the microphysical features of the downburst, we further found that ‘Zdr (differential reflectivity) column’ can be regarded as an important early warning indicator of the location of the downburst. Finally, a schematic of the formation process of the downburst according to the analyses was produced.

Keywords: downburst; wind-vector potential-temperature energy analysis; dual-polarization; microphysical feature; physical mechanism

1. Introduction A downburst refers to a downward-moving middle-air current that on reaching the surface can generate disastrous divergent or linear ground-level horizontal wind speeds >17.9 m/s [1]. After convective storms develop into a mature stage, cold downdrafts within the thunderstorm clouds can become sufficiently strong to form outflows and squall lines on reaching the ground. It can have considerable impact on the take-off and landing activities of planes. The destructive gale resulting from a downburst can cause plane crashes, human injuries and fatalities, and substantial damage to trees and crops [2–4]. Fujita [1] classified downbursts into two categories based on their spatial scales: macrobursts and microbursts. The area of influence of a macroburst is >4 km in diameter and the burst can persist for up to half an hour. The area of influence of a microburst is ≤4 km in diameter and its duration is usually less than five minutes. In addition, downbursts can also be classified as “dry downbursts” or “wet downbursts” according to amount of surface precipitation during the downburst

Atmosphere 2018, 9, 348; doi:10.3390/atmos9090348 www.mdpi.com/journal/atmosphere Atmosphere 2018, 9, 348 2 of 14 process (with 0.25 mm of precipitation used as the differentiation criterion) and radar echo intensity (with 35 dBz of echo intensity used as the differentiation criterion) [4,5]. Studies on downbursts began in the 1970s when Fujita analyzed the damage caused by an outbreak of super tornadoes that occurred in the USA on 3–4 April 1974, and investigated a plane crash that happened at JFK Airport in New York (USA) on 24 June 1976. It was reported that the main reason for both these incidents was strong downdrafts hitting the ground surface at very high speed and quickly spreading in all directions, thereby causing strong wind shear near the ground surface [6]. In 1978, under the support of the Northern Illinois Meteorological Research on Downbursts Project, Fujita et al. were able to verify the existence of downbursts by conducting the first field observational experiments using three evenly distributed Doppler radars. After analyzing the Doppler velocity field and intensity field information during multiple downburst events, they discovered the existence of a bow echo structure evident on the radar echo map during the downburst process [7,8]. This discovery constitutes a very important reference indicator for the prediction of downbursts. Although about 50 microburst events were observed during the experimental campaign, details of the low-altitude dynamics of the downbursts were not obtained because the strength of the divergent wind field was weak and because the radars were widely separated (i.e., about 60 km apart). Later, Fujita [6] obtained the three-dimensional evolutionary features of the kinematic field of a downburst via the Joint Airport Wind Shear observation experiment, which further promoted elucidation of the forcing mechanism of downbursts. Considerable research has been undertaken on downbursts based on experimental observations with radars. This research effort has not only explored the echo features presented by downbursts [6,9,10], but it has also examined the features of downburst-prediction signals, especially the three most important features of descending reflectivity cores, mid-level radial convergence, and descending horizontal troughs of Zdr [11–13]. With the development of computer technology, the effective combination of experimental observations and numerical models has provided a new research perspective for further study both of the formation mechanisms of downbursts and of the sensitivity of downburst formation to external environmental conditions. Srivastava [14] found that with a declining (increasing) lapse rate of environmental temperature, the occurrence of a downburst required a greater (lesser) amount of water vapor; thus, parent thunderstorm clouds with high moisture content will lead to stronger downdrafts. Based on simulations of downdrafts, Proctor [15,16] found the evaporation of precipitation, melting of hail, and sublimation of snow are all major driving forces behind the formation of downbursts. Knupp [17] used a three-dimensional cloud model to simulate two downburst events generated by thunderstorm clouds. It was found that both the temperature of the boundary layer and the environmental wind shear played important roles in the evolution of the microphysical features of the downbursts. Wakimoto et al. [18] conducted observational experiments and found that snowflake-like hydrometeor particles would be very likely to produce downbursts, which verified an earlier finding by Proctor [16] that was based on numerical simulation. Subsequent research related to numerical models has focused primarily on comparisons of the effects on downbursts of different boundary conditions and microphysical parameterization schemes [19–21]. The combination of observations and simulations has established a theoretical model for the formation mechanism of downbursts. However, because of the limitations of observational and simulation methods, many research conclusions remain theoretical and they have yet to be verified observationally. Therefore, such conclusions cannot be considered accurate and reasonable explanations for the evolutionary features of the physical structure of downdraft phenomena. Following the upgrading of the Weather Surveillance Radar-1988 Doppler system, the application of dual-polarization weather radars has been proven effective for revealing the microphysical features and dynamic mechanisms of downbursts. Dual-polarization weather radars provide information about the intensity change (horizontal reflectivity: Zh) and the phase (average radial velocity: V, spectral width: W) of object-backscattered electromagnetic wave signals at two different polarization states. However, they also provide information about the differences in object-backscattered echoes Atmosphere 2018, 9, 348 3 of 14

between the two different polarization states (differential reflectivity, Zdr; two-way propagation phase difference, Φdp; differential propagation phase shift, Kdp; correlation coefficient, CC; linear polarizability, Ldr). These dual-polarization variables are sensitive to hydrometeor properties such as shape, size, orientation, and phase. The integrated use of these variables can help identify hydrometeor type, which is beneficial in understanding the formation mechanism and microphysical changes of downbursts [22]. Bringi et al. [23,24] employed dual-polarization radars and, for the first time, discerned the existence of hail beneath the melting layer, thereby effectively identifying the specific location of a downburst. In 1986, using polarization information provided by dual-polarization weather radars, Fujita and co-workers [25] studied the types of precipitation particles and they analyzed the microphysical change characteristics of thunderstorm clouds before the occurrence of a downburst. Wakimoto and Bringi [26] showed that the downdraft of a microburst was related to the long and narrow axis of the hail, and they found that a ‘Zdr Hole’ appearing in the main core area of precipitation would lead to a strong downdraft. Using dual-polarization radar variables, Doviak and Zrnic [22] conducted detailed analysis and calculation both to determine the capability of the radars to identify various hydrometeors and to establish the value ranges of the various variables. Based on those results, Straka and Zrnic [27] further proposed the use of fuzzy logic algorithms to identify the phases of precipitation particles. Because fuzzy logic algorithms can describe a system using simple rules instead of formulas, they have obvious advantages when used to identify the phases of hydrometeor. Liu and Chandrasekar [28] established a fuzzy logic algorithm for the classification of hydrometeor based on the variables of the Colorado State University CHILL (CSU–CHILL) weather radar (an S-band, dual-polarization radar), and they verified the classification results obtained with the algorithm. Based on the Hydrometeor Classification Algorithm function, Mahale et al. [29] used the KOUN dual-polarization radar in Oklahoma (USA) to study a downburst event, analyzing the detail of the microphysical evolution characteristics of the process and obtaining a corresponding conceptual model. To date, only a small number of theoretical studies on downbursts have been conducted using dual-polarization radar observations, and few have explored the relationships between the meteorological environmental fields and microphysical structures, both of which affect downburst formation. Analysis of the meteorological environmental fields and microphysical structures is of pivotal importance regarding the prediction of downbursts. Therefore, this study had three primary objectives: (1) to perform qualitative analysis of the downburst process based on conventional observational data; (2) to investigate the microphysical processes of downbursts from the perspective of detection, focusing on linking the external environmental conditions to the microphysical change characteristics during the evolution of a downburst to elucidate the underlying cause of downburst development; and (3) to refine the theoretical model of the occurrence and development of downbursts. The remainder of this paper is organized as follows. Section2 introduces the data and methods used in this study. Section3 presents a detailed analysis of the case study. The conclusions are presented in Section4.

2. Data and Methods

2.1. Data Sources The radar data were obtained by the CSU–CHILL. This radar, located in Greeley (CO, USA), is an advanced, transportable dual-polarized dual-wavelength (S- and X-band) weather radar system. The facility is operated under the joint sponsorship of Colorado State University and the National Science Foundation [30]. This radar is one of the most advanced dual-polarization radar systems currently available, and it is the ﬁrst radar that measures differential reﬂectivity factors using a slow switch. Following several upgrades, the radar can be operated in three observation modes: (1) the conventional mode of alternating horizontal and vertical polarization; (2) the mode of simultaneous transmission and alternating reception; and (3) the mode of simultaneous transmission Atmosphere 2018, 9, 348 4 of 14 and simultaneous reception. This study used the CSU–CHILL dual-polarization X-band radar observation data to monitor and analyze the downburst process (see Table1 for the main parameters). X-band data were used for the analysis because they allow variables to be investigated in more detail, and because the X-band is more sensitive than the S-band to microphysical phase changes when analyzing the differential phase (Kdp) of radar pulses. Before use, quality control and attenuation correction were performed on the data [30,31].

Table 1. Main speciﬁcations of the CSU–CHILL X-band radar system.

Gain 53 dBi Beamwidth 0.3◦ Sidelobe level <36 dB Frequency 9.41 GHz ± 30 MHz PRF max 2.00 kHz Sensitivity (dual-wavelength mode) −15 dBz, 10 km Range sampling 1.5–192.0 m Dynamic range 90 dB Scan type PPI (360◦, sector), RHI, ﬁxed pointing, vertically pointing

In addition to using the CSU–CHILL radar data, our research also used the North American Regional Reanalysis (NARR) dataset to construct and analyze the meteorological environmental fields. The spatial resolution of the NARR dataset is 32.463 × 32.463 km and the temporal resolution is 3 h [32]. The dataset is derived primarily from all the observational data in the National Centers for Environmental Prediction/National Center for Atmospheric Research Global Reanalysis project, as well as relevant precipitation data, TOVS 1B radiance data, profiler data, and land surface and moisture data. All the data are processed using the Eta 32 km/45-layer model to produce the final dataset. This dataset is generally considered reliable and thus it was adopted in this study to conduct a comprehensive analysis of the atmospheric environmental fields.

2.2. Method When analyzing the atmospheric environmental fields, the NARR data were used to represent the horizontal or vertical distributions of variables such as wind direction, wind speed, temperature, humidity, and air pressure at various altitudes. The data were mainly used for the qualitative analysis of the atmospheric environmental fields, but they were also used to provide temperature and humidity and other information that the CSU-CHILL radar failed to measure. When studying the vertical atmospheric structure based on single-station observational data, the “3θ” energy analysis method can be used to analyze and study the thermal features of the atmospheric environmental fields [33,34]. Here, 3θ refers to three types of temperature: the potential temperature θ, pseudo-equivalent potential temperature θsed (calculated using the dew point temperature), and saturation temperature θ*, which assumes a saturated state of air at the current temperature. Details about the definitions and calculation methods of the three temperatures can be found in Wang et al. [33]. The main reason for adopting this method in this study was that it allows the evolution of the atmospheric environmental fields under strong-convection weather to be presented in a qualitative manner. A schematic of the 3θ is shown in Figure1. In general, potential temperature increases with altitude (showing a 45◦ inclination in the P–T diagram). A decrease (or no change or only a slight change) of the three θ curves in the leftward direction of the abscissa axis with increasing altitude indicates the vertical structure of the troposphere is extremely unstable and that accumulating energy will need to be released [35]. Atmosphere 2018, 9, 348 5 of 14

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Figure 1. Schematic of the “3θ” energy analysis method. Figure 1. Schematic of the “3θ” energy analysis method. 3. Case Study 3. Case StudyThe case study incident was a strong-convection weather process involving a downburst

accompanied by localized hail and precipitation. The downburst incident occurred from 20:20 to The20:36 case Universal study Time incident (hereafter,Figure was 1. UT) Schematic a on strong-convection 30 July of the 2017 “3 inθ” theenergy northwestern weather analysis method. processpart of Greely involving (CO, USA). a downburst accompaniedDuring bythis localizedincident, the hail area and of impact precipitation. of the downburst The downburst exceeded 4 incidentkm in diameter occurred and the from 20:20 to 20:36 Universal3. maximumCase Study Timeground (hereafter, wind speed UT) excee onded 30 20.0 July m/s. 2017 Prior in theto this northwestern incident, the National part of Weather Greely (CO, USA). Service had issued a warning of slow-moving thunderstorms in the region, which produced up to During thisThe incident, case study the area incident of impact was a of strong-conve the downburstction exceeded weather 4process km in diameterinvolving anda downburst the maximum 1.25 inches of rain in 30 min in some areas. Forecasters warned of the potential for stream and street accompanied by localized hail and precipitation. The downburst incident occurred from 20:20 to ground windflooding speed due to exceeded the heavy rain; 20.0 however, m/s. Prior the foreca to thisst did incident, not mention the Nationalthe potential Weather for downbursts. Service had issued a warning20:36 ofUniversal slow-moving Time (hereafter, thunderstorms UT) on 30 in July the 2017 region, in the which northwestern produced part up of to Greely 1.25 inches(CO, USA). of rain in 30 minDuring in3.1. some Atmospheric this areas. incident, Circulation Forecasters the Situationarea warnedof impact of of the the potential downburst for exceeded stream and4 km street in diameter flooding and due the to the maximum ground wind speed exceeded 20.0 m/s. Prior to this incident, the National Weather heavy rain; however,Analysis of thethe atmospheric forecast did circulation not mention situation the associated potential with for this downbursts. incident revealed that a Servicesubsynoptic-scale had issued horizontala warning trough of slow-moving in front of athun baroclinicderstorms northwest in the cold region, vortex which was producedthe main up to 3.1. Atmospheric1.25influential inches Circulation ofsystem rain inin 30 this Situationmin process. in some As areas.shown Foreca by thesters high- warned and low-altitude of the potential configurations for stream of theand street floodingatmospheric due tocirculation the heavy fields, rain; the however, 500-hPa upperthe foreca troughst didat 18:00 not UTmention on 30 July the waspotential slightly for ahead downbursts. of Analysisthe 700-hPa of the upper atmospheric trough (Figure circulation 2a,b). Althou situationgh the 700- associated and 850-hPa with upper this troughs incident largely revealed that a subsynoptic-scale3.1.overlapped Atmospheric each horizontalCirculation other in terms Situation trough of their in positions, front of both a baroclinic lagged behind northwest the 500-hPa cold upper vortex trough. was the main Strong-convection weather usually occurs in such forward-tilting trough structures, because it is influentialAnalysis system of in the this atmospheric process. Ascirculation shown situation by the high-associated and with low-altitude this incident configurations revealed that a of the easy for unstable convective structures to form where the dry and cold advection behind the upper subsynoptic-scale horizontal trough in front of a baroclinic northwest cold vortex was the main atmospherictrough circulation is located above fields, the thewarm 500-hPa and wet upper advect troughion in the at front 18:00 part UT of on low-altitude 30 July was trough. slightly ahead of theinfluential 700-hPaAnother important uppersystem troughin influential this process. (Figure system 2Asa,b). was shown the Although warm by the anticyclone high- the 700- and below andlow-altitude 600 850-hPa hPa that configurations upperwas centered troughs of the largely overlappedatmosphericover southeastern each circulation other Colorado. in terms fields, The ofthe location their500-hPa positions, of upperthe downburst trough both at was lagged 18:00 situated UT behind on between 30 July the thewas 500-hPa cold slightly trough upper ahead of trough. the 700-hPa upper trough (Figure 2a,b). Although the 700- and 850-hPa upper troughs largely Strong-convectionand the warm weather anticyclone. usually With occurs the downward in such forward-tiltingswing of the troughs trough (from structures, Figure 2c,d), because the it is easy overlappedanticyclone each moved other slowly in terms southward of their and positions, it expanded both toward lagged thebehind east, thewhich 500-hPa increased upper the trough. for unstableStrong-convectiontemperature convective gradient structuresweather between usually to the form coldoccurs where trough in thesuch and dry forward-tiltingthe and anticyclone, cold advection troughleading structures,to behind an increase the because upper of it trough is is locatedeasy abovebaroclinicity for unstable the warm and convectiveconsequently, and wet stru advection to cturesan increase to in form of the wind where front speed. partthe dry of low-altitudeand cold advection trough. behind Another the upper important influentialtrough system is 60located was above the warm the warm anticyclone and wet belowadvection 600 in hPa the thatfront was part centered of low-altitude over southeasterntrough. Colorado.Another The important location influential of the downburst system was wasthe warm situated anticyclone between below the 600 cold hPa trough that was and centered the warm 10 50 30 0 5640 5740 3 311 over southeastern Colorado. The location of the downburst060 was situated between the cold trough anticyclone. With the57 downward swing of the troughs (from Figure2c,d), the anticyclone moved 40 3110 3210 and the warm40 anticyclone. With the downward swing of the troughs (from Figure 2c,d), the slowly southward and5840 it expanded toward the east, which increased the temperature gradient between anticyclone moved slowly southward and it expanded toward the east, which increased the the cold trough and the anticyclone, leading to an increase of baroclinicity and consequently, to an temperature30 gradient between the cold trough and the anticyclone, leading to an increase of increase of wind speed. baroclinicity20 and consequently, to an increase of wind speed. 130 120 110 100 90 80 70 60 Longitude（°W） (a) 10 50 30 0 5640 5740 3 311 060 57 40 3110 3210 40 5840

20 130 120 110 100 90 80 70 Longitude（°W） (a)

Figure 2. Cont. Atmosphere 2018, 9, 348 6 of 14 Atmosphere 2018, 9, x FOR PEER REVIEW 6 of 14

T (K) 60

20 130 120 110 100 90 80 70

Longitude（°W） Reference Vectors

Figure 2. (a) 500-hPa and (b) 700-hPa geopotential height over North America at 18:00 UT on 30 July Figure 2. (a) 500-hPa and (b) 700-hPa geopotential height over North America at 18:00 UT on 30 July 2017, respectively; (c) composite of 700-hPa wind field and 850-hPa temperature field at 9:00 UT on 2017, respectively;30 July 2017; and (c) ( composited) similar to of (c) 700-hPa but for 18:00 wind UT field on 30 and July 850-hPa 2017. The temperature red dots in the field four at panels 9:00 UT on 30 Julyrepresent 2017; and the ( dlocation) similar of tothe (c )downburst. but for 18:00 All UTthe ondata 30 are July from 2017. the The North red American dots in the Regional four panels representReanalyis the location (NARR) of dataset. the downburst. All the data are from the North American Regional Reanalyis (NARR) dataset. 3.2. Atmospheric Environmental Conditions 3.2. AtmosphericThe root Environmental cause of strong-convection Conditions weather such as downbursts is the non-uniform change and distribution of atmospheric heat [35,36]. The occurrence of strong-convection weather is not The root cause of strong-convection weather such as downbursts is the non-uniform change and only related to relevant dynamic factors but also closely related to unstable energy distribution distribution(thermal of conditions) atmospheric and water heat [vapor35,36 ].saturation The occurrence (water vapor of strong-convectionconditions) [37]. Therefore, weather this isstudy not only relatedused to relevant the 3θ analysis dynamic method factors to butanalyze also the closely atmospheric related energy to unstable structure energy during distribution the case study (thermal conditions)incident and to waterreveal vaporthe evolution saturation of the (water atmo vaporspheric conditions)environmental [37 ].fields. Therefore, In this thismethod, study θ used the 3θrepresentsanalysis methodthe vertical to analyzedistribution the of atmospheric atmospheric heat, energy θsed structureis the potential during temperature the case calculated study incident to revealusing the the evolution dew point oftemperature the atmospheric instead of environmental the traditional condensation fields. In thislevel method,temperature,θ represents and θ* is the the potential temperature assuming a saturated state of the air at the current temperature, which is vertical distribution of atmospheric heat, θsed is the potential temperature calculated using the dew intended to compare the distribution of atmospheric water vapor. point temperature instead of the traditional condensation level temperature, and θ* is the potential As shown by the atmospheric structures at the three time points of 12:00, 15:00, and 18:00 UT on temperature assuming a saturated state of the air at the current temperature, which is intended to 30 July 2017 (Figure 3a–c, respectively), the lines of θsed and θ* are leftward tilting and especially for comparethe thealtitude distribution below 780-hPa of atmospheric (about 2.3 waterkm) which vapor. form obtuse angles with the temperature axis As(T-axis). shown These by theconfigurations atmospheric are attributed structures primaril at they three to convective time points instability of 12:00, of the 15:00,low-altitude and 18:00 air UT on 30 Julydue to 2017 ground (Figure heating,3a–c, respectively),indicating that theoveral linesl convection of θsed and wasθ* establishedare leftward below tilting the and 780-hPa especially for thealtitude. altitude Moreover, below 780-hPa convection (about started 2.3 to km)strengthen which after form 12:00 obtuse UT, as angles reflected with in the the increase temperature of the axis (T-axis).obtuse These angles configurations and the increase are attributed of the unstable-layer primarily altitude to convective [33]. In instability addition, with of the regard low-altitude to the air entire layer of the atmosphere, the average distance between the θsed and θ* lines is substantial, due to ground heating, indicating that overall convection was established below the 780-hPa altitude. indicating the atmospheric water vapor content was not high, which is representative of the essential Moreover,difference convection between started general to showery strengthen precipitation after 12:00 and UT,continuous as reflected heavy inprecipitation the increase in ofweather the obtuse anglesprocesses and the increase[35]. As shown of the by unstable-layer the vertical distributi altitudeon [33 of]. specific In addition, humidity with in regard Figure to4, thethere entire was layer of theaccumulation atmosphere, of the water average vapor distance before betweenthe onse thet ofθ sedtheand downburst,θ* lines isand substantial, although indicatingadequate the atmospherichigh-altitude water water vapor vapor content might washave notprovided high, conditions which is necessary representative for the occurrence of the essential of hail, there difference betweenwas general no obvious showery change precipitation in the water and vapor continuous content at heavylower altitudes. precipitation As the in release weather of processesunstable [35]. As shownenergy by can the occur vertical in the distribution form of rainfall of specific (hail), strong humidity winds, in or Figure thunder4, there and lighting, was accumulation it is foreseeable of water based on the temperature–humidity distribution characteristics (calculations showed that vapor before the onset of the downburst, and although adequate high-altitude water vapor might have cumulative precipitation on the ground was less than 10 mm during 18:00–21:00 UT). Because of the providedlow conditionsheight of the necessarywater vapor for transport the occurrence and the inad ofequate hail, there low-level was water no obvious vapor, we change can infer in thethat water vaporthis content process at lowerwas not altitudes. dominated As by theprecipitation release of but unstable that it had energy already can released occur unstable in the form energy of in rainfall (hail),another strong form. winds, Therefore, or thunder we should and lighting, heed more it the is foreseeable occurrence of based hail and on strong the temperature–humidity winds, as well as distributionthunder characteristics and lightning. (calculations showed that cumulative precipitation on the ground was less than 10 mm during 18:00–21:00 UT). Because of the low height of the water vapor transport and the inadequate low-level water vapor, we can infer that this process was not dominated by precipitation but that it had already released unstable energy in another form. Therefore, we should heed more the occurrence of hail and strong winds, as well as thunder and lightning.

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Figure 3. The 3θ distribution conditions at the center location: (a) 12:00; (b) 15:00; and (c) 18:00 UT on Figure 3. The 3θ distribution conditions at the center location: (a) 12:00; (b) 15:00; and (c) 18:00 UT on 30Figure July 3.2017. The Red, 3θ distribution green, and conditionsblue lines are at the the center θ, θsed ,location: and θ* curves, (a) 12:00; respectively. (b) 15:00; and Area (c )indicated 18:00 UT byon 30 July 2017. Red, green, and blue lines are the θ, θsed, and θ* curves, respectively. Area indicated by the30 July dotted 2017. green Red, box green, is the and unstable blue lines area. are the θ, θsed, and θ* curves, respectively. Area indicated by the dotted green box is the unstable area. the dotted green box is the unstable area.

Figure 4. Specific humidity (kg/kg) between 12:00 UT on 30 July 2017 and 00:00 UT on 31 July 2017 forFigure the center4. Specific location humidity of the (kg/kg)case study between downburst. 12:00 UT on 30 July 2017 and 00:00 UT on 31 July 2017 Figure 4. Specific humidity (kg/kg) between 12:00 UT on 30 July 2017 and 00:00 UT on 31 July 2017 for for the center location of the case study downburst. the3.3. centerDual-Polarization location of theRadar case Observations study downburst. 3.3. Dual-PolarizationThe configurations Radar of Observations the atmospheric environmental fields, which created the external 3.3. Dual-Polarization Radar Observations conditionsThe configurations favorable for ofthe theoccurrence atmospheric of the enviro case nmentalstudy downburst, fields, which could created qualitatively the external have Theprovidedconditions configurations a favorablecertain degree of for the the of atmospheric predictabilityoccurrence environmentalof regarding the case thestudy fields, occurrence downburst, which of created the could incident. thequalitatively external To study conditionshave the favorabledownburstprovided for a the certainprocess occurrence degree in detail, of of predictability the we case analyzed study regarding downburst,the processthe occurrence couldevolution of qualitatively thefeatures incident. based haveTo study on provided the a certaindual-polarizationdownburst degree ofprocess predictability radar in observations,detail, regarding we analyzedfocusing the occurrence onthe theprocess microphysical of theevolution incident. features features To studyof thebased thedownburst on downburst the present in the special atmospheric environment when the downburst occurred. processdual-polarization in detail, we analyzedradar observations, the process focusing evolution on the features microphysical based on features the dual-polarization of the downburst radar presentThe in hail the precipitationspecial atmospheric began environmentat 20:16 UT, whenwhich the is downburstwhen the strongoccurred. surface wind began to observations, focusing on the microphysical features of the downburst present in the special increaseThe inhail intensity. precipitation As shown began in theat 20:1620:23 UT,UT Planwhich Position is when Indicator the strong (PPI) surface radial velocitywind began map atto atmospheric0.5°increase elevation, environmentin intensity. a pair ofAs whenvery shown strong the in downburstthe positive 20:23 UTand occurred. Plan negative Position velocities Indicator was (PPI) present radial about velocity 21 km map from at Thethe0.5° CSU–CHILL hailelevation, precipitation a pairradar of station. beganvery strong atThis 20:16 indicated positive UT, which andstro ngnegative is divergent when velocities the movement strong was surface presentwith maximum wind about began 21 divergent km to from increase ◦ in intensity.windthe CSU–CHILL speed As of shown >18.0 radar m/s in station. thewas 20:23 occurringThis indicated UT Plannear strothe Position ngground, divergent Indicator that movement is, approximately (PPI) with radial maximum velocity0.2 km abovedivergent map the at 0.5 elevation,groundwind aspeed (Figure pair of of >18.05a). very The m/s strong maximum was positiveoccurring instantaneous and near negative the wind ground, speed velocities that at theis, was approximatelyground present surface about 0.2 exceeded km 21 above km 20 fromm/s. the the CSU–CHILLMultipleground (Figure PPI radar radial 5a). station. Thevelocity maximum This maps indicated atinstantaneous 20:23–20:25 strong UTwind divergent (Figure speed 5)at movement illustratethe ground the surface with wind maximum fieldexceeded structure 20 divergent m/s. of windtheMultiple speed downburst. of PPI >18.0 radial There m/s velocity was was strongly maps occurring at divergent 20:23–20:25 near themoveme UT ground, (Figurent at that low5) illustrate altitudes is, approximately the (0.5° wind elevation, field 0.2 structure kmat around above of the ground0.2the (Figure km)downburst. (Figure5a). The5a),There a maximum distinctlywas strongly convergent instantaneous divergent structure moveme wind at speednt high at low altitudes at thealtitudes ground (6.5° (0.5° elevation, surface elevation, exceededat around at around 202.4 m/s. km) (Figure 5d), and cyclonic circulation between the two (Figure 5b,c). This circulation Multiple0.2 km) PPI (Figure radial 5a), velocity a distinctly maps convergent at 20:23–20:25 structure UT (Figureat high 5altitudes) illustrate (6.5° the elevation, wind fieldat around structure 2.4 of strengthenedkm) (Figure these5d), andtwo movementscyclonic circulation and allowed between them tothe develop two (Figure vigorously. 5b,c). In◦ Thisaddition, circulation as the the downburst.height that the There downburst was strongly system reached divergent (i.e., movement around 2.4 km) at low coincided altitudes very (0.5 muchelevation, with that of at the around strengthened these two movements and allowed them to develop vigorously.◦ In addition, as the 0.2 km)unstableheight (Figure that layer5 thea), downburst aof distinctly the atmospheri system convergent creached environmental structure (i.e., arou atfields,nd high 2.4 km) altitudeswe conccoincidedluded (6.5 very thatelevation, much the atmospherically with at around that of the 2.4 km) (Figureunstable5d), and arealayer cyclonic obtained of the circulationatmospheri from the wind-vectorc betweenenvironmental the potential-temperature two fields, (Figure we 5concb,c).luded energy This that circulation analysis the atmospherically indicated strengthened the theseevolvingunstable two movements areaposition obtained of and this from allowed downburst. the wind-vector them Thus, to develop thispotential-temperature could vigorously. be used to In energyconstrain addition, analysis the as thekey indicated heightregion thatthefor the downburstdownburstevolving system position monitoring. reached of this (i.e., downburst. around 2.4 Thus, km) this coincided could verybe used much to withconstrain that ofthe the key unstable region layerfor of the atmospheric downburst monitoring. environmental fields, we concluded that the atmospherically unstable area obtained from the wind-vector potential-temperature energy analysis indicated the evolving position of this downburst. Thus, this could be used to constrain the key region for downburst monitoring. Atmosphere 2018, 9, 348 8 of 14 Atmosphere 2018, 9, x FOR PEER REVIEW 8 of 14 Atmosphere 2018, 9, x FOR PEER REVIEW 8 of 14

Figure 5. Plan Position Indicator (PPI) scans of radial wind speed (m/s) at (a) 0.5°; (b◦) 2.9°; (c) ◦5.3°; (d) ◦ FigureFigure 5. 5.Plan Plan Position Position Indicator Indicator (PPI)(PPI) scans of of radial radial wind wind speed speed (m/s) (m/s) at (a at) 0.5°; (a) 0.5 (b) ;(2.9°;b) 2.9(c) 5.3°;;(c) (d 5.3) ; 6.5°◦ elevations at 20:23–20:25 UT on 30 July 2017. Black lines represent the radar radial line. (d)6.5° 6.5 elevationselevations at at 20:23–20:25 20:23–20:25 UT UT on on30 30July July 2017. 2017. Black Black lines lines represent represent the radar the radar radial radial line. line. In-depth analysis of the internal structure of a downburst is fundamental to understanding the In-depth analysis of the internal structure of a downburst is fundamental to understanding the process.In-depth The analysis CSU–CHILL of the radar internal performed structure eight of comp a downburstlete Range–Height is fundamental Indicator to understanding(RHI) scans from the process. The CSU–CHILL radar performed eight complete Range–Height Indicator (RHI) scans from process.20:28:30 The to CSU–CHILL20:31:46 UT. As radar shown performed in the RHI eight scans complete at the azimuth Range–Height of 31°, the Indicator strong echo (RHI) region scans (Z fromh > 20:28:30 to 20:31:46 UT. As shown in the RHI scans at the azimuth of 31°, the◦ strong echo region (Zh > 20:28:3045 dBz) to extended 20:31:46 in UT. the As vertical shown direction in the RHIto a height scans of at 8.6 the km azimuth (Figure of 6a). 31 Two, the kernels strong with echo strong region 45 dBz) extended in the vertical direction to a height of 8.6 km (Figure 6a). Two kernels with strong (Zhreflectivity> 45 dBz) factors extended (>60 in dBz) the were vertical evident direction at heights to a of height 5 and of 1 km 8.6 at km horizontal (Figure6 a).distances Two kernels of 19.5 and with reflectivity factors (>60 dBz) were evident at heights of 5 and 1 km at horizontal distances of 19.5 and strong21 km, reflectivity respectively, factors from (>60 the dBz) radar. were In evidentgeneral, at th heightse maximum of 5 and reflectivity 1 km at horizontalfactors of storm distances cells ofthat 19.5 21 km, respectively, from the radar. In general, the maximum reflectivity factors of storm cells that andproduce 21 km, respectively,downbursts have from large the radar. heights In and general, rapidly the de maximumscending reflectivityreflectivity factor factors cores. of storm The cellshigher that produce downbursts have large heights and rapidly descending reflectivity factor cores. The higher producethe maximum downbursts reflectivity have large factors, heights the and faster rapidly the factors descending descend, reflectivity and the factor longer cores. the precipitation The higher the the maximum reflectivity factors, the faster the factors descend, and the longer the precipitation maximumparticles reflectivitydive downward factors, [38]. the This faster means the factors the air descend, currents andwould the plummet longer the at precipitation higher speeds, particles and particles dive downward [38]. This means the air currents would plummet at higher speeds, and divewhen downward the vertically [38]. Thisdescending means air the currents air currents approach would the plummet ground surface, at higher they speeds, would and generate when a the when the vertically descending air currents approach the ground surface, they would generate a destructive wind, that is, a downburst. In the case study incident, the center of high divergence verticallydestructive descending wind, that air is, currents a downburst. approach In the groundcase study surface, incident, they the would center generate of high adivergence destructive caused prolonged sinking that further strengthened the intensity of the downburst. The above wind,caused that prolonged is, a downburst. sinking In that the casefurther study strengthen incident,ed the the center intensity of high of divergencethe downburst. caused The prolonged above features suggest that the strong reflectivity factors at the distance of 21 km from the radar, close to sinkingfeatures that suggest further that strengthened the strong thereflectivity intensity factors of the at downburst. the distance The of 21 above km from features the radar, suggest close that to the the ground surface, reflected the presence of hail or strong precipitation, and that strong divergent strongthe ground reflectivity surface, factors reflected at the the distance presence of 21 of km hail from or strong the radar, precipitation, close to the and ground that strong surface, divergent reflected currents had appeared on the ground surface, meaning that this incident had manifested the basic thecurrents presence had of appeared hail or strong on the precipitation, ground surface, and me thataning strong that divergent this incident currents had manifested had appeared the basic on the features of a downburst. groundfeatures surface, of a downburst. meaning that this incident had manifested the basic features of a downburst.

Figure 6. Range–Height Indicator (RHI) scans of dual-polarization variables: (a) Zh; (b) Zdr; (c) Kdp; Figure 6. Range–Height Indicator (RHI) scans of dual-polarization variables: (a) Zh; (b) Zdr; (c) Kdp; Figureand ( 6.d)Range–Height correlation coefficient Indicator (CC) (RHI) at azimuth scans ofof dual-polarization31° at 20:28 UT on variables:30 July 2017. (a )ZBlackh;( boxb)Z indicatesdr;(c)Kdp ; and (d) correlation coefficient (CC) at azimuth of 31°◦ at 20:28 UT on 30 July 2017. Black box indicates andthe (d critical) correlation area in coefﬁcient which the (CC)downburst at azimuth was generated. of 31 at 20:28 UT on 30 July 2017. Black box indicates thethe critical critical area area in in which which the the downburst downburst was was generated. generated.

Atmosphere 2018, 9, 348 9 of 14

The changes of dual-polarization variables allow further analysis of the microphysical features of the downburst process and the determination of features for the prediction of the downburst occurrence. Calculation based on observational data suggested the freezing level of the storm cloud in the case study incident was located at about the 650-hPa height, equivalent to about 3.8 km above the ground. Moreover, it was observed that the ‘Zdr column’ extended to a height of about 4.7 km (Figure6b). This exceeded the height of the zero-degree layer by about 0.9 km, indicating that the strong upward movement had flushed liquid droplets to above the freezing level [39]. This likely led to the formation of supercooled water droplets and small ice particles, which—once in contact with one another—would have resulted in the freezing of the liquid droplets onto the surfaces of the ice crystals and formation of larger graupel particles, a process called accretion. Supercooled water droplets and graupel particles are important sources for the formation of hail embryos [40]. At the top of the ‘Zdr column’, above the zero-degree level, contact between ice crystals and supercooled water droplets would have led to the freezing of the liquid droplets and the formation of small ice particles, which could account for the rapid decrease of Zdr values. In addition, the height of the top of the ‘Zdr column’ can be regarded as the maximum height reachable during ascent within a storm [29]. Thus, this maximum height could represent a reference value for the evaluation of the strength of the incident process. From the perspective of dynamics, the kinetic energy of an updraft is stored as potential energy at this height. With the beginning of precipitation (hail precipitation), the upper parts of clouds begin to collapse and become downdrafts, and the potential energy is released and converted back into kinetic energy. At the top of the ‘Zdr column’ shown in Figure6d, there is a zone where the CC values decreased rapidly to <0.9. Other studies have referred to such a zone as a ‘CC Hole’ [41]. The appearance of a ‘CC Hole’ indicates that graupel particles and hailstones were forming and growing within the zone [42,43]. Therefore, based on the location of the ‘Zdr column’, the appearance of the ‘CC Hole’, and the low Kdp values within the zone, it can be deduced that hailstones were continually forming and developing despite the occurrence of the downburst [44]. The black box in Figure6 denotes the core zone of the microburst. Based on the features of Z h (>50 dBz), Zdr (3–6 dB), ◦ Kdp (2–4 /km), and CC (0.90–0.95), it can be ascertained that the microburst process involved mixed precipitation dominated by hail, confirming the conclusion of other researchers that a wet downburst is usually accompanied by the formation of hail [16,45]. Although the Zh values showed the presence of two kernels with strong reflectivity factors, the distribution of Zdr suggested the phases of the hydrometeor of the two kernels were different. The high-altitude kernel was associated with smaller values of Zdr, indicating the formation and growth of hailstones, while the low-altitude kernel was associated with larger values of Zdr, indicating a mixed precipitation process in which hailstones were melting and large raindrops were forming. When the radar scanned at the azimuth of 37◦ at 20:32 UT (Figure7), it was found that of the two kernels with strong reflectivity factors, which had existed at earlier time points, the distribution of Zh indicated only one remained at this time point. The kernel close to the ground surface had disappeared at this time. Comparison of the distribution of strong echoes (>45 dBz) between this time point and earlier time points suggests that although the echoes were still large, they had loose internal structures with blurred edges, and that their sizes and intensities were weak than azimuth at 31◦. The top height was still large but the intensity gradient had dropped significantly, with strong echoes occurring in the middle and lower parts as the height kept decreasing. At this time point, the height of the ‘Zdr column’ was less than 3.8 km above the ground, indicating that there were no strong upward movements within the storm cloud and no liquid water droplets below the freezing level being flushed to above the freezing level. As shown by the distribution features of the correlation coefficients, the ‘CC hole’ was in the middle and lower layers and the CC values distributed around it were mostly <0.95. Moreover, most importantly, based on the RHI radial velocity maps (Figure8), there was obviously no divergent movement of velocity near the ground. This indicates that the downburst formation was closely associated with the occurrence of and the extended height of the ‘Zdr column’. Although Zh was relatively high, the lack of dynamic uplift made it difficult for liquid Atmosphere 2018, 9, 348 10 of 14 droplets to develop into supercooled water droplets and ice particles. Thus, energy released during AtmosphereAtmosphere 20182018,, 99,, xx FORFOR PEERPEER REVIEWREVIEW 1010 ofof 1414 precipitation was insufficient, bringing relatively weak downward airflow. To further verify the ◦ validityairflow.airflow. ofToTo the furtherfurther conclusion, verifyverify wethethe compared validityvalidity ofof the thethe Z dr conclusion,conclusion,and radial we velocitywe comparedcompared maps obtainedthethe ZZdrdr andand atazimuths radialradial velocityvelocity of 33 ◦ andmapsmaps 35 obtainedobtainedand we discoveredatat azimuthsazimuths divergent ofof 33°33° andand movements 35°35° anandd withwewe discovereddiscovered high velocities divergentdivergent near the movementsmovements ground when withwith the highhigh ‘Zdr column’velocitiesvelocities appeared nearnear thethe andgroundground extended whenwhen overthethe ‘Z‘Z thedrdr column’column’ freezing appearedappeared level. andand extendedextended overover thethe freezingfreezing level.level.

h dr dp FigureFigure 7.7. RHIRHI scansscans ofof dual-polarizationdual-polarization variables variablesvariablesof ofof( a((a)Za)) ZZhh;(;; (b(bb)Z)) ZZdrdr;(;; ((cc)K)) KKdp;; andand (( dd)) CC CC at at the the azimuth azimuthazimuth ofof 3737°37°◦ atat 20:32 20:32 UT UT on on 30 30 July July 2017. 2017.

Figure 8. RHI scan of radial wind speed (m/s) at the azimuth of 37° at 20:33 UT on 30 July 2017. FigureFigure 8.8. RHI scan ofof radialradial windwind speedspeed (m/s)(m/s) at th thee azimuth azimuth of 37° 37◦ atat 20:33 20:33 UT UT on on 30 30 July July 2017. 2017.

4.4. ConclusionsConclusions 4. Conclusions ThisThis studystudy conductedconducted in-depthin-depth analysisanalysis ofof thethe weatherweather prprocessocess ofof aa typicaltypical downburstdownburst This study conducted in-depth analysis of the weather process of a typical downburst phenomenonphenomenon thatthat occurredoccurred onon 3030 JulyJuly 20172017 inin GrGreely,eely, ColoradoColorado (USA).(USA). BasedBased onon thethe WPEAWPEA methodmethod phenomenon that occurred on 30 July 2017 in Greely, Colorado (USA). Based on the WPEA method andand dual-polarizationdual-polarization radarradar observations,observations, wewe obtainedobtained thethe criticalcritical meteorologicalmeteorological andand and dual-polarization radar observations, we obtained the critical meteorological and microphysical microphysicalmicrophysical featuresfeatures ofof thisthis downburstdownburst processprocess andand wewe producedproduced aa schematicschematic ofof itsits formationformation features of this downburst process and we produced a schematic of its formation (Figure9). (Figure(Figure 9).9). TheThe primaryprimary conclusionsconclusions derivedderived areare asas follows.follows. The primary conclusions derived are as follows. (1)(1) Features Features ofof thethe atmosphericatmospheric environmentalenvironmental field:field: TheThe mostmost importantimportant featurefeature ofof downburstdownburst (1) Features of the atmospheric environmental ﬁeld: The most important feature of downburst occurrenceoccurrence waswas thethe unevenuneven distributiondistribution ofof heatheat inin thethe externalexternal atmosphere.atmosphere. TheThe distributiondistribution occurrence was the uneven distribution of heat in the external atmosphere. The distribution structurestructure determinesdetermines thethe degreedegree ofof instabilityinstability ofof thethe atmosphere.atmosphere. BeforeBefore thethe onsetonset ofof thethe structure determines the degree of instability of the atmosphere. Before the onset of the downburst,downburst, thethe 33θθ diagramdiagram showedshowed thethe anglesangles ofof tilttilt ofof thethe θθsedsed andand θθ** lineslines becomingbecoming largerlarger θ θ θ* relativerelativedownburst, toto thethe the T-axis,T-axis, 3 diagram andand thethe showed unstableunstable the angleslayerlayer beforebefore of tilt ofthethe the onsetonsetsed ofandof thethe downburstdownburstlines becoming waswas largeratat anan altitudealtituderelative tobelowbelow the T-axis, thethe 780-hPa780-hPa and the levellevel unstable (about(about layer 2.32.3 beforekm).km). Furthermore,Furthermore, the onset of the analysisanalysis downburst ofof thethe was windwind at field anfield altitude ofof thethe downburstdownburstbelow the 780-hPa systemsystem level showedshowed (about thatthat 2.3 thethe km). heightheight Furthermore, ofof thethe downburst,downburst, analysis of theatat aroundaround wind ﬁeld 2.42.4 of km,km, the waswas downburst highlyhighly consistentconsistent withwith thatthat ofof thethe unstableunstable layerlayer ofof thethe atmosphericatmospheric environmenenvironmentaltal fields.fields. Therefore,Therefore, aa newnew findingfinding inin thisthis paperpaper isis thatthat thethe importanimportantt locationslocations toto monitormonitor forfor downburstsdownbursts cancan bebe determineddetermined byby studyingstudying thethe atmosphericallyatmospherically unstableunstable areasareas usingusing thethe WPEAWPEA method.method.

Atmosphere 2018, 9, 348 11 of 14

system showed that the height of the downburst, at around 2.4 km, was highly consistent with that of the unstable layer of the atmospheric environmental ﬁelds. Therefore, a new ﬁnding in this paper is that the important locations to monitor for downbursts can be determined by Atmospherestudying 2018, 9 the, x FOR atmospherically PEER REVIEW unstable areas using the WPEA method. 11 of 14 (2) The release of unstable energy was found to have strong correlation with the impending weather (2) The release of unstable energy was found to have strong correlation with the impending process. The distribution and content of water vapor are very important in the occurrence weather process. The distribution and content of water vapor are very important in the of a downburst. In the case study, water vapor continuously accumulated in the mid- and occurrence of a downburst. In the case study, water vapor continuously accumulated in the high-altitude layers before the onset of the downburst, while there was no obvious change of mid- and high-altitude layers before the onset of the downburst, while there was no obvious water vapor in the lower atmosphere. Thus, it can be asserted qualitatively that the downburst change of water vapor in the lower atmosphere. Thus, it can be asserted qualitatively that the was a weather process that involved low precipitation accompanied by localized hail precipitation. downburst was a weather process that involved low precipitation accompanied by localized Based on the information from the WPEA method, and combined with the dual-polarization hail precipitation. Based on the information from the WPEA◦ method, and combined with the characteristic variables (Zh (>50 dBz), Zdr (3–6 dB), Kdp (2–4 /km)) and CC (0.9–0.95), we can dual-polarization characteristic variables (Zh (>50 dBz), Zdr (3–6 dB), Kdp (2–4°/km)) and CC better distinguish the phase distribution of the hydrometeor in the process and greatly enhance (0.9–0.95), we can better distinguish the phase distribution of the hydrometeor in the process the judgment of the possibility of the downburst. and greatly enhance the judgment of the possibility of the downburst. (3) It was clearly observed in the initial stage of the downburst that the ‘Zdr column’ thrust upward (3) It was clearly observed in the initial stage of the downburst that the ‘Zdr column’ thrust beyond the zero-degree layer. Furthermore, the strong upward movement was indicative of upward beyond the zero-degree layer. Furthermore, the strong upward movement was the continuous formation and growth of supercooled water droplets and small ice particles. indicative of the continuous formation and growth of supercooled water droplets and small ice These were similar to the conclusions from previous studies. But in this paper we further particles. These were similar to the conclusions from previous studies. But in this paper we discovered that the formation of the downburst was found closely associated with the occurrence further discovered that the formation of the downburst was found closely associated with the of and the extended height of the ‘Zdr column’. However, no downburst process was found occurrence of and the extended height of the ‘Zdr column’. However, no downburst process in the position where there is no obvious ‘Zdr column’ in the upper atmosphere. Therefore, was found in the position where there is no obvious ‘Zdr column’ in the upper atmosphere. we can regard the ‘Zdr column’ as an important early warning indicator of the location of the Therefore, we can regard the ‘Zdr column’ as an important early warning indicator of the downburst in this case. However, the validity of this conclusion needs more case studies to verify location of the downburst in this case. However, the validity of this conclusion needs more in the future. case studies to verify in the future.

4.7 km supercooled water droplets and ice particles 3.8 km Freezing Level liquid droplets

Updraft

2.4 km

Unstable Area

0.2 km

Figure 9. Schematic of the formation of the studied downburst.

Given its intensity and small spatial scale, localized strong convection can often cause Given its intensity and small spatial scale, localized strong convection can often cause disasters. disasters. However, existing observation systems have low resolution and radar monitoring can However, existing observation systems have low resolution and radar monitoring can discern discern convection only hours or minutes in advance. Through comprehensive study of the change convection only hours or minutes in advance. Through comprehensive study of the change process process of each feature variable during the evolution process of strong-convection weather, it is of each feature variable during the evolution process of strong-convection weather, it is possible to possible to determine precursory information prior to the onset of such phenomena. Moreover, the determine precursory information prior to the onset of such phenomena. Moreover, the application of application of dual-polarization weather radars can both provide fine-scale observations of the dual-polarization weather radars can both provide ﬁne-scale observations of the internal structure internal structure of downbursts and reveal their microphysical mechanisms, establishing features of downbursts and reveal their microphysical mechanisms, establishing features relevant for the relevant for the prediction of future downburst incidents. Our future work will focus on qualitative prediction of future downburst incidents. Our future work will focus on qualitative research and research and fine-scale quantification, including the investigation of other downburst cases, to explore the links among relevant parameters and to eventually establish a pre-warning and forecast indicator system.

Atmosphere 2018, 9, 348 12 of 14

ﬁne-scale quantiﬁcation, including the investigation of other downburst cases, to explore the links among relevant parameters and to eventually establish a pre-warning and forecast indicator system.

Author Contributions: H.W. conceived this research and designed the study. The paper was written by H.W. V.C. and J.H. presented some research ideas. Z.S. analyzed the data and presented some conclusions. L.W. undertook some programming work. Funding: This work was supported by the National Natural Science Foundation of China (No. 41375043, No. 41405036, No. 41505031), China Scholarship Council (No. 201608515106), and the 2015 Middle-aged Academic Leaders Research Fund of Chengdu University of Information Technology (CUIT) (J201502). Acknowledgments: The authors would like to express their sincere thanks to NCAR/UCAR and CSU for supplying the data used in this paper, and also to thank the reviewers for their constructive comments and editorial suggestions that helped improve the quality of the paper considerably. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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