Harry T. Ochs

Rain in shallow cumulus over the ocean: the RICO Campaign

Shallow, maritime cumuli are ubiquitous over much of the tropical oceans, and characterizing their properties is important to understanding weather and climate. The Rain in Cumulus over the Ocean (RICO) field campaign, which took place during November 2004–January 2005 in the trades over the western Atlantic, emphasized measurements of processes related to the formation of rain in shallow cumuli, and how rain subsequently modifies the structure and ensemble statistics of trade wind clouds. Eight weeks of nearly continuous S-band polarimetric radar sampling, 57 flights from three heavily instrumented research aircraft, and a suite of ground- and ship-based instrumentation provided data on trade wind clouds with unprecedented resolution. Observational strategies employed during RICO capitalized on the advances in remote sensing and other instrumentation to provide insight into processes that span a range of scales and that lie at the heart of questions relating to the cause and effects of rain from shallow ...

Warm-Rain Initiation: An Overview of Microphysical Mechanisms

Abstract Rain triggering mechanisms are evaluated in three microphysical steps: droplet activation on cloud condensation nuclei, droplet growth by condensation, and droplet growth by coalescence. Although considerable progress has been made since the pioneering work of Squires, crucial questions in each of the above steps remain unresolved: Under what conditions do giant particles trigger rain by acting as coalescence nuclei? What is the contribution of stochastic condensation to the growth of large droplets in regions of entrainment? What are the collection efficiencies for droplet sizes critical to the onset of colaescence growth? Such questions cannot be answered without better observations. Aircraft instruments are becoming available with the potential to measure the very largest particles and cloud droplets at the concentration of raindrops. Recent advances in sampling and analysis techniques have extended observations of cloud microstructure to smaller scales, providing new insight on the growth of ...

The Lake—Induced Convection Experiment and the Snowband Dynamics Project

Abstract A severe 5—day lake—effect storm resulted in eight deaths, hundreds of injuries, and over

Natural oscillations of small raindrops

3 million in damage to a small area of northeastern Ohio and northwestern Pennsylvania in November 1996. In 1999, a blizzard associated with an intense cyclone disabled Chicago and much of the U.S. Midwest with 30—90 cm of snow. Such winter weather conditions have many impacts on the lives and property of people throughout much of North America. Each of these events is the culmination of a complex interaction between synoptic—scale, mesoscale, and microscale processes. An understanding of how the multiple size scales and timescales interact is critical to improving forecasting of these severe winter weather events. The Lake—Induced Convection Experiment (Lake—ICE) and the Snowband Dynamics Project (SNOWBAND) collected comprehensive datasets on processes involved in lake—effect snowstorms and snowbands associated with cyclones during the winter of 1997/98. This paper outlines the goals and operations of thes...

Collisions between Small Precipitation Drops. Part II: Formulas for Coalescence, Temporary Coalescence, and Satellites

REMOTE sensing of the rate of rainfall and the characteristics of hydrometeors with polarimetric radars is based on the assumption that the eccentricity of raindrops increases predictably with drop size1–3. Field observations, however, have shown that a particular raindrop size can have a wide range of shapes4,5 because of oscillations thought to be caused by drop collisions. Here we report observations of the shapes of small raindrops in circumstances where oscillations might occur sympathetically with eddy shedding in the drops wake. The analysis of oscillations was aided by measurements of oscillation frequency, based on variations in rainbow angle and comparisons of mode energies. Our observations agree with quiescent shapes only for drops <1 mm in diameter. For 1–1.5 mm we discovered significant shape variability of distinct types corresponding to drop oscillations in axisymmetric and transverse modes. Our finding of reduced average distortion explains the mysterious shift away from quiescent distortion for small raindrops, postulated from radar observations6, and indicates the existence of an important class of raindrop oscillations—promoted not by collisions but by intrinsic aerodynamic forces.

Initial Precipitation Formation in Warm Florida Cumulus

Abstract Collisions between small precipitation drops in free fall were analyzed for sizes applicable to self-collection, the process that controls the spreading of precipitation drops to larger sizes. Results from 45 laboratory experiments were generalized using dimensionless parameters to scale the coalescence efficiency, for the temporary coalescence probability, and the satellite occurrence frequency. The coalescence efficiency for uncharged drops (ϵ0) was found to be highly correlated (ρ = 0.99) with a simple combination of factors that scale the tendency for colliding drops to bounce apart as a function of the Weber number (We) and size ratio (p). Charge-induced coalescence was scaled by the electric field between the drops, assuming charged conducting spheres. The coalescence efficiency was obtained as a function of the normalized charge using a semiempirical formula (ρ = 0.95) for the amount of charge required to eliminate bounce and temporary coalescence. The occurrence of temporary coalescence i...

The Cape Canaveral Sea and River Breezes: Kinematic Structure and Convective Initiation

The microphysical processes that lead to the development of precipitation in small, warm cumulus are examined using data from the Small Cumulus Microphysics Study near Cape Canaveral, Florida. Aircraft measurements are used to determine the concentration and size distribution of giant and ultragiant nuclei in clear air as a function of relative humidity, altitude, wind speed, and wind direction. The clear-air particle distributions show that ultragiant particles (radii extending from 10 to 150 mm) exist from the surface to cloud base in concentrations that correspond to the concentrations of raindrops observed during drizzle to moderate rainfall events. A shift of the spectra toward larger size with increasing relative humidity was observed, suggesting that the spectra are composed of deliquesced particles growing by condensation. The small cumulus clouds are shown to contain cores where the observed liquid water content was nearly adiabatic. The observed evolution of the cloud droplet distribution within the near-adiabatic cores as a function of height showed an increase in the small droplet mode associated with condensation and an increase in the concentration of larger droplets associated with growth by accretion. Droplets with radii extending to nearly 100 mm were present just above cloud base. These measurements were consistent with the clear-air measurements and provided evidence that the ultragiant nuclei can immediately act as embryos for raindrop growth by accretion upon entering cloud base. Comparisons of reflectivity computed from the cumulus core composite droplet distributions with the radar-observed reflectivity data provided independent evidence that the composite spectra reasonably represented the evolving microstructure of the cores of small cumulus clouds as they grew vertically. The analyses provide strong evidence of an efficient process for the initial development of precipitation in small Florida cumuli. This process consists of raindrop embryo formation on ultragiant nuclei followed by growth by accretion as the newly formed drops proceed upward through the adiabatic cores of the cumulus clouds. These data support the conceptual model of raindrop formation in marine clouds first proposed by Woodcock a half century ago.

The Microphysical Structure and Evolution of Hawaiian Rainband Clouds. Part I: Radar Observations of Rainbands Containing High Reflectivity Cores

Abstract This study examines complex flow patterns associated with the Cape Canaveral sea breeze and sea-breeze front using dual-Doppler radar, sounding, and surface data collected on 26 July 1991 during the Convection and Precipitation/Electrification Experiment. This case focuses on (a) the structure of the sea breeze, an associated trailing convergence line, river-induced convergence zones, and thunderstorm outflow boundaries, and (b) the development of convection where these features interacted. Variations in the direction of the sea breeze in the vicinity of irregular coastlines, such as Cape Canaveral, can lead to persistent zones of convergence within the sea-breeze air. The findings show that these zones of convergence, in turn, can locally increase the depth of the sea-breeze air and create circulations at the top of the sea breeze, which can support the development of convection. The observational study is the first to document the development and evolution of the trailing convergence line over ...

Laboratory Measurements of Small Raindrop Distortion. Part I: Axis Ratios and Fall Behavior

Abstract Radar reflectivity factors exceeding 60 dBZ are documented within shallow ( 0°C), summertime tropical rainbands offshore of the island of Hawaii. Dual-Doppler radar measurements from the Hawaiian Rainband Project are used to document the formation, evolution, and kinematic structure of the high reflectivity cores. The authors show that extremely high radar reflectivities (50–60 dBZ) can develop from echo free regions (−20 dBZ) within approximately 15 min and are preceded by 5–9 m s−1 peak updrafts. High reflectivities (>50 dBZ) typically first formed in the middle or upper part of the clouds. Over the next 10–15 min, the mature high reflectivity cores extended vertically through the cloud depth and then collapsed to the surface as the updrafts weakened. A near-upright orientation of most updrafts producing these high reflectivity cores is conceptually consistent with the idea that large raindrops grow in the highest liquid water content while falling through the updraft core. Strong...

Simple two-dimensional kinematic framework designed to test warm rain microphysical models

Abstract The resonant interactions between eddy shedding and drop oscillations postulated by Gunn for millimeter diameter raindrops were investigated in a series of laboratory measurements of axis ratio and fall behavior for water drops of d = 0.70–1.54 mm. Drops were produced at terminal velocity using a orifice-jet drop generator and allowed to fall several meters so that the initial oscillations (produced during jet break up) decayed to a negligible amplitude before the drop shape was recorded using stroboscopic photographs. The measured axis ratios had equilibrium values for the smallest sizes (d = 0.70–0.96 mm), but scattered somewhat above equilibrium at intermediate sizes (d = 1.04–1.29 mm). An order of magnitude larger scatter in axis ratio, extending above and below equilibrium, was found for the largest sizes (d = 1.40–1.54 mm) with the average axis ratio being larger than equilibrium. All but the smallest drops displayed a sideways drift varying from 1% of the terminal velocity for d = 0.82–1.0...