BULLETIN 63 Summary of METROMEX, Volume 2: Causes of Precipitation Anomalies by B. ACKERMAN, S. A. CHANGNON. JR., ©. DZURISIN, D. L GATZ, R. C. GROSH, S. D. HILBERS, F. A. HUFF, J. W. MANSELL, H. T. OCHS. Ill, M. E. PEDEN, P. T. SCHICKEDANZ, R. G. SEMONIN, and J. L. VOGEL Title: Summary of METROMEX, Volume 2: Causes of Precipitation Anomalies. Abstract: This is the last of two volumes presenting the major findings from the 1971-1975 METROMEX field operations at St. Louis. It presents climatological analyses of surface weath- er conditions, but primarily concerns those factors helping to describe the causes of the anom- alies. Volume 1 covers spatial and temporal distributions of surface precipitation and severe storms, and impacts of urban-produced precipitation anomalies. Volume 2 describes relevant surface weather conditions including temperature, moisture, and winds, all influenced by the urban area. Urban influences extend well into the boundary layer affecting aerosol distribu- tions, winds, and the thermodynamic structure, and often reach cloud base levels. Studies of modification of cloud and rain processes show urban-industrial influences on 1) initiation, local distribution, and characteristics of summer cumulus clouds; and 2) development of precipita- tion in clouds and the resulting surface rain entities. Reference: Ackerman, B., S. A. Changnon, Jr., G. Dzurisin, D. L. Gatz, R. C. Grosh, S. D. Hilberg. F. A. Huff, J. W. Mansell, H. T. Ochs, HI, M. E. Peden, P. T. Schickedanz, R. G. Semohin, and J. L. Vogel. Summary of METROMEX, Volume 2: Causes of Precipitation Anomalies, Illinois State Water Survey, Urbana, Bulletin 63, 1978. Indexing Terms: : Atmospheric chemistry, boundary layer, climatology, cloud physics, inadver- tent weather and climate change, meteorology, model studies of cloud microphysics, pollution, rainfall, severe weather, urban climate, weather modification, weather radar. STATE OF ILLINOIS HON. JAMES R. THOMPSON, Governor DEPARTMENT OF REGISTRATION AND EDUCATION JOAN G. ANDERSON, B.S., Director BOARD OF NATURAL RESOURCES AND CONSERVATION Joan G. Anderson, B.S., Director Thomas Park, Ph.D., Biology H. S. Gutowsky, Ph.D., Chemistry Stanley K. Shapiro, Ph.D., Forestry Laurence L. Sloss, Ph.D., Geology John C. Guyon, Ph.D., Southern Illinois University William L. Everitt, E.E., Ph.D., University of Illinois STATE WATER SURVEY DIVISION WILLIAM C. ACKERMANN, D.Sc, Chief URBANA 1978 (P.O. 6077-1500-5/78) Printed by authority of the State of lllinois-Ch. 127, IRS, Par. 58.29 (7-78-1500) CONTENTS PAGE Part A. Introduction 1 Part B. Description of surface weather conditions 11 Temperature analysis 11 Diurnal temperature and moisture cycles 25 Average dew points 43 Equivalent potential temperature patterns associated with rainstorms .... 63 Surface wind climatology 71 References in Part B 100 Part C. Urban boundary layer 103 Aerosol patterns 103 Summary of aerosols 123 Introduction to physical structure of the PBL 124 Thermodynamic structure of the PBL at midday 129 - Regional kinematic fields 165 References in Part C 206 Part D. Cloud and precipitation processes 212 Studies of anomalous cumulus clouds 212 Satellite-observed urban cloud distributions 229 Cloud characteristics 236 Cloud modeling 240 Radar analyses of urban effects on rainfall 265 Vertical characteristics and behavior of radar echoes 274 Surface raincell analyses 280 Deposition of aerosols 345 References in Part D 377 Part E. Recapitulation and outlook 382 Summary of Volume 2 . 382 METROMEX inadvertent weather modification hypotheses 385 Unresolved questions and recommendations 389 Project publications 391 METROMEX instrument sites 394 Abbreviations and acronyms . 395 PART A CONTENTS PAGE Project background 1 Goals of the Water Survey projects 1 General analytical approach 2 Background and adjustment to emerging findings 2 Transmission of results to users 6 Scope of this report 6 Acknowledgments 7 References in Part A 8 Part A. Introduction Stanley A. Changnon, Jr. and Richard G. Semonin Project Background This report is the second of two volumes issued by the Illinois State Water Survey to summarize the METROMEX (Metropolitan Meteorological Experiment) activities and results obtained by the Survey staff during the 1971-1977 period. This second volume focuses on two of the major research areas of the Water Survey program in METROMEX: 1) the surface weather conditions, and 2) the urban factors related to the local anomalies in clouds, rainfall, and severe storms. Volume 2 also serves as a progress report to the Department of Energy (DOE) on Contract EY-76-S-02-1199, and as part of the final report to the National Science Foundation on Grant ENV73-07796. This grant was the fourth in a series of NSF grants that began in March 1971; the DOE contract existed throughout METROMEX. Some of the research under the NSF grants per- tained to the weather anomalies and their impacts, and these results are in volume 1 of this summary. The Water Survey projects for METROMEX have major support from three sources including the National Science Foundation, the Department of Energy (ERDA and Atomic Energy Commission during the early years of METROMEX), and the State of Illinois. Some supplementary EPA funding for a minor portion of the field effort was also obtained. Most of the research reported here is a result of the 6V2 years of funding from the Department of Energy and National Science Foundation coupled with sizeable funding from the State of Illinois at a federal to state ratio of approximately 4:1. Volume 1 essentially focuses on two of the four broad major METROMEX goals: 1) the dimensionalizing of the surface precipitation and severe weather anomalies, and 2) the resulting impacts. Volume 2 of this 2-volume summarization of Water Survey METROMEX activities focuses on a third major goal: the delineation and definition of the causes of the METROMEX precipitation anomalies. It should be noted that, at this time, the research of the METROMEX data cannot totally delineate the causative mechanisms and that added research has been conducted by the Water Survey under NSF sponsorship in 1977-1978 for further studies of the METROMEX data. It should also be noted that a fourth major METROMEX goal, the translation and prediction of anomalies in other areas, has not been satisfied in existing Water Survey METROMEX research projects. Cloud modeling under NSF ENV73-07882, a 3-year grant, will be useful in the eventual translation process. However, a new research project launched by the Water Survey in 1977 is focusing on study of the past data in the Chicago area to develop comparisons with findings at St. Louis. This will test the transferability of the findings at St. Louis to another city with a different physical setting. Goals of the Water Survey Projects The general goals of the Illinois State Water Survey projects of METROMEX consisted of: 1) the delineation of any anomalies in the precipitation (quantity and quality) and in the severe weather frequencies in St. Louis and environs; 2) the definition of the causes for such anomalies; 1 3) investigations of the impacts of the weather anomalies on the local area and other urban- agricultural areas of Illinois; and 4) the transmission of all findings to potential users in the scientific community, the government, and the public. These four broad goals of the. Water Survey projects in METROMEX actually consisted of 14 specific objectives involving field operations, data collection, analyses, and research. These objectives also included the application and transmission of the results to various user groups. The 14 specific objectives and activity areas of the Water Survey's METROMEX program appear in table A-1. A flow chart depicting the 14 areas involved in the Survey's projects and how they inter- related appears in figure A-1. The means of information exchange and transmission of results indicated on this chart reflect how our METROMEX data and findings have been exchanged both with internal (other METROMEX groups) users and with external users. The goals and activities addressed specifically by the DOE and NSF support coupled with state support can be followed by examining table A-l and figure A-l. This support addressed 9 of the 14 goal-activity areas: • 2—Mapping of surface weather conditions • 3—Study of the low-level airflow • 4—Study of aerosol budgets • 7—Identification of the causes for anomalies • 8—Measurements for prediction • 10—Planning information • 11—Weather forecasting • 12—Applications to planned weather modification • 14—Transfer of knowledge General Analytical Approach Two basic approaches to the analyses of the METROMEX data were employed. The first of these is typified by the results on surface weather conditions presented here in volume 2. This approach is basically one that treats all the data from a particular source in a climatic-type evalu- ation. That is, total or very large data samples of a given event, say hourly temperatures, are treated for all months, seasons, or years of the METROMEX operations. The other basic research approach that has been employed and used extensively in this report has consisted of intensive meteorological analyses of individual periods exhibiting various precipitation conditions. This 'case study' approach has been pursued as part of the NSF, DOE, and related state support through- out METROMEX. Earlier results have appeared in separate publications (Changnon and Semonin, 1975). Background and Adjustment to Emerging Findings The major focus of the METROMEX effort was on summer (June-August) weather con- ditions. This focus was based on climatic research (Huff and Changnon, 1972) which had indicated the presence of major urban-related precipitation anomalies in this season. Much of the operational effort involving specialized field measurements was conducted during the summer months of the 1971-1975 period. Other facets of this 5-year field program featured rapid data processing and a quick initial analysis to inspect the results of each summer.