SEVERE WIND-DRIVEN HAIL EVENTS: DEPENDENCE ON CONVECTIVE

MORPHOLOGY AND LARGER-SCALE ENVIRONMENT

NICHOLAS D CARLETTAˡ

Mentors: William A. Gallus Jr.ˡ, M. A. Fowle² and D. J. Miller³

Department of Geological and Atmospheric Sciences

Iowa State University, Ames, IAˡ National Weather Service Aberdeen, SD²

National Weather Service Duluth, MN³

ABSTRACT

Severe wind events and hail events are well covered in various studies

but relatively little study has been done on wind driven hail. Both

wind and hail can cause significant damage but together they can cause

even more damage. Thus a further examination of the storms that

produce these events is warranted. By comparing the times and

locations of reported severe hail and wind, the locations of wind driven

hail can be identified. These locations can be compared to the storm

morphologies identified in Gallus et al (2008) and Duda and Gallus

(2010) to determine a specific morphology associated with these

events. Severe storms producing either severe wind or hail are often

associated with high values of CAPE and strong shear. Identifying

how closely these parameters correlate to wind driven hail events can

assist the forecasting of these events. Determining which parameters

are present in wind driven hail events that are not present in just a

severe hail event, or just a severe wind event, could prove vital to

forecasting these events. Squall lines, as well as isolated cells and

clusters of cells, were the most frequent morphologies associated with

wind-driven hail. CAPE played a role in the strength of the wind-driven

hail events as higher CAPE resulted in larger hail and/or faster wind. If

an event occurs with a supercell, the hail will be larger and the wind

will be faster for wind-driven hail events and the CAPE will also be

higher in supercells.wwwwwwwwwwwwwwwwwwwwwwwwwwww

1. Introduction

A wind-driven hail event is defined here as an

event where severe hail greater than 1 inch in

diameter occurs simultaneously with severe

wind in excess of 50 knots. Prior studies have

examined both the near-storm environments

(e.g. Rasmussen and Blanchard 1998) and

convective morphologies, most likely to be

associated with either severe wind events or hail

events (e.g., Duda and Gallus 2010), but

relatively little work has been done to

understand the conditions favorable for severe

wind-driven hail events. A study that did look at

simultaneous wind and hail was Lemon and

Parker (1996). In that study they made mention

of the importance of shear in the presence of

severe wind in a storm. Mention is also made to

a strong updraft being required for large hail to

be present. Some recent cases, such as the

Eldora Iowa event in August 2009 where hail of

3 inch diameter was blown by winds exceeding

90 knots, have demonstrated how tremendously

damaging and dangerous a thunderstorm can be

when severe wind and large hail occur together.

The present study uses the approach followed in

Gallus et al. (2008) and Duda and Gallus (2010)

to examine the convective morphologies

associated with severe wind-driven hail events.

In addition to this analysis of these events,

CAPE (Convective Available Potential Energy),

0-2 km SRH (Storm Relative Helicity), and 0-3

km SRH present for each event was also

examined. The events are also compared to 30

wind-only, 30 hail-only, and 14 wind-driven

hail events from the National Weather Service

at Aberdeen, South Dakota that have occurred

over the past decade, hereafter called known

events. This all leads to the hypothesis of this

study: Wind-driven hail events occurred most

frequently with certain storm morphology and

these storms had high values of CAPE and

SRH, as expected of a high end severe event

like wind-driven hail.

2. Methods

The present study follows the approach of Duda

and Gallus (2010), using storms from 2007 and

assigning morphologies based on Gallus et al.

(2008). The morphologies used were IC –

isolated cells, CC – cluster of cells, BL –

broken line of cells, NS – squall line with no

stratiform precipitation, TS – trailing stratiform

precipitation, PS – parallel stratiform

precipitation, LS – leading stratiform

precipitation, BE – bow echo, and NL – non-

linear convective system with storm motion

from left to right. Examples of these

morphologies are illustrated in Fig. 1. Unlike

the earlier studies that looked at flooding,

tornadoes, hail, and wind alone, the present

study focuses on severe wind-driven hail events.

To determine a wind-driven hail event, the

National Climatic Data Center’s (NCDC) Storm

Data publications were used. Since Duda and

Gallus (2010) used 2007’s storm reports and

classified the morphologies of all convective

systems in a 10-state region from April through

August, that same dataset was used for this

study. Wind-driven hail events were noted if

severe hail and severe wind were reported

within five miles of each other and within the

Figure 1: The morphologies used in the present study, taken from Duda and Gallus (2010)

same thirty minute period. In 2007, 0.75 inch

diameter hail was used to define a severe

hailstone, but that minimum criterion has

changed, so we used the new one inch diameter

criterion.

In all, 69 wind-driven hail events were found in

the 10 state region during the 2007 warm

season, occurring on 33 different days. Then

for the 2002 season, in the same region over the

same time period as 2007, 69 wind-driven hail

events were found over 38 different days. After

this dataset of cases was built, the location and

date of the events were compared to the dataset

of convective morphologies from Duda and

Gallus (2010) to assign a morphology. The

Duda and Gallus (2010) study also mentioned if

a storm was a supercell, so a note was made if

the event matched a supercell, as well. For the

69 2002 cases, data morphologies were from

Gallus et al (2008) with the same methods

above applied.

In both the Duda and Gallus (2010) and Gallus

et al (2008) studies, morphologies were

assigned based on date and state(s). For cases

where multiple morphologies were found at one

date in one state, the wind and hail reports listed

were used to identify the true morphology. If

there were still multiple events that had hail and

wind at a certain date and time, then the radar

composite archive provided by UCAR

(University Cooperation for Atmospheric

Research) was used online.

The next part of the project determined the

CAPE (Convective Available Potential Energy),

0-2 km SRH (Storm Relative Helicity), and 0-3

km SRH present, using RUC output from Iowa

State University’s online meteorology archive.

For each event a date, time, and location was

provided with the storm data so the

corresponding zero hour forecast closest to the

event was downloaded from the meteorology

archive. This data was then loaded in the

program nsharp, included in the GEMPAK

package of programs provided on the Iowa State

Meteorology computers. This program can read

the model upper air sounding data from the file

and compute CAPE, 0-2 km SRH, and 0-3 km

SRH. The latitude and longitude for the events

to be used in the nsharp program to obtain the

data were obtained with an online mapping tool.

In 2007, the online archive had RUC files every

three hours starting daily at 0000 UTC , while in

2002 there were RUC files every 6 hours

starting at 0000 UTC every day.

Then for comparison to the results from the

wind-driven hail cases, the previous methods

were used to obtain data of significant wind-

only and hail-only events in the 2007 warm

season period due to availability of archived

RUC model data. Events were taken from the

same NCDC storm data used for the previous

section. To qualify, the storm had to only have

wind or hail reports in it, not just wind and hail

either more than five miles apart or with thirty

minutes or more between them. The same

method explained above, with nsharp, was then

used.

Another point of comparison is the known

wind-driven hail events from the NWS.

Morphologies were determined with a

combination of the previously mentioned

UCAR image archive and the level II radar

archive that NCDC maintains. Then the method

with nsharp was used to determine CAPE, 0-2

km SRH, and 0-3 km SRH.

The last point of comparison used the hail and

wind breakdowns established in Gallus et al

(2008) and Duda and Gallus (2010) of hail one

to two inches and hail greater than two inches as

well as wind 50 knots to 65 knots and greater

than 65 knots. If a storm met the lower wind

speed and smaller hail category, it was

considered a category 1 event. If it met the

higher wind speed but not the larger hail it was

considered a category 2 event. If it met the

larger hail criteria but not the higher wind speed

it was considered a category 2 event. Last, if it

met both the higher wind speed and the larger

hail, it was considered a category 3 event. All

box charts were produced using the fit y by x

functions in the software package JMP.

Figure 2: Severe hail reports (sorted by size) as a function of convective morphology, taken from

Duda and Gallus (2010).

Figure 3: Severe wind reports (sorted by speed of wind in knots) as a function of morphology,

taken from Duda and Gallus (2010).

Figure 4: The percentage of convective morphologies in the data sample from 2002 and 2007 from Duda and Gallus (2010) on the

left, and the percentage of convective morphologies for wind driven hail events in 2002 and 2007 on the right.

3. Results

a. Wind and Hail from Duda and Gallus (2010)

In Duda and Gallus (2010) storm reports for

wind, hail, tornado, and flooding were classified

according to the convective morphology of the

system responsible for producing the report.

For large hail events, (Fig. 2) PS was the largest

producer in 2002 while BL was largest in 2007.

The number of NS events was much larger in

2007 than 2002. Otherwise, CC and IC also

contributed significantly to reports of hail larger

than two inches in diameter. For all hail in

2002, the top three morphologies were BL, BE,

and PS. In 2007 BL, CC, and PS were the top

three in 2007. For high wind events greater than

65 knots, BE events were largest in 2002

followed by TS. While in 2007, BE events again

were largest but followed by BL (Fig. 3). For

all severe wind events, BE were most followed

by TS, and both BL and PS were close.

b. Morphologies for 2007 and 2002 Wind-

Driven Hail

i) Morphologies for 2007

For cases of wind-driven hail, squall lines

contributed the greatest share, with TS and NS

combined accounting for 30% of all events. IC

and CC events also contributed substantially to

the total, with 19% and 16% respectively (Fig.

4). As for changes in percentage compared to

the 2007 non wind-driven hail events, TS had a

twelve point increase, NS a one point increase,

and IC a nine point increase. These results

imply squall lines have a relatively higher risk

of severe wind-driven hail compared to other

morphologies than they do for hail or wind

alone. On the other hand, CC events had a

seventeen percent decrease with BL and NL

having a one percent increase.

ii) Morphologies for 2002

For cases in 2002, clusters of cells and isolated

cells were more prevalent for wind-driven hail

events. Together they represent 58% of the

events in 2002. The average hail size and wind

speed in the 2002 data was greater, which may

be part of the reason for this difference in

morphologies. As far as changes from all the

events in 2002, CC had a seventeen point

increase, IC had a two point increase, and NS

had a two point increase (Fig. 4). All other

morphologies had decreases with the most

significant being for NL and PS, who both fell

by six points.

c. Comparisons

i) Comparing 2002 to 2007

The two years studied produced very different

storms with some similarities. For

morphologies, the difference between the two

seasons is ten points for TS, no change for NS,

no change for IC, twenty-three points for CC,

four points for PS, seven points for NL, six

points for BE, eleven points for BL, and one

point for LS. So over the two seasons, CC, IC,

and TS play the largest roles with combined

2002 and 2007 season percentages of 27%,

19%, and 18% respectively.

The values of CAPE and SRH between the two

years were very similar. For CAPE in 2002, the

Figure 5: Box plots of CAPE values from the 2002 and

2007 season. Also plotted is the set of base known cases as

well as cases in 2007 where only sever wind or only severe

hail was reported

Figure 6: Box plots of 0-3 km SRH values from the 2002

and 2007 season. Also plotted is the set of base known cases

as well as cases in 2007 where only sever wind or only severe

hail was reported

average value was 2678 J/kg and in 2007 the

average value was 2823 J/kg, but both years had

a standard deviation around 1500 J/kg. So both

years averaged a high amount of CAPE but with

significant variability in the data due to the large

standard deviations present. For 0-3 km SRH,

2002 averaged 124 m2/s

2 with a standard

deviation of 110.44 m2/s

2 and 2007 averaged

127 m2/s

2 with a standard deviation of 123.12

m2/s

2. Once again both values have a large

standard deviation because of a wide range of

values from negative to over 600 m2/s

2. As

shown in figures 5 and 6, the two years have a

similar spread of data and there is not a

statistically significant difference (Table A1).

ii) Comparing to Severe Wind-only and

Severe Hail-only Events

The wind-only events averaged CAPE of 2630

J/kg which is comparable to the values of 2002

and 2007 wind-driven hail. The hail-only

events averaged a higher 3066 J/kg of CAPE

and is also close to the average value in 2007

wind-driven hail. The average 0-3 km SRH,

though for wind-only cases, is 192 m2/s

2 and for

hail-only cases it is 116 m2/s

2. Comparing

wind-only and hail-only values to the report

based cases; only SRH between the wind-only

cases and the report based cases has a

statistically significant difference (Table A1).

iii) Comparing to known Wind-Driven Hail

Events

These known events all occurred within the past

ten years and are all in the Midwest. These

cases have an average CAPE of 2515 J/kg and

an average 0-3 km SRH of 225 m2/s

2 with once

again high standard deviations. These events

presented a CAPE similar to the 2002 cases and

not far from 2007 cases. The difference is the

SRH, which is around 100 m2/s

2 larger than

both the 2002 and 2007 average values. There

is a statically significant difference between the

known cases and the storm report based cases of

wind-driven hail (Table A1). Some of this can

be attributed to comparing the 14 cases from the

NWS to the 138 cases from storm reports.

iv) Comparing Parameters between

Morphologies

A change in CAPE and SRH was also present

across the different morphologies.. When

comparing these data sets, the greatest

differences were noticed with BE morphologies

that produced wind-driven hail events, because

they had statistically larger values of both 0-2

km and 0-3 km SRH than the other

morphologies with the exception of BL (Table

A1). The other difference was that the IC

morphologies had statistically higher CAPE

than the BE, NS, LS, and PS events. The BL,

NL, and IC events had the highest average

CAPE (Fig. 8). While the BE, BL, and CC

events had the highest SRH (Fig. 7). There is

variability in this though, since the highest

CAPE value was a CC event with 7905 J/kg

although the highest 0-3 km SRH was a BL

event with 602 m2/s

2.

Figure 7: Box plots of 0-2 km and 0-3 km SRH plotted for the different morphologies from both the 2002 and 2007 season.

The morphology of the event also seemed to

play a role in size of the hail and the speed of

the wind present. Figure 9 shows this with the

number of events for each category with respect

to hail size and wind speed across each

morphology. Based in figure 9, the CC, IC, and

TS events were the events that had a category

three event with hail over 2 inches and wind

over 65 knots. The CC, NL, IC, BL, TS, and

NS all had category two events with either hail

over 2 inches or wind over 65 knots. Category

one seems to follow a trend of the most frequent

occurrence of wind-driven hail events for the

2002 and 2007 data so that is insignificant.

d. Supercells and Dependence on Hail Size and

Wind Speed

i) Comparing Supercell Events to Non-

supercell events

Figure 10: Chart of morphologies associated the supercells

with wind-driven hail from the 2007 season.

Supercells play a role in wind-driven hail events

even though a supercell is not required for an

event to occur; however, it can increase the

Figure 8: Box plots of CAPE

plotted for the different

morphologies from both the 2002

and 2007 season.

Figure 9: Graph illustrating the

different number of cases for each

morphology that was a category one, two, or

three event based on hail size and wind

speed as described in the text above.

Figure 8: Box plots of

CAPE plotted for the different

morphologies from both the 2002

and 2007 season.

strength of the event. The average hail size for

a non-supercell is 1.26 inches and the average

wind speed is 55.53 knots. For a supercell, the

average hail size is 1.47 inches with wind of

60.16 knots. These differences are statistically

significant as well for the hail and the wind

(Table A2). Certain morphologies were also

more frequent for supercell events, CC had 38%

and BL had 31% of the events. . CC already

had a large amount of the cases, but BL did not

(Fig. 10). Following these two were IC (19%),

NL (6%), and BE (6%). So if a there is a

supercell, if it is A BL or a CC it has a better

chance of wind-driven hail occurring.

The size of the hail and speed of the wind were

not the only factors influenced by supercells,

because the CAPE was also higher. The

average CAPE for a supercell was 3140 J/kg

which is higher than the 2007 average of 2823

J/kg and the non-supercell average value of 258

J/kg. Through a t-test this was also proved

significant because when comparing supercells

with wind-driven hail to non-supercell with

wind-driven hail the value is 0.05. The SRH is

also higher for supercells with 147 m2/s

2 versus

109 m2/s

2. The CAPE is a significant difference

and the SRH is not (Table A1).

ii) Comparing Events based on Hail Size

and Wind Speed

The categories, as defined in the methodology,

have average CAPE of 2609 J/kg for one, 3188

J/kg for two, and 3487 J/kg for three. For 0-3

km SRH the average was 121 m2/s

2 for one, 141

m2/s

2 for two, and 119 m

2/s

2 for three. There is

a generally increasing trend in the value of

CAPE as the severity of the event increases but

this does not hold true with the SRH. This is

Figure 11: Box plots of CAPE and 0-3 km SRH from the 2007 season comparing supercell cases to non-supercell cases.

Figure 12: Box plots of CAPE and 0-3 km SRH from the 2002 and 2007 seasons separated based on the category determined by hail

size and wind speed.

supported by a t-test in which comparing the

CAPE of category three and two events to

category one events (Table A1). So CAPE can

be an indicator for the potential of more severe

wind-driven hail.

4. Conclusions

For severe wind-driven hail, both morphology

and CAPE seem to play some role in

determining the strength of the event. For all

the wind-driven hail events in the 2002 and

2007 seasons, the NS, TS, IC, and CC

morphologies were the most common. The

CAPE and SRH over these two periods were

very similar and both had a high variance. A

difference was noted, when the 2002 and 2007

cases were compared to the wind-only cases,

that there was a higher value for both the 0-2 km

and 0-3 km SRH in the wind-only cases. The

CAPE was higher for the hail-only events but it

was not statistically significant.

In comparing the different morphologies, they

seemed to have varying levels of CAPE and

SRH. The BL, NL, and IC morphologies

produced the highest values of CAPE. The BE,

BL, and CC events produced the highest amount

of SRH. Then the CC, IC, and NL events

produced the largest number of hail over 2

inches and/or wind over 65 knots.

Supercells with wind-driven hail produced

statistically larger hail and higher wind speeds

than cases that were not in a supercell. These

supercells were often CC, BL, or IC

morphologies. The CAPE of supercells were

larger but the SRH, while larger on average, it

was not statistically significant. For the

category breakdown of wind-driven hail events,

it was the CAPE that had a positive correlation

with the events. The CAPE had a statistically

significant difference between the top two

categories and category one, with the CAPE

getting larger as the hail size and wind speed

increased. CAPE in a number of instances

seemed to have an impact on the occurrence of

severe wind-driven hail and certain favorite

morphologies presented themselves. The high

value of variance in all of the data warrants

further research into this topic to see if these

points hold. For either, there is a high amount

of variability in wind-driven hail or there needs

to be another step beyond simultaneous wind

and hail reports at the same location to

determine an event.

5. Acknowledgements

I would like to thank all three of my mentors,

William Gallus, Michael Fowle, and Daniel

Miller, for their support and guidance on this

project. I would especially like to thank

William Gallus for helping me layout my plan

for this project and providing the morphology

data from Duda and Gallus (2010) and Gallus et

al (2008).

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APENDIX A

CAPE 0-2 km SRH 0-3 km SRH

2007 to 2002 0.572603 0.97449736 0.89110934

2007 to hail-only 0.477844 0.9678285 0.70814482

2007 to wind-only 0.541664 0.00260713 0.01891734

2007 to known 0.497312 0.01730106 0.01740117

2002 to hail-only 0.257879 0.9470008 0.76915778

2002 to wind-only 0.879883 0.00145136 0.00899765

2002 to known 0.717265 0.01113094 0.00886651

cat3 to 2 hail 0.299293 0.2144603 0.19308014

cat3 to 2 wind 0.416776 0.47271931 0.45253635

2 hail to 2 wind 0.325067 0.14284887 0.14398163

cat3 to 2007 0.45388 0.92952655 0.91325496

cat3 to 2002 0.359376 0.91337438 0.9370357

cat3 to wind 0.270725 0.27331569 0.34282419

cat3 to hail 0.67937 0.95359017 0.97546847

cat3 to control 0.307617 0.37698678 0.35664867

2 hail to 2007 0.797966 0.27037759 0.25608803

2 hail to 2002 0.605733 0.22770241 0.1870065

2 hail to wind 0.519964 0.50051994 0.7899235

2 hail to hail 0.87997 0.37055904 0.21092231

2 hail to control 0.497451 0.51693008 0.52262651

2 wind to 2007 0.243846 0.96099316 0.98286383

2 wind to 2002 0.123701 0.93932857 0.9036966

2 wind to wind 0.122916 0.02261875 0.07335663

2 wind to hail 0.666651 0.99346976 0.7580156

2 wind to control 0.185726 0.06541757 0.06774443

BE to BL 0.141003 0.29253034 0.29004228

BE to CC 0.390067 0.0531993 0.06583375

BE to IC 0.095503 0.05058691 0.039641

BE to NL 0.197853 0.02438653 0.01537897

BE to NS 0.376949 0.0504234 0.07803075

BE to PS 0.321476 0.08577723 0.06366815

BE to TS 0.375305 0.07456396 0.07631107

BL to CC 0.140166 0.26849656 0.27211247

BL to IC 0.463928 0.1708542 0.13923156

BL to NL 0.404524 0.16427055 0.12209907

BL to NS 0.058189 0.24156555 0.27689468

BL to PS 0.099293 0.2597694 0.20812559

BL to TS 0.163132 0.27698864 0.28169573

CC to IC 0.103516 0.18462438 0.14551136

CC to NL 0.205155 0.15724273 0.10957496

CC to NS 0.239309 0.26554202 0.35382638

Table A1: P-values for all

comparisons with respect to CAPE and

SRH with green being less than 0.1 and

red being greater than 0.1

CC to PS 0.267565 0.1963697 0.14335798

CC to TS 0.473196 0.43284578 0.4063685

IC to NL 0.413441 0.46961592 0.43998047

IC to NS 0.027423 0.4561835 0.34696244

IC to PS 0.053457 0.38363146 0.32532431

IC to TS 0.123817 0.26697833 0.23753357

NL to NS 0.093895 0.41600004 0.26796521

NL to PS 0.14187 0.36586963 0.30130522

NL to TS 0.231182 0.23612252 0.18884107

NS to PS 0.406612 0.32399608 0.21706594

NS to TS 0.2286 0.33795975 0.43226179

PS to TS 0.255011 0.25550958 0.19988308

Y to N 0.051217 0.14426647 0.10857432

cat 3 to cat 2 0.377764 0.19230439 0.18430051

cat 3 to cat 1 0.08878 0.27482532 0.29063652

cat 2 to cat 1 0.032483 0.18600044 0.13739561

cat 3 to cat 2 wind 0.416776 0.24347163 0.23833047

cat 3 to cat 2 hail 0.299293 0.10881367 0.0945726

cat 2 wind to cat 1 0.021778 0.23394739 0.17655923

cat2 hail to cat 1 0.139263 0.18860821 0.1520781

hail wind

SC Hail and wind 0.078852 0.00455

Table A2: P-values for the comparison

of size of hail and speed of wind between

supercell and non-supercells.