Search Results

You are looking at 1 - 10 of 91 items for

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

Abstract

Twenty-five years (1958-82) of monthly 700 ml, geopotential forecasts produced by the U.S. National Weather Service are verified and then used in a series of temperature specification experiments. The forecasts show skill with respect to climatology through the positive correlations between forecast and observed anomalies in all calendar months. Improvement over persistence is apparent in the root mean square error, the mean absolute error, and the S skill score. The verification statistics also show a temporal trend toward smaller errors during the 1958-82 period.

Stepwise screening and EOF (empirical orthogonal function) regressions are compared as alternative strategies for the specification of surface station temperatures from 700 mb heights. When the statistical significance of the model order is comparable and the equations are applied to the developmental sample of 700 mb verification grids, the screening procedure outperforms the EOF procedure according to the mean absolute error, the fraction of described variance, and a 3-category skill score. However, when the same sets of equations are applied to an independent sample consisting of the most skillful 700 mb forecasts, the results of the screening procedure show considerably more degradation. Temperature forecasts derived fromthe two procedures have comparable 3-category skill scores, but the EOF-derived forecasts are characterized by smaller mean absolute errors and higher correlations between the forecast and observed temperatures. The tendency for less degradation of the EOF-derived forecasts is attributed to their smaller variances and to the ability of the EOFs to capture modest amounts of forecast skill regardless of the region in which the skill is achieved.

Full access
John E. Walsh

Abstract

The use of a nested grid system in the numerical integration of a nonhydrostatic system of hydrodynamic equations is investigated. Computational problems introduced by the abandonment of the hydrostatic approximation are noted.

Tests in which a solitary inertial gravity wave is propagated through a periodic domain indicate that the computational noise due to the grid interaction is negligible when an upstream or a two-step Lax-Wendroff differencing scheme is used. A precipitating cumulus cloud is then simulated with several arrangements of grid points. The “nested” results differ very little from those of the corresponding “fine grid” simulation. Computational noise cannot be detected in the grid interaction zone even when this zone is in the active cloud area.

Full access
John E. Walsh

Abstract

High-latitude monthly surface temperature data for the years 1954–75 are objectively analyzed. All existing monthly temperatures from drifting ice stations are included in the analyses. The variances about the monthly means are found to be no larger in the central Arctic than in the northern land areas. Surface temperature trends computed by linear regression vary considerably by season and by geographical region within the Arctic. The spatial distribution of the recent temperature trends is interpreted in terms of the empirical orthogonal functions accounting for the largest fractions of the temperature variance.

The net area-weighted 22-year trend for the region north of 60°N is −0.02°C year−1 but this relatively small value is found to be the resultant of cooling prior to the.mid-1960's and warming thereafter. During both the cooling and warming periods, the trends are found to be largest in the 70–80°N latitude belt. Inferences about the significance of the results are made by comparing the computed trends with those excepted when the temperatures are randomly distributed about their monthly means.

Full access
John E. Walsh

Abstract

Daily data for the years 1932–75 are used in a study of the fluctuations in the arctic circulation over time scales of several days to several months. An updated set of normal sea level pressures is constructed, and a semiannual cycle is found in the high-latitude gradient of zonally averaged pressure. July is the only month in which a mean convergence of the low-level flow into the central arctic is indicated.

The high-latitude fields of sea level pressure, surface temperature and 700 mb height and temperature are represented in terms of empirical orthogonal functions in order to isolate the dominant modes of variability. The amplitudes of the functions are used to evaluate the daily persistence of arctic pressure anomalies as a function of season and to compare the persistence of arctic and midlatitude pressure anomalies. The month-to-month persistence of arctic pressure anomalies is found to be small, although the monthly persistence does exceed that expected from the lagged autocorrelations of the daily data.

Cross correlations between the anomaly fields of pressure (height) and temperature at the surface and 700 mb are evaluated at lags ranging from −8 to +8 months. The cross correlations differ substantially from zero only at 0 lag. Fluctuations in the first eigenvector of 700 mb temperature are in surprisingly good agreement with the surface temperature fluctuations reported in an earlier paper.

Full access
John E. Walsh

Abstract

The linearized Boussinesq equations with rotation, viscosity, conduction, and a mean stratification are used to model the sea breeze in two dimensions. The motion is forced by a prescribed surface temperature function.

The linear model produces a sea breeze with realistic velocities and spatial dimensions. Hydrostatic solutions are found to differ very little from nonhydrostatic solutions. The only distinguishing feature of the solution at the inertial latitude is an amplitude maximum far from the coastline. Both the phase and the amplitude depend on the mean atmospheric stability. The computed vertical heat fluxes, when summed along the coastlines of the principal land masses, indicate that the sea breeze effect can account for several percent of the globally averaged vertical flux of sensible heat at a height of several hundred meters.

The land-sea temperature difference required by the model to create a net onshore flow in opposition to a basic current agrees well with the empirical criterion defined by Biggs and Graves.

The nonlinear advection process is studied with a finite-difference model based on a series of overlapping grids. The principal effect of the nonlinear terms is a landward advection oof the sea breeze circulation

Full access
Becky Ross
and
John E. Walsh

Abstract

Daily observational data for thirty winters (1951–80) are used to test the hypothesis that anomalous distributions of snow and ice cover influence the intensification and/or trajectories of synoptic-scale cyclones. The pools of objectively chosen cases include 100 wintertime cyclonic events in the marginal snow/ice zones of each of three regions: eastern North America, the North Atlantic Ocean and the North Pacific Ocean. For each region, the errors of 24- and 48-hour force derived from a barotropic model, from persistence and from an objective analog procedure are stratified according to the concurrent anomalies of snow or ice cover. The results support the notion that the enhanced baroclinicity new the snow/ice margin contributes to stronger intensification and/or to motion parallel to the snow or ice margin in eastern North America and in the North Atlantic. A weaker signal is found in the North Pacific. The signal is qualitatively similar in the fields of 500 mb geopotential and sea level pressure, although the differences between the composites are statistically significant only in the sea level pressure fields. The results suggest that forecasts of weekly or monthly circulation patterns may, in situations of extreme snow/ice cover, be improved by consideration of observed snow/ice anomalies, if these anomalies persist through the forecast period.

Controlled experiments with the NCAR (National Center for Atmospheric Research) primitive equations forecast model show a weaker dependence on the extent of snow and ice, although qualitative similarities to the data-based results are detectable.

Full access
William H. Klein
and
John E. Walsh

Abstract

A comparison is made between two types of specification of monthly wintertime surface temperatures over the United States. The specifications are obtained by multiple regression of station temperature anomaly at each of 37 stations onto 700 mb height anomalies represented by 1) grid-point values selected by a forward stepwise screening procedure, and 2) coefficients of the dominant empirical orthogonal functions (EOF's). Various measures of skill show that specifications derived from the pointwise screening are superior in both developmental (dependent) and independent samples. The differences in the skill levels are interpreted as a disadvantage of the spatial generality inherent in the EOF representations.

Full access
John E. Walsh
and
Michael B. Richman

Abstract

No abstract available.

Full access
John E. Walsh
and
Michael B. Richman

Abstract

Thirty-one years of monthly data are used to evaluate the seasonal dependence of the associations between large-scale temperature anomalies over the United States and the North Pacific Ocean. Both station (grid-point) values and empirical orthogonal functions of temperature are used in the correlative analysis.

The North Pacific sea surface temperature (SST) anomalies correlate most highly with temperature fluctuations over the southeastern and the far western states. Correlations with the SST anomalies have opposite signs in the two portions of the United States. The associations with the SST anomalies are independent of season only in the western states. The association involving the southeastern states is strongest during the winter and insignificant during the summer. The pattern of North Pacific SST that correlates most highly with the United States temperature is an east-west SST gradient between the West Coast and 35°N, 160°W. Statistically significant fractions of temperature variance over at least some areas of the United States are described by regression onto the SST anomalies in all seasons except spring. The results imply some seasonal predictability based on North Pacific SST patterns alone, although useful predictability appears to be confined primarily to the winter.

The pattern analysis shows that the failure to rotate the dominant eigenvectors can obscure the spatial and temporal interrelationships deduced from the two sets of data fields.

Full access
John E. Walsh
and
Anthony Mostek

Abstract

Monthly meteorological data for the years 1900–77 are used in an eigenvector analysis of the anomaly patterns of surface temperature, precipitation and sea level pressure over the United States. Approximately 70% of the variance is contained in the first three of 61 temperature eigenvectors and in the first three of 25 pressure eigenvectors. Large-scale patterns of precipitation are also identified, although the compression of the data is somewhat less effective. The first eigenvector of each variable contains anomalies of the same sign over most of the United States; the second and third modes describe gradients in approximately perpendicular directions.

Cross correlations between the amplitudes of eigenvectors of different variables are statistically significant, consistent with physical expectations, and, in some cases, are seasonally dependent. The first modes of both temperature and pressure are most persistent in the summer. Persistence on the seasonal time scale is generally largest for temperature and largest when summer is the antecedent season. The seasonal persistences of the amplitudes of the temperature eigenvectors are generally consistent with the persistences of station temperatures obtained recently by Namias (1978).

The most prominent feature of the frequency spectra is a strong peak at 2.1 years in the amplitude of the third temperature eigenvector.

Full access