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Boers, Reinout

Publications and source records attributed to Boers, Reinout.

17 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS1991

Saturation point representation of cloud-top entrainment instability

Cloud-top entrainment instability was investigated using a mixing line analysis. Mixing time scales are closely related to the actual size of the parcel, so that local instabilities are largely dependent on the scales of mixing near the cloud top. Given a fixed transport velocity, variation over a small range of parcel length scales (parcel mixing velocities) turns an energy-producing mixing process into an energy-consuming mixing process. It is suggested that a single criterion for cloud-top entrainment instability will not be found due to the role of at least three factors operating more or less independently; the stability of the mixing line, the entrainment speed, and the strength of the internal boundary-layer circulation.

Boers, Reinout↗
NASA NTRS1991

Cold-air outbreak during GALE - Lidar observations and modeling of boundary layer dynamics

Two cold-air outbreaks were studied during the Genesis of Atlantic Lows Experiment. A lidar system was operated to observe the boundary layer evolution and the development of clouds. On the first day (January 30, 1986) boundary layer rise was less than 50 percent of the value for the second day (March 2, 1986). On the first day only a thin broken cloud cover formed, while on the second day a thick solid cloud deck formed - although the average moisture content was 60 percent of that on the first day. A trajectory slab model was employed to simulate the evolution of the layer over the ocean near the east Atlantic shore. The model allows for vertical gradients in conservative variables under neutrally buoyant conditions. The primary effect of these assumptions, which are based on observed thermodynamic profiles, is to reduce cloudiness to be more in line with observations. Boundary-layer depth was reasonably well predicted as was sensible and latent heat flux.

Boers, Reinout↗
NASA NTRS1990

Analysis of ER-2 radiometer and lidar observations from the 1987 marine stratus experiment

Extensive remote sensing observations of marine stratus clouds were acquired from the NASA ER-2 aircraft during the 1987 FIRE stratus field experiment. The observations included high spatial resolution imaging radiometry at eighteen wavelengths and active lidar cloud top profiling. Analysis of these data can provide results to both enhance and extend aircraft in-situ cloud physics observations. For the spectral imaging radiometry observations of the marine stratus, a potentially useful result was obtained. For near infrared wavelengths, a large number of the bidirectional reflectance observations included persistent and significant single scattering features. With few exceptions, the features were not correspondingly observed at visible wavelengths. An initial comparison to direct cloud physics measurements give a good agreement (Spinhirne and Nakajima, 1989). The analysis of lidar returns from marine stratus cloud tops were shown to be potentially valuable for study of the microphysical and dynamic characteristics of the clouds (Boers et al., 1988, Spinhirne et al., 1989). Analysis involving cloud top water content and height spectra was applied to some of the extensive lidar observations of the 1987 field experiment. Derivation of cloud top radiative parameters and comparison to in-situ observations are planned.

Spinhirne, James D.↗
NASA NTRS1990

A cloudiness transition in a marine boundary layer

Boundary layer cloudiness plays several important roles in the energy budget of the earth. Low level stratocumulus are highly reflective clouds which reduce the net incoming shortwave radiation at the earth's surface. Climatically, the transition to a small area fraction of scattered cumulus clouds occurs as the air flows over warmer water. Although these clouds reflect less sunlight, they still play an important role in the boundary layer equilibrium by transporting water vapor upwards, and enhancing the surface evaporation. The First ISCCP (International Satellite Cloud Climatology Project) Regional Experiment (FIRE) included a marine stratocumulus experiment off the southern California coast from June 29 to July 19, 1987. The objectives of this experiment were to study the controls on fractional cloudiness, and to assess the role of cloud-top entrainment instability (CTEI) and mesoscale structure in determining cloud type. The focus is one research day, July 7, 1987, when coordinated aircraft missions were flown by four research aircraft, centered on a LANDSAT scene at 1830 UTC. The remarkable feature of this LANDSAT scene is the transition from a clear sky in the west through broken cumulus to solid stratocumulus in the east. The dynamic and thermodynamic structure of this transition in cloudiness is analyzed using data from the NCAR Electra. By averaging the aircraft data, the internal structure of the different cloud regimes is documented, and it is shown that the transition between broken cumulus and stratocumulus is associated with a change in structure with respect to the CTEI condition. However, this results not from sea surface temperature changes, but mostly from a transition in the air above the inversion, and the breakup appears to be at a structure on the unstable side of the wet virtual adiabat.

Betts, Alan K.↗
NASA NTRS1990

Thermodynamic structure of the stratocumulus-capped boundary layer on 5 July 1987

The National Center for Atmospheric Research (NCAR) Electra flew a single aircraft mission on July 5 in comparatively uniform stratocumulus cloud (80 to 100 percent cloud coverage). The flight pattern, a series of north-south doglegs centered at (32N, -122W), included 15 full or partial soundings to examine layer vertical structure between altitudes of 150 and 5000 m. The air below the inversion was well-mixed in moist static energy, total water, and ozone. The inversion base measured by the aircraft ranged from 920 to 938 hPa, while the thickness of the cloud layer varied between 30 and 10 hPa. Maximum cloud liquid water contents approached 0.3 g kg(-1) (as measured by the Johnson-Williams and King probes). Comparison of soundings made in the same location show a strengthening of the inversion and increasing shear at cloud top over the observation period (16:50 to 00:22 UT). Typical windspeeds below the inversion were approximately 10 m s(-1) decreasing to 6 m s(-1) 8 hPa above cloud top. A thermodynamic budget for this case is in preparation; it will provide a comparison and contrast with the more complicated July 7 case.

Austin, Philip H.↗
NASA NTRS1990

ER-2 lidar measurements of stratocumulus cloud top structure on July 14, 1987

On July 14, 1987 NASA's ER-2 high altitude aircraft flew a mission to measure the structure of stratocumulus clouds off the coast of California. A flight pattern was executed so that the two-dimensional variability of the clouds could be detected. The technique of analysis of the lidar data to measure cloud tops follows. First each signal is searched for its maximum in return strength. This maximum is caused by scattering of the laser light off cloud particles or from the ocean surface. Next the variance of the signal return above the level of maximum backscatter is determined. Cloud top is assigned to a level (above the level of maximum backscatter) where the backscatter exceeds the average variance. This two-step process is necessary because the level of maximum backscatter does not correspond to the cloud top. Ocean surface returns are easily separated from cloud returns in this process, described in detail by Boers, Spinhirne, and Hart (1988). Analysis of the data so far has shown that there were very few breaks in the clouds. Furthermore the layer top was very flat with local oscillations not exceeding 30 m. Such small cloud top variations are still well within the range of detectability, because the precision of this technique of cloud top detection has previously been established to be 13 to 15 m. Data are presently being analyzed to compute cloud top distributions and fractional cloudiness. The aim of this research is to relate the fractional cloudiness to the mean thermodynamic structure of the boundary layer. Researchers plan to compute spectral scales of the cloud top variability in two dimensions to determine the orientation of the clouds with respect to the mean wind. Furthermore the lidar derived cloud top distribution will be used in the computation of the thermodynamic and radiation budget of the boundary layer.

Boers, Reinout↗
NASA NTRS1990

Comparison and calibration of NCAR electra instruments: July 5 and 7, 1987

Measurements of temperature, water vapor, cloud liquid water, and lidar cloud height taken by the National Center for Aeronautical Research (NCAR) Electra are analyzed for two days of the First ISCCP Regional Experiment (FIRE) stratocumulus field program. Examination of instrument time series, correlations between sensors, soundings through the layer, and lidar measurements of cloud base height are used to correct sensor offset problems, check for probe wetting, and choose the most accurate measurements of temperature, humidity, and cloud liquid water.

Austin, Philip H.↗
NASA NTRS1990

Thermodynamic structure of the stratocumulus-capped boundary layer on 7 July, 1987

As part of project First ISCCP Regional Experiment (FIRE), a mission was carried out on 7 July 1987 to study the thermodynamic structure of a boundary layer which is in transition from a clear to a cloudy state. The National Center for Aeronautical Research (NCAR) Electra flew a pattern in tight coordination with the NASA ER-2 aircraft near 122 West, 31.6 North off the coast of California. A description is given here of the thermodynamic structure. The purpose is to derive the entrainment rate and the fluxes of the thermodynamic variables. To this end researchers represent the data in conserved variable diagrams.

Boers, Reinout↗
NASA NTRS1990

A cloudiness transition in a marine boundary layer

In situ aircraft data and lidar data are used to analyze a transition in the boundary layer thermodynamic structure from a clear boundary layer through small cumulus and broken stratocumulus to a deck of solid stratocumulus. The data was collected in conjunction with a Landsat overpass on July 7, 1987 off the coast of southern California. A steady progression in mixing line stability is seen associated with the change in cloudiness. The (empirically based) stability threshold for the breakup of this stratocumulus is that the slope of the mixing line is 0.66 + or - 0.04 of the slope of the wet virtual adiabat. A simple linear parameterization is proposed for cloud fraction in terms of mixing line stability. Surface flux measurements are consistent with bulk aerodynamic estimates.

Boers, Reinout↗
NASA NTRS1989

A parameterization of the depth of the entrainment zone

A theory of the parameterization of the entrainment zone depth has been developed based on conservation of energy. This theory suggests that the normalized entrainment zone depth is proportional to the inverse square root of the Richardson number. A comparison of this theory with atmospheric observations indicates excellent agreement. It does not adequately predict the laboratory data, although it improves on parcel theory, which is based on a momentum balance.

Boers, Reinout↗
NASA NTRS1988

Lidar observations of the fine-scale variability of marine stratocumulus clouds

An Nd:YAG lidar system was flown aboard NASA's ER-2 high altitude aircraft. Observations of cloud top height were made with 70 m along-track and 7.5 m vertical-height resolution. The lidar data observed from an East Pacific stratocumulus cloud height deck revealed large cloud variability on 1-5 km scales. The cloud deck sloped upward from 700 to 1000 m in a northeast-southwest direction over a distance of 120 km. Vertical cloud top distributions were negatively skewed indicating flat-topped clouds. The dominant spectral peak of the cloud top variations was found at 4.5 km, which is 5 to 7 times the depth of the local boundary layer. No other peaks were significant in the average spectrum. The cloud layer was stable with respect to cloud top entrainment instability. The southwestern region of the study area was more prone to shear instability at cloud top than the northeastern region. The results of this study show that a lidar system is ideal to provide the topography of clouds and local boundary layer depth. This information is useful in the study of cloud top radiation and parameterization of clouds in numerical models.

Boers, Reinout↗
NASA NTRS1988

Genesis of Atlantic Lows Experiment NASA Electra Boundary Layer Flights Data Report

The objective of this research was to obtain high resolution measurements of the height of the Marine Atmospheric Boundary Layer (MABL) during cold air outbreaks using an Airborne Lidar System. The research was coordinated with other investigators participating in the Genesis of Atlantic Lows Experiment (GALE). An objective computerized scheme was developed to obtain the Boundary Layer Height from the Lidar Data. The algorithm was used on each of the four flight days producing a high resolution data set of the MABL height over the GALE experiment area. Plots of the retrieved MABL height as well as tabular data summaries are presented.

Palm, Stephen P.↗
NASA NTRS1988

Saturation point structure of marine stratocumulus clouds

An investigation of the microstructure of a Pacific stratocumulus capped boundary layer is presented. A complex structure of three branches, identified using conserved variable diagrams, is found to correspond well to a conceptual model for the unstable, radiatively cooled cloud topped boundary layer. A simple conditional sampling method was used to identify saturation point pairs for ascending and descending branches of the internal boundary layer circulation. Results indicate a primary circulation scale of 5 km and provide a reasonable cloud top entrainment rate of 1 cm/s.

Boers, Reinout↗
NASA NTRS1988

Efficiency of shear induced entrainment in convectively mixed layers

Lidar data of mixed layer height are used to compare the model of process partitioning to that of Eulerian partitioning. It is found that the Eulerian partitioning model more closely follows the data and that the process partitioning model underpredicts the mixed layer depth. The results suggest that shear production and consumption of turbulent kinetic energy at the mixed layer top are local processes.

Boers, Reinout↗