Search Results

You are looking at 1 - 10 of 67 items for

  • Author or Editor: M. A. Miller x
  • Refine by Access: All Content x
Clear All Modify Search
A. J. Miller and L. M. Leslie

Abstract

Forecast probabilities of rain were calculated up to 12 hours in advance using a Markov chain model applied to three-hourly observations from five major Australian cities. The four weather states chosen in this first study were three cloudiness states (0–2 oktas, 3–5 oktas and 6–8 oktas) and a rain state. Second-order Markov models with time-of-day dependent transition probabilities were fitted after appropriate statistical testing.

Forecasts were made using transition probabilities for summer and winter seasons. The skill of the Markov chain forecast probabilities of rain was evaluated in terms of Brier scores using to years of independent data, and compared with forecasts based upon persistence and climatology. The skill of the Markov model forecasts appreciably exceeded that of persistence and climatology and a real time trial of the procedure is being planned.

Full access
M. J. Miller and A. K. Betts

Abstract

The low-level atmospheric transformation associated with a class of traveling convective storms observed. over Venezuela is described. A strong low-level cooling is observed, confined mostly to the subcloud layer, and associated with a deeper layer of drying and acceleration of the easterly flow. A density current model is used to stratify the storm travel speeds, peak surface gusts and the accelerated flow at low levels behind the storm, and to relate these to the low-level flow ahead of the storm. There is reasonable agreement between these atmospheric data and laboratory observations of density currents. The updraft and down-draft structure is discussed using an interesting sounding cross section and trajectories from a three-dimensional numerical simulation. It appears that two distinct downdrafts exist: one driven by precipitation within the cumulonimbus cell, and a second mesoscale feature which is dynamically driven, and associated with descent over the spreading cold outflow.

Full access
A. J. Thorpe, M. J. Miller, and M. W. Moncriefe

Abstract

Abstract not available.

Full access
M. J. Miller, A. C. M. Beljaars, and T. N. Palmer

Abstract

Stimulated by the results of a simple SST anomaly experiment with the ECMWF forecast model, a study was carried out to examine the model parameterization of evaporation from the tropica] oceans. In earlier versions of the model, these fluxes were parameterized with neutral transfer coefficients in accordance with the Charnock relation with equal coefficients for momentum, heat, and moisture. Stability correction was applied using Monin-Obukhov theory. This parameterization resulted in an extremely weak coupling between atmosphere and ocean at wind speeds below 5 m s−1. The transfer coefficients for heat and moisture have now been modified for low wind speeds to bring them in accordance with the empirical scaling law for free convedion. It is shown that these revisions to the transfer coefficients at very low wind speeds (<5 m s1) have a dramatic positive impact on almost all aspects of the model's simulation of the tropics. These include much improved seasonal rainfall distributions (with the virtual elimination of a tendency to generate a double ITCZ in both winter and summer), a much improved Indian monsoon circulation, and substantially reduced tropical systematic errors. The model previously had an eagerly bias in the zonal-mean upper tropical tropospheric flow with a corresponding cold bias in the deep tropics; it is shown that the flux revision substantially reduces this. Furthermore, the revision to the fluxes greatly enhances the model's ability to represent interannual and intraseasonal variability (see also the companion paper by Palmer et al.).

Full access
A. J. MILLER, S. TEWELES, and H. M. WOOLF

Abstract

Monthly mean values of the geostrophic angular momentum transport at 500 mb. have been computed as a function of latitude and zonal wave number (1 through 10) for a 10-yr. period.

The total transport is found to be in good agreement with previous calculations; at the same time several wave numbers exhibit considerable individuality. Equatorward transport by wave 2 at high latitudes extends farther south, and is much larger in magnitude, than the transport by any of the other wave numbers. Also, the negative transport in low latitudes is in distinct contrast to the behavior of the other waves. Wave 3, on the other hand, transports momentum poleward in mid-latitudes at a rate at least twice as great as that of any other wave number.

An additional finding is that in July and August, waves 1 through 5 are relatively inactive in transporting momentum, while waves 6 through 10 accomplish substantial transport near the latitude of the summertime maximum westerlies.

Full access
Jon M. Nese, Arthur J. Miller, and John A. Dutton

Abstract

A low-order moist general circulation model of the coupled ocean-atmosphere system is reexamined to determine the source of short-term predictability enhancement that occurs when an oceanic circulation is activated. The predictability enhancement is found to originate predominantly in thermodynamic processes involving changes in the mean hydrologic cycle of the model, which arise because the mean sea surface temperature is altered by the oceanic circulation. Thus, time-dependent sea surface temperature anomalies forced by anomalous geostrophic currents in the altered mean conditions do not contribute to the dominant ocean-atmosphere feed-back mechanism that causes the predictability enhancement in the model.

Full access
A. J. Miller, H. M. Woolf, and F. G. Finger

Abstract

Results of comparisons of wind and temperature data obtained by closely spaced launchings of Japanese MT-135 and American ARCAS meteorological rocket systems are presented. In general, smoothed temperature profiles show a certain disagreement that is unexplained at this time. Perturbation profiles of wind and temperature indicate a degree of correlation that tends to substantiate the existence of small-scale features.

Full access
M. W. Downton, T. R. Stewart, and K. A. Miller

Abstract

Time series of approximate United States average annual per capita heating and cooling degree days for the years 1895–1983 are presented. The data reflect the combined effects of climate fluctuations and population shifts, and can be used in studies of historical energy demand for heating and cooling at the national level. The development of the series is described, and the resulting data are used to show the effect of the north-to-south population shift of recent decades on the country's heating and cooling needs. Finally, other research topics are suggested for which the data may prove useful.

Full access
R. S. Quiroz, A. J. Miller, and R. M. Nagatani

Abstract

Significant advances have been made recently both in observational studies and in dynamical numerical simulations of stratosphere warmings. Observed characteristics of warmings are reviewed, with discussion of the trajectory of warm cells, the vertical and horizontal scale of the warm-air systems, the time-scale of warming, circulation effects, initial zonal flow conditions before a warming, and details of the energy budget before and after warming. Distinctions are drawn between the 1973 and 1963 types of warmings, which involved a poleward advance of warm air in wave 1 and wave 2, respectively. In contrast to the warming of 1963, a strong baroclinic conversion of eddy potential to eddy kinetic energy was not discerned in 1973, but both events were preceded by extraordinarily large fluxes from the troposphere. The results of dynamical warming simulations by several investigators reflect varying degrees of success in reproducing observed features of warmings. The results of Matsuno and Newson closely resemble important features of the 1963 and 1973 warmings, respectively. Some areas of apparent disagreement are explainable in part by the difficulty of matching the phase of simulated and observed events in time and space. Factors requiring elucidation include the physical process accounting for upward energy fluxes leading to warmings, the role of wave interaction in the stratosphere, and the associated tropospheric synoptic conditions.

Full access
Rebecca L. Miller, Gary M. Lackmann, and Walter A. Robinson

Abstract

Persistent weather regimes characterized by anomalous temperature or precipitation are often associated with persistent anomalies (PAs) in the tropospheric geopotential height field. To identify PAs throughout the annual cycle, an earlier definition is modified to apply a seasonally varying magnitude threshold, based on a smoothed, daily varying climatological average of daily 500-hPa geopotential height variability. The modified index can be applied to a wide variety of analysis, reanalysis, or model-forecast gridded data. Here, the modified PA index is used to identify positive and negative Northern Hemisphere PAs in all seasons and to compute trends in PA frequency, strength, location, and duration, in the ECMWF ERA-Interim reanalysis dataset (1979–2016). Height data are detrended and anomalies are weighted with an inverse sine-of-latitude function. In addition to maxima in PA frequency identified previously (North Pacific, North Atlantic, and Russia), an additional summertime maximum appears in the Arctic; this feature has not been analyzed extensively. A composite of summertime positive Arctic PA events reveals an equivalent barotropic structure, similar to that documented for midlatitude PAs. Arctic PA frequency is greatest in summer; it exhibits no trend in frequency over the 38-yr ERA-Interim analysis period. In fact, no discernable trends in PA frequency, strength, or duration are evident in the analysis period for the primary PA regions, although there is a suggestion of a northward shift in positive PA activity in the North Pacific.

Free access