Samuel Haimov

Dynamics and chemistry of marine stratocumulus - DYCOMS II

The second Dynamics and Chemistry of Marine Stratocumulus (DYCOMS-II) field study is described. The field program consisted of nine flights in marine stratocumulus west-southwest of San Diego, California. The objective of the program was to better understand the physics a n d dynamics of marine stratocumulus. Toward this end special flight strategies, including predominantly nocturnal flights, were employed to optimize estimates of entrainment velocities at cloud-top, large-scale divergence within the boundary layer, drizzle processes in the cloud, cloud microstructure, and aerosol–cloud interactions. Cloud conditions during DYCOMS-II were excellent with almost every flight having uniformly overcast clouds topping a well-mixed boundary layer. Although the emphasis of the manuscript is on the goals and methodologies of DYCOMS-II, some preliminary findings are also presented—the most significant being that the cloud layers appear to entrain less and drizzle more than previous theoretical work led investigat...

THE TERRAIN-INDUCED ROTOR EXPERIMENT : A Field Campaign Overview Including Observational Highlights

Abstract The Terrain-Induced Rotor Experiment (T-REX) is a coordinated international project, composed of an observational field campaign and a research program, focused on the investigation of atmospheric rotors and closely related phenomena in complex terrain. The T-REX field campaign took place during March and April 2006 in the lee of the southern Sierra Nevada in eastern California. Atmospheric rotors have been traditionally defined as quasi-two-dimensional atmospheric vortices that form parallel to and downwind of a mountain ridge under conditions conducive to the generation of large-amplitude mountain waves. Intermittency, high levels of turbulence, and complex small-scale internal structure characterize rotors, which are known hazards to general aviation. The objective of the T-REX field campaign was to provide an unprecedented comprehensive set of in situ and remotely sensed meteorological observations from the ground to UTLS altitudes for the documentation of the spatiotem-poral characteristics ...

Finescale structure and microphysics of coastal stratus

Observations were made of unbroken marine stratus off the coast of Oregon using the combined capabilities of in situ probes and a 95-GHz radar mounted on an aircraft. Reflectivity and Doppler velocity measurements were obtained in vertical and horizontal planes that extend from the flight lines. Data from three consecutive days were used to examine echo structure and microphysics characteristics. The clouds appeared horizontally homogeneous and light drizzle reached the surface in all three cases. Radar reflectivity is dominated by drizzle drops over the lower two-thirds to four-fifths of the clouds and by cloud droplets above that. Cells with above-average drizzle concentrations exist in all cases and exhibit a large range of sizes. The cells have irregular horizontal cross sections but occur with a dominant spacing that is roughly 1.2‐1.5 times the depth of the cloud layer. Doppler velocities in the vertical are downward in all but a very small fraction of the cloud volumes. The cross correlation between reflectivity and vertical Doppler velocity changes sign at or below the midpoint of the cloud, indicating that in the upper parts of the clouds above-average reflectivities are associated with smaller downward velocities. This correlation and related observations are interpreted as the combined results of upward transport of drizzle drops and of downward motion of regions diluted by entrainment. The in situ measurements support these conclusions.

Finescale Vertical Structure of a Cold Front as Revealed by an Airborne Doppler Radar

Abstract In the afternoon of 24 May 2002, a well-defined and frontogenetic cold front moved through the Texas panhandle. Detailed observations from a series of platforms were collected near the triple point between this cold front and a dryline boundary. This paper primarily uses reflectivity and Doppler velocity data from an airborne 95-GHz radar, as well as flight-level thermodynamic data, to describe the vertical structure of the cold front as it intersected with the dryline. The prefrontal convective boundary layer was weakly capped, weakly sheared, and about 2.5 times deeper than the cold-frontal density current. The radar data depict the cold front as a fine example of an atmospheric density current at unprecedented detail (∼40 m). The echo structure and dual-Doppler-inferred airflow in the vertical plane reveal typical features such as a nose, a head, a rear-inflow current, and a broad current of rising prefrontal air that feeds the accelerating front-to-rear current over the head. The 2D cross-fro...

The Structure of Thermals in Cumulus from Airborne Dual-Doppler Radar Observations

Abstract A newly developed technique for airborne dual-Doppler observations with the Wyoming Cloud Radar is used to characterize the velocity fields in vertical planes across cumulus turrets. The clouds sampled were continental in nature, with high bases (near 0°C) and with depths of 2–3 km. Clear evidence was found that the clouds evolved through sequences of bubbles, or thermals, with well-defined toroidal circulations, or vortex rings. The ring core and tube diameters were about 200–600 m, leading to turret sizes of 1–2 km in the horizontal. The largest updraft speeds were observed in the ring centers, but regions of turbulent, ascending air extended behind the thermals to distances comparable with the toroid sizes. Vertical shear of ambient winds, when present, led to a tilting of the updrafts and toroids. Patterns in the reflectivity and velocity fields indicated regions of major intrusions into the thermals, accompanied by entrainment of ambient air, or recycling of larger hydrometeors, depending on...

The Cumulus, Photogrammetric, In Situ, and Doppler Observations Experiment of 2006

The finescale structure and dynamics of cumulus, evolving from shallow to deep convection, and the accompanying changes in the environment and boundary layer over mountainous terrain were the subjects of a field campaign in July–August 2006. Few measurements exist of the transport of boundary layer air into the deep troposphere by the orographic toroidal circulation and orographic convection. The campaign was conducted over the Santa Catalina Mountains in southern Arizona, a natural laboratory to study convection, given the spatially and temporally regular development of cumulus driven by elevated heating and convergent boundary layer flow. Cumuli and their environment were sampled via coordinated observations from the surface, radiosonde balloons, and aircraft, along with airborne radar data and stereophotogrammetry from two angles. The collected dataset is expected to yield new insights in the boundary layer processes leading to orographic convection, in the cumulus-induced transport of boundary layer a...

A High-Resolution Dual-Doppler Technique for Fixed Multiantenna Airborne Radar

A new technique is presented for the analysis of data collected by pairs of aircraft (AC)-mounted fixed radar beams directed ~30deg apart, leading to two-dimensional wind field syntheses in the horizontal and vertical planes. The technique was applied to the 95-GHz Wyoming Cloud Radar (WCR) configurations aboard the University of Wyoming King Air and the National Center for Atmospheric Research C-130 research AC. The construction of the data grid allows to closely follow the AC flight attitude, and thus the beam scanned surfaces, with a resolution between 30 and 45 m. A weighted least squares method is employed to solve the velocity inverse decomposition problem for every valid grid cell. The effect on the solution of the nonmeasured (cross-plane) wind component is minimized by using an external estimate of the ambient wind. The error sources are discussed, and maximum expected values for the velocity field errors are provided with reference to the radar system, the data collection process, and specific aspects of the target under investigation. These uncertainties (1-2 m/s) are quantified for two case studies that show the potential of the technique in retrieving the kinematic field in transects of cumulus and stratus clouds. The technique produces smooth fine-scale kinematic fields although each velocity point is the result of an independent calculation, and no binding constraint is imposed among neighboring points as is done in other dual-Doppler techniques

Aircraft-Induced Hole Punch and Canal Clouds: Inadvertent Cloud Seeding

Abstract The production of holes and channels in altocumulus clouds by two commercial turboprop aircraft is documented for the first time. An unprecedented dataset combining in situ measurements from microphysical probes with remote sensing measurements from cloud radar and lidar operating from the National Science Foundation (NSF)/NCAR C-130 aircraft, as well as ground-based NOAA and Colorado State University (CSU) radars, is used to describe the radar/lidar properties of a hole punch cloud and channel and the ensuing ice microphysical properties and structure of the ice column that subsequently developed. Ice particle production by commercial turboprop aircraft climbing through clouds much warmer than the regions where contrails are produced has the potential to significantly modify the cloud microphysical properties and effectively seed them under some conditions. Jet aircraft may also be producing hole punch clouds when flying through altocumulus with supercooled droplets at heights lower than their n...

Snow Growth and Transport Patterns in Orographic Storms as Estimated from Airborne Vertical-Plane Dual-Doppler Radar Data

AbstractAirborne vertical-plane dual-Doppler cloud radar data, collected on wind-parallel flight legs over a mountain in Wyoming during 16 winter storms, are used to analyze the growth, transport, and sedimentation of snow. In all storms the wind is rather strong, such that the flow is unblocked. The sampled clouds are mixed phase, shallow, and generally produce snowfall over the mountain only. The 2D scatterers’ mean motion in the vertical along-track plane below flight level is synthesized using one radar antenna pointing to nadir, and one 30° forward of nadir. This yields instantaneous cross-mountain hydrometeor streamlines.The dynamics of the orographic flow dominate the precipitation patterns across the mountain. Three patterns are distinguished: the first two contain small convective cells, either boundary layer (BL) convection or elevated convection, the latter likely due to the release of potential instability in orographically lifted air. In these patterns the cross-mountain flow is relatively un...

Ice in Clouds Experiment—Layer Clouds. Part I: Ice Growth Rates Derived from Lenticular Wave Cloud Penetrations

AbstractLenticular wave clouds are used as a natural laboratory to estimate the linear and mass growth rates of ice particles at temperatures from −20° to −32°C and to characterize the apparent rate of ice nucleation at water saturation at a nearly constant temperature. Data are acquired from 139 liquid cloud penetrations flown approximately along or against the wind direction. A mean linear ice growth rate of about 1.4 μm s−1, relatively independent of particle size (in the range 100–400 μm) and temperature is deduced. Using the particle size distributions measured along the wind direction, the rate of increase in the ice water content (IWC) is calculated from the measured particle size distributions using theory and from those distributions by assuming different ice particle densities; the IWC is too small to be measured. Very low ice effective densities, <0.1 g cm−3, are needed to account for the observed rate of increase in the IWC and the unexpectedly high linear growth rate.Using data from multiple ...