Cloud Morphology Research Articles

A large active wave trapped in Jupiter’s equator

Context. A peculiar atmospheric feature was observed in the equatorial zone (EZ) of Jupiter between September and December 2012 in ground-based and Hubble Space Telescope (HST) images. This feature consisted of two low albedo Y-shaped cloud structures (Y1 and Y2) oriented along the equator and centred on it (latitude 0.5°−1°N).Aims. We wanted to characterize these features, and also tried to find out their properties and understand their nature.Methods. We tracked these features to obtain their velocity and analyse their cloud morphology and the interaction with their surroundings. We present numerical simulations of the phenomenon based on one- and two-layer shallow water models under a Gaussian pulse excitation.Results. Each Y feature had a characteristic zonal length of ~15° (18 000 km) and a meridional width (distance between the north-south extremes of the Y) of 5° (6000 km), and moved eastward with a speed of around 20−40 m s-1 relative to Jupiter’s mean flow. Their lifetime was 90 and 60 days for Y1 and Y2, respectively. In November, both Y1 and Y2 exhibited outbursts of rapidly evolving bright spots emerging from the Y vertex. The Y features were not visible at wavelengths of 255 or 890 nm, which suggests that they were vertically shallow and placed in altitude between the upper equatorial hazes and the main cloud deck. Numerical simulations of the dynamics of the Jovian equatorial region generate Kelvin and Rossby waves, which are similar to those in the Matsuno-Gill model for Earth’s equatorial dynamics, and reproduce the observed cloud morphology and the main properties the main properties of the Y features.

Improved automatic estimation of winds at the cloud top of Venus using superposition of cross-correlation surfaces

Accurate wind observation is a key to study atmospheric dynamics. A new automated cloud tracking method for the dayside of Venus is proposed and evaluated by using the ultraviolet images obtained by the Venus Monitoring Camera onboard the Venus Express orbiter. It uses multiple images obtained successively over a few hours. Cross-correlations are computed from the pair combinations of the images and are superposed to identify cloud advection. It is shown that the superposition improves the accuracy of velocity estimation and significantly reduces false pattern matches that cause large errors. Two methods to evaluate the accuracy of each of the obtained cloud motion vectors are proposed. One relies on the confidence bounds of cross-correlation with consideration of anisotropic cloud morphology. The other relies on the comparison of two independent estimations obtained by separating the successive images into two groups. The two evaluations can be combined to screen the results. It is shown that the accuracy of the screened vectors are very high to the equatorward of 30 degree, while it is relatively low at higher latitudes. Analysis of them supports the previously reported existence of day-to-day large-scale variability at the cloud deck of Venus, and it further suggests smaller-scale features. The product of this study is expected to advance the dynamics of venusian atmosphere.

Gravitational fragmentation caught in the act: the filamentary Musca molecular cloud

Filamentary structures are common in molecular clouds. Explaining how they fragment to dense cores is a missing step in understanding their role in star formation. We perform a case study of whether low-mass filaments are close-to hydrostatic prior to their fragmentation, and whether their fragmentation agrees with gravitational fragmentation models. For this, we study the 6.5 pc long Musca molecular cloud that is an ideal candidate for a filament at an early stage of fragmentation. We employ dust extinction mapping in conjunction with near-infrared data from the NEWFIRM instrument, and 870 um dust continuum emission data from the LABOCA instrument, to estimate column densities. We use the data to identify fragments from the cloud and to determine the radial density distribution of its filamentary part. We compare the cloud's morphology with 13CO and C18O line emission observed with the APEX/SHeFI instrument. The Musca cloud is pronouncedly fragmented at its ends, but harbours a remarkably well-defined, 1.6 pc long filament in its Center region. The line mass of the filament is 21-31 Ms pc^-1 and FWHM 0.07 pc. Its radial profile can be fitted with a Plummer profile that has the power-index of 2.6 \pm 11%, flatter than that of an infinite hydrostatic filament. The profile can also be fitted with a hydrostatic cylinder truncated by external pressure. These models imply a central density of 5-10 x 10^4 cm^-3. The fragments in the cloud have a mean separation of 0.4 pc, in agreement with gravitational fragmentation. These properties, together with the subsonic and velocity-coherent nature of the cloud, suggest a scenario in which an initially hydrostatic cloud is currently gravitationally fragmenting. The fragmentation has started a few tenths of a Myr ago from the cloud ends, leaving its center yet relatively non-fragmented, possibly because of gravitational focusing in a finite geometry.

A SYSTEMATIC SURVEY FOR BROADENED CO EMISSION TOWARD GALACTIC SUPERNOVA REMNANTS

ABSTRACT We present molecular spectroscopy toward 50 Galactic supernova remnants (SNRs) taken at millimeter wavelengths in 12CO J = 2 − 1. These observations are part of a systematic survey for broad molecular line (BML) regions indicative of interactions with molecular clouds (MCs). We detected BML regions toward 19 SNRs, including 9 newly identified BML regions associated with SNRs (G08.3–0.0, G09.9–0.8, G11.2–0.3, G12.2+0.3, G18.6–0.2, G23.6+0.3, 4C–04.71, G29.6+0.1, and G32.4+0.1). The remaining 10 SNRs with BML regions confirm previous evidence for MC interaction in most cases (G16.7+0.1, Kes 75, 3C 391, Kes 79, 3C 396, 3C 397, W49B, Cas A, and IC 443), although we confirm that the BML region toward HB 3 is associated with the W3(OH) H ii region, not the SNR. Based on the systemic velocity of each MC, molecular line diagnostics, and cloud morphology, we test whether these detections represent SNR–MC interactions. One of the targets (G54.1+0.3) had previous indications of a BML region, but we did not detect broadened emission toward it. Although broadened 12CO J = 2 − 1 line emission should be detectable toward virtually all SNR–MC interactions, we find relatively few examples; therefore, the number of interactions is low. This result favors mechanisms other than supernova feedback as the basic trigger for star formation. In addition, we find no significant association between TeV gamma-ray sources and MC interactions, contrary to predictions that SNR–MC interfaces are the primary venues for cosmic ray acceleration.

Interpretation of umbrella cloud growth and morphology: implications for flow regimes of short-lived and long-lived eruptions

New numerical and analytical modeling shows that the growth of a volcanic umbrella cloud, expressed as the increase of radius with time, proceeds through regimes, dominated by different force balances. Four regimes are identified: Regime Ia is the long-time behavior of continuously-supplied intrusions in the buoyancy-inertial regime; regime IIa is the long-time behavior of continuously-supplied, turbulent drag-dominated intrusions; regime Ib is the long-time behavior of buoyancy-inertial intrusions of constant volume; and regime IIb that of turbulent drag-dominated intrusions of constant volume. Power-law exponents for spreading time in each regime are 3/4 (Ia), 5/9 (IIa), 1/3 (Ib), and 2/9 (IIb). Both numerical modeling and observations indicate that transition periods between the regimes can be long-lasting, and during these transitions, the spreading rate does not follow a simple power law. Predictions of the new model are consistent with satellite data from seven eruptions and, together with observations of umbrella cloud structure and morphological evolution, support the existence of multiple spreading regimes.

Impact of particle shape on the morphology of noctilucent clouds

Abstract. Noctilucent clouds (NLCs) occur during summer in the polar region at altitudes around 83 km. They consist of ice particles with a typical size around 50 nm. The shape of NLC particles is less well known but is important both for interpreting optical measurements and modeling ice cloud characteristics. In this paper, NLC modeling of microphysics and optics is adapted to use cylindrical instead of spherical particle shape. The optical properties of the resulting ice clouds are compared directly to NLC three-color measurements by the Arctic Lidar Observatory for Middle Atmosphere Research (ALOMAR) Rayleigh/Mie/Raman (RMR) lidar between 1998 and 2014. Shape distributions including both needle- and disc-shaped particles are consistent with lidar measurements. The best agreement occurs if disc shapes are 60 % more common than needles, with a mean axis ratio of 2.8. Cylindrical particles cause stronger ice clouds on average than spherical shapes with an increase of backscatter at 532 nm by ≈ 30 % and about 20 % in ice mass density. This difference is less pronounced for bright than for weak ice clouds. Cylindrical shapes also cause NLCs to have larger but a smaller number of ice particles than for spherical shapes.

EVALUATING THE MORPHOLOGY OF THE LOCAL INTERSTELLAR MEDIUM: USING NEW DATA TO DISTINGUISH BETWEEN MULTIPLE DISCRETE CLOUDS AND A CONTINUOUS MEDIUM

Ultraviolet and optical spectra of interstellar gas along the lines of sight to nearby stars have been interpreted by Redfield & Linsky (2008) and previous studies as a set of discrete warm, partially ionized clouds each with a different flow vector, temperature, and metal depletion. Recently, Gry & Jenkins (2014) have proposed a fundamentally different model consisting of a single cloud with nonrigid flows filling space out to 9 parsecs from the Sun that they propose better describes the local ISM. Here we test these fundamentally different morphological models against the spatially unbiased Malamut et al. (2014) spectroscopic data set, and find that the multiple cloud morphology model provides a better fit to both the new and old data sets. The detection of three or more velocity components along the lines of sight to many nearby stars, the presence of nearby scattering screens, the observed thin elongated structures of warm interstellar gas, and the likely presence of strong interstellar magnetic fields also support the multiple cloud model. The detection and identification of intercloud gas and the measurement of neutral hydrogen density in clouds beyond the Local Interstellar Cloud could provide future morphological tests.

IMPACT OF WINDS FROM INTERMEDIATE-MASS STARS ON MOLECULAR CLOUD STRUCTURE AND TURBULENCE

Observations of nearby molecular clouds detect "shells", which are likely caused by winds from young main sequence stars. However, the progenitors of these observed features are not well characterized and the mass-loss rates inferred from the gas kinematics are several orders of magnitude greater than those predicted by atomic line-driven stellar wind models. We use magnetohydrodynamic simulations to model winds launching within turbulent molecular clouds and explore the impact of wind properties on cloud morphology and turbulence. We find that winds do not produce clear features in turbulent statistics such as the Fourier spectra of density and momentum but do impact the Fourier velocity spectrum. The density and velocity distribution functions, especially as probed by CO spectral lines, strongly indicate the presence and influence of winds. We show that stellar mass-loss rates for individual stars must be $\dot m_w \gtrsim 10^{-7}$ Msun yr$^{-1}$, similar to those estimated from observations, to reproduce shell properties. Consequently, we conclude that B and A-type main sequence stars have mass-loss rates several orders of magnitude larger that those predicted by models or that young stars are more variable than expected due to magnetic activity or accretion.

UV contrasts and microphysical properties of the upper clouds of Venus from the UV and NIR VMC/VEx images

The nature of UV contrasts observed on the upper cloud deck of Venus is still not known. To constrain better the properties of particles that may cause the UV contrasts, the phase dependences of brightness of the venusian clouds measured by the ultraviolet and near-infrared channels of the Venus Express Venus Monitoring Camera (VMC) in the UV dark and bright regions are jointly analyzed. The range of small phase angles, where the glory phenomenon is observed, is of key importance, since the properties of cloud particles can be reliably estimated from the shape and position of glory. However, from more than 2500 orbits of the mission, only in ten orbits the images were taken simultaneously in UV and near-IR channels at small phase angles. Their analysis have yielded the following results. In the UV dark and bright clouds of the equatorial region near the local noon, the derived radii of cloud particles turned out to be the same and rather large, 1.3–1.6μm. No unambiguous connection between the UV contrasts and the brightness in the near-IR channel was found. In some cases, the regions that appear contrasting in UV show no difference in the near-IR brightness. This means that the properties of 1-μm mode particles are the same in these regions and only the contribution of small submicron particles differs, because the near-IR channel is weakly sensitive to the presence of particles smaller than ≈0.3μm in radius. The difference in the composition of 10% of the number of submicron particles (if sulfur and sulfuric acid compositions are considered as probable for the submicron mode) is enough to produce the observed UV contrasts. In the other cases, the UV contrasts are accompanied by the differences in near-IR brightness. This suggests that the cloud particles of the 1-μm mode contribute to these contrasts as well. However, the modeling showed that exactly the variations in the composition of submicron particles in the clouds produce a key effect on the UV contrasts observed. Moreover, a portion of submicron particles with a high refractive index, when incorporated into the sulfuric acid aerosols during the condensation process, may provide the higher refractive index (relative to that of sulfuric acid) of the 1-μm mode derived from modeling. The glory phenomenon was also observed at latitudes 30–60°S, in the transition region from the mottled clouds in dark tropics to streaky cloud morphology at higher latitudes. This allowed the sizes of cloud particles near the frequently seen UV-bright bands to be estimated. It was found that the radii of particles in the upper cloud layer decrease from 1.05–1.2 to 0.8–0.9μm with increasing latitude from ∼35°S to ∼62°S. Our present modeling also clearly showed that the high brightness in the UV-bright band is caused by an additional amount of non-absorbing (in UV) particles with Reff≈0.9μm at the cloud top and/or by the decrease of a portion of absorbing particles inside the clouds below this layer. No variations of the effect with the local solar time for the interval from 10 to 13h available in the data were detected.

Characteristics of cirrus clouds in the tropical lower stratosphere

A unique type of cloud in the tropical lower stratosphere, which we call “stratospheric cirrus”, is described in this study. Stratospheric cirrus clouds are generally detached from overshooting deep convection and are much smaller than subvisual cirrus often observed near the tropical tropopause. We analyzed two cases of stratospheric cirrus in the tropical and subtropical lower stratosphere captured by the space-borne lidar, Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP). Both cases occurred 2–3 hours after the most active phase of the nearby convective cloud clusters. Case 1 has a double-layer structure above the cold point height (CPH); the CPH and two cloud top heights are, respectively, 17.8, 18.9, and 19.9km. Case 2 has a single cloud layer where CPH and the cloud top height are, respectively, 16.5 and 18.7km. The mode radius and ice water content of the stratospheric cirrus clouds are estimated to be 4–10μm and 0.2–0.8mg/m3 based on the radar-lidar method and consideration of the cloud particle terminal velocity. Comparisons with previous numerical model simulation studies suggest that the double-layer stratospheric cirrus clouds are likely from an overshooting plume, pushed up into the stratosphere in an overshoot when warm stratospheric air is inhomogeneously mixed with cold overshooting air. The single-layer stratospheric cirrus cloud is associated with some non-negligible wind shear, so it could be a jumping cirrus cloud, although we cannot rule out the possibility that it came from an overshooting plume because of the similarity in cloud characteristics and morphology between the two cases. Guided by the case studies, an automatic algorithm was developed to select stratospheric cirrus clouds for global survey and statistical analysis. A total of four years of CALIPSO and space-borne cloud radar (CloudSat) data were analyzed. Statistical analysis suggests that stratospheric cirrus clouds occur on the order of 3.0×103 times a year between 30 °S and 30 °N. Many of the stratospheric cirrus clouds are found in the pre-monsoon season in the South and Southeast Asia, where convection is deep and intense.

The relative influence of environmental characteristics on tropical deep convective morphology as observed by CloudSat

AbstractUtilizing a previously developed CloudSat cloud object database, the sensitivity of oceanic, mature, deep convective cloud morphology to cloud‐scale environmental characteristics is examined. Convective available potential energy (CAPE), aerosol optical depth, midlevel vertical velocity, and tropospheric deep shear are all used to characterize the environment. The sensitivity of various aspects of convective morphology to each one of these environmental quantities is assessed individually. The results demonstrate that clouds tend to be invigorated by higher CAPE, aerosol, and upward midlevel vertical velocity. Stronger shear tends to make clouds wider but also shallower. The relative importance of each of these and some additional environmental measures to trends in cloud morphology are compared. It is found that aerosol, deep‐layer shear, and sea surface temperature tend to be the most influential environmental factors to convective morphology. The results are shown to be insensitive to the manner in which the environmental characteristics are defined. The potentially surprising weak sensitivity of cloud morphology to CAPE is discussed in detail.

Atmospheric movies acquired at the Mars Science Laboratory landing site: Cloud morphology, frequency and significance to the Gale Crater water cycle and Phoenix mission results

We report on the first 360 sols (LS 150° to 5°), representing just over half a Martian year, of atmospheric monitoring movies acquired using the NavCam imager from the Mars Science Laboratory (MSL) Rover Curiosity. Such movies reveal faint clouds that are difficult to discern in single images. The data set acquired was divided into two different classifications depending upon the orientation and intent of the observation. Up to sol 360, 73 Zenith movies and 79 Supra-Horizon movies have been acquired and time-variable features could be discerned in 25 of each. The data set from MSL is compared to similar observations made by the Surface Stereo Imager (SSI) onboard the Phoenix Lander and suggests a much drier environment at Gale Crater (4.6°S) during this season than was observed in Green Valley (68.2°N) as would be expected based on latitude and the global water cycle. The optical depth of the variable component of clouds seen in images with features are up to 0.047±0.009 with a granularity to the features observed which averages 3.8°. MCS also observes clouds during the same period of comparable optical depth at 30 and 50km that would suggest a cloud spacing of 2.0 to 3.3km. Multiple motions visible in atmospheric movies support the presence of two distinct layers of clouds. At Gale Crater, these clouds are likely caused by atmospheric waves given the regular spacing of features observed in many Zenith movies and decreased spacing towards the horizon in sunset movies consistent with clouds forming at a constant elevation. Reanalysis of Phoenix data in the light of the NavCam equatorial dataset suggests that clouds may have been more frequent in the earlier portion of the Phoenix mission than was previously thought.

Fabrication of hollow fiber microfiltration membrane from PVDF/DBP/DBS system via thermally induced phase separation process

Abstract Polyvinylidene fluoride (PVDF) membranes were prepared via a thermally induced phase separation (TIPS) process using mixed diluent consisting of dibutyl phthalate (DBP) and dibutyl sebacate (DBS). The compatibility between PVDF and mixed diluent was predicted via the solubility parameter theory and verified by experimental results. The effects of mixed diluent composition on cloud point temperature, crystallization temperature, crystallinity, crystal phase and morphology of the membrane were investigated. As a result, as DBS mass fraction in mixed diluent increased, the cloud point temperature and the crystallinity increased, the crystal phase transformed from β phase to α phase and the morphology of the membrane cross-section changed from spherulitic structures to a bicontinuous structure. PVDF hollow fiber membranes were prepared successfully with the PVDF/DBP/DBS system via the TIPS method. The coagulant composition had a significant influence on the water permeability. When the dimethylacetamide (DMAc) content in the coagulant was 60%, the water permeability reached 877 l/m2·h, while the rejection rate to the particles with a size of 200–500 nm was above 95%.

The relationship between mesoscale circulation and cloud morphology at the upper cloud level of Venus from VMC/Venus Express

The Venus Monitoring Camera (VMC) acquired a set of ultraviolet (UV) images during the Venus Express mission unprecedented in its duration from May 2006 to September 2013. Here we present the results of digital tracking of the cloud features in the upper cloud layer at latitudes 25–75°S using images from 257 orbits with the best spatial coverage. The method relies on analysis of correlations between pairs of UV images separated in time. The bulk of data processed allows us to clarify the reasons why the mid-latitude jet is not always present in latitudinal wind profiles. Comparing VMC images with wind velocity fields we found a relationship between cloud morphology at middle latitudes and the circulation. The vector field in middle latitudes depends on the presence of a contrast global streak in the cloud morphology tilted with respect to latitude circles. The angle of the flow deflection (the angle between the wind velocity and latitudinal circles) and the difference of the zonal velocity on the opposite sides of the streak are in direct relationship to the angle between the streak and latitude circles. During such orbits the jet bulge does not appear in the latitudinal profile of the zonal wind component. Otherwise a zonal flow with small changes of the meridional velocity dominates in middle latitudes and manifests itself as a jet bulge. The relationship between the cloud cover morphology and circulation peculiarities can be attributed to the motion of global cloud features, like the Y-feature. We prepared plots of zonal and meridional velocities averaged with respect to the entire observation period. The average zonal velocity has a diurnal maximum at 15:00 local solar time and at 40°S. The meridional velocity reaches its maximum between 13:00 and 16:00 and at 50°S. The velocities obtained by the digital method are in good agreement with results of the visual method in the middle latitudes published earlier by Khatuntsev et al. (2013).

Dynamics of polar vortices at cloud top and base on Venus inferred from a general circulation model: Case of a strong diurnal thermal tide

Polar vortices in the presence of a thermal tide are investigated using a Venusian middle atmosphere general circulation model. Around the cloud top, where the warm polar region is maintained by the thermal wind associated with a high latitude jet, the temperature contrast forms the polar vortex pattern. The cold collar and hot oval (monopole) near the pole are enhanced by the polar diurnal tide, and unstable vortices form the hot dipole and tripole. The centroid of the hot oval is displaced from the pole to around 80° by the diurnal tide. The hot dipole appears and breaks up into a tripole when transient vortical and divergent eddies with zonal wavenumbers 2 and higher are predominant within the polar hot oval region. Because the divergence and temperature are a quarter cycle out of phase with the eddy vorticity, the vortical eddies transport heat toward the cold region. Thus, the cloud-top polar vortices are mainly formed by a combination of the diurnal tide and transient baroclinic wave. At the cloud base, isotherms are almost zonally uniform and the eddy temperature structure is not apparent. In contrast, divergence and vorticity have large amplitudes within this region. The vortical eddies have a comma-shaped pattern, which is stably maintained and rotates with a period of about 5 days. The divergence and vorticity might be important in controlling cloud morphology at the cloud base via material transport.

Aerosol-cloud-precipitation interactions in WRF model: Sensitivity to autoconversion parameterization

Cloud-to-rain autoconversion process is an important player in aerosol loading, cloud morphology, and precipitation variations because it can modulate cloud microphysical characteristics depending on the participation of aerosols, and affects the spatio-temporal distribution and total amount of precipitation. By applying the Kessler, the Khairoutdinov-Kogan (KK), and the Dispersion autoconversion parameterization schemes in a set of sensitivity experiments, the indirect effects of aerosols on clouds and precipitation are investigated for a deep convective cloud system in Beijing under various aerosol concentration backgrounds from 50 to 10000 cm−3. Numerical experiments show that aerosol-induced precipitation change is strongly dependent on autoconversion parameterization schemes. For the Kessler scheme, the average cumulative precipitation is enhanced slightly with increasing aerosols, whereas surface precipitation is reduced significantly with increasing aerosols for the KK scheme. Moreover, precipitation varies non-monotonically for the Dispersion scheme, increasing with aerosols at lower concentrations and decreasing at higher concentrations. These different trends of aerosol-induced precipitation change are mainly ascribed to differences in rain water content under these three autoconversion parameterization schemes. Therefore, this study suggests that accurate parameterization of cloud microphysical processes, particularly the cloud-to-rain autoconversion process, is needed for improving the scientific understanding of aerosol-cloud-precipitation interactions.

Climatology of stratocumulus cloud morphologies: microphysical properties and radiative effects

Abstract. An artificial neural network cloud classification scheme is combined with A-train observations to characterize the physical properties and radiative effects of marine low clouds based on their morphology and type of mesoscale cellular convection (MCC) on a global scale. The cloud morphological categories are (i) organized closed MCC, (ii) organized open MCC and (iii) cellular but disorganized MCC. Global distributions of the frequency of occurrence of MCC types show clear regional signatures. Organized closed and open MCCs are most frequently found in subtropical regions and in midlatitude storm tracks of both hemispheres. Cellular but disorganized MCC are the predominant type of marine low clouds in regions with warmer sea surface temperature such as in the tropics and trade wind zones. All MCC types exhibit a pronounced seasonal cycle. The physical properties of MCCs such as cloud fraction, radar reflectivity, drizzle rates and cloud top heights as well as the radiative effects of MCCs are found highly variable and a function of the type of MCC. On a global scale, the cloud fraction is largest for closed MCC with mean cloud fractions of about 90%, whereas cloud fractions of open and cellular but disorganized MCC are only about 51% and 40%, respectively. Probability density functions (PDFs) of cloud fractions are heavily skewed and exhibit modest regional variability. PDFs of column maximum radar reflectivities and inferred cloud base drizzle rates indicate fundamental differences in the cloud and precipitation characteristics of different MCC types. Similarly, the radiative effects of MCCs differ substantially from each other in terms of shortwave reflectance and transmissivity. These differences highlight the importance of low-cloud morphologies and their associated cloudiness on the shortwave cloud forcing.

Evaluation of gridded scanning ARM cloud radar reflectivity observations and vertical doppler velocity retrievals

Abstract. The scanning Atmospheric Radiation Measurement (ARM) cloud radars (SACRs) provide continuous atmospheric observations aspiring to capture the 3-D cloud-scale structure. Sampling clouds in 3-D is challenging due to their temporal–spatial scales, the need to sample the sky at high elevations and cloud radar limitations. Thus, a suggested scan strategy is to repetitively slice the atmosphere from horizon to horizon as clouds advect over the radar (Cross-Wind Range-Height Indicator – CW-RHI). Here, the processing and gridding of the SACR CW-RHI scans are presented. First, the SACR sample observations from the ARM Southern Great Plains and Cape Cod sites are post-processed (detection mask, gaseous attenuation correction, insect filtering and velocity de-aliasing). The resulting radial Doppler moment fields are then mapped to Cartesian coordinates with time as one of the dimensions. Next the Cartesian-gridded Doppler velocity fields are decomposed into the horizontal wind velocity contribution and the vertical Doppler velocity component. For validation purposes, all gridded and retrieved fields are compared to collocated zenith-pointing ARM cloud radar measurements. We consider that the SACR sensitivity loss with range, the cloud type observed and the research purpose should be considered in determining the gridded domain size. Our results also demonstrate that the gridded SACR observations resolve the main features of low and high stratiform clouds. It is established that the CW-RHI observations complemented with processing techniques could lead to robust 3-D cloud dynamical representations up to 25–30 degrees off zenith. The proposed gridded products are expected to advance our understanding of 3-D cloud morphology, dynamics and anisotropy and lead to more realistic 3-D radiative transfer calculations.

Aerosol Replenishment and Cloud Morphology: A VOCALS Example

Abstract The impact of aerosol replenishment in a stratocumulus-topped boundary layer (STBL) on cloud morphology and albedo is examined using a large-eddy simulation (LES) model in conjunction with a prey–predator-type dynamical model following Koren and Feingold. In both the LES and the prey–predator models, the aerosol replenishment is represented as a relaxation term toward an ambient aerosol concentration with a time scale . The LESs suggest the existence of three distinct cloud regimes corresponding to different aerosol relaxation times. Specifically, for a small , the simulations are characterized by a large aerosol concentration, weak precipitation, relatively thick cloud depth, and closed cells. For a moderate , the simulated clouds exhibit open cellular patterns, in accordance with low aerosol concentration and moderate precipitation that oscillates in time. For a large , the aerosol may be depleted by intense drizzling and accordingly the cloud disappears. The critical aerosol relaxation times that separate these regimes vary with the ambient aerosol number concentration and cloud depth. Solutions from the low-order dynamical model with parameters relevant to the LESs are in general consistent with the LES results and provide further insight into the interplay among clouds, aerosol, and precipitation.

에어로솔의 대륙 층운형 구름 연직발달(Invigoration)에 미치는 영향 분석

This study examines the effect of aerosols on the vertical invigoration of continental stratiform clouds, using a dataset of Atmospheric Radiation Measurement (ARM) Intensive Operational Period (IOP, March 2000) at the Southern Great Plains (SGP) site. To provide further support to our observation-based findings, the weather research and forecasting (WRF) sensitivity simulations with changing cloud condensation nuclei (CCN) concentrations have been carried out for the golden episode over SGP. First, cross correlation between observed aerosol scattering coefficient and cloud liquid water path (LWP) with a 160-minutes lag is the highest of r = 0.83 for the selected episode, which may be attributable to cloud vertical invigoration induced by an increase in aerosol loading. Modeled cloud fractions in a control run are well matched with the observation in the perspective of cloud morphology and lasting period. It is also found through a simple sensitivity with a change in CCN that aerosol invigoration (AIV) effect on stratiform cloud organization is attributable to a change in the cloud microphysics as well as dynamics such as the corresponding modification of cloud number concentrations, drop size, and latent heating rate, etc. This study suggests a possible cloud vertical invigoration even in the continental stratiform clouds due to aerosol enhancement in spite of a limited analysis based on a few observed continental cloud cases.