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Joseph S. Hogan and Richard W. Stewart

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Richard W. Stewart and Joseph S. Hogan

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S. I. Rasool and R. W. Stewart

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The S-band occultation experiments have provided important observational data on the structure of the atmospheres of Mars and Venus. The results for Mars show a very cold region in the middle atmosphere where CO2, the main constituent of the Martian atmosphere, will saturate and may condense. Also, at the surface boundary of Mars there appears to be a temperature discontinuity between the air and ground in the local afternoon and a temperature inversion in the atmosphere in the nightside of the planet. On Venus, the Mariner 5 results show that the diurnal variation of temperature is small not only in the lower atmosphere but also in the stratosphere, implying strong zonal mixing at those levels. Also, the amplitude data from the Mariner 5 experiment has provided evidence for the existence of cloud layer or layers in the lower atmosphere where the ambient temperatures range between 350 and 450K. Such high temperatures preclude water as the constituent of these clouds. As for the upper atmospheres of both Mars and Venus, the occultation experiments indicate that the ionospheres of these planets contain up to a factor of two more electrons than can be explained in terms of the presently accepted values of the EUV flux. At the same time the exospheric temperature of Mars appears to be as low as 350–450K, about 100K lower than the closest predicted value.

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R. E. Stewart, C. A. Lin, and S. R. Macpherson

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On 22 February 1986 Nova Scotia experienced heavy precipitation in the form of snow, freezing precipitation, and rain from a storm having a central pressure no lower than 99.3 kPa. Using observations obtained during the Canadian Atlantic Storms Program (CASP) field project, the mesoscale structure of this storm was investigated. Throughout much of the storm, the lowest 1–3 km of the atmosphere over the coastline was near 0°C as a result of the diabatic process of melting and refreezing. Convergent flow aloft and the trajectories of particles undergoing terminal velocity changes contributed to enhanced precipitation near the coastline that was sometimes detected by radar as a precipitation band straddling the coastline. A mesoscale circulation, driven by melting and forced to remain linked to the coastline between the warm ocean and the cold land, is consistent with the observations.

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S. Pond, R. W. Stewart, and R. W. Burling

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Further support to Kolmogoroff's theory of local isotropy is provided by hot-wire measurements of fluctuations of downwind velocities at a height of 1 to 2 m above the sea surface. The results agree with those obtained from observations made in a tidal stream by Grant, Stewart and Moilliet (1962) and with measurements of skewness of velocity differences in air over land, made by Gurvich (1960).

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S. I. Rasool, J. S. Hogan, R. W. Stewart, and L. H. Russell

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S. Pond, G. T. Phelps, J. E. Paquin, G. McBean, and R. W. Stewart

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This paper describes results of measurements of the fluxes of momentum, moisture and sensible heat by both the eddy correlation and “dissipation” techniques. The data were collected on the R/V Flip during BOMEX (Barbados Oceanographic and Meteorological Experiment) and during a pre-BOMEX trial cruise near San Diego in February 1969. The results are mainly based on data collected by personnel from Oregon State University and the University of British Columbia. We are grateful to the University of Washington personnel who have made their data and results available to us to check some of our results and allowed us to use their temperature fluctuation data from the San Diego cruise when our equipment failed to provide such data.

The methods of determining the fluxes are discussed. The instrumentation and methods of data analysis are described. The effects of Flip's interference on the flow are described and the method of removing the interference from the results is given. The spectra of the three components of velocity fluctuations and the cospectra between the vertical velocity fluctuations w and the downstream velocity u, temperature T, and humidity q fluctuations are presented. The fluxes determined by the eddy correlation method are compared with fluxes estimated from the rates of dissipation of kinetic energy and scalar fluctuations. These fluxes are then used to evaluate the constants in the bulk aerodynamic formulas for estimating the fluxes.

The normalized velocity component spectra and the normalized uw cospectra appear to have universal forms and are similar to earlier results. The normalized wT cospectra do not appear to have a universal form. The normalized wq cospectra do appear to have a universal form and are very similar to the normalized uw cospectra. As has been found before, the dissipation and eddy correlation methods agree quite well on the average for the momentum flux. The two methods do not give the same results for the sensible heat flux for BOMEX although there is fair agreement for the small number of San Diego results. The two methods do give good agreement for the moisture flux. Comparison of the eddy correlation flux for momentum with the mean wind speed squared leads to a drag coefficient of 1.5 × 10−3. The sensible heat flux, however, does not show a good relationship with the mean wind speed times the mean sea-air temperature difference during BOMEX. For the San Diego results the relationship is fair and similar to other measurements. The moisture flux shows a strong correlation with the wind speed times the mean sea-air humidity difference. The non-dimensional aerodynamic evaporation coefficient (corresponding to the drag coefficient for momentum) was found to be 1.2 × 10−3 with an uncertainty of about 20%. This result based on direct measurements of the flux agrees rather well with some earlier indirect estimates based on evaporation pan data.

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Peter E. Goble, Nolan J. Doesken, Imke Durre, Russ S. Schumacher, Abigail Stewart, and Julian Turner

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Every day, thousands of volunteers across the United States report the amount of precipitation they have received in the past 24 hours. This study focuses on the largest of these volunteer-submitted reports for each day, using precipitation measurements from the Community Collaborative Rain, Hail and Snow Network (CoCoRaHS) from January 2010 to December 2017 as well as observations from the U.S. Cooperative Observer Program (COOP) network from January 1981 through December 2017. Results provide clarity on spatial variability, temporal variability, and seasonal cycle of contiguous U.S daily precipitation extremes (DPEs). During 2010–17, the DPEs ranged from 11 mm on 28 March 2013 in Oregon to 635 mm on 27 August 2017 in Texas during Hurricane Harvey. Coastal states are most prone to high daily precipitation totals, especially those bordering the Gulf of Mexico or Atlantic Gulf Stream. The average DPE value varies with season; it is greater than 175 mm in late August and less than 100 mm through meteorological winter. These observations also show that location of the DPE varies with season as well. For example, 28.5% of February extremes fall in Pacific states, whereas all August extremes occur east of that region. Perhaps most importantly, these findings demonstrate strength in numbers. The large daily sample size of CoCoRaHS and COOP networks forms a basis for monitoring, mapping, and categorizing DPEs, and other aspects of extreme precipitation, with considerable spatial detail.

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P. E. Goble, N. J. Doesken, I. Durre, R. S. Schumacher, A. Stewart, and J. Turner
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J. Simpson, E. Ritchie, G. J. Holland, J. Halverson, and S. Stewart

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With the multitude of cloud clusters over tropical oceans, it has been perplexing that so few develop into tropical cyclones. The authors postulate that a major obstacle has been the complexity of scale interactions, particularly those on the mesoscale, which have only recently been observable. While there are well-known climatological requirements, these are by no means sufficient.

A major reason for this rarity is the essentially stochastic nature of the mesoscale interactions that precede and contribute to cyclone development. Observations exist for only a few forming cases. In these, the moist convection in the preformation environment is organized into mesoscale convective systems, each of which have associated mesoscale potential vortices in the midlevels. Interactions between these systems may lead to merger, growth to the surface, and development of both the nascent eye and inner rainbands of a tropical cyclone. The process is essentially stochastic, but the degree of stochasticity can be reduced by the continued interaction of the mesoscale systems or by environmental influences. For example a monsoon trough provides a region of reduced deformation radius, which substantially improves the efficiency of mesoscale vortex interactions and the amplitude of the merged vortices. Further, a strong monsoon trough provides a vertical wind shear that enables long-lived midlevel mesoscale vortices that are able to maintain, or even redevelop, the associated convective system.

The authors develop this hypothesis by use of a detailed case study of the formation of Tropical Cyclone Oliver observed during . In this case, two dominant mesoscale vortices interacted with a monsoon trough to separately produce a nascent eye and a major rainband. The eye developed on the edge of the major convective system, and the associated atmospheric warming was provided almost entirely by moist processes in the upper atmosphere, and by a combination of latent heating and adiabatic subsidence in the lower and middle atmosphere. The importance of mesoscale interactions is illustrated further by brief reference to the development of two typhoons in the western North Pacific.

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