Small Cumulus Microphysics Study Research Articles

Large Eddy Simulation of Microphysics and Influencing Factors in Shallow Convective Clouds

A flight of shallow convective clouds during the SCMS95 (Small Cumulus Microphysics Study 1995) observation project is simulated by the large eddy simulation (LES) version of the Weather Research and Forecasting Model (WRF-LES) with spectral bin microphysics (SBM). This study focuses on relative dispersion of cloud droplet size distributions, since its influencing factors are still unclear. After validation of the simulation by aircraft observations, the factors affecting relative dispersion are analyzed. It is found that the relationships between relative dispersion and vertical velocity, and between relative dispersion and adiabatic fraction are both negative. Furthermore, the negative relationships are relatively weak near the cloud base, strengthen with the increasing height first and then weaken again, which is related to the interplays among activation, condensation and evaporation for different vertical velocity and entrainment conditions. The results will be helpful to improve parameterizations related to relative dispersion (e.g., autoconversion and effective radius) in large-scale models.

The onset of precipitation in warm cumulus clouds: An observational case‐study

AbstractData collected with a ground radar and an instrumented aircraft during the Small Cumulus Microphysics Study (SCMS) are analysed to examine the physical processes that control the onset of precipitation. On 10 August 1995, the Météo‐France Merlin‐IV successively sampled the core of the top cell in three convective clouds, following the cells from their initial stage of young vigorous ascending turrets up to their decaying stage. Simultaneous range–height indicator (RHI) scans were also performed by the National Center for Atmospheric Research CP2 radar.Although the three clouds showed similar values of the droplet number concentrations and were able to generate precipitation embryos, the first two collapsed after reaching an altitude of 3 km above sea level, without producing any precipitation, while the third one reached a higher level of 4 km and produced significant precipitation. This case‐study illustrates how sensitive the onset of precipitation is to cloud dynamics, revealing that in a conditionally unstable atmosphere hardly noticeable changes in cloud thermodynamics and microphysics may lead to major changes in cloud vertical development and precipitation. Copyright © 2010 Royal Meteorological Society

Another Look at Stochastic Condensation for Subgrid Cloud Modeling: Adiabatic Evolution and Effects

Abstract The theory of stochastic condensation, which models the impact of an ensemble of unresolved supersaturation fluctuations S′ on the volume-averaged droplet-size distribution f (r), is revisited in the modern context of subgrid cloud parameterization. The exact transition probability density for droplet radius driven by independent, Gaussian S′ fluctuations that are periodically renewed is derived and shown to be continuous but not smooth. The Fokker–Planck model follows naturally as the smooth-in-time approximation to this discrete-in-time process. Evolution equations for the moments of f (r) that include a contribution from subgrid S′ fluctuations are presented; these new terms are easily implemented in moment-based cloud schemes that resolve supersaturation. New, self-consistent expressions for the evolution of f (r) and mean supersaturation S in a closed, adiabatic volume are derived without approximation; quite appropriately, these coupled equations exactly conserve total water mass. The behavior of this adiabatic system, which serves as a surrogate for a closed model grid column, is analyzed in detail. In particular, a new nondimensional number is derived that determines the relative impact of S′ fluctuations on droplet spectral evolution, and the contribution of fluctuations to S is shown to be negative definite and maximal near the accommodation length and has a direct correspondence to the analysis of Cooper. Observational support for the theory of stochastic condensation is found in cloud droplet spectra from cumulus cloud fields measured during the Rain in the Cumulus over the Ocean (RICO) and Small Cumulus Microphysics Study (SCMS) campaigns. Increasing spectral broadening with increasing spatial scale is discovered and compares well with theoretical predictions. However, the observed spectra show evidence of non-Gaussian S′ fluctuations and inhomogeneous mixing, processes neglected in the current theory.

Radar Analysis of Precipitation Initiation in Maritime versus Continental Clouds near the Florida Coast: Inferences Concerning the Role of CCN and Giant Nuclei

Abstract A method of analyzing radar data is developed and applied to determine whether the X-band radar reflectivity evolution of clouds observed during summertime on the northeast Florida coast during the Small Cumulus Microphysics Study (SCMS) shows distinct differences in precipitation development that can be associated with the clouds’ maritime or continental characteristics. For this study, the entire National Center for Atmospheric Research CP2 radar dataset from SCMS was examined, and 38 clouds were used. For these clouds the evolution in X-band radar reflectivity, from the clouds’ earliest detection through precipitation, was clearly documented and met specific requirements concerning the clouds’ location relative to the coastline and direction of movement. Since cloud condensation nuclei (CCN) and giant and ultragiant nuclei (GN) measurements were not available for the specific clouds used in this study, proxies were used to partition the clouds into four groups based on the cloud location and direction of movement. Specifically, it was assumed that clouds forming over the ocean during onshore flow had maritime characteristics (group 1: low CCN, high GN), clouds forming over land during onshore flow would have modified maritime characteristics (group 2: high CCN, high GN), clouds forming over land during offshore flow would have continental characteristics (group 3: high CCN, low GN), and clouds forming over the ocean during offshore flow would have modified continental characteristics (group 4: high CCN, high GN). These assumptions are based on past measurements presented by Sax and Hudson. Then, these populations were statistically compared using the nonparametric multiresponse permutation procedure developed by Mielke et al. A comparison of groups 1 and 2 provided a test of the role of CCN concentrations in precipitation development in these cloud populations. A comparison of groups 3 and 4 provided a test of the role of GN concentrations in precipitation development in these cloud populations. The two cloud populations that were disjoint at a statistically significant level were groups 1 and 2. For these groups, the analysis showed that the median characteristic total water content of the truly maritime clouds (group 1) was about half that of the modified maritime clouds (group 2) at the time of precipitation formation. The characteristic time to precipitation formation was about 60% smaller for the truly maritime clouds. Thus, the characteristic reflectivity threshold for precipitation development was reached at a much lower altitude above cloud base in a much faster time in the truly maritime clouds. This result supports the conclusions of Hudson and Yum that precipitation development in the SCMS clouds was primarily controlled by CCN concentrations rather than GN concentrations.

Humidity Halos Surrounding Small Cumulus Clouds in a Tropical Environment

Abstract A large dataset of aircraft cloud traverses from the Small Cumulus Microphysics Study (SCMS) was used to add to the existing knowledge of humidity halo characteristics for small cumulus clouds in a tropical environment. The findings from this investigation show a larger frequency of observed humidity halos than earlier studies. Regardless of the radial direction with respect to shear, humidity halos were observed with a frequency of 77%–90%. The difference in frequency of halo occurrences between upshear and downshear regions was much smaller than previously reported observations. These findings likely resulted from the absence of a strong vertical wind shear environment. SCMS cumuli had a mean cloud diameter (i.e., in-cloud traverse distance) of 1.1 km and mean halo lengths of about 0.6, 0.7, and 0.8 cloud radii for upshear, cross-shear, and downshear regions, respectively. Humidity halos of less than one cloud radius were observed during about 70% of SCMS aircraft traverses. Approximately 98% of humidity halos had radial lengths of less than four cloud radii. Although considerable differences were not observed between upshear and downshear halo lengths for clouds of similar age, large increases in the frequency and length of halos occurred with an increase in cloud age.

The Role of Giant and Ultragiant Nuclei in the Formation of Early Radar Echoes in Warm Cumulus Clouds

Abstract Observations of the formation of the first radar echoes in small cumulus clouds are compared with results of a stochastic coalescence model run in the framework of a closed parcel. The observations were made with an instrumented aircraft and a high-powered dual-wavelength radar during the Small Cumulus Microphysics Study (SCMS) in Florida. The principal conclusion is that coalescence growth on giant and ultragiant nuclei may be sufficient to explain observations. The concentration of cloud droplets varied from under 300 cm−3 when surface winds were from the ocean, to over 1000 cm−3 when the wind direction was from the mainland. Although there is a slight tendency for the altitude of the first 0-dBZ echo to be lower on average in maritime than in continental clouds there were several cases where it was higher. The model results suggest that the lack of correlation is consistent with drops forming on giant and ultragiant nuclei. The first 0-dBZ echo was observed to form at higher altitudes in cloud...

Size Distributions and Dynamical Properties of Shallow Cumulus Clouds from Aircraft Observations and Satellite Data

Abstract In this paper aircraft observations of shallow cumulus over Florida during the Small Cumulus Microphysics Study (SCMS) are analyzed. Size distributions of cloud fraction, mass flux, and in-cloud buoyancy flux are derived. These distributions provide information on the specific contribution of clouds with a certain horizontal size and reveal, for example, which size has the largest effect on cloud fraction or vertical transport. The analysis of four flights shows that the mass flux and buoyancy flux are dominated by intermediate-sized clouds (horizontal dimension of about 1 km). The cloud fraction, on the other hand, is found to be dominated by the smallest clouds observed. These clouds are additionally found to have a negative contribution to the mass flux, yet a positive contribution to the buoyancy flux. About 200 flight intersections of cumuli with horizontal sizes larger than 500 m are used to obtain average horizontal cross-section profiles of vertical velocity, liquid water content, liquid ...

Shallow cumulus convection: A validation of large‐eddy simulation against aircraft and Landsat observations

AbstractLarge‐eddy simulation (LES) results of shallow cumulus convection are directly evaluated against in‐cloud aircraft measurements, as made during the Small Cumulus Microphysics Study (SCMS). To this purpose an LES case is first constructed, based on available observations (during SCMS). Then the simulations are directly compared with the in‐cloud measurements by using conditionally sampled fields. An advantage of the SCMS dataset is the combination of a range of different surface measurements, in‐cloud measurements by an aircraft at many levels in the cloud layer, and the availability of high‐resolution Landsat images.The results show that given the correct initial and boundary conditions the LES concept is capable of realistically predicting the bulk thermodynamic properties of temperature, moisture and liquid‐water content of the cumulus‐cloud ensemble as observed in SCMS. Furthermore, the vertical component of the in‐cloud turbulent kinetic energy and the cloud size distribution in LES were in agreement with the observations. These results support the credibility of cloud statistics as produced by LES in general, and encourage its use as a tool for testing hypotheses and developing parametrizations of shallow cumulus cloud processes.Several hypotheses which make use of conditionally sampled fields were tested on the SCMS data. The magnitudes and the decrease with height of the bulk entrainment rate following from the SCMS data confirm the typical values of the order of magnitude of 10−3 m−1 as reported in several other recent LES studies. An alternative formulation of the lateral entrainment rate as a function of the liquid‐water content and the mean lapse rate agrees well with the original form based on the conserved variables. Applying an often‐used simplified equation for the cloud vertical velocity to the aircraft measurements results in a reasonably closed budget. Copyright © 2003 Royal Meteorological Society

Comparison between Standard and Modified Forward Scattering Spectrometer Probes during the Small Cumulus Microphysics Study

Abstract Microphysical measurements performed during the Small Cumulus Microphysics Study (SCMS) experiment are analyzed in order to examine the instrumental limitations of forward scattering spectrometer probes (FSSPs). Complementary information collected with a modified version of the instrument, the Fast-FSSP, are used to address crucial issues such as the size calibration of the spectrometers and the effects on the measured spectra (distortion and broadening), of beam inhomogeneities, of variations of the sampling section, and of the coincidence of particles. Their impact on the calculation of liquid water content is evaluated by the comparison with measurements performed with a hot-wire probe and a particle volume monitor. In addition to the statistical approach that aims at evaluating the typical uncertainty of the measurements, special attention is given to the identification of circumstances under which some of the instrumental limitations combined are likely to affect significantly the accuracy o...

Measurements of Ultragiant Aerosol Particles in the Atmosphere from the Small Cumulus Microphysics Study

Abstract Ultragiant aerosol particles (UGA) are potentially important for warm rain formation because of their ability to initiate coalescence immediately upon entering a cloud, so it is desirable to obtain local estimates during any field campaign that studies warm rain. Estimates of UGA in clear air from a one-dimensional optical array probe averaged over long time periods from the Small Cumulus Microphysics Study have been published in the literature, but further analysis and comparisons to other probes, presented here, show that the data on which these estimates were based were probably contaminated by noise. A possible explanation for the noise in the probe is given, as are new upper limits, based on few or no particles detected by a two-dimensional optical array probe.

Early Radar Echoes from Ultragiant Aerosol in a Cumulus Congestus: Modeling and Observations

Abstract The growth of ultragiant aerosol (UGA) in a Lagrangian framework within a simulated three-dimensional cloud is analyzed and compared with radar and aircraft observations of a cumulus congestus collected during the Small Cumulus Microphysics Study (SCMS). UGA are ingested into the simulated cloud and grow by continuous collection; the resulting radar reflectivity factor and raindrop concentrations are evaluated at 1-min intervals. The calculations produce a substantial echo (>30 dBZ) within a short time (18 min), containing few raindrops (0.3 L−1). The calculated radar echo is very sensitive to the amount of UGA ingested into the modeled cloud and its liquid water content. The modeled radar echo and raindrop concentrations are consistent with the observations in that the differences fall within the modeling and measurement limitations and uncertainties.

Spectral Density of Cloud Liquid Water Content at High Frequencies

Abstract Aircraft measurements of liquid water content (LWC) made at sampling frequencies of 1 and 2 kHz with a particle volume monitor (PVM) probe from horizontal traverses in stratocumulus clouds during the Southern Ocean Cloud Experiment and cumulus clouds during the Small Cumulus Microphysics Study are described. The spectral density of the LWC measurements is calculated and compared to the −5/3 scaling law. The effect of PVM sampling noise is found to be small in most cases. Most measurements follow approximately the −5/3 law until cloud scales decrease below about 5 m in length. Below this length LWC variance can exceed that predicted by the −5/3 law. It is suggested that the enhanced LWC variance at small scales is related to entrainment of environmental air into the clouds, which changes primarily the droplet concentration.

Droplet Spectra Broadening in Cumulus Clouds. Part I: Broadening in Adiabatic Cores

Measurements of cloud droplet spectra performed with the Fast-Forward Scattering Spectrometer Probe during the Small Cumulus Microphysics Study (1995) are analyzed. Fifty cloud samples with narrow droplet spectra are selected. They are characterized by values of liquid water content slightly below the adiabatic value. Each observed spectrum is then compared to a narrow adiabatic spectrum predicted at the same level with the current theory of condensational growth in an adiabatic cloud cell, initialized with a reference spectrum measured right above the activation level, at cloud base. Broadening is characterized for each observed spectrum by the probability density function of condensational growth expressed as the Lagrangian integral of the ratio of supersaturation to vertical velocity, along the droplet trajectories. In particular it appears that the derived density functions show high probabilities of very low and very large values of condensational growth. The large values are related to a high relative density of big droplets in the measured spectra, higher than predicted by the adiabatic model. The contribution of the instrument to this feature is examined with a model of probe functioning. The simulations suggest that most of those big droplets are instrumental artifacts. The remaining broadening is parameterized by a linear relationship between the mean value and the standard deviation of the density function of condensational growth. This result will be used to examine the respective contributions to spectra broadening of microscale heterogeneities of the droplet concentration, in Part II, and of the mixing processes, in Part III of this series.

Initial Precipitation Formation in Warm Florida Cumulus

The microphysical processes that lead to the development of precipitation in small, warm cumulus are examined using data from the Small Cumulus Microphysics Study near Cape Canaveral, Florida. Aircraft measurements are used to determine the concentration and size distribution of giant and ultragiant nuclei in clear air as a function of relative humidity, altitude, wind speed, and wind direction. The clear-air particle distributions show that ultragiant particles (radii extending from 10 to 150 mm) exist from the surface to cloud base in concentrations that correspond to the concentrations of raindrops observed during drizzle to moderate rainfall events. A shift of the spectra toward larger size with increasing relative humidity was observed, suggesting that the spectra are composed of deliquesced particles growing by condensation. The small cumulus clouds are shown to contain cores where the observed liquid water content was nearly adiabatic. The observed evolution of the cloud droplet distribution within the near-adiabatic cores as a function of height showed an increase in the small droplet mode associated with condensation and an increase in the concentration of larger droplets associated with growth by accretion. Droplets with radii extending to nearly 100 mm were present just above cloud base. These measurements were consistent with the clear-air measurements and provided evidence that the ultragiant nuclei can immediately act as embryos for raindrop growth by accretion upon entering cloud base. Comparisons of reflectivity computed from the cumulus core composite droplet distributions with the radar-observed reflectivity data provided independent evidence that the composite spectra reasonably represented the evolving microstructure of the cores of small cumulus clouds as they grew vertically. The analyses provide strong evidence of an efficient process for the initial development of precipitation in small Florida cumuli. This process consists of raindrop embryo formation on ultragiant nuclei followed by growth by accretion as the newly formed drops proceed upward through the adiabatic cores of the cumulus clouds. These data support the conceptual model of raindrop formation in marine clouds first proposed by Woodcock a half century ago.

Observations of microphysics pertaining to the development of drizzle in warm, shallow cumulus clouds

AbstractThe evolution of drizzle in small cumulus clouds in a subtropical environment was studied. Observations were made using three instrumented aircraft, a ground‐based radar, and a 95 GHz airborne Doppler radar during the Small Cumulus Microphysics Study in east central Florida in summer 1995. Data from six clouds, from two days, are examined in this paper. Both sets of clouds were less than 2 km in depth at observation times; in‐cloud temperatures were > 10 °C.Droplet spectra observed in these clouds were generally bi‐modal. The sizes of the droplets within the large mode are consistent with growth through condensation. It remains unclear how droplets within the small mode formed, but their existence appears to be tied to entrapment and mixing.Drizzle drops (drops with diameters > 50 μm) were found at all levels in the clouds. The penetration‐averaged concentrations of drizzle drops were less than 15 L−1, except in the uppermost regions of clouds on one of the days where drizzle drop concentrations exceeded 100L−1. In general, the presence of drizzle drops in the upper regions of the clouds is consistent with models of droplet growth through condensation and stochastic collection. The existence of drizzle at low and mid‐levels may be due to collection initiated by ultra‐giant aerosols, or to the redistribution of drizzle drops from neighbouring or earlier cloud elements.

Evolution of small cumulus clouds in Florida: observations of pulsating growth

Observations have been made in six small cumulus clouds using instrumented aircraft, a ground-based radar, and a 95 GHz airborne Doppler radar. The clouds occurred on two days during the Small Cumulus Microphysics Study in east-central Florida, summer 1995. Cloud tops were below 3 km and in-cloud temperatures were warmer than 10°C. Maximum observed reflectivity factors were less than 0 dBZ. The evolution of the kinematics of the observed clouds was tracked using measurements from both radars. High-resolution cross-sections of reflectivity and vertical Doppler velocity from the airborne radar appear remarkably similar to fine-scale models of convection reported in the literature. In general, each cloud resembled a collection of individual bubbles ascending through the boundary layer. During the growth phase of a bubble, a positive correlation existed between vertical velocity and reflectivity. As bubbles penetrated further into the inversion, entrainment/detrainment led to a weakening or, in some cases, a reversal of this correlation. Growth of subsequent bubbles ascending through remnants of earlier bubbles were aided by an increase in the amount of moisture in the environment resulting from earlier detrainment of cloudy air, and thus were able to achieve higher altitudes than their predecessors.

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.

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

A recent paper by Lawson and Blyth (1998) [Lawson, R.P., Blyth, A.M., 1998. A comparison of optical measurements of liquid water content and drop size distribution in adiabatic regions of Florida cumuli. Atmos. Res. 47–48, 671–690.] is critiqued for its conclusions dealing with the measurement with several different aircraft microphysics probes of liquid water content (LWC) in `adiabatic cores' of isolated and relatively small cumulus clouds found during the 1995 Small Cumulus Microphysics Study (SCMS) experiment in Florida. The criteria used in that paper to identify the cores were found to be unreliable, leading to identifications of adiabatic cloud cores that contained sub-adiabatic LWC. The analysis in the present paper of 1000-Hz particulate volume monitor (PVM) LWC data for SCMS Cu leads to the following conclusions: (1) A new probe, the cloud droplet spectrometer (CDS), located on the same aircraft and operated at 1 Hz, overestimates maximum LWC in the Cu, and the claim of Lawson and Blyth (1998) that the 1-Hz CDS data show often precisely LWC equivalent to the predicted adiabatic LWC in those Cu is not supportable. (2) The high-frequency PVM data show adiabatic LWC in aircraft passes close to cloud base, and maximum LWC that is slightly sub-adiabatic in small cloud parcels in passes through the active updraft regions of taller Cu. (3) The taller Cu show internally sharp LWC gradients that are interpreted as nonuniform local mixtures of LWC and interfaces resulting from the entrainment process. (4) The understanding of the observed high-resolution LWC structure requires a better understanding of Cu dynamics associated with cloud growth, and entrainment, and mixing processes.

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.