Robert D. Kelly

BOREAS in 1997: Experiment overview, scientific results, and future directions

The goal of the Boreal Ecosystem-Atmosphere Study (BOREAS) is to improve our understanding of the interactions between the boreal forest biome and the atmosphere in order to clarify their roles in global change. This overview paper describes the science background and motivations for BOREAS and the experimental design and operations of the BOREAS 1994 and BOREAS 1996 field years. The findings of the 83 papers in this journal special issue are reviewed. In section 7, important scientific results of the project to date are summarized and future research directions are identified.

An airborne 95 GHz dual-polarized radar for cloud studies

A 95 GHz dual-polarization radar system was developed and flown on the University of Wyoming King Air research aircraft, from which it measured reflectivity, depolarization, and Doppler-derived velocity mean and standard deviation of a variety of clouds. This paper describes the radar and a data acquisition system that uses commercially available digitizers, signal processors, and signal generators. The authors also describe the tradeoffs between spatial resolution and ability to estimate reflectivity and velocity. This paper presents the first known airborne measurements of clouds made at 95 GHz; these are thought to be the most highly resolved millimeter-wave cloud images made to date. Depolarization, measured in terms of the linear depolarization ratio (LDR), was especially high in the melting band and in regions containing pristine ice crystals. These measurements demonstrate the advantages that high-spatial-resolution airborne millimeter-wave radars offer for the study of cloud microphysical properties. >

Horizontal Roll and Boundary-Layer Interrelationships Observed over Lake Michigan

Abstract On 7, 9 and 10 January 1981 some 35 hours of organized horizontal roll convection in the boundary layer over lake Michigan were probed with two airplanes and two radars from the University of Chicago Lake Snow Project. During this time, roll wavelength varied from 1.5 to 13.7 km, roll depth from 0.9 to 2.1 km, and roll aspect ratio from 1.0 to 9.1; the roll axes changed orientation to remain within ±10° of the inversion-level geostrophic wind direction, and the maximum transverse wind speed varied from 1 to 2 m s−1. Boundary-layer parameters such as wind speed, wind direction, over-water fetch, surface-layer instability, and upstream stability, also varied over the same period. The roll orientations, and the variations of roll wavelength and maximum roll transverse wind speed with Rayleigh number and Richardson number, are examined with reference to theoretical models of roll convection. The observations suggest that roll orientation, roll wavelength, and roll transverse wind speed depend primari...

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.

A Single Doppler Radar Study of Horizontal-Roll Convection in a Lake-Effect Snow Storm

Abstract The boundary-layer structure and circulation over Lake Michigan were probed with a Doppler radar and an aircraft during a lake-effect snow storm on 9 December 1978. Evidence is presented that observed wind-parallel echo bands and satellite cloud bands resulted from horizontal-roll convection in the boundary layer. The organization of precipitation within the roll field is discussed, and the roll geometry is compared with cases of dry horizontal-roll convection reported in the literature.

A comparison of surface sensible and latent heat fluxes from aircraft and surface measurements in FIFE 1987

Surface fluxes of sensible and latent heat over a tall grass prairie in central Kansas, as measured by 22 surface stations during FIFE 1987, are compared with values gained indirectly by simple linear extrapolation of aircraft-measured flux profiles to the surface. The results of 32 such comparisons covering the period June 26 to October 13, 1987, indicate that (1) the flux profiles were generally linear, as expected in a convective boundary layer; 2) in general, aircraft-derived surface fluxes were 30% less than the surface averages for sensible heat and 10% less for latent heat; (3) high-pass filters, with cutoffs at 5 km, acted to reduce the uncertainty of the aircraft flux estimates, without changing the magnitude or sign of the disagreement between the aircraft and the surface fluxes; (4) undersampling of high frequencies by the aircraft could have caused as much as a 15% underestimate of the surface sensible heat fluxes; and (5) the aircraft and surface Bowen ratios also disagreed, with the greatest disagreement occurring during the dry periods with decreased plant activity (IFC 4).

Satellite remote sensing of surface energy and mass balance: Results from FIFE

In FIFE a diverse, interdisciplinary group of scientists are cooperating to understand how the earths land-surface vegetation and atmospheric boundary layer interact to affect weather and climate. The focus of this research is the interception of solar and long-wave radiation by a vegetated surface, and the subsequent thermodynamic and biological control of sensible and latent heat release. In addition, the science teams are investigating the use of satellite remote sensing to monitor the components of the surface energy balance at point, regional and global scales. Field experiments were conducted in the summer of 1987 and again in the summer of 1989. Analyses of the 1987 data are reasonably mature, whereas the 1989 data analyses are in the early stages. In this paper we summarize the results of the analyses to date. The analyses have shown that the hypotheses linking energy balance components to surface biology and remote sensing are reasonable at a point level, and that satellite remote sensing can potentially provide useful estimates of the surface energy budget. GOES data have been used to estimate solar insolation at the earths surface to an accuracy of about 21.6 W m−2 and about 8.2 W m−2 for photosynthetically active radiation. The ratio of near-infrared to red reflectance has also shown to be linearly related to measured CO2 flux. Radiometric temperatures estimated from remote sensing are highly correlated to the canopy aerodynamic temperatures; however, the remote sensing estimation of H to useful accuracies (± 100 W m−2) appears to be a more difficult problem than the accurate estimation of LE. The investigation of atmospheric scattering and absorption effects on satellite remote sensing of surface radiance shows that the magnitude of atmospheric opacity variations within the FIFE site and with season can have a large effect on satellite measured values of surface radiances. However, comparisons of atmospherically corrected TM radiances with surface measured radiances agreed to within about 2% in the visible and near-infrared wavelengths and to 6% in the mid-infrared. Looking at the frequency of cloud-free (<10%) acquisitions actually achieved during FIFE shows that for land-surface climatology, multiple polar orbiters may be required to achieve the minimum desired acquisition frequency: once each 5 days.

Intercomparison results for FIFE flux aircraft

Three atmospheric research aircraft were used to explore the atmospheric boundary layer during FIFE: the National Research Council of Canada Twin Otter, the National Center for Atmospheric Research (NCAR) King Air, and the University of Wyoming King Air. The aircraft were used to measure the mean and turbulent structure of the boundary layer and its variation with height, time, and space. These measurements are important to FIFE because they are being used to scale up point surface observations to landscape scales and because they can be used to relate satellite radiance measurements to boundary layer processes. Because the aircraft were used in coordinated flight patterns to investigate changes within and between intensive field campaigns, wing-to-wing intercomparisons were made so that measurements from one aircraft could be related to another. Intercomparisons were flown on 4 days in 1987 and 3 days in 1989. The eddy correlation measurements of the mixed layer fluxes of moisture and sensible heat were of particular interest to FIFE. Sensible heat fluxes agreed within 15 W m−2 and moisture fluxes agreed within 21 W m−2. Mean wind component differences were within 1.0 m s−1, air temperature within 0.3°C, and mixing ratio within 2 g kg−1. Standard deviations showed similar good agreement, with mean differences generally less than 0.1 m s−1 for the wind components and 0.03°C for potential temperature. Intercomparisons between the NCAR King Air and the Twin Otter showed better agreement in 1989 than in 1987. Overall, the results suggest that data from the FIFE boundary layer aircraft will need little correction to account for instrument biases and spurious fluctuations. Researchers should consider these intercomparison results before using combined sets of boundary layer aircraft data.

Evolution of small cumulus clouds in Florida : observations of pulsating growth

Observations have been made in six small cumulus clouds using instrumented aircraft, a ground-based radar, and a 95 GHz airborne Doppler radar. The clouds occurred on two days during the Small Cumulus Microphysics Study in east-central Florida, summer 1995. Cloud tops were below 3 km and in-cloud temperatures were warmer than 10°C. Maximum observed reflectivity factors were less than 0 dBZ. The evolution of the kinematics of the observed clouds was tracked using measurements from both radars. High-resolution cross-sections of reflectivity and vertical Doppler velocity from the airborne radar appear remarkably similar to fine-scale models of convection reported in the literature. In general, each cloud resembled a collection of individual bubbles ascending through the boundary layer. During the growth phase of a bubble, a positive correlation existed between vertical velocity and reflectivity. As bubbles penetrated further into the inversion, entrainment/detrainment led to a weakening or, in some cases, a reversal of this correlation. Growth of subsequent bubbles ascending through remnants of earlier bubbles were aided by an increase in the amount of moisture in the environment resulting from earlier detrainment of cloudy air, and thus were able to achieve higher altitudes than their predecessors.

Airborne radar and in-situ observations of a shallow stratus with drizzle

Abstract Observations were made of a shallow stratus of upslope origin using an aircraft equipped with insitu probes and with a vertically-pointing radar of 3-mm wavelength. A cloud layer of 300 m thickness was found below the inversion; an additional layer of 100 m thickness was located within the inversion. The coldest temperature within the cloud was -2°C and the cloud contained no ice particles. Drizzle drops up to 180 Am were present in both cloud layers. The observations reveal precipitation and air motion structures of approximately 1 km horizontal dimensions. The origin of this organization appears to be weak convection. In addition, mixing played an important role in forming the cloud droplet and drizzle drop size distributions.