Forward Scattering Spectrometer Probe Research Articles

Assessing Cloud-Phase Conditions

Abstract In situ microphysics measurements made during the First and Third Canadian Freezing Drizzle Experiments (CFDE I and III, respectively) have been used to assess the relative responses to ice and liquid hydrometeors for several common instruments. These included the Rosemount icing detector, 2D-C monoscale and 2D-C grayscale probes, forward-scattering spectrometer probes (FSSP) on three measurement ranges, Nevzorov liquid water content (LWC) and total water content probes, and King LWC probes. The Nevzorov LWC and King LWC probes responded to between 5% and 30% of the ice water content, with an average response of approximately 20%. The average FSSP measurements of droplet spectra were dominated by ice particles for sizes greater than 35 μm, independent of the measurement range used, when the ice-crystal concentrations exceeded approximately 1 L−1. In contrast, the FSSP measurements of the droplet spectra less than 30 μm appeared free of ice-crystal contamination, independent of the ice-crystal con...

A GCM parameterization for bimodal size spectra and ice mass removal rates in mid-latitude cirrus clouds

This study describes a parameterization for bimodal size spectra in mid-latitude cirrus, based on 996 size distributions (SD) taken from 17 flights in non-convective cirrus during ARM and FIRE intensive observation periods (IOPs). Based on recent work and results given here, the Forward Scattering Spectrometer Probe (FSSP) appears to be a viable instrument for approximating the concentration and sizes of small ice crystals for the cirrus studied here. This, in combination with new methodologies presented, allows the small particle mode of the SD ( D≲100 μm) to be characterized. The large mode was characterized from laser imaging 2DC probe measurements. Recent work indicates that the thermal single scattering properties of ice clouds are not adequately determined by effective diameter ( D eff) and ice water content (IWC) alone, and that information on SD dispersion (e.g. degree of bimodality) is also required. This parameterization satisfies this need, providing three gamma SD parameters for each mode of the bimodal distribution. Three of these six parameters are approximated as constant, while the other three are expressed in terms of cloud IWC and/or temperature ( T). These simple inputs make the parameterization convenient for large scale models, such as Global Climate Models (GCMs). In addition, this parameterization provides a physically rigorous means for parameterizing the mass sedimentation rates from ice clouds in a microphysical sense. Accurate knowledge of mass removal rates is critical for predicting cirrus IWCs and radiative properties. Finally, the data in this study was used to parameterize D eff as a function of T for use in schemes parameterizing bulk short-wave single scattering properties. Uncertainties in estimating D eff drop considerably if most mid-latitude cirrus crystals are irregular, as recent studies suggest.

The cloud, aerosol and precipitation spectrometer: a new instrument for cloud investigations

A new airborne particle spectrometer has been developed with the same measurement capabilities of the Forward Scattering Spectrometer Probes (FSSP) models 100 and 300 (FSSP-300 and FSSP-100), two-dimensional optical imaging probe (2D-OAP), the Multiangle Aerosol Spectrometer Probe (MASP) and hot-wire liquid water probe, but with a single integrated system. The cloud, aerosol and precipitation spectrometer (CAPS) measures particles from 0.35 μm to 1.55 mm in diameter and liquid water content (LWC) from 0.01 to 3 g m −3. In addition to combining five probes into one, it measures airspeed at the sample volume and transmits a data stream that requires no special interfaces to communicate with most computers.

COMPARISONS OF CVI AND FSSP MEASUREMENTS OF CLOUD DROPLET NUMBER CONCENTRATIONS IN MARINE STRATOCUMULUS CLOUDS

Abstract Comparisons of counterflow virtual impactor (CVI) and forward scattering spectrometer probe (FSSP) measurements of cloud droplet number concentrations obtained by two different aircraft in marine boundary layer stratocumulus clouds are presented. The cut sizes of the CVIs have been calculated using a mathematical model (DLH50) and using Stokes' theory (DS50). For most of the cases the agreement between DLH50 and DS50 is within 3%. Based on the results obtained with the mathematical model the agreement between CVI and FSSP concentrations is within 25% for most of the cases analyzed in this study and also within 10% for several of the cases. The reasonable agreement between CVI and FSSP concentrations supports the assumption that a droplet releases a single residual particle upon evaporation. Furthermore, in this study it is shown that droplet shattering can qualitatively be identified and it occurs often when there are elevated concentrations of drizzle droplets in the clouds. Poor agreement betwe...

Fine Structure of Cloud Droplet Concentration as Seen from the Fast-FSSP Measurements. Part I: Method of Analysis and Preliminary Results

Abstract A statistical analysis of a series of droplet arrival times measured by the Fast Forward-Scattering Spectrometer Probe (FSSP) during aircraft flights in cumulus clouds was conducted. The main purpose of the analysis was to determine whether droplet concentration fluctuates at small scales on the order of a few centimeters or whether these fluctuations are negligible as compared with the mean concentration. In the analysis, the series of droplet arrival times is regarded as a generalized Poisson random process with time-dependent (or space dependent) parameters. The method developed is based on the representation of droplet concentration in a cloud along the aircraft track as the sum of three components: average droplet concentration in a cloud, large-scale fluctuations of droplet concentrations described by the Fourier series, and a small-scale noncoherent fraction of concentration fluctuation characterized by the energy spectrum and the correlation function. The efficiency of the method to estim...

Physical features of the atmospheric aerosol determined with an aureolemeter and a FSSP probe in the Mediterranean Lampedusa island

In order to investigate the influence of the atmospheric aerosol on the ultraviolet radiation on earth, the measurement campaign Photochemical Activity and Ultraviolet Radiation (PAUR II) Modulation was carried out in the central Mediterranean Sea during the period May–June 1999. Two sites were chosen for measurements: the island of Crete (Greece), and the island of Lampedusa (Italy). The aerosol features over the Lampedusa island, as well as the dust coming from Sahara desert, were investigated by measurements of direct and diffuse solar irradiance carried out with an aureolemeter. The columnar volume size distributions of the aerosol showed a four-modal shape in a less turbid atmosphere when the aerosol optical depth was less than 0.2 at λ=500 nm, and a tri-modal shape in a turbid atmosphere when the aerosol optical depth at the same wavelength was greater than 0.5; the background aerosol turned out to be mainly composed of sea salt. The increase of the aerosol optical depth and of the particles density with radius about 1 μm has been found to be strictly related to the passage of Saharan dust in the time periods 14–22 May and 1–3 June, 1999. The columnar volume of particles obtained by the aureolemeter has been compared with the columnar volume of particles retrieved by in situ measurements carried out with a forward scattering spectrometer probe (FSSP) aboard a light aircraft flying over the island. Although the above two techniques refer to aerosol columns of different height and operate with different resolutions, their relevant results are in good agreement, especially during days with lower aerosol content. The two volume radius distributions have been also compared and their behaviours show a satisfactory agreement, mainly for particles with radius greater than 1 μm.

Measurements Of Fine-Scale Structure At The Top Of Marine Stratocumulus

During the Dynamics and Chemistry of the MarineStratocumulus (DYCOMS) experiment in July–August 1985, the NCAR Electra aircraft flew a series of flight legs just at the top of the marinestratocumulus cloud decks that cap the mixed layer off the coast of southernCalifornia. Because of the corrugated structure of the cloud-top, the aircraft, which was flown at a nearly constant level and adjusted only to maintain its altitude at the average cloud-top height, was alternately within and above the clouds – roughly half the time in each domain. These legs were used to examine the structure of the cloud-top by compositing the segments on either side of the cloud/clear-air interface, which was identified by the transitions of liquid water measured by the Forward Scattering Spectrometer Probe (either increasing or decreasing) through a threshold of 0.04 × 10-3 kg m-3.An equivalent vertical distance (EVD) from the cloud-top was obtained from the horizontal flight legs by estimating the average slope of the cloud-top from the cloud-top radiation temperature. The results show that a near discontinuity occurs in variables across cloud top over an EVD of 0.3 m, but that above this, the air has already been modified by boundary-layer air. Thus, cloud-top is not the limit of mixing of boundary-layer air. This mixing may extend to tens of metres or more. The bulk Richardson number in the vicinity of cloud-top increases from near zero within the cloud to about 1.2 at an EVD of 3–6 m above cloud. Fluctuations of the three velocity components within cloud are nearly equal; above cloud the vertical component structure function is about half the horizontal components. The scalar structure functions are about an order of magnitude higher above cloud than in cloud. The structure parameters of temperature and humidity measured just below cloud-top agree reasonably well with predicted values based on a previously-developed model for the clear convective boundary layer. Above cloud, the scalar structure parameters are much larger, but their interpretation is questionable, since this region does notcontain isotropic turbulence.

Microphysical and radiative properties of inhomogeneous stratocumulus: Observations and model simulations

Detailed microphysical observations made from aircraft during the Atlantic Stratocumulus Transition Experiment are used to examine the optical properties of marine stratocumulus. Special attention is given to the relationship between the droplet size distribution (droplet-number concentration, effective radius, and liquid-water content) and the optical extinction parameter. The extinction parameter calculated with measurements from the forward-scattering spectrometer probe (FSSP) differs only slightly from the extinction parameter obtained from the total droplet spectrum (measured with two optical probes, FSSP and 260X). In contrast, the results for the effective radius and the liquid-water content show differences up to 40% between the two spectra. A Monte Carlo radiative-transfer model is initialized with optical properties calculated from the observed microphysics. Monte Carlo simulations of radiative fluxes are in good agreement with observations for well-mixed boundary-layer cloud fields.

Microphysical and radiative properties of inhomogeneous stratocumulus: Observations and model simulations

AbstractDetailed microphysical observations made from aircraft during the Atlantic Stratocumulus Transition Experiment are used to examine the optical properties of marine stratocumulus. Special attention is given to the relationship between the droplet size distribution (droplet‐number concentration, effective radius, and liquid‐water content) and the optical extinction parameter. The extinction parameter calculated with measurements from the forward‐scattering spectrometer probe (FSSP) differs only slightly from the extinction parameter obtained from the total droplet spectrum (measured with two optical probes, FSSP and 260X). In contrast, the results for the effective radius and the liquid‐water content show differences up to 40% between the two spectra. A Monte Carlo radiative‐transfer model is initialized with optical properties calculated from the observed microphysics. Monte Carlo simulations of radiative fluxes are in good agreement with observations for well‐mixed boundary‐layer cloud fields.

ACE-2 HILLCLOUD. An overview of the ACE-2 ground-based cloud experiment

The ACE‐2 HILLCLOUD experiment was carried out on the island of Tenerife in June–July 1997 to investigate the interaction of the boundary layer aerosol with a hill cap cloud forming over a ridge to the north‐east of the island. The cloud was used as a natural flow through reactor to investigate the dependence of the cloud microphysics and chemistry on the characteristics of the aerosols and trace gases entering cloud, and to simultaneously study the influence of the physical and chemical processes occurring within the cloud on the size distribution, chemical and hygroscopic properties of the aerosol exiting cloud. 5 major ground base sites were used, measuring trace gases and aerosols upwind and downwind of the cloud, and cloud microphysics and chemistry and interstitial aerosol and gases within the cloud on the hill. 8 intensive measurement periods or runs were undertaken during cloud events, (nocturnally for seven of the eight runs) and were carried out in a wide range of airmass conditions from clean maritime to polluted continental. Polluted air was characterised by higher than average concentrations of ozone (>50 ppbv), fine and accumulation mode aerosols (>3000 and >1500 cm−3, respectively) and higher aerosol mass loadings. Cloud droplet number concentrations N, increased from 50 cm−3 in background maritime air to >2500 cm−3 in aged polluted continental air, a concentration much higher than had previously been detected. Surprisingly, N was seen to vary almost linearly with aerosol number across this range. The droplet aerosol analyser (DAA) measured higher droplet numbers than the corrected forward scattering spectrometer probe (FSSP) in the most polluted air, but at other times there was good agreement (FSSP=0.95 DAA with an r2=0.89 for N<1200 cm−3). Background ammonia gas concentrations were around 0.3 ppbv even in air originating over the ocean, another unexpected but important result for the region. NO2 was present in background concentrations of typically 15 pptv to 100 pptv and NO˙3 (the nitrate radical) was observed at night throughout. Calculations suggest NO˙3 losses were mainly by reaction with DMS to produce nitric acid. Low concentrations of SO2(∼30 pptv), HNO3 and HCl were always present. HNO3 concentrations were higher in polluted episodes and calculations implied that these exceeded those which could be accounted for by NO2 oxidation. It is presumed that nitric and hydrochloric acids were present as a result of outgassing from aerosol, the HNO3 from nitrate rich aerosol transported into the region from upwind of Tenerife, and HCl from sea salt aerosol newly formed at the sea surface. The oxidants hydrogen peroxide and ozone were abundant (i.e., were well in excess over SO2 throughout the experiment). Occasions of significant aerosol growth following cloud processing were observed, particularly in cleaner cases. Observations and modelling suggested this was due mainly to the take up of nitric acid, hydrochloric acid and ammonia by the smallest activated aerosol particles. On a few occasions a small contribution was made by the in‐cloud oxidation of S(IV). The implications of these results from HILLCLOUD for the climatologically more important stratocumulus Marine Boundary Layer (MBL) clouds are considered.

CUT SIZE MINIMIZATION AND CLOUD ELEMENT BREAK-UP IN A GROUND-BASED CVI

A ground-based Counterflow Virtual Impactor (CVI) was optimized to achieve nearly complete in situ segregation of cloud droplets and ice crystals (with subsequent evaporation, releasing dissolved gaseous and non-volatile material) from their surrounding carrier gas and interstitial aerosol particles. With a one-dimensional numerical model, the CVI cut size D 50 was reduced to 4 μm from 7 μm in an earlier design (Anderson et al., 1993). This could be achieved by a velocity increase to 225 m s −1 inside the wind tunnel forming part of the ground-based CVI, and by minimizing all dimensions contributing to the stagnation length L stag (distance from the wind intersection plane tunnel/CVI to the stagnation plane inside the CVI that cloud elements have to reach to be sampled). CVI and high-speed wind tunnel were designed and constructed according to the modeling results. Subsequent calibrations verified the calculated lower cut sizes D 50 and quantified the slope of the collection efficiency curve in terms of cut sharpness S cut. With the new CVI lower cut sizes between 4 and 6 μm can be achieved. A cloud chamber experiment was performed with CVI measurements supplemented by a Forward Scattering Spectrometer Probe (FSSP). It could be demonstrated that significant drop break-up is caused by wind tunnel velocities well beyond 150 m s −1. For a reduced wind tunnel velocity of 150 m s −1 a reasonable cut size of at least 5 μm could be maintained, while avoiding break-up. The demonstration of break-up should have consequences for any cloud sampling technique featuring high relative velocities of cloudy air past the inlet. In particular, in-cloud retrieval of cloud nuclei concentrations on high-speed airborne platforms could be affected to a significant extent.

Remote Sensing of Cirrus Radiative Parameters during EUCREX’94. Case Study of 17 April 1994. Part I: Observations

Abstract During the intensive European Cloud and Radiation Experiment 1994 (EUCREX’94) conducted off the coast of Brittany (France) over the Atlantic Ocean during April 1994, natural cirrus have been analyzed from in situ and remote sensing measurements. The authors have particularly studied the case of 17 April 1994. For this day a cirrus bank is described by a complete dataset, that is, classic airborne thermodynamical measurements, microphysical (forward scattering spectrometer probe) and OAP-2D2-C (optical array probe-cloud) probes manufactured by Particle Measuring System, and radiative (Barnes Precision Radiation Thermometer, Eppley pyranometers, and upward- and downward-looking pyrgeometers) measurements above and below the cloud. More specific airborne instruments were used such as upward backscatter lidar with polarization capabilities (LEANDRE) on board the Avion de Recherches Atmospheriques et Teledetection and the Polarization and Directionality of the Earth’s Reflectances (POLDER) radiometer ...

Horizontal structure of marine boundary layer clouds from centimeter to kilometer scales

Horizontal transects of cloud liquid water content (LWC) measured at unprecedented 4‐cm resolution are statistically analyzed scale‐by‐scale. The data were collected with a Particulate Volume Monitor (PVM) probe during the winter Southern Ocean Cloud Experiment (SOCEX) on July 26, 1993, in a broken‐stratocumulus/towering‐cumulus cloud complex. Two scaling regimes are found in the sense that two distinct power laws, k−β are needed to represent the wavenumber spectrum E(k) over the full range of scales r ≈ 1/k. Detailed numerical simulations show that the scale break at 2–5 m is not traceable to the normal variability of LWC in the PVM's instantaneous sampling volume (1.25 cm3) driven by Poissonian fluctuations of droplet number and size. The two regimes therefore differ physically. The non‐Poissonian character of the small‐scale LWC variability is consistent with a similar finding by Baker [1992] for droplet number concentration obtained from Forward Scattering Spectrometer Probe (FSSP) data: at scales of a few centimeters, spatial droplet distributions do not always follow a uniform Poisson law. With β = 0.9 ± 0.1, the small‐scale (8–12 cm ≲ r ≲ 2–5 m) regime is stationary: jumps in LWC are highly variable in size and rapidly cancel each other, leading to short‐range correlations. By contrast, the large‐scale (5 m ≲ r ≲ 2 km) variability with β = 1.6 ± 0.1 is nonstationary: jumps are generally quite small, conveying a degree of pixel‐to‐pixel continuity and thus building up long‐range correlations in the low‐pass filtered signal. The large‐scale structure of the complex SOCEX cloud system proves to be multifractal, meaning that large jumps do occur on an intermittent basis, that is, on a sparse fractal subset of space. Low‐order, hence more robust, multifractal properties of the SOCEX clouds are remarkably similar to those of their First ISCCP Regional Experiment (FIRE) and Atlantic Stratocumulus Transition EXperiment (ASTEX) counterparts, and also to those of passive scalars in fully developed turbulence. This is indicative of a remarkable similarity in the micro‐physical and macrophysical processes that determine cloud structure in the marine boundary layer at very remote locales, especially since the particular SOCEX cloud system investigated here was rather atypical. Interesting differences are also found: in the scaling ranges on the one hand, and in higher‐order moments on the other hand. Finally, we discuss cloud‐radiative effects of the large‐ and small‐scale variabilities.

In situ measurements of effective diameter and effective droplet number concentration

The effective diameter of cloud droplets is usually derived from measurements of droplet size distribution measured by Particle Measuring Systems (PMS) probes. The disadvantage of this method is that PMS probes have a truncated size range. During the RACE project, an alternative method to measure the effective diameter used a cloud extinction meter and King and Nevzorov hot wire liquid water content and total water content (LWC/TWC) probes installed on the National Research Council (NRC) Twin Otter. The effective diameter was derived from direct in situ measurements of the extinction coefficient (ε) and liquid water content (W) as Deff=k1W/ε. This method of calculation of Deff is free of problems related to deriving Deff from the truncated particle size distribution. Since measurements of ε and W cover the whole size range of cloud particles, this method gives an accurate value of Deff. This method can also be successfully applied for mixed and ice phase clouds, since the Nevzorov TWC probe provides measurements of total (ice plus liquid) water content. Effective number concentration was calculated as Neff = k2ϵ3/W2. Comparisons of Deff and Neff, calculated by this method, and directly from PMS Forward Scattering Spectrometer Probe (FSSP) spectra, are favorable in the subset of conditions when the FSSP is considered to measure the spectra fully and accurately.

Validation of droplet spectra and liquid water content measurements

Abstract Various airborne devices have been recently developed to improve measurements of cloud microphysics. Data from the Small Cumulus Microphysics Study (SCMS) provide a unique data set for evaluation of the probe performances. In this paper we present an analysis of data collected during two SCMS flights where five instruments were tested: the Forward Scattering Spectrometer Probe (FSSP), the Fast-FSSP, the Cloud Drop Spectrometer (CDS), the Particulate Volume Monitor (PVM) and the CSIRO hot wire probe.

Parameterizations of the microphysical and shortwave radiative properties of boundary layer stratus from ground‐based measurements

In earlier work a δ2‐stream radiative transfer model was used in conjunction with ground‐based measurements to retrieve the rnicrophysical and shortwave radiative properties of boundary layer stratus. The effective radii output by the retrieval scheme for optically thick clouds have now been parameterized as a function of the cloud liquid water path, the transmission ratio, and the cosine of the solar zenith angle. The parameterization enables estimation of stratus cloud microphysical properties using ground‐based measurements that are readily available at a number of locations. To validate the retrieved and parameterized cloud microphysics, ∼5 hours of data were analyzed and compared to collocated in situ measurements made by a Forward Scattering Spectrometer Probe. On average, the retrieved values of the cloud droplet effective radius and the cloud droplet number concentration differed from the corresponding aircraft measurements by 7% and 27%, respectively, while the parameterized values differed from the aircraft measurements by 15% and 32%, respectively. Averaging all of the data to 30‐min resolution significantly reduced the differences between the aircraft data and the retrieved and parameterized results, suggesting that both the aircraft and the ground‐based data are capable of characterizing the cloud microphysics over this temporal scale. The parameterizations of the stratus shortwave radiative properties were generally within 5% of Slingo's four‐band, model‐derived parameterization when absorption above cloud top was incorporated into the Slingo parameterization.

The Nevzorov Airborne Hot-Wire LWC–TWC Probe: Principle of Operation and Performance Characteristics

Abstract The Nevzorov liquid water content (LWC) and total water content (TWC) probe is a constant-temperature, hot-wire probe designed for aircraft measurements of the ice and liquid water content of clouds. The probe consists of two separate sensors for measurements of cloud liquid and total (ice plus liquid) water content. Each sensor consists of a collector and a reference winding. The reference sensors are shielded from impact with cloud particles, specifically to provide an automatic compensation for convective heat losses. This results in a potentially improved sensitivity over uncompensated probes such as the King LWC probe. The Nevzorov probe has been used in four Canadian field experiments on the National Research Council (NRC) Convair580 since 1994. Intercomparison of Nevzorov LWC, TWC, King, and two PMS Forward Scattering Spectrometer Probes show good agreement in liquid clouds, although the Nevzorov probe displays distinct advantages in low-LWC situations due to a more stable baseline. The se...

Improvements of Droplet Size Distribution Measurements with the Fast-FSSP (Forward Scattering Spectrometer Probe)

Abstract The basics of single particle measurements are discussed and illustrated with measurements of the droplet size distribution with an optical spectrometer, the Forward Scattering Spectrometer Probe (FSSP), and its improved version, the Fast-FSSP. The various sources of uncertainties are successively analyzed: the statistical significance of an incomplete sampling of the particle population, counting losses due to coincidence of particles and electronic dead time of the counter, artificial broadening of the size distributions by the coincidences, inhomogeneities of the sensitive volume of the probe, ambiguities of the Mie scattering curve, errors on the sampled volume, and, finally, the uncertainties on the size calibration of the instrument. It is demonstrated with examples of data collected with the Fast-FSSP that additional parameters, such as the pulse duration and the interarrival times between detections, are crucial for improving the sizing of the particles and the retrieval of the spatial ev...

A Quantitative Comparison of Ground-Based FSSP and PVM Measurements

Abstract Cloud microphysical measurements of two ground-based field campaigns are analyzed in order to compare Particle Volume Monitor (PVM) measurements and Forward Scattering Spectrometer Probe (FSSP)-derived integrated quantities (mainly liquid water content LWC) for different types of drop size distributions. It is shown that in case of size distributions containing mostly small drops with diameters D 25 μm) are present in the drop size distribution, which considerably increases LWC, discrepancies between PVM and FSSP measurements are established. The most likely explanation for the differences is that the sensitivity of...

A comparison of optical measurements of liquid water content and drop size distribution in adiabatic regions of Florida cumuli

Airborne measurements of liquid water content (LWC) and drop size distribution were made in adiabatic regions of small, growing cumulus clouds during the Small Cumulus Microphysics Study (SCMS). A new instrument, the cloud drop spectrometer (CDS), which measures LWC and also drop size from an ensemble of drops, was flown for the first time in the field. Measurements from other sensors, including a Particle Measuring Systems (PMS) forward scattering spectrometer probe (FSSP), the `fast' FSSP (FFSSP) developed by the Centre National De Recherches Meteorologiques (CNRM), and a Gerber Scientific airborne particulate volume monitor (PVM-100A), are compared with the CDS data collected in adiabatic and other regions. The CDS appeared to reliably measure very close to the predicted value of LWC in regions identified as being adiabatic. In addition, the drop size distribution measured by the CDS compared very well with the FSSP and FFSSP measurements, except where the 3–200 μm range of the CDS allowed it to measure larger drops than the nominal 3–45 μm range of the FSSP, and the 2.7 to 38.4 μm range of the FFSSP.