Brad Baker

An overview of microphysical properties of Arctic clouds observed in May and July 1998 during FIRE ACE

Microphysical data were collected by the NCAR C-130 research aircraft during the First International Satellite Cloud Climatology Project (ISCCP) Regional Experiment Arctic Cloud Experiment (FIRE ACE). Boundary layer clouds 100 to 400 m thick were observed on 11 of the 16 missions. The all-water clouds varied from being adiabatic and homogeneous with monomodal drop spectra to subadiabatic and inhomogeneous with bimodal drop spectra and drizzle. The subadiabatic clouds were observed to be actively mixing near cloud top. The adiabatic clouds provided a test of the performance of the liquid water content (LWC) probes but only in low LWC conditions. A mixed-phase boundary layer cloud displayed striking variability in the hydrometeor fields on a horizontal scale of 10 km and a vertical scale of 100 m. Cloud Particle Imager (CPI) data showed separate regions with small supercooled cloud drops, supercooled drizzle (at −25°C) and graupel particles. A deep stratus cloud with its base at 2 km (+2°C) and top at 6 km (−25°C) contained drizzle near cloud top and (lower in the cloud) very high (2500 to 4000 L−1) concentrations of ice particles in conditions that did not meet all the Hallett-Mossop criteria. CPI data showed that an Arctic cirrus cloud was composed of very high (∼100,000 L−1) concentrations of small ice particles interspersed with single, large (mostly bullet rosette) crystals. The data showed that the cirrus cloud was inhomogeneous on scales down to tens of meters. The average ice particle concentrations measured in the cirrus by the FSSP and CPI probes were several hundred to a few thousand per liter, much higher than commonly found in the literature.

In situ observations of the microphysical properties of wave, cirrus, and anvil clouds. Part II: Cirrus clouds

A Learjet research aircraft was used to collect microphysical data, including cloud particle imager (CPI) measurements of ice particle size and shape, in 22 midlatitude cirrus clouds. The dataset was collected while the aircraft flew 104 horizontal legs, totaling over 15 000 km in clouds. Cloud temperatures ranged from 28° to 61°C. The measurements show that cirrus particle size distributions are mostly bimodal, displaying a maximum in number concentration, area, and mass near 30 m and another smaller maximum near 200–300 m. CPI images show that particles with rosette shapes, which include mixed-habit rosettes and platelike polycrystals, constitute over 50% of the surface area and mass of ice particles 50 m in cirrus clouds. Approximately 40% of the remaining mass of ice particles 50 m are found in irregular shapes, with a few percent each in columns and spheroidal shapes. Plates account for 1% of the total mass. Particles 50 m account for 99% of the total number concentration, 69% of the shortwave extinction, and 40% of the mass in midlatitude cirrus. Plots and average equations for area versus particle size are shown for various particle habits, and can be used in studies involving radiative transfer. The average particle concentration in midlatitude cirrus is on the order o f1c m 3 with occasional 10-km averages exceeding 5 cm 3 . There is a strong similarity of microphysical properties of ice particles between wave clouds and cirrus clouds, suggesting that, like wave clouds, cirrus ice particles first experience conversion to liquid water and/or solution drops before freezing.

Improvement in Determination of Ice Water Content from Two-Dimensional Particle Imagery. Part I: Image-to-Mass Relationships

Ice water content in natural clouds is an important but difficult quantity to measure. The goal of a number of past studies was to find average relationships between the masses and lengths of ice particles to determine ice water content from in situ data, such as those routinely recorded with two-dimensional imaging probes. The general approach in these past studies was to measure maximum length L and mass M of a dataset of ice crystals collected at a ground site. Linear regression analysis was performed on the logarithms of the data to estimate an average mass-to-length relationship of the form M L. Relationships were determined for subsets of the dataset based on crystal habit (shape) as well as for the full dataset. In this study, alternative relationships for determining mass using the additional parameters of width W, area A, and perimeter P are explored. A 50% reduction in rms error in the determination of mass relative to using L alone is achieved using a single parameter that is a combination of L, W, A, and P. The new parameter is designed to take into account the shape of the ice particle without the need to classify the crystals first. An interesting result is that, when applied to the test dataset, the same reduction in rms error is also shown to be achievable using A alone. Using A alone facilitates the reanalysis and improvement of the determination of ice water content from large existing datasets of two-dimensional images, because A is simply the number of occulted pixels in the digital images. Possible sources of error in this study are investigated, as is the usefulness of first segregating the particles into crystal habits.

Biogenic volatile organic compound emissions from a lowland tropical wet forest in Costa Rica

Atmospheric Environment 36 (2002) 3793–3802 Biogenic volatile organic compound emissions from a lowland tropical wet forest in Costa Rica Chris Geron a, *, Alex Guenther b , Jim Greenberg b , Henry W. Loescher c , Deborah Clark d , Brad Baker e a National Risk Management Research Laboratory, United States Environmental Protection Agency, Research Triangle Park, NC 27711, USA b National Center for Atmospheric Research, Boulder, CO 80303, USA c School of Forest Resources and Conservation, University of Florida, Gainesville, FL 32611, USA d Department of Biology, University of Missouri-St. Louis, St. Louis, MS 63121, USA e South Dakota School of Mines and Technology, Rapid City, SD 57701, USA Received 10 August 2001; accepted 17 April 2002 Abstract Twenty common plant species were screened for emissions of biogenic volatile organic compounds (BVOCs) at a lowland tropical wet forest site in Costa Rica. Ten of the species examined emitted substantial quantities of isoprene. These species accounted for 35–50% of the total basal area of old-growth forest on the major edaphic site types, indicating that a high proportion of the canopy leaf area is a source of isoprene. A limited number of canopy-level BVOC flux measurements were also collected by relaxed eddy accumulation (REA). These measurements verify that the forest canopy in this region is indeed a significant source of isoprene. In addition, REA fluxes of methanol and especially acetone were also significant, exceeding model estimates and warranting future investigation at this site. Leaf monoterpene emissions were non-detectable or very low from the species surveyed, and ambient concentrations and REA fluxes likewise were very low. Although the isoprene emission rates reported here are largely consistent with phylogenetic relations found in other studies (at the family, genus, and species levels), two species in the family Mimosaceae, a group previously found to consist largely of non-isoprene emitters, emitted significant quantities of isoprene. One of these, Pentaclethra macroloba (Willd.) Kuntze, is by far the most abundant canopy tree species in the forests of this area, composing 30–40% of the total basal area. The other, Zygia longifolia (Humb. & Bonpl.) Britton & Rose is a common riparian species. Our results suggest that the source strength of BVOCs is important not only to tropical atmospheric chemistry, but also may be important in determining net ecosystem carbon exchange. Published by Elsevier Science Ltd. Keywords: Isoprene; Biogenic volatile organic compounds; Relaxed eddy accumulation; Pentaclethra macroloba; Palmae; La selva biological station 1. Introduction Isoprene emission from vegetation is the world’s largest known source of non-methane volatile organic *Corresponding author. Tel.: +1-919-541-4639; fax: +1- E-mail address: geron.chris@epa.gov (C. Geron). 1352-2310/02/

Light Scattering by Single Natural Ice Crystals

- see front matter Published by Elsevier Science Ltd. PII: S 1 3 5 2 - 2 3 1 0 ( 0 2 ) 0 0 3 0 1 - 1 compounds (NMVOCs). Its importance is further amplified by its high reactivity with the hydroxyl radical compared to other abundant atmospheric NMVOCs. It is estimated that over 90% of isoprene emission is from vegetation (Guenther et al., 1995). Modeling (Guenther et al., 1995) and limited measurement (Guenther et al., 1999, 1996) studies have suggested that at least 50% of the global annual isoprene flux is from tropical

Microphysical and Optical Properties of Atmospheric Ice Crystals at South Pole Station

During the South Pole Ice Crystal Experiment, angular scattering intensities (ASIs) of single ice crystals formed in natural conditions were measured for the first time with the polar nephelometer instrument. The microphysical properties of the ice crystals were simultaneously obtained with a cloud particle imager. The observations of the scattering properties of numerous ice crystals reveal high variability of the ASIs in terms of magnitude and distribution over scattering angles. To interpret observed ASI features, lookup tables were computed with a modified ray tracing code, which takes into account the optical geometry of the polar nephelometer. The numerical simulations consider a wide range of input parameters for the description of the ice crystal properties (particle orientation, aspect ratio, surface roughness, and internal inclusions). A new model of surface roughness, which assumes the Weibull statistics, was proposed. The simulations reproduce the overwhelming majority of the observed ASIs features and trace very well the quasi-specular reflection from crystal facets. The discrepancies observed between the model and the experimental data correspond to the rays, which pass through the ice crystal and are scattered toward the backward angles. This feature may be attributed to the internal structure of the ice crystals that should be considered in modeling refinements.

Drop Size Distributions and the Lack of Small Drops in RICO Rain Shafts

Abstract In early February 2001 (during the austral summer), over 900 000 digital images of ice crystals were recorded at the South Pole using two ground-based cloud particle imagers (CPIs). Of these, 721 572 crystals >50 μm were classified into crystal habits. When sorted by number, 30% of the crystals were rosette shaped (mixed-habit rosettes, platelike polycrystals, and rosette shapes with side planes), 45% were diamond dust (columns, thick plates, and plates), and 25% were irregular. When sorted by area, rosette shapes comprised 50%, diamond dust 30% and irregular 20%. By mass, the percentages were 57% rosette shapes, 23% diamond dust, and 20% irregular. Particle size distributions as a function of maximum dimension and equivalent radius are compared with previous studies. Particles are generally found to be slightly larger than previous austral wintertime studies. In 2002, a polar nephelometer (PN) that measures scattering phase function was incorporated with one of the CPIs. Correlated measurements ...

Improvement in Determination of Ice Water Content from Two-Dimensional Particle Imagery. Part II: Applications to Collected Data

Abstract Data from the new two-dimensional stereo (2D-S) probe are used to evaluate drop size distributions in rain shafts observed during the Rain in Shallow Cumulus over the Ocean (RICO) experiment. The 2D-S takes images of both precipitation drops and cloud droplets with 10-μm resolution. These are the first reported measurements of rain to include sizes smaller than 100 μm. The primary result is that there are almost no hydrometeors smaller than about 100 μm in these rain shafts. The measured low concentration of small hydrometeors implies that their rate of production is slow relative to their removal rate. Algorithms for removing the spurious effects of splashing precipitation and noisy photodiodes on 2D probes are also described.

Analysis of sesquiterpene emissions by plants using solid phase microextraction.

In Part I of this two-part series, a new relationship for ice particle mass M was derived based on an expanded dataset of photographed ice particles and melted drops. The new relationship resulted in a reduction of nearly 50% in the rms error in M. In this paper, new relationships for computing particle mass and ice water content from 2D particle imagery are compared with other relationships previously used in the literature. Comparison of the old and new relationships, when applied to data collected in natural clouds, shows that results using the old relationships differ from the new relationships by up to a factor of 3, depending on particle size and shape. One of the new relationships can be applied to existing (archived) datasets of two-dimensional images, provided that the number of occulted pixels in each image (i.e., projected area) is available.

The effects of precipitation on cloud droplet measurement devices.

Solid phase microextraction (SPME) was characterized for the sampling and analysis of sesquiterpenes (SQTs) emitted by plants. Constant mixing ratio SQT standards were produced using a capillary diffusion system. Polydimethylsiloxane SPME fibers were characterized with respect to relative absorption of SQTs, and the effects of sample linear velocity and sample relative humidity on SQT absorption. SPME was then utilized to measure SQT emissions from gray pine (Pinus sabiniana) and ponderosa pine (Pinus ponderosa). Total SQT emission rates at a photosynthetic photon flux density of 1200 micromol m(-2)s(-1) and 28 degrees C ranged 0.025-0.050 microg Cm(-2)h(-1) (alpha-farnesene) and 0.450-3.325 microg Cm(-2)h(-1) (alpha-farnesene, beta-farnesene, and alpha-bergamotene) for gray pine and ponderosa pine, respectively.