Christopher W. Fairall

Measurement of Stratus Cloud and Drizzle Parameters in ASTEX with a Kα-Band Doppler Radar and a Microwave Radiometer

Abstract Data are used from a Kα-band radar and microwave radiometer along with a droplet model to determine the droplet parameters of drizzle and clouds. Drizzle droplet parameters are determined from the zeroth, first, and second moments of the Doppler spectrum. Cloud droplet parameters are determined from the zeroth moment of the Doppler spectrum and the measured integrated liquid water. Measurements of stratus clouds were made during the Atlantic Stratocumulus Transition Experiment (ASTEX) on the island of Porto Santo in the Madeira Islands, Portugal. Potential applications of this technique would be in the long-term monitoring of stratus clouds and in determining the vertical profiles of cloud liquid water, number of cloud droplets, and modal radius.

Direct Covariance Flux Estimates from Mobile Platforms at Sea

This paper describes two methods for computing direct covariance fluxes from anemometers mounted on moving platforms at sea. These methods involve the use of either a strapped-down or gyro-stabilized system that are used to compute terms that correct for the 1) instantaneous tilt of the anemometer due to the pitch, roll, and heading variations of the platform; 2) angular velocities at the anemometer due to rotation of the platform about its local coordinate system axes; and 3) translational velocities of the platform with respect to a fixed frame of reference. The paper provides a comparison of fluxes computed with three strapped-down systems from two recent field experiments. These comparisons shows that the direct covariance fluxes are in good agreement with fluxes derived using the bulk aerodynamic method. Additional comparisons between the ship system and the research platform FLIP indicate that flow distortion systematically increases the momentum flux by 15%. Evidence suggests that this correction is appropriate for a commonly used class of research vessels. The application of corrections for both motion contamination and flow distortion results in direct covariance flux estimates with an uncertainty of approximately 10%‐20%.

Stratocumulus Cloud Properties Derived from Simultaneous Satellite and Island-based Instrumentation during FIRE

Abstract Cloud parameters derived from visible and infrared window data from the Geostationary Operational Environmental Satellite (GOES) are compared to corresponding properties determined from instrumentation on San Nicolas Island off the coast of California during the First ISCCP Regional Experiment (FIRE) marine stratocumulus intensive field observations period in July 1987. Mean cloud amounts derived by applying the hybrid bispectral threshold method to the GOES data were 5% less than the island ceilometer measurements. Examination of the satellite imagery revealed that the apparent bias can be explained by the persistence of the clouds over the northwest part of the island during periods of clearing around the island. Diurnal variations in the cloud cover were very significant; minimum cloudiness occurred during the late afternoon and maximum cloudiness early in the morning. The satellite retrievals track this variation quite well. Satellite-derived mean cloud-top height is 53 m lower than that obse...

Radar Observations of Humidity Variability in and above the Marine Atmospheric Boundary Layer

Abstract Humidity variability at the top of the marine atmospheric boundary layer and in the overlying free troposphere was examined using data collected during the marine stratocumulus phase of the First Regional Experiment (FIRE) of the International Satellite Cloud Climatology Program. A time series of the humidity structure-function parameter Cq2 derived from Doppler wind profiler reflectivity data is compared to a concurrent time series of specific humidity q. Both q and its vertical gradient were calculated from rawinsonde data obtained from sondes launched within 500 m of the profiler. Time-height correlation analysis between log(Cq2) and log(∂q/∂z)2 shows that the two time series are highly correlated at and just above the inversion base, with r approximately equal to 0.7. The correlation is slightly lower in the free troposphere where r is about 0.5 (a value of r greater than 0.2 is significant at the 95% confidence level). There is also correlation between log(Cq2) and log(q), which is maximized...

Ocean–Cloud–Atmosphere–Land Interactions in the Southeastern Pacific: The VOCALS Program

The present paper describes the Variability of the American Monsoon Systems (VAMOS) Ocean–Cloud–Atmosphere–Land Study (VOCALS), an international research program focused on the improved understanding and modeling of the southeastern Pacific (SEP) climate system on diurnal to interannual time scales. In the framework of the SEP climate, VOCALS has two fundamental objectives: 1) improved simulations by coupled atmosphere–ocean general circulation models (CGCMs), with an emphasis on reducing systematic errors in the region; and 2) improved estimates of the indirect effects of aerosols on low clouds and climate, with an emphasis on the more precise quantification of those effects. VOCALS major scientific activities are outlined, and selected achievements are highlighted. Activities described include monitoring in the region, a large international field campaign (the VOCALS Regional Experiment), and two model assessments. The program has already produced significant advances in the understanding of major issue...

Ice pack and lead surface energy budgets during LEADEX 1992

During a 1-month deployment for the Arctic Leads Experiment (LEADEX) in March and April 1992 on the Arctic ice cap roughly 200 km north of Prudhoe Bay, Alaska, surface-based mean meteorological and flux instruments plus a variety of remote sensors were operated at the main base camp. Identical systems were also deployed by helicopter on the upwind and downwind edges of several Arctic leads, two of which we describe in this paper. At the base camp the diurnal amplitude for sensible heat flux was +10 W m -2 about a mean of-3 W m -2, net radiation was +30 W m -2 about a mean of -15 W m 2, and net surface energy flux was +20 W m -2 about a mean of-12 W m -2. The mean latent heat flux was +1 W m -2 with a diurnal variation of about +1.5 W m -2. Mean values for the momentum and sensible heat transfer coefficients were C o = (1.20 + 0.20) x 10 3 and Cn = (0.75 + 0.25) x 10 -3 at a 10-m reference height with only modest diurnal variations. Two lead deployments were examined. Lead 3 was approximately 1 km across. Only limited meteorological data were obtained for about 6 hours at the end of April 7 and beginning of April 8 when the lead was covered with about 10 cm of ice. Downwind of the lead, the sensible heat flux increased to about 170 W m -2 and the stress doubled, suggesting an ice-covered lead 10-m drag coefficient of 2.2 x 10 -3. More than 36 hours of data were obtained upwind and downwind of lead 4, which varied in width from 80 to 120 m. Doppler minisodars upwind and downwind of the lead indicated a doubling in the depth (5 to 10 m) of the shear-driven turbulent surface layer downwind of the lead and an intensification of intermittent wave interactions exceeding 60 m (sodar range maximum). Three prominent waves with strong downward motion were observed in this period, apparently causing increases in the downwind stress magnitude. Various sources of data were used to compute estimates over a 36-hour period of the net surface heat flux Qg over the lead, the adjacent pack ice, and any open water that might have occurred in the lead. The results indicate that once significant ice forms, the sun is increasingly more effective in reducing the surface freezing rate and in shutting off convective mixing in the ocean under the lead. Over the period of observations the average net surface heat flux was -75 W m -2 over the pack ice, -130 W m -2 over the lead, and -250 W m 2 over the open water.

Doppler radar measurements of turbulence in marine stratiform cloud during ASTEX

Abstract A cloud-sensing Doppler radar is used with a vertically pointing antenna to measure the vertical air motion in clouds during the Atlantic Stratocumulus Transition Experiment. The droplet fall velocity contamination was made negligible by using only measurements during the time the reflectivity was below − 17 dBZ. During one day of measurements, the daytime character of the vertical velocity variance is different than that of the nighttime case. In the upper part of the cloud, the variance had a distinct maximum for both day and night; however, the nighttime maximum was about twice as large as the daytime case. Lower down in the cloud, there was a second maximum, with the daytime variance larger than the nighttime case. The skewness of the vertical velocity was negative near cloud top in both the day and night cases, changing to positive skewness in the lower part of the cloud. This behavior near cloud top indicates that the upper part of the cloud is behaving like an upside-down convective bounda...

An Eight-Month Sample of Marine Stratocumulus Cloud Fraction, Albedo, and Integrated Liquid Water

Abstract As part of the First International Satellite Cloud Climatology Regional Experiment (FIRE), a surface meteorology and shortwave/longwave irradiance station was operated in a marine stratocumulus regime on the northwest tip of San Nicolas island off the coast of Southern California. Measurements were taken from March through October 1987, including a FIRE Intensive Field Operation (IFO) held in July. Algorithms were developed to use the longwave irradiance data to estimate fractional cloudiness and to use the shortwave irradiance to estimate cloud albedo and integrated cloud liquid water content. Cloud base height is estimated from computations of the lifting condensation level. The algorithms are tested against direct measurements made during the IFO; a 30% adjustment was made to the liquid water parameterization. The algorithms are then applied to the entire database. The stratocumulus clouds over the island are found to have a cloud base height of about 400 m, an integrated liquid water content ...

Surface-Based Remote Sensing of Marine Boundary-Layer Cloud Properties

Abstract Surface-based measurements are used to define some of the important macrophysical and optical properties of marine clouds. These measurements were taken during five different marine field programs. A progression is made from a midlatitude marine stratocumulus regime with an average cloud fraction of 0.7 and a median cloud base of 460 m to a marine tropical regime with an average cloud fraction of 0.2 and a median cloud base of 1050 m. Measurements of the solar transmission coefficient taken during the Atlantic Stratocumulus Transition Experiment (ASTEX) were used in a radiative transfer algorithm to produce values of albedo, absorption, and optical depth. A microwave radiometer provided measurements of the liquid water path (LWP). For a given LWP, the ASTEX optical depths averaged a factor of 2 smaller than the optical depths observed during the marine stratocumulus phase of the First International Cloud Climatology Program Regional Experiment (FIRE) at San Nicolas Island, off the coast of southe...

A Stochastic Model of Gravity-Wave-Induced Clear-Air Turbulence

Abstract We examine the consequences of using a vertical wavenumber spectral model to describe variations of vertical profiles of atmospheric variables (horizontal and vertical wind, temperature, and other scalars) about a mean profile. At high wavenumbers the model exhibits a wavenumber to the -3 dependence, which is characteristic of a continuum of internal gravity waves whose amplitudes are controlled by a breaking process. By employing a random phase between wavenumber amplitude components, a reverse fourier transform of the spectrum yields simulated profiles of velocity and thermal variability as well as shear and Brunt–Vaisala frequency variability. Individual components of the vertical shear of the horizontal wind and the Brunt–Vaisala frequency exhibit Gaussian distributions; the square of the magnitude of the shear exhibits a Rice–Nakagami distribution. Assuming regions with Ri < 0.25 are turbulent, we can examine a number of aspects of the occurrence of clear-air turbulent breakdown in the strat...