Review of Illinois summer precipitation conditions.

164 Pages·2006·4.2 MB·English

BULLETIN 64
Review of Illinois
Summer Precipitation Conditions
by STANLEY A. CHANGNON, JR., and FLOYD A. HUFF
Title: Review of Illinois Summer Precipitation Conditions
Abstract: This report summarizes key findings regarding all aspects of precipitation in the con-
vective season gleaned from 30 years of research at the Illinois State Water Survey. Examples of
information that dimensionalize midwestern precipitation phenomena extracted from published
papers, reports, and grant-contract studies have been included. These examples are accompanied by
extensive graphs, maps, and tables to provide a useful information document pertinent to the areas
of forecasting, modification of summer convective elements, or midwestern precipitation condi-
tions in general. As an equally important purpose, the report provides listings of nearly a hundred
supporting references that serve as a guide to more extensive information. The material covers
general climatic aspects of the state, ranging from average annual, monthly, and daily rain distri-
butions through severe rainstorms and droughts. More detailed information on relationships of
precipitation with atmospheric moisture and various weather types, on organized storms and sub-
storm entities, on vertical and horizontal characteristics of individual storm cells, and on external
and internal cloud characteristics is also included.
Reference: Changnon, Stanley A. Jr., and Floyd A. Huff. Review of Illinois Summer Precipitation
Conditions. Illinois State Water Survey, Urbana, Bulletin 64, 1980.
Indexing Terms: climatology, convection, droughts, heavy rainstorms, hydrometeorology, meso-
meteorology, radar meteorology, rainfall, synoptic weather conditions, weather modification.
STATE OF ILLINOIS
HON. JAMES R. THOMPSON, Governor
INSTITUTE OF NATURAL RESOURCES
FRANK H. BEAL, M.U.P., Director
BOARD OF NATURAL RESOURCES AND CONSERVATION
Frank H. Beal, M.U.P., Chairman
Walter E. Hanson, M.S., Engineering
Thomas Park, Ph.D., Biology
Laurence L. Sloss, Ph.D., Geology
H. S. Gutowsky, Ph.D., Chemistry
Lorin I. Nevling, Ph.D., Forestry
William L. Everitt, E.E., Ph.D.,
University of Illinois
John C. Guyon, Ph.D.,
Southern Illinois University
STATE WATER SURVEY DIVISION
STANLEY A. CHANGNON, JR., M.S., Chief
URBANA
1980
Printed by authority of the State of Illinois
(4-80-1500)
P.O.19331
CONTENTS
PAGE
Introduction 1
Acknowledgments 2
General climatic aspects 3
Average anual, monthly, and daily distributions 3
Large-scale paterns and downwind studies 1
Diurnal distributions 15
Rain events (storms) over fixed areas 21
Introduction and definitions 21
Ilustrations of storms 21
Precipitation paterns based on storm motions 25
Storm characteristics 34
Sampling requirements for storm rainfal 42
Severe rainstorms 43
Droughts 43
Rainfal relations to atmospheric conditions 54
Relationship of atmospheric moisture and precipitation 54
Synoptic weather typing 57
Pre-rain conditions and forecasting 79
Prediction of July-August rainfal 84
Organized storm systems and subsystem entities 86
Wel-organized lines of cels 86
Les organized groups of cels 9
Individual storm cels 101
Introduction 101
Vertical characteristics 104
Echo growth 105
Behavior of echo tops 106
Merging echo tops 109
Echo volumes. 1
Echo tops and severe weather 13
Modeling studies 13
Radar reflectivity values aloft 16
Horizontal cel characteristics 17
Radar echoes 17
Raincels 18
Motion of cels 125
Merging 126
Cloud characteristics 137
Cloud dimensions 137
Cloud base updrafts 137
In-cloud characteristics. 139
Near the frezing level 139
At the —10°C level 143
Airflow trajectories in and around convective clouds 14
Tracer releases on 1 August 1972 14
Tracer releases on 23 July 1973 14
Tracer releases on 12 August 1973 14
Sumary 150
Cloud frequencies 150
References 158
Review of Illinois
Summer Precipitation Conditions
by Stanley A. Changnon, Jr., and Floyd A. Huff
ABSTRACT
This report summarizes key findings regarding all aspects of precipitation
in the convective season gleaned from 30 years of research at the Illinois State
Water Survey. Examples of information that dimensionalize midwestern pre-
cipitation phenomena extracted from published papers, reports, and grant-
contract studies have been included. These examples are accompanied by ex-
tensive graphs, maps, and tables to provide a useful information document
pertinent to the areas of forecasting, modification of summer convective
elements, or midwestern precipitation conditions in general. As an equally
important purpose, the report provides listings of nearly a hundred support-
ing references that serve as a guide to more extensive information.
The material covers general climatic aspects of the state, ranging from
average annual, monthly, and daily rain distributions through severe rain-
storms and droughts. More detailed information on relationships of precipita-
tion with atmospheric moisture and various weather types, on organized
storms and substorm entities, on vertical and horizontal characteristics of in-
dividual storm cells, and on external and internal cloud characteristics is also
included.
INTRODUCTION
The emergence of various programs in Illinois presenting most information in the form of graphs,
relating to the study of precipitation in the convec- maps, and tables.
tive season has shown the need for a document A variety of topics are treated within the realm
that will touch on key findings of prior Water Sur- of precipitation. They have been chosen both to
vey research. Studies of forecasting and of modifi- allow greater understanding for midwestern sci-
cation of summer convective elements in Illinois entists and to dimensionalize the phenomena for
can be aided by a host of findings from 30 years of those unfamiliar with midwestern conditions.
research by scientists of the Illinois State Water Importantly, results presented do not represent
Survey. Findings from a variety of published all pertinent results, but only those rather hurriedly
papers, state reports, and grant-contract reports and arbitrarily chosen from an extremely large
have been chosen for presentation herein. These library of information. Apologies are offered to
serve two purposes. One is to present examples those who perceive that important documents or
of information that dimensionalize the phenom- information have not been included.
ena. The second is to provide listings of the sup- The material presented begins on a larger or
porting references that allow one to find further, state scale, both in time and space, and progresses
more extensive information than can be brought to smaller time and space elements by the end of
forward in a single summary document. Thus, the report. English and metric units are used inter-
this is both a guide and an information document, changeably throughout this report. Most of the
1
material has been abstracted directly from Water The final division of this report deals with avail-
Survey reports and technical papers over a span able information on various external and internal
of 25 to 30 years. English units were used exclu- cloud characteristics from Survey studies. It should
sively in the earlier publications, whereas a change be noted that a variety of useful climatic results
to the metric system has occurred in recent years. relevant to summer rainfall in the Midwest can be
For the purpose of this report, it was deemed un- found in High Plains Climatography (Changnon et
necessary and too time-consuming to convert al., 1975).
all of the many numerical values to a single mea-
surement system.
The initial division of the report is entitled
"General Climatic Aspects." Various information
(maps and graphs) is presented to reveal the
Acknowledgments
average daily, monthly, seasonal, and annual
distributions of rainfall and rain-related phenom- The various research projects upon which this
ena (hail, thunderstorms, tornadoes, etc.). A report is based have been carried out by numerous
special section is devoted to their diurnal distri- individuals within the Atmospheric Sciences Sec-
butions followed by a section devoted to field tion, under the general direction of William C.
studies of rain events, or storms, over fixed areas. Ackermann, now Chief Emeritus of the Illinois
Next, certain conditions related to summer State Water Survey. Material assembled in this
droughts in Illinois are presented. report has been abstracted from a large number
of reports and research papers published by
The next major division of the report deals with
Survey scientists. The various research projects
"Rainfall Relations with Atmospheric Condi-
have drawn financial support from a wide range
tions." Here, the relationships of atmospheric
of sources. In addition to the state of Illinois,
moisture to precipitation and the occurrence of
precipitation research at various times has been
precipitation with various weather types are ex-
partially supported by such federal agencies as the
amined, as is information on pre-rain conditions
National Science Foundation, Department of
related to forecasting.
Energy, Armed Forces (Army, Navy, Air Force),
The third major division of the report deals with
and the Department of the Interior. Much of the
organized storm or substorm entities, treated for
hail research has been supported by the Crop-
their lifetimes and not over a fixed area. Within
Hail Insurance Actuarial Association.
this section, information on organized lines and
Credit is extended to the Survey's Graphic
poorly organized groups of cells are presented.
Arts Unit under the guidance of John W. Brother,
The fourth major division of the report deals
Jr., and especially to William Motherway, Jr.,
with information available on individual storm
for their considerable efforts in locating or redraw-
cells. Within this division, considerable informa-
ing the numerous figures from previous publica-
tion is presented in a section on the vertical charac-
tions. Thanks are also due J. Loreena Ivens and
teristics of individual storm cells. Another section
Patricia A. Motherway for editing the final manu-
focuses on the horizontal characteristics of these
script, and Marilyn J. Innes for preparing the
phenomena as seen aloft by radar and at the sur-
camera-copy.
face.
2
GENERAL CLIMATIC ASPECTS
Average Annual, Monthly, and Daily Distributions
Figure 1 presents the pattern of average annual
precipitation in Illinois, based on 1901-1944 data
(Changnon, 1958). Basically, a north-to-south dis-
tribution is shown ranging from 32 inches in the
north to 46 inches in the hill area of extreme
southern Illinois. Previous research (Huff et al.,
1975) has shown that this hill area has an effect on
warm season rainfall leading to 10 to 15% increas-
es, on the average.
Figure 2 presents average precipitation patterns
for the 3-month seasons as defined for the Illinois
climatic regime. The winter (December-February)
pattern is dominated by a north-south latitudinal
distribution of precipitation, whereas the summer
distribution (figure 2b) reveals a generally flat
pattern, ranging from 9.5 to 11.5 inches. Notably,
the summer distribution indicates slightly higher
rainfall, on an average, in western Illinois. This
reflects a greater incidence of nocturnal thunder-
storm activity during summer (June-August) in
western Illinois. Changnon (1957) shows that 70 to
80% of the summer rain comes from thunder-
storms (see figure 4c).
A further subdivision of the warm season rainfall
into that for the July-August period, when the
greatest benefit of rainfall is realized in agricultural
crop production, is shown in figure 3 (Huff and
Vogel, 1977). Again, a relatively flat pattern is
shown with the highest amounts, those greater Figure 1. Average annual Illinois precipitation, inches,
than 18 cm, found in the western extremes of derived from 1901-1944 data
Illinois, the lowest being slightly less than 15 cm in
northeastern Illinois. in central Illinois; and Evansville (EVV) in south-
Figure 4a presents the pattern based on the western Indiana (considered representative of
average number of days with thunderstorms in Il- southern Illinois). A latitudinal variation is shown
linois indicated by the 1901-1944 data (Changnon, in most months, although in summer the Spring-
1957). This shows a pattern similar to the annual field frequency of days and hours of thunder-
rainfall (figure 1), ranging from 38 thunder days in storms exceed those in southern and northern Illi-
the northeast to 58 days in the south. Figure 4b nois. Also shown in table 1 are the average number
shows the annual average precipitation from thun- of hours with thunder on days when thunder oc-
derstorms. Tables 1 and 2 present additional infor- curs. This reveals the typical duration on a thun-
mation on the monthly and seasonal distribu- derstorm day. In the warm season, thunderstorm
tions of thunderstorm events throughout Illinois events tend to be longer than in any other season
(Changnon, 1968). Shown in table 1 are frequen- of the year. Table 2 presents extremes in the hour-
cies of thunderstorm days and hours derived from ly duration of thunderstorm events between three
1948-1961 data at three stations. These include stations; these yield some estimate of their range
Moline (MLI) in northern Illinois; Springfield (SPI) at a point.
3
Figure 2. Average seasonal precipitation, inches
4
Figure 5 reveals the temporal frequency of
thunderstorm days within the state of Illinois, as
determined by state records over a 50-year period
(Changnon, 1962a). Several interesting facts are
shown, including 1) that within any given month
there are periods of marked high and low frequen-
cies of thunderstorm activity, including a maxi-
mum in early June, late July, and mid-August, and
2) that the distribution is not uniform through-
out the summer season. Studies of singularities in
precipitation days (Changnon, 1960) have shown
that there are periods during June, July, and Au-
gust when, on the average, precipitation events are
more or less common than on days just before or
after (see figure 12a).
Figure 6 presents a pattern based on the number
of hail days in an average 20-year period in Illinois
(Changnon, 1963a). This shows that hail is gener-
ally more frequent in central Illinois than in other
parts, although there are isolated parts of western
and northwestern Illinois with high frequencies.
Most hail occurs during March-May, with decreas-
ing frequencies in the summer months, June
through August. A further inspection of the sum-
mer hail events is shown in figure 7 (Changnon,
1962b). Here the number of severe hail days, de-
fined as those producing more than $10,000 in
crop or property damage, are shown. In this in-
stance, damaging hail events are shown to be most Figure 3. Normal rainfall (cm) for July-August in Illinois
frequent in northern Illinois, with a secondary
maximum in central Illinois averaging slightly more
show that hail is not a frequent event at a point,
than one every third year. Basically, figures 6 and 7
and that damaging hail is very infrequent at a point.
Table 1. Average Number of Days and Hours of Thunderstorms during 1948-1961
5
c. Average percent of normal monthly precipitation from thunderstorms during summer
Figure 4. Annual and seasonal thunderstorm information
6