Particle Fall Speed Research Articles

Relative pollen productivity and fall speed estimates for southern African savanna taxa

Understanding the characteristics of pollen dispersal and deposition of different plant taxa is crucial to accurately reconstructing past landscapes using fossil pollen data. Quantitative reconstruction of past landscapes from pollen data using the Prentice-Sugita approach requires estimates of fall speed and relative pollen productivity for all taxa modelled. This study presents estimates of pollen productivity and fall speeds for key southern African savanna taxa, providing a basis for the improved interpretation of fossil pollen records from this extensive and heterogeneous biome. The work was carried out in 5 steps. (1) Modern pollen assemblages from 34 surface sediment samples were analysed. (2) Vegetation around each sampling site was surveyed in concentric circles to a radius of 50 m, and data from existing park surveys were analysed to extend the survey distance to 5 km. (3) Fall speeds for the main pollen taxa were estimated using Stoke’s Law of particle settling velocity. (4) Vegetation data were weighted using three different distance-weightings, one incorporating the different particle fall speeds. (5) Extended R-Value analysis was carried out on the pollen and distance-weighted plant abundance datasets using HUMPOL software to estimate relevant source area and relative pollen productivity for the main pollen taxa present. Results showed the Relevant Source Area of Pollen surrounding the sites to be 600–900 m radius, and Poaceae/Cyperaceae were found to be twice as productive (PPE 2.03) as the arboreal taxa analysed (PPE 0.50–0.99). The problems encountered in calculating pollen productivity estimates in savanna environments are discussed and improvements for future studies are suggested.

Observations of Precipitation Size and Fall Speed Characteristics within Coexisting Rain and Wet Snow

Abstract Ground-based measurements of particle size and fall speed distributions using a Particle Size and Velocity (PARSIVEL) disdrometer are compared among samples obtained in mixed precipitation (rain and wet snow) and rain in the Oregon Cascade Mountains and in dry snow in the Rocky Mountains of Colorado. Coexisting rain and snow particles are distinguished using a classification method based on their size and fall speed properties. The bimodal distribution of the particles’ joint fall speed–size characteristics at air temperatures from 0.5° to 0°C suggests that wet-snow particles quickly make a transition to rain once melting has progressed sufficiently. As air temperatures increase to 1.5°C, the reduction in the number of very large aggregates with a diameter > 10 mm coincides with the appearance of rain particles larger than 6 mm. In this setting, very large raindrops appear to be the result of aggregrates melting with minimal breakup rather than formation by coalescence. In contrast to dry snow and rain, the fall speed for wet snow has a much weaker correlation between increasing size and increasing fall speed. Wet snow has a larger standard deviation of fall speed (120%–230% relative to dry snow) for a given particle size. The average fall speed for observed wet-snow particles with a diameter ≥ 2.4 mm is 2 m s−1 with a standard deviation of 0.8 m s−1. The large standard deviation is likely related to the coexistence of particles of similar physical size with different percentages of melting. These results suggest that different particle sizes are not required for aggregation since wet-snow particles of the same size can have different fall speeds. Given the large standard deviation of fall speeds in wet snow, the collision efficiency for wet snow is likely larger than that of dry snow. For particle sizes between 1 and 10 mm in diameter within mixed precipitation, rain constituted 1% of the particles by volume within the isothermal layer at 0°C and 4% of the particles by volume for the region just below the isothermal layer where air temperatures rise from 0° to 0.5°C. As air temperatures increased above 0.5°C, the relative proportions of rain versus snow particles shift dramatically and raindrops become dominant. The value of 0.5°C for the sharp transition in volume fraction from snow to rain is slightly lower than the range from 1.1° to 1.7°C often used in hydrological models.

Contrails to Cirrus—Morphology, Microphysics, and Radiative Properties

Abstract This work is two pronged, discussing 1) the morphology of contrails and their transition to cirrus uncinus, and 2) their microphysical and radiative properties. It is based upon the fortuitous occurrence of an unusual set of essentially parallel contrails and the unanticipated availability of nearly simultaneous observations by photography, satellite, automated ground-based lidar, and a newly available database of aircraft flight tracks. The contrails, oriented from the northeast to southwest, are carried to the southeast with a component of the wind so that they are spread from the northwest to southeast. Convective turrets form along each contrail to form the cirrus uncinus with fallstreaks of ice crystals that are oriented essentially normal to the contrail length. Each contrail is observed sequentially by the lidar and tracked backward to the time and position of the originating aircraft track with the appropriate component of the wind. The correlation coefficient between predicted and actual time of arrival at the lidar is 0.99, so that one may identify both visually and satellite-observed contrails exactly. Contrails generated earlier in the westernmost flight corridor occasionally arrive simultaneously with those formed later closer to the lidar to produce broader cirrus fallstreaks and overlapping contrails on the satellite image. The minimum age of a contrail is >2 h and corresponds to the longest time of travel to the lidar. The lag between the initial formation of the contrail and its first detectability by Moderate-Resolution Imaging Spectroradiometer (MODIS) is ≈33 min, thus accounting for the distance between the aircraft track and the first detectable contrail by satellite. The lidar also provides particle fall speeds and estimated sizes, optical extinction coefficients, optical thickness (τ = 0.35), and ice water path (IWP = 8.1 g m−2). These values correspond to the lower range of those found for midlatitude cirrus by Heymsfield et al. The ice water per meter of length along the cloud lines is 103–104 times that released by typical jet aircraft. The synthesis of these findings with those of prior investigators provides confidence in the present results. Various authors find that contrail-generated cirrus such as reported here contribute to net regional warming.

A 3D stochastic cloud model for investigating the radiative properties of inhomogeneous cirrus clouds

AbstractThe three‐dimensional structure of clouds is known to be important for determining their radiative effects, but it is difficult to obtain this structure directly from observations. In this paper a stochastic model is described that is capable of simulating the structural properties unique to cirrus: fallstreak geometry and shear‐induced mixing. We first present an analysis of time–height cloud radar sections from southern England to extract the cirrus parameters of interest. It is found that horizontal power spectra of the logarithm of ice‐water content (estimated from radar reflectivity factor and temperature) typically exhibit a spectral slope of around −5/3 near cloud top that decreases with depth into the cloud, to values as low as −3.5 in some cases. This decrease can be explained by wind shear coupled with a spread of particle fall speeds leading to a homogenization that acts preferentially at smaller scales. The power spectra exhibit a distinct scale break, becoming flat at scales larger than around 50 km (the ‘outer scale’). The orientation of the fallstreaks may be predicted from the profile of mean wind and mean ice fall speed. We then describe the stochastic model, which takes as input profiles of the mean and fractional standard deviation of ice‐water content, spectral slope, outer scale and wind speed. It first generates an isotropic 3‐D fractal field by performing an inverse 3‐D Fourier transform on a matrix of simulated Fourier coefficients with amplitudes consistent with the observed 1‐D spectra. Random phases for the coefficients allow multiple realizations of a cloud with the same statistical properties to be generated. Then horizontal slices from the domain are manipulated in turn to simulate horizontal displacement and changes to the spectra with height. Finally the field is scaled to produce the observed mean and fractional standard deviation of ice‐water content. Vertical 2‐D slices extracted from the domain are very similar in appearance to cloud radar observations. Radiative‐transfer calculations using the independent column approximation are used to show that the different fallstreak orientation resulting from different wind shears can change mean top‐of‐atmosphere radiative fluxes by in excess of 45 W m−2 in the short‐wave and 15 W m−2 in the long‐wave. The effect of wind shear to induce horizontal mixing causes an additional but smaller radiative effect. We also investigate the biases that would be expected from the assumptions made in the radiation schemes of general‐circulation models (GCMs). It is found that there is some compensation between the errors arising from the assumptions of horizontal homogeneity and maximum‐random overlap; if a GCM were to improve the overlap assumption but still assume clouds to be horizontally homogeneous then the total error in cloud radiative effect would be likely to increase. Copyright © 2005 Royal Meteorological Society.

Deriving Mixed-Phase Cloud Properties from Doppler Radar Spectra

In certain circumstances, millimeter-wavelength Doppler radar velocity spectra can be used to estimate the microphysical composition of both phases of mixed-phase clouds. This distinction is possible when the cloud properties are such that they produce a bimodal Doppler velocity spectrum. Under these conditions, the Doppler spectrum moments of the distinct liquid and ice spectral modes may be computed independently and used to quantitatively derive properties of the liquid droplet and ice particle size distributions. Additionally, the cloud liquid spectral mode, which is a tracer for clear-air motions, can be used to estimate the vertical air motion and to correct estimates of ice particle fall speeds. A mixed-phase cloud case study from the NASA Cirrus Regional Study of Tropical Anvils and Cloud Layers- Florida Area Cirrus Experiment (CRYSTAL-FACE) is used to illustrate this new retrieval approach. The case of interest occurred on 29 July 2002 when a supercooled liquid cloud layer based at 5 km AGL and precipitating ice crystals advected over a ground measurement site. Ground-based measurements from both 35- and 94-GHz radars revealed clear bimodal Doppler velocity spectra within this cloud layer. Profiles of radar reflectivity were computed independently from the liquid and ice spectral modes of the velocity spectra. Empirical reflectivity- based relationships were then used to derive profiles of both liquid and ice microphysical parameters, such as water content and particle size. Although the ice crystals extended down to a height of 4 km, the radar measurements were able to distinguish the base of the cloud liquid at 5 km, in good agreement with cloud-base measurements from a collocated micropulse lidar. Furthermore, radar-derived cloud liquid water paths were in good agreement with liquid water paths derived from a collocated microwave radiometer. Results presented here demonstrate the ability of the radar to both identify and quantify the presence of both phases in some mixed-phase clouds. They also demonstrate that, in terms of radar reflectivity, the ice component of mixed-phase clouds typically dominates the radar signal, while in terms of mean Doppler velocity, the liquid component can make a significant contribution. The high temporal resolution, 94-GHz Doppler radar observations were able to reveal a periodic cloud-top updraft that, combined with horizontal wind speeds, suggests a horizontal scale for the in-cloud circulations.

The Sensitivity of Radiative Fluxes to Parameterized Cloud Microphysics

Abstract The sensitivity of modeled radiative fluxes to the specification of cloud microphysical parameterizations of effective radius and fallout are investigated using a single-column model and measurements from the Atmospheric Radiation Measurement (ARM) Program. The single-column model was run with data for the 3-month period of June–August 2000 at the ARM Southern Great Plains site forced with operational numerical weather prediction data. Several different packages of cloud microphysical parameterizations were used in the single-column model. The temporal evolution of modeled cloud amount as well as surface radiative fluxes from a control run compare well with ARM measurements. Mean ice particle fall speeds varied significantly with respect to the assumed ice particle habit. As particle fall speeds increased, the overall cloud fraction, cloud height, and grid-averaged ice water path decreased. The outgoing longwave radiation (OLR) differs by up to 4 W m−2 over the range of fall speeds examined, whil...

Aggregation and Scaling of Ice Crystal Size Distributions

Abstract Ice particle size distributions (PSDs) can be scaled onto a single exponential distribution for a wide range of observed conditions as demonstrated using data from Atmospheric Research Measurement (ARM) cirrus uncinus, Tropical Rainfall Measurement Mission (TRMM) tropical anvils, and First International Satellite Cloud Climatalogy Project (ISCCP) Regional Experiment (FIRE1) midlatitude cirrus field programs. The successful scaling of the PSDs is the result of the dominance of the aggregation process. The PSD is found to be a function of mean particle size and precipitation rate only. A correlation between precipitation rate and particle mass and fall speed relations is demonstrated and made use of in a semiempirical model of ice cloud that predicts the evolution of PSDs.

3D Wind Field Retrieval Applied to Snow Events Using Doppler Radar

Abstract The variational analysis method of Laroche and Zawadzki for the retrieval of the 3D wind field from single–Doppler radar data is adapted to analyze stratiform snow events. Modifications include prefiltering of noisy radial velocity data using a velocity azimuth display (VAD) analysis, the use of snow particle fall speeds derived from extended VAD analyses, and the usage of high-elevation angle data. Arctic cases were collected during the Beaufort and Arctic Storms Experiment with X-band radars located at Inuvik and Tuktoyaktuk, Northwest Territories, Canada. These were compared with C-band radar data acquired by the King City Doppler radar in southern Ontario, Canada, a midlatitude site. Retrieved vertical airspeeds were verified using data from a research aircraft. Mean horizontal speeds were verified using VAD analyses, and horizontal wind speed anomalies were checked using Mesonet station data and VAD residual analyses. The results indicate that retrieved vertical velocities are in relative ag...

Application of the 2D-video-distrometer for weather radar data inversion

Conversion of radar reflectivity Z to precipitation rate R is the basic task in weather radar data processing. The relationship Z = a∗Rb depends on precipitation fine structure, like material type, shape and size distribution. In order to apply the right constants for Z-R conversion, ancillary precipitation characteristics are required. The 2D-Video Distrometer (2DVD) offers the capability of measuring particle shapes and sizes (front and side views) with precision in the order of 0.2 mm, as well as the particle's fall speed, with an error typically less than 5%, over an active catchment area of approx. 100 cm2. Over the last eight years an extensive data base has been collected with such distrometers, in different climates and seasons. Significant samples have been extracted from this data base, in order to check the validity of so far used Z-R relationships and their extreme cases. It is found, that tropical rain contains more smaller, showery or convective events in moderate climate more bigger drops than expected by present parameter limits. The overall result of this study is that traditional literature models and parameter limits may be exceeded in nature, and in particular that higher ZDR values may be observed than expected, caused by an excess of large drops.

A Method for Estimating Particle Fall Velocities from Vertically Pointing Doppler Radar

Abstract A method is presented that estimates particle fall velocities from Doppler velocity and reflectivity measurements taken with a vertically pointing Doppler radar. The method is applicable to uniform, stratified clouds and is applied here to cirrus clouds. A unique aspect of the technique consists of partitioning the Doppler velocities into discrete cloud height and cloud reflectivity bins prior to temporal averaging. The first step of the method is to temporally average the partitioned Doppler velocities over an hour or two to remove the effects of small-scale vertical air motions. This establishes relationships between particle fall velocity and radar reflectivity at various levels within the cloud. Comparisons with aircraft in situ observations from other experiments show consistency with the remote-sensing observations. These results suggest that particle fall speeds can be determined to within 5–10 cm s−1 by means of this technique.

Vertical Motion Characteristics of Tropical Cyclones Determined with Airborne Doppler Radial Velocities

Abstract Vertical motions in seven Atlantic hurricanes are determined from data recorded by Doppler radars on research aircraft. The database consists of Doppler velocities and reflectivities from vertically pointing radar rays collected along radial flight legs through the hurricane centers. The vertical motions are estimated throughout the depth of the troposphere from the Doppler velocities and bulk estimates of particle fallspeeds. Portions of the flight tracks are subjectively divided into eyewall, rainband, stratiform, and “other” regions. Characteristics of the vertical velocity and radar structure are described as a function of altitude for the entire dataset and each of the four regions. In all of the regions, more than 70% of the vertical velocities range from −2 to 2 m s−1. The broadest distribution of vertical motion is in the eyewall region where ∼5% of the vertical motions are >5 m s−1. Averaged over the entire dataset, the mean vertical velocity is upward at all altitudes. Mean downward mot...

The evolution of the dehydration in the Antarctic stratospheric vortex

In 1994 an intensive program of balloon‐borne frost point measurements was performed at McMurdo, Antarctica. During this program a total of 19 frost point soundings was obtained between February 7 and October 5, which cover a wide range of undisturbed through strongly dehydrated situations. Together with several soundings from South Pole station between 1990 and 1994, they give a comprehensive picture of the general development of the dehydration in the Antarctic stratospheric vortex. The period of dehydration typically starts around the middle of June, and a rapid formation of large particles leads to a fast dehydration of the vortex. The evaporation of falling particles leads to rehydration layers, which have significantly higher water vapor concentrations than the undisturbed stratosphere. Through the formation of these rehydration layers in the early stages of the dehydration we can estimate a particle fall speed of ⅓ km/d and thus a mean particle size of 4 μm. Ice saturation was observed over McMurdo in only two cases and only well after the onset of the dehydration. From the inspection of synoptic maps it then follows that a small cold region inside the vortex seems to be sufficient to dehydrate the entire vortex. Above 20 km the dehydration is completed by the end of July. From the descent of the upper dehydration edge we can estimate a mean descent rate inside the vortex of 1.5 km/month. In McMurdo we observed occasional penetration of the vortex edge in cases where the vortex edge was close to McMurdo, however, these cases seem to have little effect on the bulk of the vortex. A sounding from November 3, 1990, at South Pole shows that the dehydration may persist into November and indicates that there is no significant transport into the vortex throughout winter and early spring.

Effects of Variations in Precipitation Size Distribution and Fallspeed Law Parameters on Relations between Mean Doppler Fallspeed and Reflectivity Factor

Abstract A description is given of the influence of variations in the form of the hydrometeor particle size distribution and of variations in the form of the particle fallspeed law on relations between mean Doppler fallspeed vD and reflectivity factor Z. It is shown that for rain, variations in distribution shape can produce errors in vD of 20%–25%. The resultant errors in raindrop distribution parameters deduced from vD and Z are calculated. Uncertainties regarding hydrometeor phase (and thus in the particle fallspeed law) are shown to produce much larger errors in the value of vD deduced from Z of up to a factor of 4. These results are deduced theoretically, tested with experimental raindrop size spectra, and demonstrated with experimental Doppler radar data for a tropical thunderstorm. It is concluded that the use of theoretical or empirical vD-Z relations to deduce particle size distributions will produce large errors in most meteorological situations. However, in those cases where a reasonable assump...

A New Relationship between Mean Doppler Velocity and Differential Reflectivity

Abstract A new relationship has been established linking the vertical mean Doppler velocity of raindrop spectra and the accompanying differential reflectivities. It is based upon the specific radar combination of a vertically pointing Doppler and a polarization radar scanning at low elevations, and was derived from consideration of two extensive disdrometer datasets containing 40 000 raindrop-size distributions. Following a detailed error analysis of the new relation, the discussion is directed towards its possible application and limitation. It is shown that under optimum conditions the velocity errors are as low as 0.3 m s−1. This results in Doppler radar-derived drop-size distributions having liquid-water contents with uncertainties of only about 40%. Compared to standard particle fall speed estimations of the Rogers' type, this means an improvement in accuracy of more than a factor of three. Although the new technique is not suitable for operational use, it can provide fresh quantitative insight into ...

Theoretical study of the cloud physics of Mei‐Yu

The paper theoretically explains some of the microphysical phenomena observed in Mei‐Yu. In the melting layer the theory shows that most of the changes from the top to the bottom of the melting layer in the particle number density and the particle size distribution slope can be explained by assuming a constant precipitation rate and increasing particle fall speed due to melting. However, the theory implies that some aggregation is going on during melting. In the warm‐rain region, binary interaction of coalescence and breakup are the prominent microphysical processes. In general, coalescence dominates over breakup. But in these cases the two processes will approximately arrive at equilibrium when the precipitation reaches the Earth's surface.

Microphysics in an oceanic mesoscale convective system during the Taiwan area mesoscale experiment

The microphysical processes in both the melting layer and the warm‐rain region of an oceanic mesoscale convective system observed during the Taiwan Area Mesoscale Experiment (TAMEX) are studied using airborne observations. Two flight cases are used, one flown in the transitional region and the other in the stratiform region. In the melting layer the particle number density decreases by an order of magnitude from the top to the bottom of the layer. This is caused by the sharp increase of particle fall speed as snowflakes melt into rain. The ice water content is higher in the transitional region, thus cooling the environment more efficiently during melting. In the warm‐rain region the microphysical process is dominated by coalescence process. Large raindrops are generated and the corresponding median diameter of all raindrops increases by a factor of 2 to 3 from the bottom of the melting layer to the ground level. The breakup process is more efficient in the transitional region.

Hindered settling of semidilute monodisperse and polydisperse suspensions

AbstractA light‐extinction principle was used to measure the fall speeds of the interfaces that develop during gravity sedimentation of monodisperse, bidisperse, and tridisperse suspensions of noncolloidal particles with small particle Reynolds numbers in the semidilute total particle volume fraction range 0.0003 ⩽ c0 ⩽ 0.15. For monodisperse suspensions, the hindered settling velocity of the interface at the top of the suspension was found to be well represented by the correlation of Richardson and Zaki, provided that the isolated particle fall speed was chosen by linear extrapolation of the data to c0 = 0, with 90% confidence intervals on the exponent of n = 5.0 ± 0.1. For dilute monodisperse, bidisperse, and tridisperse suspensions, the hindered settling velocities of the interfaces showed agreement within experimental uncertainty with the theory of Batchelor, which predicts that the settling velocity decreases linearly with increasing particle concentration and which is based upon pairwise particle interactions.

Doppler Radar Study of the Trailing Anvil Region Associated with a Squall Line

Abstract The kinematic structure and reflectivity distribution within a region of widespread precipitation associated with a summertime midlatitude (Illinois) squall line, as revealed by an analysis of Doppler radar data, are presented and discussed. The squall line moved in a southeasterly direction while active convection forming on its leading edge moved in a northeasterly direction. Decaying thunderstorms and their anvils merged to form the extensive region of stratiform precipitation which trailed the squall line. An extension of the VAD (Velocity Azimuth Display) method, or the EVAD (Extended VAD) method, has been developed for the analysis of single-Doppler radar data. In contrast to the VAD method, which requires knowledge of the particle fall velocities (or assumptions regarding it) to calculate the divergence of the horizontal wind, the EVAD method yields the vertical distributions of both the particle fall speed and the divergence. Also presented are results from a multiple-Doppler (MDOP) analy...

Airborne Doppler radar velocity measurements of precipitation seen in ocean surface reflection

Airborne Doppler radar measurements over the ocean reveal mirror images of precipitation echoes in reflection from the ocean surface at their virtual positions “below” the surface. Because the detected vertical component of the precipitation motion changes sign on reflection, the velocity field in the reflection appears shifted by amounts depending on the mean reflectivity‐weighted hydrometeor fall speed, wind speed, and viewing angle. The shift of the zero‐velocity isotach, for example, from the upwind side of the aircraft in the direct echoes to the downwind side in the mirror image is readily observed. With two views of each precipitation volume, one direct and the other in mirror image, one may solve for both a horizontal wind component and the net mean particle fall speed. These components may thereby be mapped in an appreciable volume. Measurements of divergence may be made by either flying or scanning the beam in a circle. The availability of mean fall speed with radar reflectivity provides a promising two‐parameter system for improved measurements of rainfall rate. The airborne radar also provides measurements of the normalized radar backscatter cross section at all angles from nadir to glancing incidence, thus providing a variety of estimates of ocean surface properties and winds. The availability of direct Doppler wind measurements using echoes from either precipitation or chaff provides a much needed “ground truth” system for the radar used as a scatterometer or for corresponding observations from space.

The Downwind Spread of an Initially Vertical Column of Particles in a Sheared Environment

The effects of particle fallspeeds on the downwind spread of initially vertical columns or curtains are examined in environments with wind shear. Sets of equations describing the column width as a function of time and distance below column top are derived by assuming, first, that the particles fall at a constant rate and, second, that particle fallspeed changes with time. These predictions are compared with measurements of a seeding curtain within a non-turbulent stratus cloud with high wind shear (0.017 s−1). The comparison implies that differential fallspeed effects in a non-turbulent sheared environment can account for much of the spread of the curtains.