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.

Abstract

A continental stratus cloud layer was studied by advanced ground-based remote sensing instruments and aircraft probes on 30 April 1994 from the Cloud and Radiation Testbed site in north-central Oklahoma. The boundary layer structure clearly resembled that of a cloud-topped mixed layer, and the cloud content is shown to be near adiabatic up to the cloud-top entrainment zone. A cloud retrieval algorithm using the radar reflectivity and cloud droplet concentration (either measured in situ or deduced using dual-channel microwave radiometer data) is applied to construct uniquely high-resolution cross sections of liquid water content and mean droplet radius. The combined evidence indicates that the 350–600 m deep, slightly supercooled (2.0° to −2.0°C) cloud, which failed to produce any detectable ice or drizzle particles, contained an average droplet concentration of 347 cm⁻³, and a maximum liquid water content of 0.8 g m⁻³ and mean droplet radius of 9 μm near cloud top. Lidar data indicate that the K_a-band radar usually detected the cloud-base height to within ∼50 m, such that the radar insensitivity to small cloud droplets had a small impact on the findings. Radar-derived liquid water paths ranged from 71 to 259 g m⁻² as the stratus deck varied, which is in excellent agreement with dual-channel microwave radiometer data, but ∼20% higher than that measured in situ. This difference appears to be due to the undersampling of the few largest cloud droplets by the aircraft probes. This combination of approaches yields a unique image of the content of a continental stratus cloud, as well as illustrating the utility of modern remote sensing systems for probing nonprecipitating water clouds.

Abstract

The local structure and evolution of the convective boundary layer (CBL) are studied through measurements obtained with a volume-imaging radar, the turbulent eddy profiler (TEP). TEP has the unique ability to image the temporal and spatial evolution of both the velocity field and the local refractive index structure-function parameter, C̃ ² _n . Volumetric images consisting of several thousand pixels are typically formed in as little as 1 s. Spatial resolutions are approximately 30 m by 30 m by 30 m.

CBL data obtained during an August 1996 deployment at Rocks Springs, Pennsylvania, are presented. Measurements of the vertical C̃ ² _n profile are shown, exhibiting the well-known bright band near the capping inversion at z _i , as well as intermittent plumes of high C̃ ² _n . Horizontal profiles show coherent 100-m-scale C̃ ² _n and vertical velocity (w) structures that correspond to converging horizontal velocity vectors. To quantify the scales of structures, the vertical and streamwise horizontal correlation distances are calculated within the TEP field of view.

To study the statistics and scales of larger structures, effective volumes larger than the TEP field of view are constructed through Taylor’s hypothesis. Statistics of C̃ ² _n and w time series are compared to an appropriately scaled large eddy simulation (LES). While w time series comparisons agree very well, the LES C̃ ² _n predictions agree only with some of the measured data. Finally, the scales of C̃ ² _n structures in the TEP time series measurements are calculated and compared to the scales in the LES spatial domain. Good agreement is found only near the capping inversion layer, the area of largest structures. This study highlights the unique capabilities of the TEP instrument, and shows what are believed to be the first statistical comparisons of measured C̃ ² _n data with LES derived results.