Richard L. Pfeffer

Comparison of Methodologies for Probabilistic Quantitative Precipitation Forecasting

Abstract Twenty-four-hour probabilistic quantitative precipitation forecasts (PQPFs) for accumulations exceeding thresholds of 0.01, 0.05, and 0.10 in. are produced for 154 meteorological stations over the eastern and central regions of the United States. Comparisons of skill are made among forecasts generated using five different linear and nonlinear statistical methodologies, namely, linear regression, discriminant analysis, logistic regression, neural networks, and a classifier system. The predictors for the different statistical models were selected from a large pool of analyzed and predicted variables generated by the Nested Grid Model (NGM) during the four cool seasons (December–March) from 1992/93 to 1995/96. Because linear regression is the current method used by the National Weather Service, it is chosen as the benchmark by which the other methodologies are compared. The results indicate that logistic regression performs best among all methodologies. Most notable is that it performs significantly...

Further calculations of eddy heat fluxes and temperature variances in baroclinic waves using weakly non-linear theory

Abstract Drazins weakly non-linear theory of baroclinic instability is recast in a framework which allows us to account more clearly for experimentally observed changes of eddy heat flux and temperature variance with wave scale and rotation. The theory deals with a single slightly unstable self interacting mode in a viscous baroclinic fluid with a continuous density stratification. Viscosity is accounted for only in Ekman layers. The results of the calculations are found to compare somewhat more favorably with the experimental determinations of eddy heat flux and temperature variances than previous results obtained by the present authors.

Barotropic flow over bottom topography— experiments and nonlinear theory

Abstract Barotropic flow over finite amplitude two-wave bottom topography is investigated both experimentally and theoretically over a broad parameter range. In the experiments, the fluid is contained in a vertically oriented, rotating circular cylindrical annulus. It is forced into motion relative to the annulus by a differentially rotating, rigid, radially sloping lid in contact with the top surface of the fluid. The radial depth variation associated with the slope of the lid, and an equal and opposite slope of the bottom boundary, simulates the effect of the variation of the Coriolis parameter with latitude (β) in planetary atmospheres and in the ocean. The dimensionless parameters which control the fluid behavior are the Rossby number (ϵ), the Ekman number (E), the β parameter, the aspect ratio (δ), the ratio of the mean radius to the gap width (α) and the ratio of the topographic height to the mean fluid depth (η). The Rossby and Ekman numbers are varied over an order of magnitude by conducting experiments at different rotation rates of the annulus. Velocity measurements using photographs of tracer particles suspended in the fluid reveal the existence of a stationary, topographically forced wave superimposed on an azimuthal mean current. With successively larger rotation rates (i.e. lower ϵ and E) the wave amplitude increases and then levels off, the phase displacement of the wave upstream of the topography increases and the azimuthal mean velocity decreases and then levels off. Linear quasigeostophic theory accounts qualitatively, but not quantitatively, for the phase displacement, predicts the wave amplitude poorly and provides no basis for predicting the zonal mean velocity. Accordingly, we have solved the nonlinear, steady-state, quasigeostrophic barotrophic vorticity equation with both Ekman layer and internal dissipation using a spectral colocation method with Fourier representation in the azimuthal direction and Chebyshev polynomial representation in the radial direction. For boundary conditions at the side walls, we specified zero velocity. Side wall boundary layers then appear explicitly in the numerical solution. At the bottom and top of the fluid, we specified that the vertical velocity at the mean height of each boundary is the sum of two components—one forced by Ekman suction in the absence of topography and the other by the condition that there can be no flow normal to the rigid boundary. We justify this choice by the smallness of the Ekman number and of the radial and azimuthal slopes of the topography. We have found that the use of three Fourier components and seven Chebyshev polynomials is sufficient to account qualitatively for the experimental results, although small quantitative discrepancies suggest that further investigation of the neglect of effects originally considered to be small is needed.

A Fluid Experiment of Large-Scale Topography Effect on Baroclinic Wave Flows

The effects of topography on baroclinic wave flows are studied experimentally in a thermally driven rotating annulus of fluid.Fourier analysis and complex principal component (CPC) analysis of the experimental data show that, due to topographic forcing, the flow is bimodal rather than a single mode. Under suitable imposed experimental parameters, near thermal Rossby numberROT =0.1 and Taylor numberTa = 2.2 × 107, the large-scale topography produces low-frequency oscillation in the flow and rather long-lived flow pattern resembling blocking in the atmospheric cir-culation. The ‘blocking’ phenomenon is caused by the resonance of travelling waves and the quasi-stationary waves forced by topography.The large-scale topography transforms wavenumber-homogeneous flows into wavenumber-dispersed flows, and the dispersed flows possess lower wavenumbers.

Velocity and temperature measurement with thermistor anemometers in a thermally stratified rotating fluid

The implementation of a new scheme for a thermistor anemometer for the measurement of both velocity and temperature in a two-dimensional horizontal flow is described. The anemometer is composed of a cluster of four bead thermistors, consisting of a central heater bead and three surrounding sensor beads. Empirical relationships for the dependence of flow direction on sensor/heater spacing, fluid flow speed, and fluid, heater and sensor temperatures are presented which permit in situ calibration of large numbers of thermistor anemometers. Fabrication details, calibration procedures and results of measurements from a large number of anemometers under actual experimental conditions in a thermally stratified, rotating fluid flow are included.

Some Effects of Rotation Rate on Planetary-Scale Wave Flows

A series of experiments were performed in a rotating annulus of fluid to study effects of rotation rate on planetary-scale baroclinic wave flows. The experiments reveal that change in rotation rate of fluid container causes variation in Rossby number and Taylor number in flows and leads to change in flow patterns and in phase and amplitude of quasi-stationary waves. For instance, with increasing rotation rate, amplitude of quasi-stationary waves increases and phase shifts upstream. On the contrary, with decreasing rotation rate, amplitude of quasi-stationary waves decreases and phase shifts downstream. In the case of the earth’s atmosphere, although magnitude of variation in earth’s rotation rate is very small, yet it causes a very big change in zonal velocity component of wind in the atmosphere and of currents in the ocean, and therefore causes a remarkable change in Rossby number and Taylor number determining regimes in planetary-scale geophysical flows. The observation reveals that intensity and geographic location of subtropic anticyclones in both of the Northern and Southern Hemispheres change consistently with the variation in earth’s rotation rate. The results of fluid experiments are consistent, qualitatively, with observed phenomena in the atmospheric circulation.