The authors have analyzed the scaling behavior of marine boundary layer (MBL) clouds using high-resolution temperature (T) and liquid water content (LWC) fluctuations from aircraft measurements collected over the Pacific Ocean during the Physics of Stratocumulus Top (POST) research campaign in summer of 2008. As an extension of the past studies for scale-invariant properties of MBL clouds, the authors studied the variability of scaling exponents with height. The results showed that both LWC and T have two distinct scaling regimes: the first one displays scale invariance over a range from about 1-5 m to at least 7 km, and the second one goes from about 0.1-1 to 1-5 m. For the large-scale regime (r > 1-5 m), turbulence in MBL clouds is multifractal, while scale break and scaling exponents vary with height, most significantly in the cloud-top region. For example, LWC spectral exponent beta increases from 1.42 at cloud base to 1.58 at cloud top, while scale break decreases from similar to 5 m at cloud base to 0.8 m at cloud top. The bifractal parameters (H-1, C-1) for LWC increase from (0.14, 0.02) at cloud base to (0.33, 0.1) at cloud top while maintaining a statistically significant linear relationship C-1 approximate to 0.4H(1) - 0.04 in MBL clouds. From near surface to cloud top, (H-1, C-1) for T also increase with height, but above cloud top H-1 increases and C-1 decreases with height. The results suggest the existence of three turbulence regimes: near the surface, in the middle of the boundary layer, and in the cloud-top region, which need to be distinguished.

Buoyancy reversal by evaporative cooling in entrainment holes has a minimal influence on stratocumulus (Sc) observed during the Physics of Stratocumulus Top (POST) aircraft field study held off the California coast in 2008. High-resolution temperature and microphysics measurements show only small differences for Sc with and without buoyancy reversal predicted by mixing fraction analysis that relates mixtures of cloudy air and free-atmospheric air to buoyancies of the mixtures. The reduction of LWC due to evaporation in the holes is a small percentage (average similar to 12%) of liquid water diluted in the Sc by entrainment from the entrainment interface layer (EIL) located above unbroken cloud top where most mixing, evaporation, and reduction of the large buoyancy jump between the cloud and free atmosphere occur. Entrainment is dominated by radiative cooling at cloud top.

Correction factors C-f are derived for ice-crystal volume and effective radius Re, measured by Forward Scattering Spectrometer Probe (FSSP) and Particulate Volume Monitor (PVM) that are known to over-estimate both parameters for nonspherical particles. Correction factors are based on ice-crystal volume and the projected area of randomly oriented model ice crystals with column, rosette, capped-column, and dendrite habits described by Takano and Liou. In addition, C-f are calculated for oblate and prolate spheroids. To test C-f, both probes are compared to small, predominately solid hexagonal ice-crystal plates and columns generated in the Colorado State University (CSU) Dynamic Cloud Chamber (DCC). The tendency of heat released by the PVM (placed inside the chamber) to evaporate ice crystals and the smaller upper size range of the PVM than the size range of the FSSP caused large differences in the probes' outputs for most comparisons in the DCC. Correction factors improved the accuracy of Re measured by the FSSP for the solid hexagonal crystals, and both probes produced similar results for the projected area and ice water content when crystal sizes fell within the probes' size ranges. The modification for minimizing ice-crystal shattering and the application of C-f for forward scatter probes such as the FSSP suggests the probes' improved usefulness for measuring small ambient ice crystals.

High spatial resolution measurements of temperature and liquid water content, accompanied by moderate-resolution measurements of humidity and turbulence, collected during the Physics of Stratocumulus Top experiment are analyzed. Two thermodynamically, meteorologically and even optically different cases are investigated. An algorithmic division of the cloud-top region into layers is proposed. Analysis of dynamic stability across these layers leads to the conclusion that the inversion capping the cloud and the cloud-top region is turbulent due to the wind shear, which is strong enough to overcome the high static stability of the inversion. The thickness of this mixing layer adapts to wind and temperature jumps such that the gradient Richardson number stays close to its critical value. Turbulent mixing governs transport across the inversion, but the consequences of this mixing depend on the thermodynamic properties of cloud top and free troposphere. The effects of buoyancy sorting of the mixed parcels in the cloud-top region are different in conditions that permit or prevent cloud-top entrainment instability. Removal of negatively buoyant air from the cloud top is observed in the first case, while buildup of the diluted cloud-top layer is observed in the second one.

An aircraft field study (POST; Physics of Stratocumulus Top) was conducted off the central California coast in July and August 2008 to deal with the known difficulty of measuring entrainment rates in the radiatively important stratocumulus (Sc) prevalent in that area. The Center for Interdisciplinary Remotely‐Piloted Aircraft Studies Twin Otter research aircraft flew 15 quasi‐Lagrangian flights in unbroken Sc and carried a full complement of probes including three high‐data‐rate probes: ultrafast temperature probe, particulate volume monitor probe, and gust probe. The probes' colocation near the nose of the Twin Otter permitted estimation of entrainment fluxes and rates with an in‐cloud resolution of 1 m. Results include the following: Application of the conditional sampling variation of classical mixed layer theory for calculating the entrainment rate into cloud top for POST flights is shown to be inadequate for most of the Sc. Estimated rates resemble previous results after theory is modified to take into account both entrainment and evaporation at cloud top given the strong wind shear and mixing at cloud top. Entrainment rates show a tendency to decrease for large shear values, and the largest rates are for the smallest temperature jumps across the inversion. Measurements indirectly suggest that entrained parcels are primarily cooled by infrared flux divergence rather than cooling from droplet evaporation, while detrainment at cloud top causes droplet evaporation and cooling in the entrainment interface layer above cloud top.

The role of large sea‐salt condensation nuclei generated by wind blowing over the ocean surface is evaluated by applying a Lagrangian parcel model to a range of conditions based on observations made during NCAR research flight RF12 of the Rain in Cumulus over the Ocean (RICO) trade wind cumulus (Cu) study in the Caribbean near Antigua. The model utilizes droplet condensation growth, a simplified droplet sedimentation scheme, and quasi‐stochastic coalescence to calculate drizzle rates 1100 m above Cu base. The calculations are repeated without the sea‐salt solution droplets to permit calculation of a drizzle rate enhancement factor (Df) owing to the large nuclei. The model predicts a small effect of the large nuclei on the RF12 drizzle rate, as well as suggesting the same for other RICO flights in agreement with radar studies of the same Cu that also show at most a small effect on precipitation due to the large nuclei. These findings are contrary to those some other studies of the Cu. The present study agrees with several previous studies that large nuclei affect the drizzle rate for wind speeds greater than about 10 m s−1, that the rate increases as wind speed increases, and that the rate increases as droplet concentration becomes larger at constant wind speed. Df values are fit with an analytical expression relating drizzle rate with wind speed and in‐cloud droplet concentration.

High resolution measurements of temperature and cloud water, collected during Physics of Stratocumulus Top experiment are investigated. Two case studies presented here illustrate differences between "classical" stratocumulus capped with a sharp inversion and dry layer above and one type of "nonclassical" stratocumulus with weak inversion with moist air above. Entrainment and tranport into the cloud deck are investigated by means of statistical analysis of LWC and temperature fluctuations. It comes, that in "classical" case downdrafts with depleted water content are characterized with reduced temperature (effect of evaporative cooling, presumably negative buoyancy), while in this "non classical" case such downdrafts are of increased temperature, suggesting that in this case evaporative cooling is not a driving mechannism of downward transport.

Our knowledge of boundary layer cloud processes is insufficient to resolve pressing scientific problems. Boundary layer clouds often have liquid-water paths (LWPs) less than 100 gm{sup 2}, which are defined here as being 'thin' Clouds with Low Optical Water Depths (CLOWD). This type of cloud is common globally, and the Earth's radiative energy balance is particularly sensitive to small changes in their optical properties. However, it is difficult to retrieve accurately their cloud properties via remote sensing because they are tenuous and often occur in partly cloudy skies. This interferes with our ability to obtain the routine, long-term statistics needed to improve their representation in climate models. To address this problem, in-situ data are needed to investigate cloud processes and to evaluate and refine existing retrieval algorithms. Coordinated by the ARM Aerial Facility (AAF), the Routine AAF CLOWD Optical Radiative Observations (RACORO) field campaign conducted long-term, systematic flights in boundary layer, liquid-water clouds over the ARM Southern Great Plains (SGP) site between 22 January and 30 June 2009. This was the first time that a long-term aircraft campaign was undertaken for systematic in-situ sampling of cloud properties. Using the CIRPAS Twin Otter aircraft equipped with a comprehensive set of instrumentsmore » to measure solar and thermal radiation, cloud microphysics, aerosol properties and atmospheric state, the RACORO team logged an unprecedented 59 flights and 259 research hours above the SGP site. Data gathered during the RACORO campaign will provide researchers with a statistically relevant data set of boundary-layer cloud and aerosol properties for future study. These data can be used to validate retrieval algorithms and support process studies and model simulations of boundary layer clouds and, in particular, CLOWD-type clouds. In addition to cloud observations, complementary clear-sky flight patterns were conducted to map the surface albedo, characterize the aerosol and cloud condensation nuclei, and study boundary layer turbulence. For RACORO to operate as a routine, long-term program, flight operations had to be kept as simple as possible to achieve its objectives, which required an operating paradigm different from typical, short-term, intensive aircraft field programs. This poster summarizes RACORO operations, measurements and instruments.« less

The generation of large sea-salt particles from whitecaps generated by wind over the ocean is well documented (W oodcock, 1953). It is, however, not clear to what degree these large hygroscopic particles act as condensation nuclei that initiate precipitation in small cumulus clouds (Cu). Blyth et al (2003) suggested that in SCMS (Small Cumulus Microphysics Study) cumuli giant and ultra-giant (UGN; sea-salt particles > than 2-um dry radius) affected the appearance of initial precipitation, while Goeke et al (2007) concluded that sea salt had no effect for the same cumuli. Investigations of small trade-wind cumuli during RICO (Rain in Cumulus over the Ocean) likewise found a negligible effect of sea-salt particles on initial precipitation (ColonRobles et al, 2006; Hudson and Mishra, 2007; Knight et al, 2008). An observational study (Gerber et al, 2008) following the microphysical evolution with height of RICO cumuli found a significant number of large drizzle drops associated with UGN concentrations below cloud base. A simple coalescence parcel model constrained by near cloud-base microphysics was in approximate agreement with the observed “drizzle tail” in the droplet size spectrum near mean cloud top. However, a conclusion could not be reached in this study as to the role of this drizzle in precipitation initiation at cloud base. The present study expands the results given in Gerber et al (2008) by performing a modeling sensitivity analysis where droplet spectra and subcloud sea-salt spectra are varied about measured spectra obtained from RICO flight RF12. Drizzle rates are estimated at 1100 m and ~250 m below cloud top. The goal is to compare these new results to all other RICO flights, and to parameterize the drizzle rate as a function of wind speed over the ocean and incloud droplet concentration.

The vertical evolution of microphysics in trade-wind cumuli (Cu) observed from the NCAR C-130 research aircraft during one flight of the RICO (Rain in Cumulus Over the Ocean) study is analyzed. Conditional sampling of > 200 Cu traversed on this flight is used to chose Cu for which the aircraft penetrated single and growing Cu turrets about 250-m below cloud top where maximum LWC is often found and where radar has often observed initial stages of precipitation. The vertical evolution of the sampled set of Cu was assumed to follow Lagrangian behavior. The entrainment rate, entrained parcel scales, mixing mechanisms, and effects on the droplet size distribution are measured and evaluated. A parcel model is applied over the 1100-m maximum Cu height of the traverses to determine the relationship between the observed large number of small droplets and the fewer ultra-giant sea-salt nuclei (UGN) in order to assess the role of these nuclei in evolving the size spectrum and in causing a growing "drizzle tail". New insight on these topics is obtained by using the PVM (Particle Volume Monitor) probe to measure incloud microphysics with 10-cm resolution.The results include the following: Entrainment causes primarily dilution of the drops without significant size changes, thus either extreme inhomogeneous mixing or more likely homogeneous mixing resulting from mixing with cool and humid entrained air take place. ne entrained parcels are surprisingly small following lognormal behavior and decaying rapidly upon entering the Cu, as a result super-adiabatic drops are not evident. The entrained parcels are consistent with the Bragg-scattering "mantle echo" often observed by radar in small Cu. The FSSP (Forward Scattering Spectrometer Probe) droplet spectra are nearly constant with height. These "self-preserving" spectra are a result of an approximate balance between dilution by entrainment of droplets originating at cloud base, droplet activation on entrained CCN (cloud condensation nuclei), and detrainment and coalescence losses. Sea-salt nuclei follow Woodcock's wind dependence, and are shown with the parcel model to play an important role in forming the observed drizzle that increases with cloud height. Accretion is the dominant coalescence mechanism near cloud top in these Cu.

parameters. The droplet concentration in the RF01 case varies in the range up to 300 cm -3 , with the mean of 156 cm -3 and standard deviation of 27 cm -3 resulting in a variance coefficient 1 , VC, of about 18%. The liquid water content fluctuates from 0 to about 0.5 gm -3 with the mean of 0.2 gm -3 and standard deviation of 0.06 gm -3 (VC of ~30%).

Shallow, maritime cumuli are ubiquitous over much of the tropical oceans, and characterizing their properties is important to understanding weather and climate. The Rain in Cumulus over the Ocean (RICO) field campaign, which took place during November 2004–January 2005 in the trades over the western Atlantic, emphasized measurements of processes related to the formation of rain in shallow cumuli, and how rain subsequently modifies the structure and ensemble statistics of trade wind clouds. Eight weeks of nearly continuous S-band polarimetric radar sampling, 57 flights from three heavily instrumented research aircraft, and a suite of ground- and ship-based instrumentation provided data on trade wind clouds with unprecedented resolution. Observational strategies employed during RICO capitalized on the advances in remote sensing and other instrumentation to provide insight into processes that span a range of scales and that lie at the heart of questions relating to the cause and effects of rain from shallow maritime cumuli.

Aircraft flights through stratocumulus clouds (Sc) during the Dynamics and Chemistry of Marine Stratocumulus II (DYCOMS-II) study off the California coast found narrow in-cloud regions with less liquid water content (LWC) and cooler temperatures than average background values. The regions are named cloud holes and are assumed to be a result of water evaporated by the entrainment of dryer air from above the Sc. While such features have been noted previously. this study provided a unique opportunity to investigate in Much greater detail the nature of the holes. as well as their relationship to the entrainment rate. because high-speed temperature and LWC probes with maximum spatial resolution of 10 cm were flown together for the first time. Nine long-duration flights were made through mostly unbroken Sc for which conditional sampling was used to identify the location and size of the holes. The holes are concentrated near cloud top. their average width near cloud top is about 5 in, their relative length distribution is nearly constant for all flights. and they can penetrate hundreds of meters deep into the Sc before being lost by mixing. Entrainment velocities at cloud top are estimated from measurements of fluxes of reduced LWC and vapor mixing ratios in holes, the fraction of cloud area covered by holes, and the total water jump between cloud top and the free atmosphere. Rates as large as 10 mm s(-1) are found for nocturnal flights, and these rates are about 3 times larger than for daytime flight segments. The rates correlate best with the size of the buoyancy jump above the Sc; the present conditional-sampling approach for measuring the rates gives larger rates than the "flux jump" rates determined by others for the same flights by a factor of about 2. The stability criterion for all Sc predicts thinning and breakup of the Sc, which does not occur. The minimal amount of cloud-top evaporative cooling caused by entrainment contributes little to the top-down convection dominated by radiative cooling during nocturnal flights: however, evaporative cooling caused by the mixing of holes as they subduct with the large-scale eddy circulation in the Sc may contribute. but with an as-of-yet unknown amount.