James E. Dye

Observations and Parameterizations of Particle Size Distributions in Deep Tropical Cirrus and Stratiform Precipitating Clouds: Results from In Situ Observations in TRMM Field Campaigns

Abstract This study reports on the evolution of particle size distributions (PSDs) and habits as measured during slow, Lagrangian-type spiral descents through deep subtropical and tropical cloud layers in Florida, Brazil, and Kwajalein, Marshall Islands, most of which were precipitating. The objective of the flight patterns was to learn more about how the PSDs evolved in the vertical and to obtain information of the vertical structure of microphysical properties. New instrumentation yielding better information on the concentrations of particles in the size (D) range between 0.2 and 2 cm, as well as improved particle imagery, produced more comprehensive observations for tropical stratiform precipitation regions and anvils than have been available previously. Collocated radar observations provided additional information on the vertical structure of the cloud layers sampled. Most of the spirals began at cloud top, with temperatures (T) as low as −50°C, and ended at cloud base or below the melting layer (ML)....

Particle size distributions in Arctic polar stratospheric clouds, growth and freezing of sulfuric acid droplets, and implications for cloud formation

Particle size and volume measurements obtained with the forward scattering spectrometer probe (FSSP), model 300 during January and February 1989 in the Airborne Arctic Stratospheric Experiment are presented and used to study processes important in the formation and growth of polar stratospheric cloud (PSC) particles. Comparisons of the observations with expected sulfuric acid droplet deliquescence suggest that in the Arctic a major fraction of the sulfuric acid droplets remain liquid until temperatures at least as low as 193 K. Arguments are presented to suggest that homogeneous freezing of the sulfuric acid droplets might occur near 190 K and might play a role in the formation of PSCs. The first suggestion of nitric acid trihydrate (NAT) particles appears near saturation ratios of HNO3 with respect to NAT of 1 (about 195 K) as an enhancement, of the large particles on the tail of the sulfuric acid droplet size distribution. The major increases in number and volume indicative of the main body of the NAT cloud are not seen in these Arctic investigations until 191 to 192 K, which corresponds to an apparent saturation ratio of HNO3 with respect to NAT of about 10, unlike the Antarctic where clouds were encountered at saturation ratios near 1. A decrease in the number of particles was observed in regions in which the airmass was denitrified, i.e. NOy, the sum of all reactive nitrogen species, was reduced. This was especially true for the larger particles on the upper tail of the sulfate size distribution. The loss of these largest particles supports the idea that denitrification may be the result of the preferential nucleation and growth of NAT on only the largest sulfate particles, which then sediment out of the airmass.

Evaluation of the Forward Scattering Spectrometer Probe. Part I: Electronic and Optical Studies

Abstract Laboratory studies of the Forward Scattering Spectrometer Probe (FSSP), were conducted to better understand the operation, to determine limitations and to define the measurement accuracy of the instrument for airborne cloud physics research. The studies included electronic cheeks of the instrument sensitivity to simulated particles of different sizes, airspeeds and arrival rates; measurement of important aspects of the optical configuration; and intercomparisons of six different FSSPs in a small wind tunnel with a droplet spray. The tests demonstrated measurement differences between various probes in several areas as well as areas in which there was reasonable agreement. Part of the differences can be attributed to different feature and design specifications of different probes as changes were made to improve the FSSP. Areas in which care needs to be taken in the calibration and processing of data from the FSSP are identified.

Evaluation of the Forward Scattering Spectrometer Probe. Part II: Corrections for Coincidence and Dead-Time Losses

Abstract Cloud particle concentrations measured by the Forward Scattering Spectrometer Probe (FSSP) can be underestimated when particles are either coincident or pass through the sensing area of the probe during the electronic dead-time. In the absence of any corrections, the differences between actual and measured concentrations can typically exceed 15% when aircraft mounted probes measure droplet concentrations > 500 cm−3. The sources of counting losses are described and correctional procedures derived and demonstrated.

Interpretation of measurements made by the forward scattering spectrometer probe (FSSP‐300) during the Airborne Arctic Stratospheric Expedition

An improved forward scattering spectrometer probe, the FSSP-300, was developed for the Airborne Arctic Stratospheric Expedition. The 300 measures particles in the size range 0.3 μm to 20 μm and has a greater sensitivity and faster time response than its predecessor, the FSSP-100X. An intensive characterization of this probes operating characteristics has been made and its limitations evaluated. Measurements from this probe are affected by Mie scattering ambiguities and index of refraction uncertainties, nonuniform laser intensity, uncertainties in sample volume definition, and time response roll-off. Correction algorithms have been developed to account for some of the probe limitations. After applying these corrections, the uncertainties in number and mass concentration are on the order of 25% and 60%, respectively.

Chemical loss of ozone in the arctic polar vortex in the winter of 1991-1992.

In situ measurements of chlorine monoxide, bromine monoxide, and ozone are extrapolated globally, with the use of meteorological tracers, to infer the loss rates for ozone in the Arctic lower stratosphere during the Airborne Arctic Stratospheric Expedition II (AASE II) in the winter of 1991-1992. The analysis indicates removal of 15 to 20 percent of ambient ozone because of elevated concentrations of chlorine monoxide and bromine monoxide. Observations during AASE II define rates of removal of chlorine monoxide attributable to reaction with nitrogen dioxide (produced by photolysis of nitric acid) and to production of hydrochloric acid. Ozone loss ceased in March as concentrations of chlorine monoxide declined. Ozone losses could approach 50 percent if regeneration of nitrogen dioxide were inhibited by irreversible removal of nitrogen oxides (denitrification), as presently observed in the Antarctic, or without denitrification if inorganic chlorine concentrations were to double.

Distributions of NO, NOx, NOy, and O3 to 12 km altitude during the summer monsoon season over New Mexico

During late July and August 1989, 12 flights of the National Center for Atmospheric Research Sabreliner jet aircraft were made over New Mexico when the region was dominated by either synoptic high pressure or moist “monsoon” flow. In the latter case, sampling was made within and about deep convective clouds which were sometimes electrically active. A summary of the measurements of the species listed in the title and their ratios are given. These distributions include signatures from deep convection, lightning production of odd nitrogen, aircraft exhaust emissions, and possible stratospheric input. The averages and range of these distributions are considered to be more representative of typical summer conditions over the region compared to flights that are often restricted more to fair weather situations. Coherence between the O3 and the NOy observations is compared to results from other ground-based and aircraft programs and possible contributing factors are discussed. Because the measurements were made with then newly developed instrumentation, its capabilities and shortcomings are summarized.

Microphysical observations of tropical clouds

Abstract The results from airborne in situ sampling of convective tropical storms in the Amazon and Kwajalein are presented. Three cases from the Amazon and two from Kwajalein are compared and provide examples of the much larger dataset that was obtained from field campaigns in these two regions during 1999. The strength of the updraft was a major factor in determining the microphysical characteristics of hydrometeors. Weak updrafts exhibited a well-developed warm rain process by the time droplets had reached the freezing level. Stronger updrafts (>5 m s−1) contained smaller droplets or ice particles at cloud midlevels than regions with the weaker updrafts. Significant supercooled liquid water was found only at temperatures warmer than −12°C, although traces of liquid water were observed at temperatures as cold as −18°C. In deep stratiform anvil regions, aggregation was observed to be a major growth mechanism. These clouds did not contain appreciable amounts of supercooled water. Clouds with similar updra...

The potential of cirrus clouds for heterogeneous chlorine activation

The ER-2 data from ascents and descents through layers of cirrus clouds are utilized to study the heterogeneous reactions of ClONO 2 with H 2 O, of HOCl and ClONO 2 with HCl, and their potential role for the activation of chlorine in the tropopause regions which could affect ozone there. Lacking measured data for the three chlorine containing molecules their abundances as a function of altitude have been calculated from a 2D model. The aerosol surface data measured by a Forward Scattering Spectrometer Probe (FSSP-300) on the ER-2 were corrected for the expected asphericity of cirrus cloud particles by means of a T-matrix method. The results indicate considerable potential of cirrus clouds for chlorine activation. If ClONO 2 and HCl are present in the tropopause region they are likely to be quickly converted to active chlorine by cirrus clouds.

A model evaluation of noninductive graupel‐ice charging in the early electrification of a mountain thunderstorm

The role of noninductive graupel-ice charge separation in the early electrification of the July 31, 1984, New Mexico mountain thunderstorm is assessed with a three-dimensional kinematic cloud model along with multiple Doppler radar and in situ measurements. Observations of the early electrification rate and the electric field distribution are consistent with modeled values that result when the noninductive mechanism works under the influence of convective motions and precipitation growth. An increase in ice particle concentrations and sizes, arising from vigorous precipitation growth, accelerates graupel-ice collision rates and hence the noninductive charging rate. Growing graupel particles experience increasing fall speed as they rise toward the top of the updraft. The resulting vertical flux convergence of graupel containing charge from previous noninductive collisions is a significant factor in the growth of the main negative charge density. This implies that a combination of air motion, precipitation interaction, and sedimentation contributes to the rapid intensification of storm electric fields. The linear electrification phase, which begins with the cessation of convective growth, is caused by a roughly constant noninductive charging rate and by the separation of negatively charged graupel and positively charged smaller ice particles by differential sedimentation, downdrafts, and horizontal advection in vertically sheared flow. When the sign reversal temperature for noninductive charging is assumed to be −10°C, the model results are characterized by a main negative charge in middle levels and provide the best overall agreement with the in situ field measurements in the July 31 storm. For a sign reversal temperature of −21°C the model results are characterized by a main positive charge center in middle levels, and the electric field polarity is opposite to the polarity measured at low and middle levels of the storm. The model and observational data, combined with findings of some laboratory studies, support the hypothesis that the actual reversal temperature in the July 31 storm is around −10°C. When the model includes the inductive graupel-droplet charging mechanism in addition to the noninductive mechanism, the effect of inductive charging is secondary to that of noninductive charging. The net effect of adding induction is dissipative. For example, the maximum field strength at the location of the aircraft measurements is slightly less than in the case where the noninductive mechanism acts alone. Weak charge screening layers were found to develop on the boundary of the modeled cloud.