Showing papers in "Monthly Weather Review in 1974"

Theoretical Skill of Monte Carlo Forecasts

Abstract: The theoretical skill of Monte Carlo approximations to the stochastic dynamic forecasting technique proposed by Epstein is examined by means of an extension of earlier atmospheric predictability studies that used the test-field model of two-dimensional turbulence. The fundamental statistical hydrodynamical concept of an ensemble of phase paths evolving in a dynamical phase space is reviewed and used to define the statistical properties of a finite Monte Carlo sample. The application of a linear regression step to arrive at a final best estimate of the state of the atmosphere is also discussed. The resulting forecasts approach the climatological mean at forecast times so late that all skill has been lost. For an ideal case with an observing resolution, hopefully achievable in the 1980s with satellite-based sensors, it is found that the. Monte Carlo procedure leads to the greatest improvement in mean-square vector wind forecast skill in the 6- to 10-day range. For another case corresponding roughly...

A Three-Dimensional Numerical Model of the Sea Breezes Over South Florida

Abstract: An eight-level three-dimensional primitive equation model which includes a detailed boundary layer parameterization scheme has been used to describe the initiation and evolution of sea-breeze convergence patterns over south Florida as a function of the surface heat and momentum fluxes and of the large-scale synoptic forcing. A minimum grid spacing of 11 km was used. Model results are presented for several different initial conditions and the results, when compared against cumulus cloud and shower patterns, demonstrate that the dry sea-breeze circulations are the dominant control on the locations of thunderstorm complexes over south Florida on undisturbed days. It is also shown that, in contrast to the differential roughness, the differential heating between land and water over south Florida is the primary determinant of the magnitudes of convergence. The values of surface roughness, however, indirectly influence convergence patterns by affecting the intensity of the vertical turbulent transport o...

Various Vertical Coordinate Systems Used for Numerical Weather Prediction

Abstract: For numerical weather prediction with primitive equations (the Eulerian hydrodynamic equations modified by the assumption of hydrostatic equilibrium), various coordinate systems are used to represent the vertical structure of the atmosphere. In this paper, we review the essential features of prediction equations, satisfying the conservation of mass and total energy, in various vertical coordinate systems. We formulate the equations of horizontal motion, hydrostatic balance, mass continuity, and thermodynamics using a generalized vertical coordinate in which any variable that gives a single-valued monotonic relationship with a geometric height can be used as a vertical coordinate. Conditions to conserve total energy in a generalized vertical coordinate are investigated. Various prediction schemes using pressure, height, and potential temperature as a vertical coordinate are derived from the set of basic equations in the generalized coordinate system. These three coordinate systems are unique in th...

Application of Gravity Current Model to Analysis of Squall-Line Gust Front

Abstract: An analysis of a severe gust front is made to examine its three-dimensional structure. The data used are from the storm measurement network operated by the National Severe Storms Laboratory (NSSL). The important structural features shown in the analysis include: i) intense wind and thermal gradients at the leading edge of the air mass; ii) retardation of the cold-air front near the ground; and iii) vertical protrusion of the cold air to 1700 m at the front of the cold air mass, forming a bulge called a head. Upstream of the head, the depth of the cold air was almost constant at 3350 m; iv) a well defined circulation cell with a horizontal axis was found within the head; this wind field was associated with a core of maximum horizontal wind speed of 34 m sec−1 only 175 m above the ground, upward motion of 5–10 m sec−1 within the frontal zone, and weaker downward motion on the rear side of the head. In these structural characteristics, as well as in some substructural properties, this gust frontal a...

A multi-level spectral model. I. Formulation and hemispheric integrations

Abstract: The formulation of a multi-level spectral model suitable for simulation of atmospheric flow on a hemispheric or global scale is presented. The derived primitive equations are employed together with spectral-grid transform procedures in the multi-level domain. An efficient semi-implicit time integration scheme is detailed and results of numerical integrations initialized from analytic fields and Southern Hemisphere data sets are presented. A simple initializing device of divergence dissipation is suggested and shown to be most effective in eliminating spurious large-scale inertia-gravity oscillations.

Fourier Series on Spheres

Abstract: Spectral methods for the numerical solution of problems in spherical (and spheroidal) geometries are discussed. A new class of expansions based on special Fourier series are shown to lead to efficient and accurate simulations. A detailed exposition is given of the properties of surface harmonic series and transform methods and their relation to the new Fourier series on spheres. With resolution 1/N in both longitude and latitude, spectral methods using surface harmonics require 0(N) arithmetic operations per retained mode per time step while those based on Fourier series on spheres require only 0(logN) operations per retained mode per time step.

Heat and Moisture Budget Analyses Using BOMEX Data

Abstract: Large-scale heat and moisture budgets over the tropical Atlantic Ocean are examined during Phase 3 (22–30 June 1969) of the Barbados Oceanographic and Meteorological Experiment (BOMEX). From the satellite cloud photographs of ATS-3, the analyzed period is subdivided into an undisturbed part and a disturbed part. During the undisturbed period, downward motion predominates from the surface to about 500 mb and a large apparent heat sink and apparent moisture source are found near the top of the trade inversion layer. The upward heat flux due to cumulus clouds is confined below the 700-mb level. On the other hand during the relatively disturbed period, upward motion takes place at low levels and the heat flux due to cumulus convection extends to at least 500 mb. Values of the total heat flux estimated by large-scale budgets agree well with those obtained independently by bulk aerodynamic computations.

Frequencies of Cyclones and Cyclogenesis for North America, 1951–1970

Abstract: The mean frequency of cyclonic events for North America is determined for five months based on the 20-year period, 1951–1970. Monthly maps are used to present the data and display the distribution of cyclonic frequencies. The frequencies of cyclogenesis are also determined and used to construct mean cyclone tracks. A subjective comparison between these data and cyclone frequencies from earlier studies reveals differences which do not appear to form a consistent pattern. Although some differences can be attributed to differences in the methods of analysis, others suggest significant variation between these years and the early 1900's.

Strong Downslope Winds at Boulder, Colorado

Abstract: Twenty wind storms, with maximum gusts exceeding 33 m sec−1, are analyzed. At the height of the storms, mean hourly speeds in Boulder were 20 m sec−1 with mean maximum gusts of 36 m sec−1. At the same time wind speeds were about 50% lower along the lee slopes to the west and within 10 km to the east into the plains. The phenomena are non-stationary, and the result is a great diversity in surface wind speed patterns at the stations. Observational data show that the storms are the result of a deep lee trough or pressure jump. Air is accelerated down the slopes toward the local pressure minimum beneath the disturbance and is decelerated again rapidly to the east. Air mass characteristics that lead to the development of the disturbance are a stable layer or inversion at a critical level above mountain top and relatively strong winds at that level; strong winds in the upper troposphere are not necessary.

A Surge of Maritime Tropical Air—Gulf of California to the Southwestern United States

Abstract: This synoptic study, for the period 13–16 July 1972, involved the use of surface, radiosonde, and radar observations, as well as satellite pictures. Isentropic analysis indicated that the depth of the moisture with this surge of tropical air was of the order of 8000 to 12,000 ft. A unique feature of this type of surge is its resemblance to a giant sea-breeze effect, where the main advective forces result from the low-level pressure gradient between the desert thermal low and the relatively higher pressures over the cooler Gulf of California. This effect is emphasized by the lack of upper-air support, as shown in the mean vector winds from 10,000 to 20,000 ft for the period of concern. Satellite photographs and film loops give a dramatic picture of the movement of the cloud mass initially associated with the surge. They also suggest that one of the mechanisms that may be a factor in the development of the cloudy, showery area at the mouth of the Gulf of California is an easterly wave. These extens...

Hurricane-Spawned Tornadoes

Abstract: Hurricane-spawned tornadoes are most frequent at the time when hurricanes initially cross land and undergo rapid filling. This paper presents data composite information on all available rawinsonde and pibal reports surrounding this type of tornado genesis in the United States and Japan. Information has also been gathered on tropical storms entering land which did not produce tornadoes. The most important difference between storms which produce tornadoes and those which do not is a very large increase of the vertical shear of the horizontal wind between the surface and 4–5 thousand feet. This averages about 40 knots for the tornado cases, but is much less in the cases which do not produce tornadoes. Differences in vertical stability are observed to be small. An overland hurricane dissipation model is proposed whereby the boundary layer frictional inflow towards the hurricane center occurs without the usual ocean sensible heat gain and is not, as over the ocean, isothermal. Over land the inward spi...

The detection of dust storms over land and water with satellite visible and infrared measurements

Abstract: The position of a large area of dust was successfully located over the Sahara Desert and off the west coast of Africa with infrared measurements from the Temperature Humidity Infrared Radiometer (THIR) and visible information from the Image Dissector Camera System (IDCS) on Nimbus 4 between 20 and 25 April 1970 Over land, near midday, the dust areas were associated with lower equivalent blackbody temperatures (TBB's) than in the surrounding areas due to the mixing of the dust with the cooler air just above the surface Near midnight dust areas appeared to be located where the TBB's were higher than the surrounding regions The TBB contrast was insufficient for dust detection over the ocean, but the contrast in the visible was great enough to track the movement of the dust for several days off the African west coast Measurements from the THIR 65-72 micron channel were useful for detecting the presence of cirrus clouds which could have caused a misinterpretation of the 105-125 micron TBB's

Changes in the Characteristics of Typhoons Crossing the Island of Taiwan

Abstract: Twenty-two typhoons (1960–72) are examined to determine the effect of Taiwan on the intensity and movement of tropical cyclones crossing the island. The results show an average intensity (maximum surface wind) decrease of over 40% and a distinct northward deflection as the storms approach the island with a southward deflection after passage. Forecast rules for typhoons approaching or crossing Taiwan are presented.

Some Further Results Concerning the Vertical Structure of Atmospheric Tidal Motions Within the Lowest 30 Kilometers

Abstract: Long records of 2- and 4-times-daily rawinsonde data have been analyzed in order to obtain further information concerning the vertical structure of atmospheric tidal motions within the lowest 30 km. From the results of this study it is apparent that topographically induced diurnal tidal motions extend upward to at least 28 km, the highest level at which there exist sufficient data f or isolating them. At stratospheric levels the diurnal tide exhibits large seasonal variations with complete phase reversals in some regions. In contrast to the diurnal tide, the semidiurnal tide appears to be largely independent of topography and varies little with season. There is no evidence of a node below 30 km.

Seasonal Rainfall in Southwestern Australia and the General Circulation

Abstract: Seasonal rainfall data at 56 stations in southwestern Australia covering the period 1876–1970 are analysed. The square-root transformation is used. Principal component analysis is used for an exploratory study; the first eigenvector and coefficient are particularly informative. Rainfall patterns of recent years are then individually compared with features of the general circulation. In winter there is good spatial correlation over the region except in a small area near Hopetoun where reverse anomalies often occur. Two fundamental types of winter rain are postulated: continuous rains due to widespread uplift in mid-troposphere associated with winds from north of west, and showery rains associated with airstream instability and enhanced by topography and coastal convergence. Rainfall of early winter is mainly of the first type, closely related to the intensity of the westerlies and probably also to the quasi-biennial osculation of the equatorial stratosphere. Rainfall of late winter is more of the ...

Credible Interval Temperature Forecasting: Some Experimental Results

Abstract: This paper describes the results of an experiment involving credible interval temperature forecasts. A credible interval is an interval of values of the variable of concern, in this case maximum or minimum temperature, accompanied by a probability which expresses a forecaster's “degree of belief” that the temperature will fall in the given interval. The experiment was designed to investigate the ability of fore-casters to express the uncertainty inherent in their temperature forecasts in probabilistic terms and to compare two approaches (variable-width and flied-width intervals) to credible interval temperature forecasting. Four experienced weather forecasters participated in the experiment, which was conducted at the National Weather Service Forecast Office in Denver, Colorado. Two forecasters made variable-width, fixed-probability forecasts using 50% and 75% intervals, while the other two forecasters made fixed-width, variable-probability forecasts using 5°F and 9°F intervals. On each occasion ...

A Multiple Structured Frontal Zone-Jet Stream System as Revealed by Meteorologically Instrumented Aircraft

Abstract: The distribution of wind, temperature, ozone, and turbulence within a multiple structured frontal zone-jet stream system is described using a combination of direct horizontal measurements by meteorologically-instrumented research aircraft and conventional aerological soundings. Results document the spatial continuity of these zones and associated intrusions of ozone-rich stratospheric air. Aircraft turbulence measurements show the zones with strong vertical wind shear and near-critical values of Richardson number are preferred regions of CAT encounter. Diagnostic calculations of the terms of the gradient thermal wind equation illustrate the importance of the sign and magnitude of the air trajectory curvature and its vertical derivative in maintaining regions of strong vertical wind shear and small Richardson number in the presence of weak horizontal thermal gradient.

Thermodynamic Classification of Tropical Convective Soundings

Abstract: Soundings from a land tropical experiment (VIMHEX II) are classified according to local area rainfall into four regimes depicting the transition from dry to highly disturbed days. The thermodynamic structure for each of the four regimes is presented, and the implications for convective parameterization theories and the prediction problem are discussed in terms of the potential energy available for parcel convection for different entrainment rates.

Quasi-Biennial and Long-Term Fluctuations in the Centers of Action

Abstract: Quasi-biennial and long-term fluctuations in the latitude, longitude and pressure of the Azores High, Pacific High, Icelandic Low, and Aleutian Low have been determined for the period 1899–1967. Within the quasi-biennial framework, an increase in pressure of the Azores High is associated with an increase in pressure of the Pacific High and Aleutian Low, but a decrease in pressure of the Icelandic Low. Based on data for the period 1951–1967, there is a significant tendency for the Azores High to be relatively far west, and the pressure relatively low, at the time of quasi-biennial west wind maximum in the low tropical stratosphere, and for the Pacific High to be relatively far south at this same time. The latter appears in agreement with the finding that the centroid of the north polar vortex is displaced farther toward the Pacific Ocean at the time of quasi-biennial west wind maximum. The most impressive long-term trends are the indicated 5° latitude northward displacement of the Pacific High and...

A Stable Centered Difference Scheme Free of Two-Grid-Interval Noise

Abstract: A procedure leading to a stable space-centered, leapfrog time-differencing scheme, capable of preventing the false two-grid-interval noise that arises when two large-scale solutions are separated an different elementary subgrids, is presented.

Short-Range Prediction in Isentropic Coordinates with Filtered and Unfiltered Numerical Models

Abstract: Two limited-area prediction models in isentropic coordinates, one based on potential vorticity conservation and one using the primitive equations of motion, are tested on 50 synoptic cases chosen from winter and spring of 1972/73. Both models disregard diabatic processes, but incorporate variable terrain height. Numerical instabilities caused by overturning or entwining coordinate surfaces appear to be no problem in this approach, and the gravitational noise generated at the lower boundary in the primitive equation model is shown to remain well within acceptable limits. Skill scores based on displacement errors of surface cyclones and on correlations between predicted and observed sea-level pressure gradients indicate that neither model at this stage can compete with the six-level hemispheric model used by the National Weather Service. Of the two isentropic models, the one using primitive equations shows better skill than the potential vorticity model.

Time Spectral Analysis of Mid-Latitude Disturbances

Abstract: The method of time series analysis is used to study rawinsonde data in middle latitudes The spectra reveal baroclinically unstable traveling waves with a preferred period of four to five days and a large amount of variance at lower frequencies The preferred period of the longer-period waves is roughly 30 days, although the character of the disturbances in this range is highly variable For the waves of periods between three and seven days, phase relations are derived for the northward component of the wind, the temperature, and the height of constant pressure surfaces The phase relations are characteristic of traveling baroclinic waves and possess reasonable statistical significance No stable phases were obtained for the eastward component of the wind or for the longer-period oscillations

Temporal Variations in Seasonal Rainfalls in Southwestern Australia

Abstract: Trends and oscillations that have affected seasonal rainfalls in southwestern Australia since 1880 are revealed, and the most likely accompanying circulation changes are discussed. Results include a general increase of rainfall in early winter and a decrease in late winter, although in a small area around Hopetoun the trends have been reverse. The Southern Oscillation is correlated with winter rainfall in the region, and extremes of the SO are associated with more widespread rainfall anomalies. Methods of analysing time series for oscillations are described in detail. Quasi-biennial oscillations have affected the rainfalls of all four seasons; that of early winter has been less persistent than the others, but is probably related to the stratopsheric QBO. Some consequences concerning the past behavior of the latter are mentioned. A 4-year oscillation has affected late summer rainfall. It is concluded that rainfall patterns are useful in investigating past circulation changes, but their usefulness ...

On the Measurement of In-Cloud and Wet-Bulb Temperatures from an Aircraft

Abstract: Measurements of air temperature with an immersion thermometer from an aircraft are invariably affected by the increased temperature of the decelerated air in the vicinity of the element. For dry air and a dry thermometer this effect is well known and usually taken into account. However, the evaporation of water from an element which has been wetted either intentionally (as in a wet-bulb thermometer) or unintentionally (by cloud or rain droplets) reduces this temperature increase. The psychrometric equation generalized for high-speed flow is used to calculate the aerodynamic correction factor for a wet temperature sensor. As an example of the magnitude of the evaporation effect, the temperature difference between a wet and a dry thermometer in a saturated airstream moving at 70 m sec−1>1C. Aircraft measurements in clouds from 3 different temperature sensors are discussed. The temperature differences between an exposed and a protected thermometer are found to be as large as 1C in conditions where t...

Computation of Non-Divergent Streamfunction and Irrotational Velocity Potential from the Observed Winds

Abstract: An iterative scheme is presented to compute streamfunction and velocity potential from the observed winds. From the computed fields of streamfunction and velocity potential, the wind field is reconstructed and the reconstructed wind field is compared with the observed wind field. Such comparisons are made for the earlier methods also. The results of intercomparison among all the methods show that the root-mean-square vector error between the observed and the reconstructed total wind is minimum in the present method.

Monsoonal Influences on the Annual Variation of Tropical Cyclone Development over the Indian and Pacific Oceans

Abstract: Except for parts of the northwest Pacific, Indian Ocean and Pacific Ocean hurricanes always begin to develop in troughs located at high enough latitudes for low-level westerly winds to be observed on their equatorial sides. These near-equatorial troughs coincide with axes of maximum sea surface temperature, and in this respect they resemble continental heat troughs. Oceanic near-equatorial troughs follow the march of the sun. Generally, the farther they are from the equator the more often do hurricanes begin to form in them. The meridional rate of trough movement depends on the rate of ocean surface heating. Seas east and equatorward of the continents. Through the spring, continental heating rapidly diminishes the equatorward winds of winter, reducing evaporational cooling of the sea surface and mixed-layer depth. The consequent sea surface temperature rise brings the near-equatorial troughs poleward and starts the hurricane season only. In the height of summer, monsoon winds by increasing evapor...

A Sample Skill Score for Probability Forecasts

Abstract: A sample skill score (SSS), which is based upon a new partition of the probability, or Brier, score (PS) recently described by Murphy, is formulated. The SSS is defined simply as the difference between the PS for the sample relative frequencies, a term in this partition, and the PS for the forecast Probabilities. Thus, the SSS is a natural measure of the “skill” of probability forecasts. In addition, the other two terms in the partition of the PS form a useful partition of the SSS. Specifically, the SSS represents the difference between measures of the resolution and the reliability of such forecasts. The nature and properties of the SSS are examined. In this regard, the SSS is shown to be a strictly proper scoring rule (i.e., the SSS discourages hedging on the part of forecasters. The SSS is a difference skill score and is based upon sample relative frequencies, while the scoring rules used heretofore to measure the “skill” of probability forecasts have been ratio skill scores and have been base...

A Numerical Model of Trade Wind Weather on Oahu

Abstract: A single-layer mesoscale model, originally developed for the study of wintertime take-effect storms, is applied to the airflow over the Island of Oahu, Hawaii. Terrain effects, land roughness, island heating, and surface evaporation are all included. Convective precipitation and latent heating are parameterized in terms of larger-scale motions resolved on the 3-km × 3-km grid. Experiments were conducted with an atmospheric structure consistent with typical trade conditions as well as with variations from normal wind speed and inversion height. Satisfactory agreement was achieved with observed patterns of inversion height, cloud base height, temperature, relative humidity, wind flow, and precipitation under typical conditions. Mixing of dry air through the inversion from above was deduced to be very important. In most experiments the model generated a hydraulic jump to the lee of the mountains. Sea breeze effects were only partially simulated. Under some conditions the blocking effect of the Islan...

The Tropical Rainstorm

Abstract: The tropical rainstorm over land is extremely variable in both temporal and areal distribution. Usually only one rainstorm (defined to deposit > 10 mm of rain) occurs a day at one place. This paper is restricted to the tropical rainstorm as it occurs over Southeast Asia, Colombia, Panama, and Costa Rica. The size and duration of the rainstorms are discussed. A typical tropical rainstorm is about 30 km in diameter and the core rainfall exceeds 25 mm. More than 60% of the daily rain falls from the rainstorm within a period of one hour. The rainstorms are separated from each other with the area in between receiving little or no rain. Some effects of terrain on the size of the rainstorm and the time of day it occurs are discussed.

Spatial Variation of Precipitation over the Canadian Prairies

Abstract: Previous analyses of precipitation trends over the Canadian prairies have shown that the trends through the years have differed for different valleys. This result prompted an analysis of precipitation to discover further information on the spatial variations. Use was made of the coefficient of correlation. The results showed, as expected, that the correlation decreased rapidly with distance. It has also shown that the rate of decrease varied with the direction from station to station. Both the rate of decrease with distance and the direction of the least rapid decrease varied from month to month for the months, April to September, examined in the study. The results can be interpreted to show the mean direction of the storm tracks over the prairies.