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C. A. Pearson

A photoelectric telephotometer, which measured the illumination produced at a fixed distance from a light source of known candle power, was developed to record directly the total horizontal atmospheric transmission of light at night over a two sea-mile path. The apparatus consisted of a multiplier phototube, a DC amplifier, batteries for operation, and a one-milliampere recorder. It can be set up easily for field use. A 1000-watt projection-type lamp was used as a light source. By means of a standard lamp, the photoelectric telephotometer was calibrated in a laboratory light tunnel to record illumination in sea-mile candles. In the field the photoelectric telephotometer was compared with a visual telephotometer. Transmission measurements showed good agreement between the photoelectric and the visual methods.

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C. A. Pearson, M. J. Koomen, and R. Tousey

Measurements of the visual transmission of the atmosphere were made at night across Chesapeake Bay over a period of two years. A visual telephotometer was used to measure the illumination produced at a distance of 8.77 sea miles by a series of calibrated light sources. With this range it was possible to measure atmospheric transmission values between 0.4 and 0.9 per sea mile corresponding to daylight visual ranges from 4 to 40 sea miles with an accuracy of ± 2 percent. Under stable atmospheric conditions the data obtained at night were in good agreement with direct observations of the daylight visual range made before sunset and after the following dawn.

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B. A. White, A. M. Buchanan, C. E. Birch, P. Stier, and K. J. Pearson

Abstract

The organization of deep convection and its misrepresentation in many global models is the focus of much current interest. A new method is presented for quantifying convective organization based on the identification of convective objects and subsequent derivation of object numbers, areas, and separation distances to describe the degree of convective organization. These parameters are combined into a “convection organization potential” based on the physical principle of an interaction potential between pairs of convective objects. This technique is applied to simulated and observed fields of outgoing longwave radiation (OLR) over the West African monsoon region using data from Met Office Unified Model simulations and satellite observations made by the Geostationary Earth Radiation Budget (GERB) instrument. The method is evaluated by using it to quantify differences between models with different horizontal grid lengths and representations of convection. Distributions of OLR, precipitation and organization parameters, the diurnal cycle of convection, and relationships between the meteorology in different states of organization are compared. Switching from a configuration with parameterized convection to one that allows the model to resolve convective processes at the model grid scale is the leading-order factor improving some aspects of model performance, while increased model resolution is the dominant factor for others. However, no single model configuration performs best compared to observations, indicating underlying deficiencies in both model scaling and process understanding.

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Brodie C. Pearson, Alan L. M. Grant, Jeff A. Polton, and Stephen E. Belcher

Abstract

This study uses large-eddy simulation to investigate the structure of the ocean surface boundary layer (OSBL) in the presence of Langmuir turbulence and stabilizing surface heat fluxes. The OSBL consists of a weakly stratified layer, despite a surface heat flux, above a stratified thermocline. The weakly stratified (mixed) layer is maintained by a combination of a turbulent heat flux produced by the wave-driven Stokes drift and downgradient turbulent diffusion. The scaling of turbulence statistics, such as dissipation and vertical velocity variance, is only affected by the surface heat flux through changes in the mixed layer depth. Diagnostic models are proposed for the equilibrium boundary layer and mixed layer depths in the presence of surface heating. The models are a function of the initial mixed layer depth before heating is imposed and the Langmuir stability length. In the presence of radiative heating, the models are extended to account for the depth profile of the heating.

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Brodie C. Pearson, Alan L. M. Grant, Jeff A. Polton, and Stephen E. Belcher

Abstract

The differences between the conclusions of Noh and Choi and of Pearson et al., which are largely a result of defining different length scales based on different quantities, are discussed. This study shows that the layer over which Langmuir turbulence mixes (nominally h TKE) under a stabilizing surface buoyancy flux should be scaled by a combination of the Langmuir stability length L L and initial/nocturnal boundary layer depth h 0 rather than by the Zilitinkevich length.

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Henry Chang, Helga S. Huntley, A. D. Kirwan Jr., Daniel F. Carlson, Jean A. Mensa, Sanchit Mehta, Guillaume Novelli, Tamay M. Özgökmen, Baylor Fox-Kemper, Brodie Pearson, Jenna Pearson, Ramsey R. Harcourt, and Andrew C. Poje

Abstract

We present an analysis of ocean surface dispersion characteristics, on 1–100-m scales, obtained by optically tracking a release of O(600) bamboo plates for 2 h in the northern Gulf of Mexico. Under sustained 5–6 m s−1 winds, energetic Langmuir cells are clearly delineated in the spatially dense plate observations. Within 10 min of release, the plates collect in windrows with 15-m spacing aligned with the wind. Windrow spacing grows, through windrow merger, to 40 m after 20 min and then expands at a slower rate to 50 m. The presence of Langmuir cells produces strong horizontal anisotropy and scale dependence in all surface dispersion statistics computed from the plate observations. Relative dispersion in the crosswind direction initially dominates but eventually saturates, while downwind dispersion exhibits continual growth consistent with contributions from both turbulent fluctuations and organized mean shear. Longitudinal velocity differences in the crosswind direction indicate mean convergence at scales below the Langmuir cell diameter and mean divergence at larger scales. Although the second-order structure function measured by contemporaneous GPS-tracked surface drifters drogued at ~0.5 m shows persistent r 2/3 power law scaling down to 100–200-m separation scales, the second-order structure function for the very near surface plates observations has considerably higher energy and significantly shallower slope at scales below 100 m. This is consistent with contemporaneous data from undrogued surface drifters and previously published model results indicating shallowing spectra in the presence of direct wind-wave forcing mechanisms.

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Paul Xavier Flanagan, Rezaul Mahmood, Natalie A. Umphlett, Erin Haacker, C. Ray, William Sorensen, Martha Shulski, Crystal J. Stiles, David Pearson, and Paul Fajman

Abstract

During early 2019, a series of events set the stage for devastating floods in eastern Nebraska, western Iowa, and southeastern South Dakota. When the floodwaters hit, dams and levees failed, cutting off towns while destroying roads, bridges, and rail lines, further exacerbating the crisis. Lives were lost and thousands of cattle were stranded. Estimates indicate that the cost of the flooding has topped $3 billion as of August 2019, with this number expected to rise. After a warm and wet start to winter, eastern Nebraska, western Iowa, and southeastern South Dakota endured anomalously low temperatures and record-breaking snowfall. By March 2019, rivers were frozen, frost depths were 60–90 cm, and the water equivalent of the snowpack was 30–100 mm. With these conditions in place, a record-breaking surface cyclone rapidly developed in Colorado and moved eastward, producing heavy rain toward the east and blizzard conditions toward the west. In areas of eastern Nebraska, western Iowa, and southeastern South Dakota, rapid melting of the snowpack due to this rain-on-snow event quickly led to excessive runoff that overwhelmed rivers and streams. These conditions brought the region to a standstill. In this paper, we provide an analysis of the antecedent conditions in eastern Nebraska, western Iowa, and southeastern South Dakota and the development of the surface cyclone that triggered the historic flooding, along with a look into the forecast and communication of flood impacts prior to the flood. The study used multiple datasets, including in situ observations and reanalysis data. Understanding the events that led to the flooding could aid in future forecasting efforts.

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D. H. Lenschow, I. R. Paluch, A. R. Bandy, R. Pearson Jr., S. R. Kawa, C. J. Weaver, B. J. Huebert, J. G. Kay, D. C. Thornton, and A. R. Driedger III

A combined atmospheric chemistry-meteorology experiment, the Dynamics and Chemistry of the Marine Stratocumulus (DYCOMS), was carried out during the summer of 1985 over the eastern Pacific Ocean using the NCAR Electra aircraft. The objectives were to 1) study the budgets of several trace reactive species in a relatively pristine, steady-state, horizontally homogeneous, well-mixed boundary layer capped by a strong inversion and 2) study the formation, maintenance and dissipation of marine stratocumulus that persists off the California coast (as well as similar regions elsewhere) in summer. We obtained both mean and turbulence measurements of meteorological variables within and above the cloud-capped boundary layer that is typical of this region. Ozone was used successfully as a tracer for estimating entrainment rate. We found, however, that horizontal variability was large enough for ozone that a correction needs to be included in the ozone budget for the horizontal displacement due to turns even though the airplane was allowed to drift with the wind. The time rate-of-change term was significant in both the ozone and radon budgets; as a result, a considerably longer time interval than the two hours used between sets of flight legs would be desirable to improve the measurement accuracy of this term.

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