UNIVERSITY OF WASHINGTON DEPARTMENT OF ATMOSPHERIC SCIENCES SEATTLE, WASHINGTON 98105 CONTRIBUTIONS FROM THE CLOUD PHYSICS LABORATORY Field Research in Cloud Physics in the Olympic Mountains, Winter 1967-68 by P. V. Hobbs, W. D. Scott, D. A. Burrows, L. F. Radke and J. D. Locatelli SEPTEMBER 1968 i ABSTRACT A field program in cloud physics was carried out at the research station in the Olympic Mountains from January 22 to March 31, 1968. During this period measurements were made of cloud condensation nuclei, ice nuclei, Aitken particles, light scattering coefficient of the air, ozone, and airborne sodium- containing particles. Observations were also made of the types and concentrations of ice and water particles in the clouds that formed at station level. On a number of occasions simul- taneous measurements were obtained of the atmospheric electric field, the net charges on precipitation particles, and the charges that these particles communicated to an ice surface with which they collided. 11 TABLE OF CONTENTS Page Abstract List of Figures ^y List of Tables vn Introduction 1 SECTION 1 SYNOPTIC MEASUREMENTS 5 SECTION 2 CLOUD CONDENSATION NUCLEI, LIGHT SCATTERING COEFFICIENT, SODIUM-CONTAINING PARTICLES, AND AITKEN NUCLEI MEASUREMENTS 8 2.1 Cloud condensation nuclei 8 2.2 Light scattering measurements 18 2.3 Sodium-containing particles 23 2.4 Aitken nuclei 30 SECTION 3 ICE NUCLEI AND OZONE MEASUREMENTS 32 3.1 Locations 32 3.2 Instrumentation 34 3.3 Comparison between ice nucleus counts in the Olympic Mountains and in Seattle 36 3.4 The effect on the concentrations of ice nuclei in the Olympics of the transport of air from the Puget Sound and Seattle 36 3.5 The effects of snowfall on the concentration of ice nuclei and ozone 52 3.6 The effect of fronts on the concentrations of ice nuclei and ozone 54 3.7 Increases in ice nucleus concentration accompanying westerly flow 55 3.8 Twelve-hour average variations in ice nucleus counts and ozone 56 SECTION 4 CLOUD AND PRECIPITATION PARTICLES 64 4.1 Types of particles 64 4.2 Liquid water content 64 4.3 Concentrations of ice particles 65 SECTION 5 ELECTRICAL MEASUREMENTS 69 5.1 Charging of an ice sphere moving through natural snowfall 69 5.2 Electrical charges on individual ice particles in the air 81 Ill 5. 3 Individual charges received by a surface exposed to natural cloud and precipitation particles 101 APPENDICES Appendix A 106 Appendix B 115 Appendix C 127 REFERENCES 135 ACKNOWLEDGMENTS 137 IV LIST OF FIGURES Figure Fig. 1 Location of the Olympic Mountains. 2 Fig. 2 Schematic diagram of the automatic cloud condensation nucleus counter. 9 Fig. 3 Concentrations of cloud condensation nuclei at 1% supersaturation and light scattering coefficient b during March 28, 1968. 12 s Fig. 4 Concentrations of cloud condensation nuclei at 1% supersaturation and light scattering coefficient b during March 29, 1968. 13 S Fig. 5 Twelve-hour variations in cloud condensation nuclei counts and light scattering coefficient b during s February. 15 Fig. 6 Twelve-hour variations in cloud condensation nuclei counts and light scattering coefficient b during s March. 16 Fig. 7 Concentration of cloud condensation nuclei at 1% supersaturation during February 7, 1968. 17 Fig. 8 Concentration of cloud condensation nuclei at 1Z supersaturation and light scattering coefficient b during March 26, 1968. s 19 Fig. 9 Variations of concentration of cloud condensation nuclei with supersaturation in different types of air mass. 20 Fig. 10 Diurnal variation in light scattering coefficient b for February 13, 1968. 24 S Fig. 11 Flame spectrometer. 26 Fig. 12 Size spectra of sodium-containing particles in the air measured on March 13, 1968. 29 Fig. 13 Location of ice nucleus counter and ozone recorder in the Olympics. 33 Fig. 14 Location of ice nucleus counter at the University of Washington. 35 Fig. 15 Average ice-nucleus spectra curves for the Olympic Mountains and Seattle. 37 v Page Synoptic, Ice Nuclei, and Ozone Data for Olympic 40 Mountains (Blue Glacier). (Key to Figures 16-26 found in Table 1. ) 41 42 43 44 45 46 47 48 49 50 Variations in ice nucleus count in the Olympic Mountains on March 28, 1968. 53 Twelve-hour averages of Synoptic, Ice Nuclei, and 58 Ozone Data for Olympic Mountains (Blue Glacier). (Key to Figures 28-29 found in Table 2. ) 59 Concentrations of ice particles and ice nuclei. 6P Net charge on rotating ice sphere and atmospheric electric field (March 14). 71 Net charge on rotating ice sphere and atmospheric electric field (March 14). 72 Net charge on rotating ice sphere and atmospheric electric field (March 15). 73 Net charge on rotating ice sphere and atmospheric electric field (March 15). 74 Net charge on rotating ice sphere and atmospheric electric field (March 15). 75 VI Figure Page Fig. 36 Net charge on rotating ice sphere and atmospheric electric field (March 15). 76 Fig. 37 Free charge on ice sphere as a function of velocity of sphere through the air. (Data from Runs 10, 11 and 12, March 15). 79 Fig. 38 Charge on ice sphere as a function of velocity of sphere through the air. (Data from Run 13, March 16). 80 Fig. 39 Ratio of number of positive to number of negative charges. 98 Fig. 40 Log-probability plots for positive charge on particles. 99 Fig. 41 Log-probability plots for negative charges on particles. 100 vii LIST OF TABLES Key to Figures 16-26. Synoptic, Ice Nuclei and Ozone Data for Olympic Mountains (Blue Glacier) Key to Figures 28-29. Twelve-hour averages of Synoptic, Ice Nuclei and Ozone Data for Olympic Mountains (Blue Glacier) Concentrations of ice particles. Tables 4-11 are the results of measurements of electrical charges on individual ice particles in the air. Tables 12-13 are the results of experiments of the individual charges received by a surface exposed to natural cloud and precipitation par- ticles. 1 INTRODUCTION The University of Washington's research station in the Olympic Moun- tains is situated at an altitude of 2025 meters above mean sea level and is about 2-1/2 km NNE of Mt. Olympus (Fig. 1). Mt. Olympus (2430 m) is the highest peak on the Olympic Peninsula. The peninsula itself is the first mountainous area encountered by storms that move in from the Pacific Ocean. Consequently, precipitation on the western slopes of the peninsula is (150 high to 200 inches per year), while in the rainshadow area some 50 km to the northeast of Mt. Olympus the precipitation falls to as low as 17 inches per year. The research station provides an ideal site for many types of cloud and physics meteorological investigations. For example, since the area to the west and southwest of Mt. Olympus is virtually free of towns and other human habitation, the air that flows over the station in the pre- vailing westerly or southwesterly airstreams is comparatively free of man-made contaminants and may be considered to be unmodified maritime air. In easterly flows, on the other hand, the air is continental in character. Measurements of the concentrations and types of particulates in the air for these two distinct cases can provide valuable information on some of the basic differences between maritime and continental air masses. Another factor which has proved to be of considerable value is the location of the research station on an isolated ridge, with the land falling off steeply to valleys on the west and east sides; on clear days the Pacific Ocean may be seen from the research station. Moreover, in addition to the orographic clouds that commonly form over the mountains, isolated cumulus clouds often build up in the valleys which surround the 124 122 120 124 122 120 Fig. I Location of the Olympic Mountains. 3 station and it is possible to study the life-histories of these clouds and investigate the effects that they have on measurements being made at the research station. Wave clouds in the lee of Mt. Olympus are also common. In winter ice crystal clouds, supercooled clouds, and mixed clouds often extend down to ground level in the neighborhood of the sta- tion. This makes it possible to carry out in-cloud measurements and experiments at the station. The cloud physics group at the University of Washington first car- ried out a field program at the research station in the Olympic Mountains during the winter of 1965-66 (see Hobbs et al. 1966) A longer and much more detailed program was carried out from January 22 to March 31, 1968, and it is with this program that the present report is concerned. The main purpose of this report is to give a general description of the meas- urements that were taken, to present examples of some of the experimental results, and to point out a few of the interesting facts that have al- ready emerged from a preliminary analysis of the data. Theories and speculations have been kept to a minimum. More detailed accounts of several aspects of the work will be submitted for publication in the near future. For convenience the report has been divided into the following five sections: Section 1: Synoptic measurements Section 2: Cloud condensation nuclei, light scattering coefficient, sodium-containing particles, and Aitken nuclei measure- ments Section 3: Ice nuclei and ozone measurements 4 Section 4: Cloud and precipitation particles Section 5: Electrical measurements SECTION 1 SYNOPTIC MEASUREMENTS Regular synoptic measurements of wet and dry bulb temperatures, maximum and minimum temperatures, wind speed and direction, total miles of wind past the station, and cloud and weather conditions were made daily at 0900, 1200, 1500, 1800 and 2100 throughout the program.