REPORT OF INVESTIGATION 81 Studies of Selected Precipitation Cases from METROMEX STANLEY A. CHANGNON and RICHARD G. SEMONIN, Editors Title: Studies of Selected Precipitation Cases from METROMEX. Abstract: In the analysis of METROMEX field data, nine rain periods (case days) in 1972 and 1973 were chosen for in-depth investigations aimed to better define the urban-related mechanisms that cause the summer rainfall in and east of St. Louis and Wood River to be greater than that in sur­ rounding areas. The nine cases were selected to represent a variety of summer precipitation condi­ tions in the St. Louis area. These investigations identified several urban-related conditions that con­ tributed to the observed precipitation alterations. Principal findings were that the urban areas: 1) initiated and thus increased the number of raincells, 2) intensified rain and severe weather from the urban-initiated cells by their subsequent merger with each other or a later system, 3) intensified rain and severe weather from existing cells passing over the urban areas by the effect of their heat, moisture, and/or aerosol plumes, 4) produced these effects only when rain-conducive synoptic con­ ditions existed, and 5) operated with a 'domino' effect in which urban cells beget more urban cells on the same day and tend to reinforce developments in a multi-day sequence in the same locale. The case studies also revealed missing or inadequate measurements critical for METROMEX or other field projects and provided guidance for direction of the research. References: Changnon, Stanley A., and Richard G. Semonin, Editors. Studies of Selected Pre­ cipitation Cases from METROMEX. Illinois State Water Survey, Urbana, Report of Investigation 81, 1975. Indexing Terms: Atmospheric chemistry, boundary layer, climatology, cloud physics, inadver­ tent weather and climate change, meteorology, pollution, rainfall, severe weather, urban climate, weather modification, weather radar. STATE OF ILLINOIS HON. DANIEL WALKER, Governor DEPARTMENT OF REGISTRATION AND EDUCATION RONALD E. STACKLER, J.D., Director BOARD OF NATURAL RESOURCES AND CONSERVATION Ronald E. Stackler, J.D., Chairman Robert H. Anderson, B.S., Engineering Thomas Park, Ph.D., Biology Laurence L. Sloss, Ph.D., Geology H. S. Gutowsky, Ph.D., Chemistry William L. Everitt, E.E., Ph.D., University of Illinois John C. Guyon, Ph.D., Southern Illinois University STATE WATER SURVEY DIVISION WILLIAM C. ACKERMANN, D.Sc, Chief URBANA 1975 Printed by authority of the State of Illinois—Ch. 127, IRS, Par. 58.29 (11-75-1500) CONTENTS PAGE A. Introduction 1 Approach 1 Site geography 3 B. Data 5 Types and acquisition 5 Abbreviations for data types, sites, and measurements ... 7 Acknowledgments 7 C. Storms of 11 August 1972 11 D. Isolated storm of 14 July 1973 68 E. Severe storms of 23 July 1973 90 F. Squall zone storms of 25-26 July 1973 127 G. Local showers on 7 August 1973 162 H. Rain periods on 9 August 1973 175 I. Air mass storms of 10 August 1973 191 J. Squall line of 12 August 1973 232 K. Complex lines of 13 August 1973 268 L. Summary and recommendations 322 Summary 322 Recommendations 326 Studies of Selected Precipitation Cases from METROMEX Stanley A. Changnon and Richard G. Semonin, Editors A. INTRODUCTION Evaluation of our progress in the analysis of METROMEX data collected during 1971-1973 led to a decision to pursue some descriptive case studies of nine precipitation periods in 1972 and 1973. These studies are each in-depth investigations, although they do not represent the most elaborate study that could be envisioned. They are, however, sufficiently extensive to describe the likely causes of the precipitation, or lack of precipitation, in each case. This report is the second in a series of Water Survey reports devoted to METROMEX (Metropolitan Meteorological Experiment). The primary goal of these case studies was to focus the entire METROMEX field and research effort by attempting to define better the various urban factors leading to measurable changes in rainfall and severe weather in the St. Louis area. These studies will help to achieve the overall goal of METROMEX which is to define, understand, and evaluate urban-induced precipitation alteration due to man's activities. These extensive analyses also revealed several missing features of the field project, and this knowledge has been useful in redesigning the field effort in 1974-1975 and in directing the analytical procedures for concluding METROMEX. These procedures include: 1) identification of those variables essential for the description of a rain event; 2) evaluation of the quality of the available data with regard to describing a rain event; 3) establishment of a means for cataloging and storing essential data; and 4) establishment of a working data exchange between the various cooperating groups within METROMEX. Approach Nine cases (rain periods lasting usually 12 to 24 hours) were chosen to represent a variety of summer precipitation conditions in the St. Louis area. This report is organized to present the cases in their chronological order. Five of the cases are from a 7-day period in August 1973 and in­ clude a case with relatively small air mass showers and a very complex day with a relative severe squall line system followed by a cold frontal rain system. Other cases were chosen from August 1972 and July 1973. Since the majority of the summer precipitation in the St. Louis area is associated with squall lines, cold fronts, and squall areas, it is not surprising that the frequency of these synoptic events dominates the case studies. Two air mass situations are reported, one dealing with a post cold-frontal thunderstorm and the other with warm sector showers. Six squall lines are studied, two of which were associated with frontal systems and four in which no front impacted on the storm situation. Studies of a cold frontal passage and its associated precipitation and of a squall area with no front in the vicinity are also included. This type of classification is from the general synoptic condition viewpoint and the indi­ vidual case studies will describe the macroscale weather conditions in greater detail. Table A-1 lists the nine case days and briefly describes their precipitation events. Different Survey scientists performed these nine case studies. In most instances, two scientists were assigned to each case and they are the authors of the case texts which follow. The scientists were asked to follow a certain organizational format in order to obtain some uniformity, but any special emphases in their analyses were left largely to their discretion. 1 Table A-1. Case Days Studied Date Precipitation events 11 August 1972 Local area showers developed forming a meso thun­ derstorm cluster just east of St. Louis with 2-inch rains 14 July 1973 Isolated thunderstorms developed over the urban area and merged with existing storm to form a cluster east of St. Louis with 1-inch rains 23 July 1973 Organized line of thunderstorms moved through the area, producing severe weather, but dissipating as it moved east 25-26 July 1973 Showers and storms developed in low-lands north­ west of St. Louis and over the Alton urban area, and organized into a thunderstorm cluster with 3-inch rainfalls 7 August 1973 Small air mass showers developed west of St. Louis and over the Alton area, and only a few produced ra­ dar echoes that reached the ground 9 August 1973 Two rain systems occurred, one in the morning with showers throughout the area and one in the evening as a cold-front rain system dissipated while approach­ ing the urban area 10 August 1973 Isolated air mass thunderstorms developed in the area, including two over the urban area, one of which de­ veloped into a cluster over the city with very heavy rainfall rates 12 August 1973 Large thunderstorm producing record rain rates and large hail developed over the urban area and became the start of a squall line that developed there and moved eastward 13 August 1973 Isolated storm developed over the urban area and then merged with a severe weather-producing squall line that passed; a cold front line of storms passed later Other goals of this effort were to familiarize the senior METROMEX scientists with all types of data being collected and to encourage interaction between scientists who had been specializing in specific research areas (rain, synoptic weather, atmospheric chemistry, etc.). Each case is presented in the following format: 1) Synoptic weather conditions including descriptions of the relevant macroscale and mesoscale conditions 2) Precipitation morphology including descriptions of the clouds and precipitation ele­ ments that made up the case 3) Surface and atmospheric measurements including relevant surface temperatures, humidity, winds, and aerosols plus any cloud physics data, low-level airflow information, soundings, and pollution (thermal and aerosol) conditions, as detected by aircraft and remote sensors . 4) Air and rainwater chemistry involving surface and airborne measurements of pollutants and special tracers released to follow atmospheric motions 2 5) Storm summary presenting composites of all the case elements, deriving conclusions as to the urban role in affecting rainfall and severe weather, and identifying recommenda­ tions for future field activities and research In most respects, each case is a separate, self-contained scientific paper. Each is complete in the format described above and includes an introduction describing the rationale for choosing the case for study as well as its own list of references. Before proceeding into the texts of the nine cases, some background material is presented. First, the physical scene is set by a description of the study site geography. This includes the sur­ face elevation and major physical features (hills, plains, and rivers). Also presented is information about the major land uses of the St.