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G. R. Stegen
,
C. H. Gibson
, and
C. A. Friehe

Abstract

Vertical fluxes of momentum and sensible heat have been measured above the sea surface by the direct dissipation method. Measurements were made over the open ocean from the Scripps Floating Instrument Platform (FLIP) during the Barbados Oceanographic and Meteorological Experiment (BOMEX). The results are compared with simultaneous measurements of the fluxes by the profile, dissipation, and eddy correlation methods.

The momentum flux was inferred from the rate of viscous dissipation ε above the sea surface. The dissipation was determined by integrating the velocity derivative spectra after correcting the spectra for filter response. The friction velocity (u *) corrected for diabatic effects was 17.4 cm sec−1, corresponding to a shear stress τ=0.35 dyn cm−2. Profile measurements by the University of Washington gave the same value of u * in agreement with the present results. Measurements of momentum flux by Oregon State University (OSU) and the University of British Columbia using dissipation and eddy correlation methods gave somewhat higher values. Correction of the Kolmogoroff inertial subrange constant used in the OSU dissipation calculations gives fluxes in good agreement with the present work.

The sensible heat flux was inferred from the rate of dissipation χ of temperature variance. The temperature derivative spectra were corrected for instrument response and integrated to obtain values of χ. The average value of the sensible heat flux was 0.74 mW cm−2, in reasonable agreement with the profile and eddy correlation measurements. A value of sensible beat flux of 2.8 mW cm−2 has been reported by OSU using the dissipation technique. Correction of the temperature inertial subrange constant used by OSU lowered their heat flux to 1.1 mW cm−2.

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K. F. Schmitt
,
C. A. Friehe
, and
C. H. Gibson

Abstract

Anomalous results concerning the micrometeorological temperature field in the boundary layer over the ocean have been obtained in many recent experiments. These include lack of an inertial-convective subrange in temperature spectra, unusually large values for the scalar universal subrange constant, underestimation of the sensible heat flux by the bulk aerodynamic formula, gross imbalance of dissipation and production terms in the temperature variance budget equation, and dissimilarities of the temperature and humidity statistics and time traces. Empirically it has been observed that such results occur for unstable conditions when the temperature time series is characterized by a peculiar waveform, termed a “cold spike”, which has no counterpart in the humidity field and has not been observed over land.

To explain these results, it is proposed that surfaces of the small temperature sensors (thermistors, thermocouples and resistance wires) commonly used in marine boundary layer experiments become contaminated with salt spray when used over the ocean. Under typical ocean conditions (relative humidity > 70%), the results of Twomey (1953) indicate that the spray will exist as saline drops on the probe surfaces. Water will evaporate from or condense on the saline drops as the humidity around the sensor decreases or increases, respectively. The latent heat of vaporization associated with the evaporation and condensation processes will cool and heat the sensor, and therefore generate erroneous temperature signals. Evidence is presented that most of the anomalous temperature results observed over the ocean, including “cold spikes”, may be due to the spray-induced humidity sensitivity of such temperature sensors.

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A. S. Frisch
,
D. H. Lenschow
,
C. W. Fairall
,
W. H. Schubert
, and
J. S. Gibson

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 boundary layer, with the downdrafts smaller in area and more intense than the updrafts. In the lower part of the cloud, the behavior of the motion is more like a conventional convective boundary layer, with the updrafts smaller and more intense than the downdrafts. The upside-down convective forcing in the upper part of the cloud is due to radiative cooling, with the daytime forcing less because of shortwave warming.

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E. C. Massoud
,
H. Lee
,
P. B. Gibson
,
P. Loikith
, and
D. E. Waliser

Abstract

This study utilizes Bayesian model averaging (BMA) as a framework to constrain the spread of uncertainty in climate projections of precipitation over the contiguous United States (CONUS). We use a subset of historical model simulations and future model projections (RCP8.5) from the Coupled Model Intercomparison Project phase 5 (CMIP5). We evaluate the representation of five precipitation summary metrics in the historical simulations using observations from the NASA Tropical Rainfall Measuring Mission (TRMM) satellites. The summary metrics include mean, annual and interannual variability, and maximum and minimum extremes of precipitation. The estimated model average produced with BMA is shown to have higher accuracy in simulating mean rainfall than the ensemble mean (RMSE of 0.49 for BMA versus 0.65 for ensemble mean), and a more constrained spread of uncertainty with roughly a third of the total uncertainty than is produced with the multimodel ensemble. The results show that, by the end of the century, the mean daily rainfall is projected to increase for most of the East Coast and the Northwest, may decrease in the southern United States, and with little change expected for the Southwest. For extremes, the wettest year on record is projected to become wetter for the majority of CONUS and the driest year to become drier. We show that BMA offers a framework to more accurately estimate and to constrain the spread of uncertainties of future climate, such as precipitation changes over CONUS.

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Shashi K. Gupta
,
Nancy A. Ritchey
,
Anne C. Wilber
,
Charles H. Whitlock
,
Gary G. Gibson
, and
Paul W. Stackhouse Jr.

Abstract

Climatological averages of surface radiation budget parameters, namely, the shortwave and longwave surface radiative fluxes, have been derived for each month of the year on a global scale. These climatological averages were derived from an 8-yr (96 month) time series of monthly average fluxes. The monthly averages were computed using fast radiation parameterizations and satellite data from the International Satellite Cloud Climatology Project and the Earth Radiation Budget Experiment. Results are presented as time series of hemispheric and global averages and as geographical distributions and time–latitude cross sections of climatological averages. The spatial/temporal variabilities of the results were found to be clearly related to the corresponding variabilities of meteorological and other inputs to the parameterizations. Numerous comparisons of the present results were made with available surface measurements for the purpose of validation. In most cases, the differences were found to be within the uncertainties of the measurements. In some cases, where they were large, the differences were attributable to identifiable deficiencies in the meteorological inputs and/or the surface measurements. However, large differences remained unexplained in a few cases. Anomalies of shortwave and longwave surface fluxes during the 1986/87 El Niño–Southern Oscillation episode show a strong relationship with corresponding top-of-atmosphere anomalies derived from an independent data source. Comparisons with results from several general circulation models showed large differences, but, in most cases, these were attributable to well-recognized deficiencies in model simulations. Global annual average downward and net shortwave fluxes were found to be about 185 and 161 W m−2, respectively. These values are 10–20 W m−2 lower than those obtained from the general circulation models, but they are in good agreement with other satellite-derived estimates. Global annual average downward and net longwave fluxes were found to be about 348 and −48 W m−2, respectively, which are about 10–15 W m−2 higher than corresponding values from general circulation models. Atmospheric shortwave absorption derived from the present results is 10–15 W m−2 larger than from the general circulation models, but it is in good agreement with another estimate based on satellite data.

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