Sea Level Pressure Fields Research Articles

The GISS Model of the Global Atmosphere

A model description and numerical results are presented for a global atmospheric circulation model developed at the Goddard Institute for Space Studies (GISS). The model version described is a 9-level primitive-equation model in sigma coordinates. It includes a realistic distribution of continents, oceans and topography. Detailed calculations of energy transfer by solar and terrestrial radiation make use of cloud and water vapor fields calculated by the model. The model hydrologic cycle includes two precipitation mechanisms: large-scale supersaturation and a parameterization of subgrid-scale cumulus convection. Results are presented both from a comparison of the 13th to the 43rd days (January) of one integration with climatological statistics, and from five short-range forecasting experiments. In the extended integration, the near-equilibrium January-mean model atmosphere exhibits an energy cycle in good agreement with observational estimates, together with generally realistic zonal mean fields of winds, temperature, humidity, transports, diabatic heating, evaporation, precipitation, and cloud cover. In the five forecasting experiments, after 48 hr, the average rms error in temperature is 3.9K, and the average rms error in 500-mb height is 62 m. The model is successful in simulating the 2-day evolution of the major features of the observed sea level pressure and 500-mb height fields in a region surrounding North America.

Supplementary Notes on Sea-Surface Temperature Anomalies and Model-Generated Meteorological Histories1

In seasonal computations, the Mints-Arakawa two-level model is found to be sensitive to a minor alteration in the computational program. Effects of the program change on monthly mean sea level pressure fields are small in the first month but large in the second and third months, although the meteorological histories generated by both the original and modified programs are equally credible. The inherited effects of a transient (1-mo) sea-surface temperature (SST) anomaly on the computed monthly mean sea level pressure fields over a period of a season are about as large in absolute magnitude as those generated in the model by a persistent (seasonal) SST anomaly. The effects of a transient SST anomaly in the North Pacific Ocean on monthly and seasonal temperature and precipitation in the eastern United States may be large enough to produce a change of one or two class intervals in these predicted weather elements. The model-generated precipitation in the equatorial region is also found to be sensitive to the sea-surface temperature field in the North Pacific.

Objective Sea-Level Pressure Analysis for Data-Sparse Areas

A computer procedure, described in an earlier study, uses the wind speed field near the ocean surface in combination with a small number of observations of pressure and wind velocity to specify the maritime sea-level pressure field. An improved version was used to analyze the pressure distribution over the North Pacific Ocean for eleven synoptic times in February 1967. Independent knowledge of the central pressures of lows is shown to reduce the analysis errors for very sparse data coverage. The application of planned remote sensing of sea-level wind speeds is shown to make a significant contribution to the quality of the analysis especially in the high gradient midlatitudes and for sparse coverage of conventional observations (such as over Southern Hemisphere oceans). Uniform distribution of the available observations of sea-level pressure and wind velocity yields results far superior to those derived from a random distribution.

The Adjustment of Surface Wind and Pressure by Sasaki's Variational Matching Technique

Abstract The sea level pressure and surface wind fields operationally produced at Fleet Numerical Weather Central are adjusted by using numerical variational analysis. The analysis region is a global band extending from 40S to 6ON and the successive corrections method is used to generate the initial or input fields. These fields are then adjusted within the framework of the variational method by requiring that they satisfy certain governing dynamical equations. A detailed study of this method is made in the Atlantic Ocean region on 4 January 1971. There is convincing evidence that small-scale wind information is incorporated into the pressure field and that the adjusted wind field has been modified to account for ageostrophic motion.

Objective Analysis of Sea-Level Winds and Pressures Derived from Simulated Observations of a Satellite Radar-Radiometer and Actual Conventional Data

A computer-based procedure is developed that combines a field of simulated satellite-derived wind speeds with a limited amount of conventional surface data so as to recover the surface pressure field and the vector wind field over the North Pacific Ocean. Wind speeds are determined from an objective analysis of high spatial density ship observations in order to simulate the anticipated output of a proposed satellite-mounted radar-radiometer system. The simulated speeds are combined with the conventional network for various spatial distributions of ship data. The average rms departure of sea-level pressure fields analyzed by deleting from 75-94% of the available ship observations from the maximum data analysis is from 3.0-4.0 mb. Comparison of the wind components implied by the isobaric patterns to those of the withheld ship observations yields average rms differences of from 8.7-9.4 kt for a range of 75-94% data deletion.

The Use of Linear Filtering as a Parameterization of Atmospheric Diffusion

A simple linear filter is adapted for use in numerical models of the large-scale circulation to act in place of an explicit horizontal diffusion term in the equations. The filter can be shown to be ideally suited for this purpose in the sense that it can be made increasingly scale-dependent as the order of the filter is increased. The one-dimensional filter of order n is constructed from n three-point symmetrical operators and involves 2n/1 grid points. It is capable of eliminating two-grid-interval waves completely, yet allowing little or no damping of longer waves. In one space dimension, the use of the n = 1 order filter can be shown to be equivalent to the incorporation of a one-dimensional Fickian diffusion term in the differential equation. For any order n, the use of the one-dimensional filter is equivalent to the incorporation of a one-dimensional linear diffusion of order 2n. It is therefore apparent that as n increases, the ability of the filter to discriminate in its response to short- and long-wavelength components becomes increasingly sensitive. The damping properties of linear diffusion are examined by means of the two-dimensional, horizontal Fickian equation and compared with the response of the order n = 8 filter in two dimensions. With typical values for the space and time increments and with K=4×109 cm2 sec−1, K−2> reduces two-grid-length waves by a factor e in 15 hr, four-grid-length waves in 30 hr, and ten-grid-length waves in 147 hr (6.1 days). The filter, applied each time step, removes the two-grid-length waves immediately and reduces three-grid-length waves by a factor e in 0.8 hr. This compares with ∼20 hr for K∇2. For four-grid-length waves, the effect of the filter is nearly the same as that of K∇2, but for longer waves the damping effect is drastically reduced. The filter requires 5 × 105 days to damp 10-grid-interval waves by a factor e and 3 × 1010 days for 20-grid-interval waves. Thus, waves shorter than four grid lengths are effectively eliminated, while longer waves are essentially unaffected, except in so far as nonlinear interactions among the longer waves produce a cascade to shorter wavelengths. If the grid length is properly chosen for the physical system being studied and the equations adequately model the large-scale dynamics, the filter will automatically handle the cascade to smaller scale and should represent the effect of viscous damping that takes place in the atmosphere. Some desirable properties of the filter are demonstrated by a one-dimensional example with no dynamics in which the order n = 8 filter is applied successively ten thousand times to grid-point data of the sea level pressure field.

A numerical experiment to predict a mean meridional pressure profile ab initio by Lagrangian integration

The question is posed: Consider an atmosphere initially at rest with a uniform mean sea level pressure field but possessing a mean baroclinicity defined by that which is observed during a selected month. How would this atmosphere readjust mass as a function of time and latitude as exhibited by the developing field of surface pressure if the initial pressure gradient field was maintained at all heights as input data and if baroclinic changes were ignored?

Tornadoes Associated with Cyclones of Tropical Origin-Practical Features

The frequency of reports of tornadoes associated with tropical cyclones is shown to have increased during the past decade to a level worthy of professional attention. Tornado incidence is demonstrated to be related to the direction of movement of the tropical cyclone, and to the intensity of the cyclone as it approaches the coast. A heading slightly cast of north is most favorable for tornado occurrence. Most tornadoes have occurred with tropical cyclones from the Gulf of Mexico which reached hurricane intensity, or with intensifying tropical depressions. Compact spatial distributions of tornadoes are disclosed upon relating occurrences to five selected stages in the life history of the associated tropical cyclone. The best relationships on location within the parent cyclone are obtained with respect to true azimuth, and are superior to those obtained using an orientation from heading along the tropical cyclone track. By eliminating the tornadoes associated with the extratropical stage of the tropical cyclones, the tornado occurrences were placed in a compact pattern in the northeastern sector and within a specified normalized distance from the center of the parent cyclone. The tornadoes connected with the extratropical stage are found widely scattered within southerly quadrants of the parent cyclone. Rather wide variations are found in the sea level pressure field in the neighborhood of tornado occurrences, but the averages established may have some utility. Tornadoes are shown to have occurred with both inner rainbands and the outer rainband, as well as outer convective cells, with the stronger cells of the outer rainband the most likely spawner of tornadoes. The forecasters' conception of the “bar” of the hurricane is disclosed as the approximate outer limit for tornadoes. Several parameters normally associated with midlatitude tornadoes are found to be mostly inapplicable to tornadoes within tropical cyclones. Some evidence is presented suggesting convective instability as a contributing factor. Sectional distribution, severity and frequency of tornadoes associated with tropical cyclones are determined to allow comparison with midlatitude tornadoes. Over 80 per cent of tropical cyclone associated tornadoes in the Carolinas and Virginia occurred with parent cyclones which had a Florida coast landfall. Precepts considered of practical value for forecasting tornadoes with tropical cyclones are summarized.