Cynthia H. Twohy

Predicting global atmospheric ice nuclei distributions and their impacts on climate

Knowledge of cloud and precipitation formation processes remains incomplete, yet global precipitation is predominantly produced by clouds containing the ice phase. Ice first forms in clouds warmer than -36 °C on particles termed ice nuclei. We combine observations from field studies over a 14-year period, from a variety of locations around the globe, to show that the concentrations of ice nuclei active in mixed-phase cloud conditions can be related to temperature and the number concentrations of particles larger than 0.5 μm in diameter. This new relationship reduces unexplained variability in ice nuclei concentrations at a given temperature from ∼103 to less than a factor of 10, with the remaining variability apparently due to variations in aerosol chemical composition or other factors. When implemented in a global climate model, the new parameterization strongly alters cloud liquid and ice water distributions compared to the simple, temperature-only parameterizations currently widely used. The revised treatment indicates a global net cloud radiative forcing increase of ∼1 W m-2 for each order of magnitude increase in ice nuclei concentrations, demonstrating the strong sensitivity of climate simulations to assumptions regarding the initiation of cloud glaciation.

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 ...

A General Approach for Deriving the Properties of Cirrus and Stratiform Ice Cloud Particles

A new approach is described for calculating the mass (m) and terminal velocity (Vt) of ice particles from airborne and balloon-borne imaging probe data as well as its applications for remote sensing and modeling studies. Unlike past studies that derived these parameters from the maximum (projected) dimension (D) and habit alone, the ‘‘two-parameter approach’’ uses D and the particle’s projected cross-sectional area ( A). Expressions were developed that relate the area ratio ( Ar; the projected area of an ice particle normalized by the area of a circle with diameter D) to its effective density ( re) and to Vt. Habit-dependent, power-law relationships between re and Ar were developed using analytic representations of the geometry of various types of planar and spatial ice crystals. Relationships were also derived from new or reanalyzed data for single ice particles and aggregates observed in clouds and at the ground. The mass relationships were evaluated by comparing calculations to direct measurements of ice water content (IWC). The calculations were from Particle Measuring Systems (PMS) 2D-C and 2D-P probes of particle size distributions in ice cloud layers on 3 days during an Atmospheric Radiation Measurement (ARM) field campaign in Oklahoma; the direct measurements were from counterflow virtual impactor (CVI) observations in ice cloud layers during the field campaign. Agreement was generally to within 20%, whereas using previous mass‐ dimension relationship approaches usually produced larger differences. Comparison of ground-based measurements of radar reflectivity with calculations from collocated balloon-borne ice crystal measurements also showed that the new method accurately captured the vertical reflectivity structure. Improvements in the accuracy of the estimates from the earlier mass‐dimension relationships were achieved by converting them to the new form. A new, more accurate mass‐dimension relationship for spatial, cirrus-type crystals was deduced from the comparison. The relationship between Vt and Ar was derived from a combination of theory and observations. A new expression accounting for the drag coefficients of large aggregates was developed from observational data. Explicit relationships for calculating Vt as a function of D for aggregates with a variety of component crystals were developed.

Measurement of Condensed Water Content in Liquid and Ice Clouds Using an Airborne Counterflow Virtual Impactor

Abstract Condensed water content (CWC) measured using a counterflow virtual impactor (CVI) with a Lyman-α hygrometer downstream is compared with that measured by other airborne instruments (a hot-wire probe, a PMS FSSP, and a PMS 2D-C). Results indicate that the CVI system provides a reliable measurement of CWC in both liquid- and ice-phase clouds and that the CVI measures CWC contained in both large and small hydrometeors; this means that the condensed water present in both phases and virtually all hydrometeor sizes can be measured with a single device. Small ice contents of a few milligrams per cubic meter present in cirrus clouds can also be measured by the technique.

Effective Ice Particle Densities Derived from Aircraft Data

Abstract In this study, aircraft data are used to derive effective ice particle densities. This density is defined as the ice particle mass divided by the volume of an equivalent diameter sphere. Measured ice particle size distributions and total ice water contents are used to derive effective ice densities for ice particle populations (ρe) as a function of particle size [ρe(D)]. The density values are critical for modeling and remote sensing applications. The method uses particle size distributions (PSDs) measured by several particle spectrometers to compute the total particle volume per unit volume of air, assuming that the particles are spheres. Simultaneous direct measurements of ice water content from a counterflow virtual impactor (CVI) yield values for the number of grams of ice per unit volume of air, enabling the overall effective ice density for a population to be calculated. The measured PSD together with the CVI measurements are used to derive mass–dimension relationships. The methods are appl...

Evaluation of the aerosol indirect effect in marine stratocumulus clouds: Droplet number, size, liquid water path, and radiative impact

Received 9 June 2004; revised 24 September 2004; accepted 10 February 2005; published 20 April 2005. [1] Data from nine stratocumulus clouds in the northeastern Pacific Ocean were analyzed to determine the effect of aerosol particles on cloud microphysical and radiative properties. Seven nighttime and two daytime cases were included. The number concentration of below-cloud aerosol particles (>0.10 mm diameter) was highly correlated with cloud droplet number concentration. Droplet number concentrations were typically about 75% of particle number concentration in the range of particle concentrations studied (� 400 cm � 3 ). Particle number was anticorrelated with droplet size and with liquid water content in drizzle-sized drops. Radiative impact also depends upon cloud liquid water content and geometric thickness. Although most variability in these macroscopic properties of the clouds could be attributed to variability in the large-scale environment, a weak anticorrelation between particle concentration and cloud geometric thickness was observed. Because of these variations, no correlation between calculated cloud optical thickness or albedo and particle concentration was detectable for the data set as a whole. For regions with comparable liquid water contents in an individual cloud, higher particle concentrations did correspond to increased cloud optical thickness. These results verify that higher particle concentrations do directly affect the microphysics of stratiform clouds. However, the constant liquid water path assumption usually invoked in the Twomey aerosol indirect effect may not be valid.

Prevalence of ice-supersaturated regions in the upper troposphere: Implications for optically thin ice cloud formation

In situ measurements of water vapor and temperature from recent aircraft campaigns have provided evidence that the upper troposphere is frequently supersaturated with respect to ice. The peak relative humidities with respect to ice (RHI) occasionally approached water saturation at temperatures ranging from −40°C to −70°C in each of the campaigns. The occurrence frequency of ice supersaturation ranged from about 20% to 45%. Even on flight segments when no ice crystals were detected, ice supersaturation was measured about 5–20% of the time. A numerical cloud model is used to simulate the formation of optically thin, low ice number density cirrus clouds in these supersaturated regions. The potential for scavenging of ice nuclei (IN) by these clouds is evaluated. The simulations suggest that if less than about 5 × 10-3 to 2 × 10-2 cm-3 ice nuclei are present when these supersaturations are generated, then the cirrus formed should be subvisible. These low ice number density clouds scavenge the IN from the supersaturated layer, but the crystals sediment out too rapidly to prevent buildup of high supersaturations. If higher numbers of ice nuclei are present, then the clouds that form are visible and deposition growth of the ice crystals reduces the RHI down to near 100%. Even if no ice clouds form, increasing the RHI from 100% to 150% between 10 and 10.5 km results in a decrease in outgoing longwave radiative flux at the top of the atmosphere of about 8 W m-2. If 0.02–0.1 cm-3 IN are present, the resulting cloud radiative forcing reduces the net radiative flux several watts per square meter further. Given the high frequency of supersaturated regions without optically thick clouds in the upper troposphere, there is a potential for a climatically important class of optically thin cirrus with relatively low ice crystal number densities. The optical properties of these clouds will depend very strongly on the abundance of ice nuclei in the upper troposphere.

Analysis of the physical state of one Arctic polar stratospheric cloud based on observations

During the Arctic Airborne Stratospheric Expe- dition, simultaneous measurements of aerosol size distribution and NOy (HNO 3 + NO + NO 2 + 2.N20 5) were made along ER-2 flight paths. The flow characteristics of the NOy instrument allow us to derive the condensed NOy amount (assumed to be HNO3) present during polar stratospheric cloud (PSC) events. Analysis of the January 24 th flight indicates that this condensed HNO 3 amount does not agree well with the aerosol volume if the observed PSCs are composed of solid nitric acid trihydrate (NAT), as is generally assumed. However, the composition agrees well with that predicted for liquid H2SO4/I-INO3/I-I2 ¸ solution droplets using a new Aerosol Physical Chemistry Model (APCM). The agreement corresponds in detail to variations in temperature and humidity. The weight percentages of H2SO 4, HNO 3, and H20 derived from the measurements all correspond to those predicted for ternary, liquid solutions.

Deep convection as a source of new particles in the midlatitude upper troposphere

downwind of the cirrus anvil, with maximum concentrations of 45,000 per standard cm 3 . Volatility and electron microscope measurements indicated that most of the particles were likely to be small sulfate particles. The enhancement extended over at least a 600-km region. Multivariate statistical analysis revealed that high CN concentrations were associated with surface tracers, as well as convective elements. Convection apparently brings gas-phase particle precursors from the surface to the storm outflow region, where particle nucleation is favored by the extremely low temperatures. Simple calculations showed that deep convective systems may contribute to a substantial portion of the background aerosol in the upper troposphere at midlatitudes. INDEX TERMS: 0305 Atmospheric Composition and Structure: Aerosols and particles (0345, 4801); 0320 Atmospheric Composition and Structure: Cloud physics and chemistry; 0322 Atmospheric Composition and Structure: Constituent sources and sinks; 0365 Atmospheric Composition and Structure: Troposphere—composition and chemistry; 0368 Atmospheric Composition and Structure: Troposphere—constituent transport and chemistry;

Improved Representation of Ice Particle Masses Based on Observations in Natural Clouds

Abstract The mass–dimensional relationship put forth by Brown and Francis has been widely used for developing parameterizations for representing ice cloud microphysical properties. This relationship forms the cornerstone for past and forthcoming retrievals of ice cloud properties from ground-based and spaceborne active and passive sensors but has yet to be rigorously evaluated. This study uses data from six field campaigns to evaluate this mass–dimensional relationship in a variety of ice cloud types and temperatures and to account for the deviations observed with temperature and size, based on properties of the ice particle ensembles. Although the Brown and Francis relationship provides a good match to the observations in a mean sense, it fails to capture dependences on temperature and particle size that are a result of the complex microphysical processes operative within most ice cloud layers. Mass–dimensional relationships that provide a better fit to the observations are developed.