Forward Scattering Spectrometer Probe Research Articles

Microphysics of clouds at Kleiner Feldberg

During a field measuring campaign at Kleiner Feldberg (Taunus) in 1990, microphysical characteristics of clouds have been measured by Forward Scattering Spectrometer Probes (FSSP). The aim was to study the influence of aerosol and meteorological factors on droplet size and number. The results are: More mass in the accumulation size range of the aerosol leads to more droplets in stratocumulus clouds and to higher soluble masses in droplets of stratus clouds. However, the aerosol distribution was coarser in the stratus clouds compared to the stratocumulus clouds. Within the first 200 m from cloud base, the droplets grow while their number decreases. The growth results in a stable size of about 14 µm diameter over a large distance from cloud base in many stratocumulus clouds. Two types of mixing processes were observed: processes with reductions in the number of droplets (inhomogeneous mixing) and with reductions in the size of the droplets (homogeneous mixing).

Calibration of the Forward-scattering Spectrometer Probe: Modeling Scattering from a Multimode Laser Beam

Scattering calculations using a more detailed model of the multimode laser beam in the forward-scattering spectrometer probe (FSSP) were carried out by using a recently developed extension to Mie scattering theory. From this model, new calibration curves for the FSSP were calculated. The difference between the old calibration curves and the new ones is small for droplet diameters less than 10 micrometers, but the difference increases to approximately 10% at diameters of 50 micrometers. When using glass beads to calibrate the FSSP, calibration errors can be minimized, by using glass beads of many different diameters, over the entire range of the FSSP. If the FSSP is calibrated using one-diameter glass beads, then the new formalism is necessary to extrapolate the calibration over the entire range.

Entrainment in cumulus clouds, II: Drop size variability

AbstractA new analysis method for examining the effects of entrained subsaturated air on cloud drop sizes is described. This, independently, gives information about height changes following dilution with subsaturated air, and hence about whether the cloud parcels received the subsaturated air when higher or lower than their present level. The physical details of the entrainment mechanisms are first discussed to provide a basis for interpreting the data.Drop‐size distributions, measured by a Forward Scattering Spectrometer Probe and obtained from aircraft cloud samples at constant altitude in convective clouds in Montana, are used to analyse these cloud processes. The volume averaged drop radii for each parcel (i.e. each measured spectrum), in several clouds for several horizontal traverses in each, are plotted against concentration, given here in the form of drop numbers per unit mass of air. This concentration from parcel to parcel depends on the fraction of subsaturated air which has been entrained. The entity‐type entrainment mixing mechanism provides a satisfactory explanation for all the examples found in this study, where the average drop radius changes very little with drop‐number concentration from parcel to parcel. Alternative mechanisms are discussed and found to be insufficient.

Observations of Cloud Microstructure at the Centimeter Scale

Current conceptual models of the processes that modify the droplet spectrum in convective clouds starts with entrainment of environmental air followed by turbulent mixing of these parcels into progressively finer filaments. Thus, one would expect, for scales larger than those at which molecular diffusion predominates, cloud structures should be observed with very sharp transitions between parcels with differing histories. Recent advances in microphysical measurements allow exploration of these processes at centimeter scales. In the present work, observations from an aircraft-mounted forward-scattering spectrometer probe, modified for faster response than the conventional probes, are presented. Cloud structures with very sharp interfaces are reported to exist on the smallest observable scale. These observations are intended to provide input into current efforts to model the evolution of the droplet spectrum.

Real-Time Detection of Performance Degradation of the Forward-scattering Spectrometer Probe

Abstract A method for detecting malfunctions during collection of data with the forward-scattering spectrometer probe (FSSP) is discussed. Droplet spectra measured with the probe are not sufficient to alert operators of probe failures, such as those caused by fogging or icing of the optics. Exact solutions of the coincidence process and application of these formulas, using additional parameters provided by the FSSP, yield estimates of the transit time of the droplets through the laser beam. Comparison of these estimates with expected values derived from aircraft speed can be used to detect a variety of FSSP malfunctions. An example of such an analysis using data from a flight through supercooled cloud is presented.

Sensitivity of Remotely Sensed Effective Radius of Cloud Droplets to Changes in LOWTRAN Version

Abstract A method of retrieving effective radius and optical depth of stratocumulus from narrowband near-infrared radiances was recently presented by Rawlins and Foot. Their retrieval scheme used LOWTRAN 5 to calculate atmospheric corrections, and the results showed that the remotely retrieved effective radii were significantly larger than those measured in situ with a PMS Forward Scattering Spectrometer Probe. LOWTRAN 7 has recently been developed and is used in this work for all atmospheric corrections. In this note, the major differences between LOWTRAN 5 and LOWTRAN 7 at near-infrared wavelengths are discussed, and it is shown that the retrieved effective radii using LOWTRAN 7, for two flights during the FIRE87 campaign, are in much better agreement with the in situ measurements.

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 probe's 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.

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.

Turbulent Entrainment and Mixing in Clouds: A New Observational Approach

Abstract A statistical analysis of cloud droplet interarrival times, measured using an aircraft-mounted forward-scattering spectrometer probe, is used to deduce spatial scales of inhomogeneity of droplet concentration in cumulus clouds. The analysis often indicates inhomogeneity of the droplet concentration at small scales (about 1 cm) within regions that appear homogeneous when viewed at larger scales. An explanation of the small-scale inhomogeneity is offered, and the larger-scale homogeneity of the droplet concentration is interpreted in terms of the processes of entrainment and mixing.

The Collection Efficiency of a Modified Mohnen Slotted-Rod Cloud-Water Collector in Summer Clouds

A modified Mohnen slotted-rod collector was used to collect cloud-water samples in summer clouds over the northeastern United States. Cloud-droplet-size distributions were measured with a forward-scattering spectrometer probe (FSSP) mounted on the National Oceanic and Atmospheric Administration (NOAA) King Air research aircraft. Cloud-droplet-volume distributions and liquid water content were determined for each cloud-water sample through analyses of the FSSP data. The theoretical collection efficiency of the slotted-rod collector was calculated as a function of droplet size for flight conditions encountered during each cloud-water sampling. The mass ratio was then calculated for each cloud sample by ratioing the actual collected water mass to the maximum possible collectable water mass. Mass-ratio values higher than unity were obtained having an average of 1.40±0.27. This could be due to an underestimation of the liquid water content by the FSSP or to the collection of large hydrometeors by the slotted rod. The modified Mohnen slotted rod collected fast representative cloud-water samples in sufficient quantities for chemical analysis of the sampled cloud water.

Comparison of techniques for measurements of fog liquid water content

During the Po Valley Fog Experiment 1989, four different methods for the measurement of the liquid water content in fog have been operated side by side. In order to check the performance of the PVM-100 (Particulate Volume Monitor) under field conditions, the data were compared to the results of the measurements with two FSSP-100 (Forward Scattering Spectrometer Probes), a Johnson Williams heated wire probe, and a Lyman-α instrument. Accuracy, limitations, and calibrations of these instruments are discussed. Results of fog chamber calibrations of a PVM-100 are shown. The PVM-100 was concluded to be the best instrument under laboratory and field conditions. DOI: 10.1034/j.1600-0889.1992.t01-1-00013.x

Measurements and modelling of cloudwater deposition to moorland and forests

The objective of the study was to measure the size dependence of cloudwater deposition and associated average ionic fluxes to vegetated surfaces. Measurements were made over a forest canopy at Dunslair heights in south Scotland and a moorland site, Great Dun Fell, in northern England. Measurements were made using the gradient and eddy correlation techniques. Eddy correlation measurements were made using an ultrasonic anemometer, a Knollenberg forward scattering spectrometer probe (to measure liquid water fluxes and fluxes of droplets in 1 μm size intervals) and a GSI particulate volume monitor (to measure liquid-water fluxes). Measurements were made at Great Dun Fell of the size dependence of droplet deposition velocity, using the gradient technique with two Knollenberg probes. Simultaneous gradient and eddy correlation measurements were also made at Great Dun Fell of average cloud-water fluxes, together with chemical analysis of cloud water composition, using a continuous analysis system. At Dunslair Heights, eddy correlation measurements were made using both the Knollenberg and Gerber Scientific Instruments (GSI) probes, while simultaneous gradient measurements using two GSI probes were also attempted. Samples of cloud water were collected at Dunslair Heights, using passive string collectors for chemical analysis by ion chromatography. The major findings of the study were: 1. 1. The droplet deposition velocities measured by the two techniques were similar. 2. 2. The deposition velocities were a strong function of droplet size. Considerable resistance to deposition was evident for droplets of less than 5 μm radius. Deposition velocities for particles from about 6 to 8 μm exceeded those for momentum. 3. 3. Except when the droplets were very small or the winds very light, bulk cloud-water deposition velocities were about 80% or more of the momentum deposition velocities to forests.

A Method for Validating FSSP Measurements Using Observational Data

A validation method of measurements from the forward-scattering spectrometer probe (FSSP) allowing preliminary analysis and verification of rough data is presented. It is based on the comparison of observed spectral distributions from undiluted cloudy air with theoretical adiabatic ones. The latter are obtained using an ascending adiabatic cloud-parcel model and the corresponding cloud-base conditions. A two-step approach is used to correct observed spectral distributions and is illustrated with data collected during the 1985 Joint Hawaii Warm Rain Project (JHWRP). The procedure provides an alternate way of obtaining the response matrix for an FSSP and can be used to validate the laboratory-derived response matrix. The results obtained (spectral characteristics and liquid water contents) agree well with the ones calculated from the laboratory characterization, which was made after the experiment, taking into account the laser beam nonuniformities.

Use of Rotating Pinholes and Reticles for Calibration of Cloud Droplet Instrumentation

Calibration devices for the Forward Scattering Spectrometer Probe (FSSP) and the Optical Array Probe (OAP) were developed. The device used with the FSSP is a rotating pinhole calibrator. It utilizes light diffracted by a pinhole of a known diameter to simulate scattered light from a water droplet. This device can be used to calibrate the FSSP, measure the FSSP's optical collection angles and for instrument alignment and troubleshooting. The device used with the OAP is a rotating reticle calibrator. Chrome disks of a known diameter on the reticle are used for calibration of the OAP and for determining the OAP's response to out-of-focus particles in the probe volume.

Size Calibration Corrections for the Forward Scattering Spectrometer Probe (FSSP) for Measurement of Atmospheric Aerosols of Different Refractive Indices

The response of the Forward Scattering Spectrometer probe (FSSP) is affected by the optical properties of measured particles. The manufacturer's size calibration data are specifically applicable to nonabsorbing water droplets. Response functions of the FSSP probe are calculated for different complex refractive indices corresponding to different types of atmospheric aerosols under various relative humidity Conditions. Based on the results of these response calculations, new corrected size calibrations are determined for six relative humidity values (0%, 50%, 70%, 80%, 90% and 99%) and for three atmospheric aerosol types (Rural, Urban and Maritime). Sample calculations with these corrected size calibration data show that a significant underestimation of the aerosol size/volume distribution can result, especially for dry atmospheric aerosols, if the manufacturer's size calibration data are used.

Corrections for the Effects of Particle Trajectory and Beam Intensity Profile on the Size Spectra of Atmospheric Aerosols Measured with a Forward Scattering Spectrometer Probe

Abstract Distortion of the size spectra measured with a forward mattering spectrometer probe (FSSP) under different transit time modes—“inhibit”, “normal”, and “delayed”—was evaluated using the theoretical analyses by Baumgardner and Spowart and the results of the response time and beam intensity profile measurements of the NOAA FSSP. The Baumgardner and Spowart work is extended to correct the FSSP atmospheric aerosol data collected under the “inhibit” or “delayed” mode. A correction algorithm is developed using the non-negative least squares (NNLS) method to reconstruct the original size distribution from a distorted one measured with an FSSP under the inhibit or delayed mode. A lognormal fit to the corrected size spectra was able to successfully recover from the original size distributions from the distorted artificial ones obtained from the theoretical simulation of the FSSP performance. When the actual test flight data for atmospheric aerosols measured with the NOAA FSSP under the inhibit and delayed ...

Evaluation of the Forward Scattering Spectrometer Probe. Part III: Time Response and Laser Inhomogeneity Limitations

Abstract The electronic response time of the Forward Scattering Spectrometer Probe (FSSP) has been evaluated and is shown to affect the measurement of droplet size. The size of droplets are underestimated when airspeeds exceed 50 m s−1 and size distributions are broadened as a result of nonuniformities in the laser beam of the probe. A processing scheme that requires a detailed knowledge of a probe's optical and electronic characteristics is described that compensates for the measurement errors caused by the probe's electronic and optical limitations.

A survey of particle measurements in the Arctic from the Forward Scattering Spectrometer Probe MODEL 300

A new instrument, the Forward Scattering Spectrometer Probe (FSSP) 300, was used on the NASA ER‐2 during the Airborne Arctic Stratospheric Expedition (AASE) to count and size particles in the size range of approximately 0.4 to 24 μm diameter. A brief description of the instrument is presented and observations for each ER‐2 flight are summarized. Measurements were made in both type I and type II polar stratospheric clouds as well as in the background sulfuric acid particles. Total particle concentrations up to 16 cm−3 with total particle volumes up to 10 μm3 cm−3 were observed in some of the polar stratospheric cloud events.

The polar stratospheric cloud event of January 24, 1989, Part 1. Microphysics

Rapid adiabatic cooling induced by synoptic forcing led to polar stratospheric cloud (PSC) formation on January 24, 1989, at altitudes sampled by the ER‐2 aircraft. Particle characteristics measured by the Forward Scattering Spectrometer Probe (FSSP) on the ER‐2 were compared to those calculated using a theoretical FSC microphysics model. Although calculations were sensitive to local changes in cooling rate, generally favorable agreement was found, that in particle surface area being especially important since this parameter dictates heterogeneous chemical rates. The overall model performance suggests that the current approach for simulating Type 1 (nitric acid trihydrate) PSC formation under rapid cooling conditions is well founded and can be used to study the effects of heterogeneous chemistry on stratospheric composition.

Observed particle evolution in the polar stratospheric cloud of January 24, 1989

Particle size distributions measured from the NASA ER‐2 with the new Forward Scattering Spectrometer Probe (FSSP) 300 in a type I polar stratospheric cloud (PSC) on January 24, 1989 show a volume mode near 0.8 μm diameter. The large increase in particle concentration and volume after cloud entry did not occur until the apparent saturation ratio of nitric acid with respect to nitric acid trihydrate reached 10, but at ratios near 1 subtle changes in the size distribution suggest some type I particles were present. Particle concentrations in cloud of 15 to 17 cm−3 were greater than the CN concentrations of 5 to 7 cm−3 just outside of cloud, suggesting nucleation on more than just sulfate particles. Some particles > 4 μm diameter were observed in a region which was saturated with respect to ice.