John R. Anderson

The Latitude-Height Structure of 40–50 Day Variations in Atmospheric Angular Momentum

Abstract Quasi-periodic variations in the relative angular momentum of the atmosphere on time scales of around 40–50 days have been observed by Langley et al. (1981). A description of the two-dimensional (latitude-height) structure of the winds responsible for these changes is constructed here from five years of NMC twice-daily global analyses. Using cross-spectral and amplitude-phase eigenvector techniques, we find these variations are associated with wavelike motions in the tropical upper troposphere that propagate and downward in phase within the tropics. A coherently connected midlatitude Northern Hemisphere component is also present whose phase is essentially independent of height. We believe the tropical component to be the zonally averaged part of the motions described by Madden and Julian (1971, 1972). The Northern Hemisphere midlatitude component may be a direct response to the tropical motions or both motions may be the common response to an as yet unidentified tropical forcing.

North Pacific Cold-Season Surface Cyclone Activity: 1975–1983

Abstract An eight year sample of cold-season (1 October through 31 March) extratropical cyclones in the, Pacific Ocean basin is used to study central pressure changes and life cycle characteristics. We find that over 90% of the cyclones passing through the area of the Kuroshio Current intensify in this region. Corresponding percentages in excess of 60% extend from the Kuroshio, south of 45°N, eastward to 130°W. Mean 24-h central pressure falls of all cyclones exceed 9 mb through the entire basin west of 140°W in the latitude band 30° to 50°N. A statistical analysis of 24-h central pressure changes is performed on all cyclones within our domain. A frequency distribution of 1996 cases of 24-h maximum deepening reveals statistically significant departures from a Gaussian distribution, with the coefficient of skewness substantially negative. We also find similarly significant departures from normal in a frequency distribution of all 24-h central pressure changes, in spite of the fact that this distribution wo...

Numerical Simulations of Microburst-producing Storms: Some Results from Storms Observed during COHMEX

Abstract In the first part of this paper, the characteristics of microburst-producing storms are examined with a three-dimensional cloud model using soundings from the Cooperative Huntsville Meteorological Experiment (COHMEX). With a grid resolution of 500 m, it is shown that the general characteristics of observed vertical velocities, vertical draft sizes, water contents, radar reflectivities, and surface outflow strengths can be simulated. In addition, observed microburst precursors such as midlevel convergence and descending precipitation cores can also be simulated. Using a grid resolution of 250 m, the observed structure of a particularly well-documented storm on 20 July 1986 during COHMEX is simulated, including a hail shaft 1–2 km wide that descended to the ground. In the second part of this paper, the influence of microphysical processes in the production of low-level downdrafts in simulated COHMEX storms is investigated. It is shown that low-level downdrafts are in some cases stronger and deeper ...

A Diagnostic Study of Pacific Basin Circulation Regimes as Determined from Extratropical Cyclone Tracks

Abstract The interannual and intraseasonal track variability of cold season extratropical cyclones in the Pacific basin is examined using an 8 year cyclone track dataset. An EOF technique incorporating VARIMAX rotation in time is used to objectively describe the regime nature of the variations. Based upon this analysis we conclude that the cyclone behavior can be classified into six major regime types, corresponding to the positive and negative amplitude excursions of each of the first three rotated EOFS. Each of these rotated EOFs explains approximately equal fractions of the total variance. A study of the cyclone tracks for individual extreme periods confirms the existence of times where each of these patterns dominate. The average 500 mb height fields for these extreme periods have been examined and are generally consistent with the cyclone track anomalies. The resultant regime description shows strong interannual variability; however, there appears to be little obvious correlation with the ENSO signal...

A Three-Dimensional Numerical Analysis of Colliding Microburst Outflow Dynamics

Abstract A parameter study of colliding microburst outflows is performed using a high-resolution three-dimensional model. The colliding microburst pairs me simulated in a domain of 18 km × 16 km × 4.25 km with 50-m resolution. Microburst pairs are examined in varying space and time separations, and the authors find that for certain geometries strong elevated wind fields are generated from the interactions between outflows. For a narrow range of space-time geometries, this elevated wind field is extremely divergent. An examination of the F-factor aircraft hazard parameter reveals that both the divergent wind fields and microburst downdraft cores are regions of danger to jet aircraft. Trajectory analysis reveals that the air composing the elevated jets can be traced back to the shallow outflow formed beneath each microburst core. An analysis of the parcel kinetic energy budget indicates that the pressure domes beneath and between the microbursts are the primary mechanisms for directing energy into the eleva...

Temporal Variations of the Tropical 40-50 Day Oscillation

Abstract In recent years there has been a great deal of interest in a quasi-periodic tropical oscillation of zonal winds, which was first reported by Madden and Julian. An attempt to determine the temporal variation of the oscillation parameters is presented here. Using a 4-year duration global time series and a 25-year station time series, we find that although the nonseasonal variations are large, any seasonal cycle in the oscillation amplitude and frequency must be very small. The small seasonal signal in the oscillation frequency seems to argue against explanations for the time scale based on Doppler-shifted traveling waves.

A Numerical Study of Traveling Microbursts

Abstract An analysis of traveling microbursts in unidirectionally sheared environments is undertaken using a three-dimensional numerical model with 50-m resolution in a 19 × 12 × 4 km domain. For each run, the cooling source is centered at a height of 2 km and travels in an eastward direction of Cm, where Cm = 3, 6, 9, 12, and 15 m s−1. Environmental winds above 2 km are equal to Cm and decay linearly to 0 m s−1 below 2 km. The authors examine the kinetic energy budget of each run, focusing on the dynamic features that are not found in a static microburst simulation. As the source speed Cm increases from 0 to 9 m s−1, the magnitude of the surface horizontal winds increase in the direction of source movement. An examination of the dynamic pressure equation shows that rotationally induced pressure work forces are primarily responsible for increasing surface horizontal winds for the moving-source microbursts. In a similar form to previous studies of vertical perturbations in a sheared environment, elevated h...

The Presence of Linear Wavelike Modes in a Zonally Symmetric Model of the Tropical Atmosphere

Abstract The linear, zonally symmetric modes of the basic state of a Hadley cell are examined. We find that the inclusion of the divergent basic state leads to the formation of a new class of slowly oscillating modes, some of which have periods in the range of 40–50 days. The modes have many features in common with the observed tropical 40–50-day oscillation; however, an explanation for the observed fluctuations in convective cloudiness remains a topic for future work.

The Walker Circulation with Observed Zonal Winds, a Mean Hadley Cell, and Cumulus Friction

Abstract The term Walker Circulation is used to refer to the zonal overturning across the equatorial Pacific driven by enhanced convection over the Indonesian region. In this work, an attempt is made to simulate the Walker Circulation using a linear model that includes a cumulus friction parameterization. The work of Geisler is extended by including a realistic mean zonal wind field obtained from the FGGE dataset and a prescribed mean Hadley cell that is computed from an analytical streamfunction. The model is forced by a stationary tropical heat source. The sensitivity of the model circulation to changes in the basic state is examined. Model results show that the inclusion of a nonzero mean zonal wind field tends to enhance the extratropical response in the winter hemisphere. Including a cumulus friction parameterization tends to damp the zonal wind response near the heating center and also lower the level of zero zonal wind in the model Walker Circulation. Including a mean Hadley cell in the basic state...

The Response of the Tropical Atmosphere to Low Frequency Thermal Forcing

Abstract The tropical response to a localized thermal forcing with approximately 45-day period is investigated for several models of increasing complexity consisting of two equivalent shallow water system and two fully stratified systems. The fully stratified models appear to be able to reproduce a number of observed features of the tropical 40–50 day oscillation including the modulation of the subtropical jet and the eastward and poleward propagation of zonal wind anomalies.