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J. T. Priestley and R. J. Hill

Abstract

Three different instrument systems are compared in their ability to either directly or indirectly measure humidity, temperature, and refractive-index fluctuations. Each system consists of a basic instrument—a Lyman-α hygrometer, an infrared absorption hygrometer or a radio refractometer—configured with its own fine-wire resistance thermometer. All measurements were obtained at a height of 5.2 m in the atmospheric surface layer. We present time series from these instruments, power spectra of humidity, temperature, and radio refractive index, as well as temperature-humidity cospectra, phase spectra, and coherence spectra. The temperature and humidity are either very well correlated or anticorrelated. The temperature-humidity cospectra have the inertial subrange power law up to wavenumbers where instrumental effects interfere. The refractive-index structure parameters calculated from the humidity and temperature fluctuations measured by the Lyman-α and its fine wire agree substantially with the structure parameters determined from the refractometer. The degradation of cospectra caused by sensor separation, the space averaging by the infrared absorption hygrometer, and the flushing-distance problem of the refractometer are illustrated.

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R. B. Hill and J. A. Johnson

Abstract

The three-dimensional circulation produced in a homogeneous ocean by a steady wind stress is investigated when the bottom topography contains a discontinuity in gradient, such as occurs at the edge of the continental shelf. A shear layer is formed above the edge of the shelf in which upwelling is significant. The vertical transport in the shear layer can be sufficient to produce a surface convergence zone in the vicinity of the shelf.

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Reginald J. Hill and Gerard R. Ochs

Abstract

The temperature structure parameter at two heights over a horizontally homogeneous site was measured using optical scintillometers and resistance-wire thermometers. The scintillometers gave systematically greater values than did the wire thermometers, as would be expected on the basis of the likely systematic errors of these two different instruments. Eddy correlation measurements of heat and momentum fluxes, as well as momentum flux derived from surface roughness and wind speed, allow investigation of the Monin-Obukhov similarity of the temperature structure parameter. The similarity is confirmed and compared with previous experiments. The data are mostly for unstable conditions. Disagreement between two previous datasets is resolved.

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Reginald J. Hill, Gerard R. Ochs, and James J. Wilson

Abstract

The first experimental test of obtaining heat and momentum fluxes from measurements of the profile of the temperature structure parameter C T 2 is performed. The parameter C T 2 is obtained from resistance-wire thermometers as well as from optical-scintillation measurements; the latter produces averaged values over a 606-m propagation path. Use of optical scintillation can produce spatial averaging over propagation paths as long as several kilometers. Deducing the fluxes requires the Monin-Obukhov similarity theory of the horizontally homogeneous atmospheric surface layer. The results show agreement with eddy-correlation measurements of the fluxes for homogeneous and stationary micrometeorological conditions. The accuracy of optical scintillometers must, however, be improved to obtain reliable fluxes by this method.

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Aaron J. Hill, Gregory R. Herman, and Russ S. Schumacher

Abstract

Using nine years of historical forecasts spanning April 2003–April 2012 from NOAA’s Second Generation Global Ensemble Forecast System Reforecast (GEFS/R) ensemble, random forest (RF) models are trained to make probabilistic predictions of severe weather across the contiguous United States (CONUS) at Days 1–3, with separate models for tornado, hail, and severe wind prediction at Day 1 in an analogous fashion to the Storm Prediction Center’s (SPC’s) convective outlooks. Separate models are also trained for the western, central, and eastern CONUS. Input predictors include fields associated with severe weather prediction, including CAPE, CIN, wind shear, and numerous other variables. Predictor inputs incorporate the simulated spatiotemporal evolution of these atmospheric fields throughout the forecast period in the vicinity of the forecast point. These trained RF models are applied to unseen inputs from April 2012 to December 2016, and their forecasts are evaluated alongside the equivalent SPC outlooks. The RFs objectively make statistical deductions about the relationships between various simulated atmospheric fields and observations of different severe weather phenomena that accord with the community’s physical understandings about severe weather forecasting. Using these quantified flow-dependent relationships, the RF outlooks are found to produce calibrated probabilistic forecasts that slightly underperform SPC outlooks at Day 1, but significantly outperform their outlooks at Days 2 and 3. In all cases, a blend of the SPC and RF outlooks significantly outperforms the SPC outlooks alone, suggesting that use of RFs can improve operational severe weather forecasting throughout the Day 1–3 period.

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R. B. Fritz, R. J. Hill, J. T. Priestley, and W. P. Schoenfeld

Abstract

Rain rate during light precipitation in winter was measured with high temporal resolution optical systems at a site in Illinois. In addition to quasi-periodic variations, a clearly sinusoidal oscillation in rain rate was found imbedded in the general precipitation. The phase shift in the occurrence of the oscillation at two sensors, with the simultaneous recording of sinusoidal fluctuations of the attenuation of a millimeter wave signal, allows simulation of this particular rain pattern by a simple model. The basic mechanism that can produce a rain event with such a sinusoidal pattern is not clearly understood.

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M. C. Buijsman, Y. Uchiyama, J. C. McWilliams, and C. R. Hill-Lindsay

Abstract

The Regional Oceanic Modeling System (ROMS) is applied in a nested configuration with realistic forcing to the Southern California Bight (SCB) to analyze the variability in semidiurnal internal wave generation and propagation. The SCB has a complex topography with supercritical slopes that generate linear internal waves at the forcing frequency. The model predicts the observed barotropic and baroclinic tides reasonably well, although the observed baroclinic tides feature slightly larger amplitudes. The strongest semidiurnal barotropic to baroclinic energy conversion occurs on a steep sill slope of the 1900-m-deep Santa Cruz Basin. This causes a forced, near-resonant, semidiurnal Poincaré wave that rotates clockwise in the basin and is of the first mode along the radial, azimuthal, and vertical directions. The associated tidal-mean, depth-integrated energy fluxes and isotherm oscillation amplitudes in the basin reach maximum values of about 5 kW m−1 and 100 m and are strongly modulated by the spring–neap cycle. Most energy is locally dissipated, and only 10% escapes the basin. The baroclinic energy in the remaining basins is orders of magnitudes smaller. High-resolution coastal models are important in locating overlooked mixing hotspots such as the Santa Cruz Basin. These mixing hotspots may be important for ocean mixing and the overturning circulation.

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John A. Knaff, Charles R. Sampson, Patrick J. Fitzpatrick, Yi Jin, and Christopher M. Hill

Abstract

In 1980 the Holland tropical cyclone (TC) wind profile model was introduced. This simple model was originally intended to estimate the wind profile based on limited surface pressure information alone. For this reason and its relative simplicity, the model has been used in many practical applications. In this paper the potential of a simplified version of the Holland B parameter, which is related to the shape of the tangential wind profile, is explored as a powerful diagnostic tool for monitoring TC structure. The implementation examined is based on the limited information (maximum wind, central pressure, radius and pressure of the outer closed isobar, radii of operationally important wind radii, etc.) that is typically available in operational models and routine analyses of TC structure. This “simplified Holland B” parameter is shown to be sensitive to TC intensity, TC size, and the rate of radial decay of the tangential winds, but relatively insensitive to the radius of maximum winds. A climatology of the simplified Holland B parameter based on historical best-track data is also developed and presented, providing the expected natural ranges of variability. The relative simplicity, predictable variability, and desirable properties of the simplified Holland B parameter make it ideal for a variety of applications. Examples of how the simplified Holland B parameter can be used for improving forecaster guidance, developing TC structure tools, diagnosing TC model output, and understanding and comparing the climatological variations of TC structure are presented.

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Russ S. Schumacher, Aaron J. Hill, Mark Klein, James A. Nelson, Michael J. Erickson, Sarah M. Trojniak, and Gregory R. Herman

Abstract

Excessive rainfall is difficult to forecast, and there is a need for tools to aid Weather Prediction Center (WPC) forecasters when generating Excessive Rainfall Outlooks (EROs), which are issued for the contiguous United States at lead times of 1–3 days. To address this need, a probabilistic forecast system for excessive rainfall, known as the Colorado State University Machine Learning Probabilities (CSU-MLP) system, was developed based on ensemble reforecasts, precipitation observations, and machine-learning algorithms, specifically random forests. The CSU-MLP forecasts were designed to emulate the EROs, with the goal being a tool that forecasters can use as a “first guess” in the ERO forecast process. Resulting from close collaboration between CSU and WPC and evaluation at the Flash Flood and Intense Rainfall Experiment, iterative improvements were made to the forecast system and it was transitioned into operational use at WPC. Quantitative evaluation shows that the CSU-MLP forecasts are skillful and reliable, and they are now being used as a part of the WPC forecast process. This project represents an example of a successful research-to-operations transition, and highlights the potential for machine learning and other postprocessing techniques to improve operational predictions.

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Malcolm J. Roberts, H. Banks, N. Gedney, J. Gregory, R. Hill, S. Mullerworth, A. Pardaens, G. Rickard, R. Thorpe, and R. Wood

Abstract

Initial results are presented from a 150-yr control and an 80-yr transient simulation of a new global coupled climate model with an ocean model resolution of ⅓°, which is sufficient to permit ocean eddies to form. With no spinup procedure or flux correction, the coupled model remains close to radiative equilibrium, and the enhanced ocean resolution allows an improved ocean state to be simulated; this includes a general decrease in sea surface temperature errors compared to climatology and more realistic large-scale flows compared to previous lower-resolution models. However, the improvements in the atmospheric and coupled model climatology are less pronounced, with small improvements in atmospheric circulation counterbalanced by an El Niño–Southern Oscillation cycle that has peak power at too short a period and with too little power on longer time scales. With the model using exactly the same atmospheric component as a lower-resolution counterpart, the comparison gives some insight into the impact of ocean resolution on climate and suggests that a corresponding increase in atmospheric resolution may be needed before major changes to the coupled climatology are seen.

The transient climate change simulation shows some important regional differences in response compared to previous lower-resolution models. A less pronounced weakening to the meridional overturning in the North Atlantic leads to a smaller decrease in northward heat transport and enhances the surface temperature increase in the northern Europe–Atlantic region by 10% over the lower-resolution model. This may be connected to processes involved in deep-water formation in the Labrador and Nordic Seas.

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