Search Results

You are looking at 1 - 10 of 35 items for

  • Author or Editor: G . Heymsfield x
  • Refine by Access: All Content x
Clear All Modify Search
Andrew J. Heymsfield and Alice G. Palmer

Abstract

Relationships between radar reflectivity and ice water content are derived from pendmtions into thundemonn anvils in Montana on seven days during the Cooperative Convective Precipitation Experiment (CCOPE), using aircraft data and radar reflectivity, based upon an approach which minimizes the errors in converting measured crystal size to mass. Other sources of error do exist, particularly when measurements are taken in the vicinity of convective cells. The effects of truncation of the measured size spectrum due to sampling volume limitations are discussed.

Ice water content (IWC) values predicted from the curves for most of the cases investigated are about the same for a given value of the radar reflectivity factor. Derived curves differ significantly in some cases from those applied to thunderstorm anvils in the past. A sensitivity study is performed to develop an improved mass-diameter relationship for anvil crystals.

The choice of Z-IWC relationship has a major effect upon the estimate of the mass transported into the anvil, as demonstrated from one of the cases where wind fields were measured using Doppler radar.

Full access
R. Meneghini, L. Liao, and G. M. Heymsfield

Abstract

The High-Altitude Imaging Wind and Rain Airborne Profiler (HIWRAP) dual-frequency conically scanning airborne radar provides estimates of the range-profiled mean Doppler and backscattered power from the precipitation and surface. A velocity–azimuth display analysis yields near-surface estimates of the mean horizontal wind vector υ h in cases in which precipitation is present throughout the scan. From the surface return, the normalized radar cross section (NRCS) is obtained, which, by a method previously described, can be corrected for path attenuation. Comparisons between υ h and the attenuation-corrected NRCS are used to derive transfer functions that provide estimates of the wind vector from the NRCS data under both rain and rain-free conditions. A reasonably robust transfer function is found by using the mean NRCS (⟨NRCS⟩) over the scan along with a filtering of the data based on a Fourier series analysis of υ h and the NRCS. The approach gives good correlation coefficients between υ h and ⟨NRCS⟩ at Ku band at incidence angles of 30° and 40°. The correlation degrades if the Ka-band data are used rather than the Ku band.

Open access
A. J. Heymsfield and R. G. Knollenberg

Abstract

Particle size spectra were measured during 20 hr of sampling in cirrus generating cells (uncinus, stratus, spissatus, thunderstorm anvil) and the particle concentration, mean crystal length, ice water content, reflectivity factor, and precipitation rate were calculated from these spectra. Average values of the physical properties in the generating cells were:

  1. Ice crystal concentration: 10,000–25,000m−3

  2. Mean crystal length: 0.6–1.0 mm

  3. Particle habit: bullet, rosette, column (75%)-plate (25%)

  4. Ice crystal density. 0.6–0.9 gm m−3

  5. Ice water content. 0.15–0.25 gm −3

  6. Reflectivity factor: 5.0–20.0 mm6−3

  7. Precipitation rate. 0.5–0.7 mm hr−1

Growth was found to be downward, reaching a maximum ice water content just below the base of the generating cell. The maximum ice water contents (not including the thunderstorm anvil) were found in cirrus uncinus. Liquid water was not found throughout the cirrus sampling by measurement with the Johnson-Williams hot wire liquid water meter; however, we believe that liquid water is present as a transient phase.

Full access
C. G. Schmitt and A. J. Heymsfield

Abstract

Representations for the surface area of ice particles in terms of the projected area have been developed using two different methods. The first method uses ice particles that are imaged in situ and geometric calculations that are based on the outline of the two-dimensional image of the particle. The second method uses computer-generated ice particle shapes and calculates the total surface area analytically. The results of the second method compare reasonably well with the results of the first method. Surface area estimates for individual particles were combined with particle size distribution and projected area measurements from the Cirrus Regional Study of Tropical Anvils and Cirrus Layers (CRYSTAL)–Florida Area Cirrus Experiment (FACE) field project to give total surface area estimates for observed ice particle populations. Population surface area estimates were also made from balloon-borne replicator data collected during the First International Satellite Cloud Climatology Project (ISCCP) Regional Experiment, phase II (FIRE-II). A relationship between the particle population surface area and projected area (cloud extinction) has been derived. The total particle surface area for particle populations is estimated to be between 8 and 10 times the projected area or between 4 and 5 times the extinction and has a small dependence on the properties of the particle size distribution for particles observed in random orientations.

Full access
R. Meneghini, L. Liao, and G. M. Heymsfield

Abstract

An important objective in scatterometry is the estimation of near-surface wind speed and direction in the presence of rain. We investigate an attenuation correction method using data from the High-Altitude Imaging Wind and Rain Airborne Profiler (HIWRAP) dual-frequency scatterometer, which operates at Ku and Ka band with dual conical scans at incidence angles of 30° and 40°. The method relies on the fact that the differential normalized surface cross section, δσ 0 = σ 0(Ka) − σ 0(Ku), is relatively insensitive to wind speed and direction and that this quantity is closely related to the magnitude of the differential path attenuation, δA = A(Ka) − A(Ku), arising from precipitation, cloud, and atmospheric gases. As the method relies only on the difference between quantities measured in the presence and absence of rain, the estimates are independent of radar calibration error. As a test of the method’s accuracy, we make use of the fact that the radar rain reflectivities just above the surface, as seen along different incidence angles, are approximately the same. This yields constraint equations in the form of differences between pairs of path attenuations along different lines of sight to the surface. A second validation method uses the dual-frequency radar returns from the rain just above the surface where it can be shown that the difference between the Ku- and Ka-band-measured radar reflectivity factors provide an estimate of differential path attenuation. Comparisons between the path attenuations derived from the normalized surface cross section and those from these surface-independent methods generally show good agreement.

Open access
C. G. Schmitt and A. J. Heymsfield

Abstract

Ice crystal aggregates imaged by aircraft particle imaging probes often appear to be fractal in nature. As such, their dimensional properties, mass, and projected area can be related using fractal geometry. In cloud microphysics, power-law mass (m)– and area (A)–dimensional (D) relationships (e.g., m = aDb) incorporate different manifestations of the fractal dimension as the exponent (b). In this study a self-consistent technique is derived for determining the mass and projected area properties of ice particles from fractal geometry. A computer program was developed to simulate the crystal aggregation process. The fractal dimension of the simulated aggregates was estimated using the box counting method in three dimensions as well as for two-dimensional projected images of the aggregates. The two- and three-dimensional fractal dimension values were found to be simply related. This relationship enabled the development of mass–dimensional relationships analytically from cloud particle images. This technique was applied to data collected during two field projects. The exponent in the mass–dimensional relationship, the fractal dimension, was found to be between 2.0 and 2.3 with a dependence on temperature noted for both datasets. The coefficient a in the mass–dimensional relationships was derived in a self-consistent manner. Temperature-dependent mass–dimensional relationships have been developed. Cloud ice water content estimated using the temperature-dependent relationship and particle size distributions agreed well with directly measured ice water content values. The results are appropriate for characterizing cloud particle properties in clouds with high concentrations of ice crystal aggregates.

Full access
G. M. Heymsfield, B. Geerts, and L. Tian

Abstract

Orbital Tropical Rainfall Measuring Mission (TRMM) precipitation radar (PR) products are evaluated by simultaneous comparisons with high-resolution data from the high-altitude ER-2 Doppler radar (EDOP) and ground-based radars. The purpose is not to calibrate any radar or to validate surface rainfall estimates, but rather to evaluate the vertical reflectivity structure, which is important in TRMM rain-type classification and estimation of latent heating profiles. The radars used in this study have considerably different viewing geometries and resolutions, demanding nontrivial mapping procedures in common earth-relative coordinates. Mapped vertical cross sections and mean profiles of reflectivity from the PR, EDOP, and ground-based radars are compared for six cases. These cases cover a stratiform frontal rainband, convective cells of various sizes and stages, and a hurricane.

For precipitating systems larger than the PR footprint size, PR reflectivity profiles compare very well with high-resolution measurements thresholded to the PR minimum reflectivity, and derived variables such as brightband height and rain types are accurate, even at off-nadir PR scan angles. Convective rainfall is marked by high-horizontal reflectivity gradients; therefore its reflectivity distribution is spread out because of the PR antenna illumination pattern and by nonuniform beamfilling effects. In these cases, rain-type classification may err and be biased toward the stratiform type, and the average reflectivity tends to be underestimated. The limited sensitivity of the PR implies that large portions of the upper regions of precipitation systems remain undetected. This implication applies to all cases, but the discrepancy is larger for smaller cells for which limited sensitivity is compounded by incomplete beamfilling. These findings have important implications for gridded TRMM products such as monthly mean rainfall.

Full access
C. G. Schmitt and A. J. Heymsfield

Abstract

Cirrus clouds in mid- and high latitudes are frequently composed of bullet rosette– and column-shaped ice crystals, which can have hollow ends. Bullet rosette–shaped ice crystals are composed of a number of bullets radiating from a central point. Research has shown that the light-scattering properties of ice particles with hollow ends are different from the scattering properties of solid ice particles. Knowledge of the frequency of occurrence of hollow particles is important to more accurately calculate the radiative properties of cirrus clouds.

This note presents the results of a survey of cirrus cloud ice crystal replicas imaged from balloon-borne Formvar (polyvinyl formal) replicators. Fifty percent to 80% of the replicated bullet rosette– and column-shaped particles had hollow ends. In bullets longer than 150 μm in length, the length of the hollows of the bullets averaged 88% of the total length of the bullet. The combined length of both hollow portions of column-shaped ice crystals varied from 50% of the length of the column for 30-μm-long columns to 80% of the length of the columns longer than 200 μm. Asymmetry parameter values estimated from cirrus cloud aircraft particle size distributions are higher by 0.014 when hollow crystals are considered. This difference leads to a 2.5 W m−2 increase for hollow crystals at the surface for a 0.5 optical depth cloud, demonstrating the importance of the incorporation of hollow particle scattering characteristics into radiative transfer calculations.

Full access
C. G. Schmitt and A. J. Heymsfield

Abstract

Ice crystal terminal velocities govern the lifetime of radiatively complex, climatologically important, low-latitude tropopause cirrus clouds. To better understand cloud lifetimes, the terminal velocities of low-latitude tropopause cirrus cloud particles have been estimated using data from aircraft field campaigns. Data used in this study were collected during the Cirrus Regional Study of Tropical Anvils and Cirrus Layers–Florida Area Cirrus Experiment (CRYSTAL-FACE) and the Pre-Aura Validation Experiment (Pre-AVE). Particle properties were measured with the NCAR video ice particle sampler (VIPS) probe, thus providing information about particles in a poorly understood size range. Data used in this study were limited to high-altitude nonconvective thin clouds with temperatures between −56° and −86°C.

Realistic particle terminal velocity estimates require accurate values of particle projected area and mass. Exponential functions were used to predict the dimensional properties of ice particles smaller than 200 microns and were found to predict ice water content measurements well when compared to power-law representations. The shapes of the particle size distributions were found to be monomodal and were well represented by exponential or gamma functions. Incorporating these findings into terminal velocity calculations led to lower values of mass-weighted terminal velocities for particle populations than are currently predicted for low-latitude ice clouds. New parameterizations for individual particle properties as well as particle size distribution properties are presented and compared to commonly used parameterizations. Results from this study are appropriate for use in estimating the properties of low-latitude thin and subvisible cirrus at temperatures lower than −56°C.

Full access
C. G. Schmitt, J. Iaquinta, and A. J. Heymsfield

Abstract

Cirrus clouds in the midlatitude and Arctic regions are often composed of bullet rosette–shaped ice crystals. Bullet rosette–shaped ice crystals are composed of a number of bullets radiating from a central point. The bullets that make up the rosette will grow to be hollow in some conditions. To understand better the radiative impact of cirrus clouds, the authors have used a ray-tracing code to calculate the scattering properties of solid and hollow bullet rosettes at visible wavelengths. Results show that hollow bullet rosettes exhibit a broader forward-scattering peak than do solid bullet rosettes. This difference results in an asymmetry parameter that is as much as 0.08 lower for hollow bullet rosettes than for solid rosettes. The effective asymmetry parameter of spheres with the same particle volume and total surface area of the bullet rosettes has also been calculated. Asymmetry parameter estimates for equivalent spheres were similar to those calculated using the ray tracing. Asymmetry parameter calculations were used in combination with direct aircraft measurements from the Atmospheric Radiation Measurement Program intensive operational period in March of 2000. Asymmetry parameter estimates were used with particle size distributions for three cirrus cloud flights for which the observed large particles were predominantly bullet rosettes. Calculated asymmetry parameter values (0.80–0.84) agreed poorly with published cirrus parameterizations (0.75–0.84) when applied to the same aircraft data. Differences lead to 4.5–9 W m−2 differences in reflected and transmitted visible light energy for a cloud of 0.5 optical depth.

Full access