Finescale structure and microphysics of coastal stratus

ABSTRACT 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

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

Lenticular 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 (I WC) 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 [WC and the unexpectedly high linear growth rate. Using data from multiple penetrations through a narrow (along wind) and thin wave cloud with relatively flat airflow streamlines, growth rate calculations are used to estimate where the ice particles originate and whether the ice is nucleated in a narrow band or over an extended period of time. The calculations are consistent with the expectation that the ice formation occurs near the leading cloud edge, presumably through a condensation-freezing process. The observed ice concentration increase along the wind is more likely due to a variation in ice growth rates than to prolonged ice nucleation.Keywords: Probes, Aerosols, Microphysical properties, Nuclei, Cirrus, Liquid water content, Snow crystals, Nucleation, Tunnel, Counterflow virtual impacto

Airborne Radar

International audienceGround-based radar systems have been used to observe clouds and precipitation since the 1940s. While weather radars that use centimeter waves can observe precipitation several hundred kilometers away, radars that are dedicated to cloud observations use millimeter waves and have limited ranges of just a few tens of kilometers. Airborne radars have the advantages that they can perform measurements close to the region of interest and they provide radar information on regions that ground-based radars cannot access. There is no such thing as a standard airborne radar system; all systems are tailored for use on specific research aircraft, although some of them are designed to be modular so that they can be mounted on various aircraft. Airborne radar systems use frequencies ranging from those in the X band to those in the W band. Radars that use shorter wavelengths are preferred due to spatial restrictions on antenna size in aircraft, but C-band systems are also being considered for installation in large aircraft. Besides reflectivity (the backscatter signal), the radial motions of scattering particles can be measured and used to retrieve atmospheric motion. In addition, several airborne radars are able to measure dual-polarization backscatter signals that can be employed to identify different types of hydrometeors

Coincident in situ and triple-frequency radar airborne observations in the Arctic

The dataset collected during the Radar Snow Experiment (RadSnowExp) presents the first-ever airborne triple-frequency radar observations combined with almost perfectly co-located and coincident airborne microphysical measurements from a single platform, the National Research Council Canada (NRC) Convair-580 aircraft. The potential of this dataset is illustrated using data collected from one flight during an Arctic storm, which covers a wide range of snow habits from pristine ice crystals and low-density aggregates to heavily rimed particles with maximum size exceeding 10ĝmm. Three different flight segments with well-matched in situ and radar measurements were analyzed, giving a total of 49ĝmin of triple-frequency observations. The in situ particle imagery data for this study include high-resolution imagery from the Cloud Particle Imager (CPI) probe, which allows accurate identification of particle types, including rimed crystals and large aggregates, within the dual-frequency ratio (DFR) plane. The airborne triple-frequency radar data are grouped based on the dominant particle compositions and microphysical processes (level of aggregation and riming). The results from this study are consistent with the main findings of previous modeling studies, with specific regions of the DFR plane associated with unique scattering properties of different ice habits, especially in clouds where the radar signal is dominated by large aggregates. Moreover, the analysis shows close relationships between the triple-frequency signatures and cloud microphysical properties (particle characteristic size, bulk density, and level of riming)