Search Results

You are looking at 1 - 10 of 46 items for

  • Author or Editor: John Williams x
  • Refine by Access: All Content x
Clear All Modify Search
John G. Williams

Abstract

Calculations and measurements show that the transmissivity parameter t is an increasing function of the optical air mass m. An equation for estimating t for several model atmospheres is presented, and the direct beam flux calculated with the estimates is compared with that obtained using a constant value of t. The equation gives significantly better results for hourly or instantaneous values, and for daily totals on some surfaces.

Full access
John K. Williams and J. Vivekanandan

Abstract

Dual-wavelength ratio (DWR) techniques offer the prospect of producing high-resolution mapping of cloud microphysical properties, including retrievals of cloud liquid water content (LWC) from reflectivity measured by millimeter-wavelength radars. Unfortunately, noise and artifacts in the DWR require smoothing to obtain physically realistic values of LWC with a concomitant loss of resolution. Factors that cause inaccuracy in the retrieved LWC include uncertainty in gas and liquid water attenuation coefficients, Mie scattering due to large water droplets or ice particles, corruption of the radar reflectivities by noise and nonatmospheric returns, and artifacts due to mismatched radar illumination volumes. The error analysis presented here consists of both analytic and heuristic arguments; it is illustrated using data from the Mount Washington Icing Sensors Project (MWISP) and from an idealized simulation. In addition to offering insight into design considerations for a DWR system, some results suggest methods that may mitigate some of these sources of error for existing systems and datasets.

Full access
Eirwen Williams and John H. Simpson

Abstract

The use of acoustic Doppler current profilers (ADCPs) to measure turbulent parameters via the variance method involves uncertainties due to instrument noise and flow-related errors in measurement. For weak flows, the uncertainty in Reynolds stress measurements arises mainly from instrument noise and is proportional to the square of the velocity standard deviation, while the uncertainty in the corresponding estimates of the rate of production of turbulent kinetic energy (TKE) is proportional to the cube of the velocity standard deviation. For stronger flows, the principal determining parameter is the number of individual independent velocity measurements over which the variance is calculated. These results are validated by detailed analyses of two datasets from an RD Instruments 1.2-MHz Workhorse ADCP, using a ping rate of 2 Hz with ensemble averaging at 0.5 Hz, and a ping rate of 10 Hz with ensemble averaging at 1 Hz, respectively. While increasing ping rate generally reduces the effects of instrument noise, it will not alleviate the influence of flow-related noise once the sampling interval is less than the autocovariance time scale of the turbulence. Using the fast ping rate, the uncertainty in the Reynolds stress due to instrument noise is reduced by a factor of more than 3 to ∼0.02 Pa; in higher energy environments there is a reduction in the uncertainty of about 30%. The observational and theoretical estimates for the reduction in the uncertainty using the fast ping rate are in good agreement.

Full access
John G. Williams and Werner H. Terjung

Abstract

Grid-point data on the MSL pressure and 500 mb height fields over western North America are filtered with the eigenvectors of their covariance matrix. The filtering allows virtually all the information in 134 grid-point data for each day of the 8-year sample to be represented by 15 eigenvector coefficients. The three-dimensional eigenvector patterns are meteorologically coherent, and the filtering process can be used as a screen for bad data or for anomalous circulation patterns.

Full access
John L. Williams III and Reed M. Maxwell

Abstract

Feedbacks between the land surface and the atmosphere, manifested as mass and energy fluxes, are strongly correlated with soil moisture, making soil moisture an important factor in land–atmosphere interactions. It is shown that a reduction of the uncertainty in subsurface properties such as hydraulic conductivity (K) propagates into the atmosphere, resulting in a reduction in uncertainty in land–atmosphere feedbacks that yields more accurate atmospheric predictions. Using the fully coupled groundwater-to-atmosphere model ParFlow-WRF, which couples the hydrologic model ParFlow with the Weather Research and Forecasting (WRF) atmospheric model, responses in land–atmosphere feedbacks and wind patterns due to subsurface heterogeneity are simulated. Ensembles are generated by varying the spatial location of subsurface properties while maintaining the global statistics and correlation structure. This approach is common to the hydrologic sciences but uncommon in atmospheric simulations where ensemble forecasts are commonly generated with perturbed initial conditions or multiple model parameterizations. It is clearly shown that different realizations of K produce variation in soil moisture, latent heat flux, and wind for both point and domain-averaged quantities. Using a single random field to represent a control case, varying amounts of K data are sampled and subsurface data are incorporated into conditional Monte Carlo ensembles to show that the difference between the ensemble mean prediction and the control saturation, latent heat flux, and wind speed are reduced significantly via conditioning of K. By reducing uncertainty associated with land–atmosphere feedback mechanisms, uncertainty is also reduced in both spatially distributed and domain-averaged wind speed magnitudes, thus improving the ability to make more accurate forecasts, which is important for many applications such as wind energy.

Full access
Gareth P. Williams and John B. Robinson

Abstract

The effects of Ekman layers on generalized Eady waves (i.e., height-varying static stability and shear) are examined. The non-constancy of N(z) and u z modify the classical Eady results but do not introduce any new effects. Thus, a short-wave cutoff is always found for flows with double Ekman layers but never for flows with a single Ekman layer.

By comparing the analytical solutions with a numerically simulated annulus wave, we are able to categorize the latter quite accurately.

Full access
Gareth P. Williams and John B. Robinson

Abstract

We test the hypothesis that the atmospheric circulations of Jupiter are a manifestation of large-scale convective instability brought about primarily by the presence of an internal heat source. This is done by examining the nature of convection in an unstable rotating atmosphere through numerical integration of the Boussinesq equations. The general properties of convection are obtained from solutions with laboratory-scale parameters while particular Jovian characteristics are studied through calculations with planetary-scale parameters.

In the Jupiter calculations, physical and theoretical constraints on parametric freedom produce a desirably under-determined system in which there remain more observational criteria to be explained than free parameters to manipulate.

The solutions indicate that a tropical westerly jet can be produced by an axisymmetric flow provided that the atmosphere is relatively shallow (d<500 km). A strong equatorial westerly flow can occur provided that there is a strong diffusion of the tropical jet. The strength of such a diffusion is of a magnitude that suggests that it can only realistically be brought about by large-scale non-axisymmetric disturbances. The axisymmetry of the convective rolls, i.e., their longitudinal stability, is controlled by the latitudinal variation of Ω cosθ. This differential rotation suppresses the organization of large-scale convective motion poleward of 45° while toward the equator such motions can set in strongly.

The banded structure and zonal velocity field of the most realistic theoretical solution resemble the observed, having five zones (ω>O) and four belts (ω<O) each with its characteristic differential zonal motion. The square-shaped form of the mean vertical velocity variation with latitude produces sharply bounded zones of uniform intensity.

Calculations to test the stability of the axisymmetric flow to longitudinal perturbations indicate that ovals and streaks are the natural form of the disturbance elements.

Full access
Kerry Emanuel, Ragoth Sundararajan, and John Williams

Changes in tropical cyclone activity are among the more potentially consequential results of global climate change, and it is therefore of considerable interest to understand how anthropogenic climate change may affect such storms. Global climate models are currently used to estimate future climate change, but the current generation of models lacks the horizontal resolution necessary to resolve the intense inner core of tropical cyclones. Here we review a new technique for inferring tropical cyclone climatology from the output of global models, extend it to predict genesis climatologies (rather than relying on historical climatology), and apply it to current and future climate states simulated by a suite of global models developed in support of the most recent Intergovernmental Panel on Climate Change report.

This new technique attacks the horizontal resolution problem by using a specialized, coupled ocean-atmosphere hurricane model phrased in angular momentum coordinates, which provide a high resolution of the core at low cost. This model is run along each of 2,000 storm tracks generated using an advection-and-beta model, which is, in turn, driven by large-scale winds derived from the global models. In an extension to this method, tracks are initiated by randomly seeding large areas of the tropics with weak vortices and then allowing the intensity model to determine their survival, based on large-scale environmental conditions. We show that this method is largely successful in reproducing the observed seasonal cycle and interannual variability of tropical cyclones in the present climate, and that it is more modestly successful in simulating their spatial distribution. When applied to simulations of global climate with double the present concentration of carbon dioxide, this method predicts substantial changes and geographic shifts in tropical cyclone activity, but with much variation among the global climate models used. Basinwide power dissipation and storm intensity generally increase with global warming, but the results vary from model to model and from basin to basin. Storm frequency decreases in the Southern Hemisphere and north Indian Ocean, increases in the western North Pacific, and is indeterminate elsewhere. We demonstrate that in these simulations, the change in tropical cyclone activity is greatly influenced by the increasing difference between the moist entropy of the boundary layer and that of the middle troposphere as the climate warms.

Full access
Gareth P. Williams and R. John Wilson

Abstract

The stability and genesis of the vortices associated with long solitary divergent Rossby waves-the Rossby vortices–are studied numerically using the single-layer (SL) model with Jovian parameters. Vortex behavior depends on location and on balances among the translation, twisting, steepening, dispersion and advection processes. Advection is the main preserver of vortices. The solutions provide an explanation for the origin, uniqueness and longevity of the Great Red Spot (GRS).

In midlatitudes, stable anticyclones exist in a variety of sizes and balances: from the large planetary-geostrophic (PG) and medium intermediate-geostrophic (IG) vortices that propagate westward, to the small quasi-geostrophic (QG) vortices that migrate equatorward. These vortices all merge during encounters. Geostrophic vortices in the f 0-plane system adjust toward symmetry by rotating; those on the sphere adjust by rotating and propagating. Stable cyclones exist mainly at the QG scale or on the f 0-plane.

In low latitudes stable anticyclones exist only when a strong equatorial westerly jet and a significant easterly current are present to eliminate the highly dispersive equatorial modes. The permanence of a GRS-like, low-latitude vortex in a Jovian flow configuration is established by a 100-year simulation. At the equator, stable anticyclones exist only when they have the Hermite latitudinal form and the Korteweg-DeVries longitudinal form and amplitude range as prescribed by Boyd (1980). Soliton interactions occur between equatorial vortices of similar order.

Vortices can be generated at the equator by the collapse of low-latitude anticyclones. In mid or low latitudes, unstable easterly jets generate vortices whose final number depends mainly on the interaction history. Stochastically forced eddies cascade by wave interactions into zonal currents and by eddy mergers into a single Rossby vortex that thrives on the turbulence. Directly forced ageostrophic jets can make vortex drift more westerly and can change it from free state values of −10 m s−1 to forced state values of −5 m s−1 (as the GRS) or of +5 m s−1 (as the Large Ovals).

Full access
John R. Mecikalski, John K. Williams, Christopher P. Jewett, David Ahijevych, Anita LeRoy, and John R. Walker

Abstract

The Geostationary Operational Environmental Satellite (GOES)-R convective initiation (CI) algorithm predicts CI in real time over the next 0–60 min. While GOES-R CI has been very successful in tracking nascent clouds and obtaining cloud-top growth and height characteristics relevant to CI in an object-tracking framework, its performance has been hindered by elevated false-alarm rates, and it has not optimally combined satellite observations with other valuable data sources. Presented here are two statistical learning approaches that incorporate numerical weather prediction (NWP) input within the established GOES-R CI framework to produce probabilistic forecasts: logistic regression (LR) and an artificial-intelligence approach known as random forest (RF). Both of these techniques are used to build models that are based on an extensive database of CI events and nonevents and are evaluated via cross validation and on independent case studies. With the proper choice of probability thresholds, both the LR and RF techniques incorporating NWP data produce substantially fewer false alarms than when only GOES data are used. The NWP information identifies environmental conditions (as favorable or unfavorable) for the development of convective storms and improves the skill of the CI nowcasts that operate on GOES-based cloud objects, as compared with when the satellite IR fields are used alone. The LR procedure performs slightly better overall when 14 skill measures are used to quantify the results and notably better on independent case study days.

Full access