Search Results

You are looking at 1 - 10 of 28 items for

  • Author or Editor: H. L. Johnson x
  • Refine by Access: All Content x
Clear All Modify Search
Richard H. Johnson and Diana L. Bartels

Abstract

The vertical structure of a midtropospheric mesovortex that developed during the decay of a midlatitude mesoscale convective system over Kansas and Oklahoma on 23–24 June 1985 is documented. Surface, rawinsonde, wind profiler, and dual-Doppler data are used to define its structure.

As has been observed in other midlatitude and tropical cases, the mesovortex occurred within the trailing stratiform precipitation region of a squall-line system. It was associated with a preexisting synoptic-scale short-wave trough, and as the circulation developed, it deformed the back edge of the stratiform precipitation region into a hooklike pattern. The ∼100–200-km mesovortex was confined to the midtroposphere (3–8 km), with a maximum amplitude just above the 0°C level. The vortex axis sloped toward the northeast, but its orientation changed hour by hour over the 2-h period of dual-Doppler coverage. Overall, the mesovortex was warm core, although its thermal structure was complex and apparently significantly influenced at the analysis time by a descending rear-inflow jet entering the rear portion of the stratiform region. The warmest anomaly was found at low levels (near 850 mb) with a shallow cool anomaly in the midtroposphere near the 0°C level and a weak warm anomaly aloft. At 500 mb the warmest air was shifted to the north of the vortex center, where the atmosphere was also relatively dry, while at 400 mb (near the top of the mesovortex) the warmest air coincided with the vortex center.

Although the mesovortex was sampled for only a brief portion of its lifetime, the observations suggest a close coupling between synoptic, mesoscale, and even microscale (cloud) processes in its formation. A vorticity budget based on the sounding data during the decaying stage of the storm indicates that convergence production of vorticity associated with a mesoscale updraft-downdraft couplet in the stratiform precipitation region was a critical factor in intensifying the circulation at that time. Vorticity production by tilting played a minor role during this period due to relatively weak environmental wind shear. The weak shear, however, likely contributed to the longevity of the mesovortex.

Full access
Richard H. Johnson and David L. Priegnitz

Abstract

An observational study of the thermodynamic and kinematic structure and properties of monsoon convective systems and their large-scale environment over the southern South China Sea during the field phase (December 1978) of the Winter Monsoon Experiment (WMONEX) has been carried out. The primary observations used are from three Soviet research vessels positioned in a triangular array off the north coast of Borneo during the period 6–28 December. Computations of thermodynamic fields, divergence and vertical motion have been made for the duration of the field phase based on six-hourly rawinsonde releases at the ship sites.

Analysis of the data indicates that the degree of convective activity over the southern South China Sea is modulated by long-period synoptic forcing (monsoon surges, easterly waves) and also significantly by diurnal forcing (land-sea breeze circulations). A diurnal cycle of convection persists in the region whether the synoptic-scale forcing is weak or strong. Convection over water to the north of Borneo regularly evolves on a diurnal basis from a small group of cumulonimbus clouds into a uniform mesoscale precipitation area having the characteristic structure of those observed in recent years over the tropical eastern Pacific and Atlantic oceans. In their mature stage the precipitation systems contain mesoscale anvil clouds commonly extending from near 500 mb to the tropopause covering a 104–105 km2 area. The ship observations provide direct evidence of mesoscale updraft motion within the anvil clouds and mesoscale down-draft below extending to near the surface.

Full access
A. K. Wåhlin and H. L. Johnson

Abstract

The Atlantic overturning circulation has conventionally been pictured in the meridional–vertical plane, but a significant densification of the water masses involved also occurs as the surface branch of the circulation flows in boundary currents around the subpolar gyre and northern marginal seas. Here an analytical model of the heat and salt budget for an idealized coastal boundary current in a marginal sea is presented. The boundary current exchanges heat and freshwater with the atmosphere as well as with the interior of the basin through eddy and Ekman transports. Its along-coast volume transport is assumed to be constant and independent of buoyancy; it is set, for example, by the wind forcing. Because the atmospheric fluxes of heat and freshwater are different, the temperature and salinity of the boundary current adjust on different length scales. The size of these length scales compared with the circumference of the basin determines the properties of the water that flows over the sill. Furthermore, the relative size of the two length scales determines the evolution of the density as the current moves around the basin. If temperature and salinity adjust on the same length scale (or if the density forcing is represented by a single component), then the density will increase or decrease monotonically from the inflow to the outflow. However, when the adjustment length scale for temperature is shorter than that for salinity, a warm and salty inflow can cool significantly before it freshens. As a result, the density first increases to a local maximum before decreasing again. Therefore, when salinity as well as temperature is included in the buoyancy forcing, the outflow from the basin can be significantly denser than for the equivalent single-component density forcing and can be more sensitive to the forcing parameters. The relevance and implications for the Nordic seas are discussed.

Full access
NEIL L. FRANK and H. M. JOHNSON

Abstract

Stronger tropical depressions are associated with a recognizable vortical cloud pattern. This note summarizes the number and history of vortical cloud systems observed over the tropical Atlantic during the 1967 hurricane season. The appearance of a “vortical cloud” does not necessarily imply the depression will intensify.

Full access
H. M. Johnson and Ronald L. Holle
Full access
William L. Smith, Lyle H. Horn, and Donald R. Johnson

Abstract

TIROS II channel 2 and channel 4 measurements of infrared radiation are statistically correlated with corresponding radiometersonde measurements of radiation divergence within various layers of the atmosphere. Estimates of the correlation coefficient greater than 0.60 are obtained for all layers extending from the surface of the earth to levels beyond 600 mb.

The results of this study indicate that the satellite measured flux is most significantly related to the infrared cooling profile within the surface to 600-mb layer. It is shown that good estimates of the infrared cooling profile might be obtained from satellite measurements if the general sky condition is also known.

Full access
Matthew Lybanon, Conrad H. Johnson, Pavel Pistek, and Richard L. Crout

Abstract

The U.S. Navy's GEOSAT active microwave altimeter provides detailed oceanographic and atmospheric information. It measures global oceanic wind speeds and significant wave height, sea ice edge in the polar regions, and dynamic topography related to mesoscale ocean circulation. The Naval Ocean Research and Development Activity processed near-real-time GEOSAT data to monitor oceanic processes from July 1985 to January 1989. We found that the combination of topographic information from GEOSAT, synoptic sea-surface-temperature information from infrared imagery, and local information from bathythermographs provides valuable information on Gulf Stream circulation. The size of the area involved, the intensity of currents, and the rapidity with which changes occur previously limited our technical ability to observe the Gulf Stream and its attendant spin-off eddies. Long-term study with the information sources described above has given a more complete picture of the Gulf Stream region's mesoscale circulation than ever before achieved.

Full access
O. Johnson, W. L. Clink, and G. H. Gilbert

Abstract

A description is given of the method used at the Suffield Experimental Station, Alberta, for the prediction of anomalous blast areas for the 100-ton explosion of 3 August 1961. Some indication is given of the change that would have occurred in the anomalous areas with increasing surface temperatures on the morning of the trial, and a comparison is made of the predicted area of anomalous blast with reports of audibility of the explosion.

Full access
E. M. Wilkins, Y. Sasaki, and H. L. Johnson

Abstract

Adiabatic thermals rising through an environment which has formed a friction layer by rotating in contact with a roughness plate are studied in the laboratory. The effects of frictionally induced motions on the growth of thermals and vortexes are assessed by comparing the friction situations with identical ones in which the roughness plate has been removed.

The analysis shows that convergence created by the friction layer gives rise to additional upward motion at the center of the cloud tank, and this tends to offset the rotational suppression of the thermal's growth. Entrainment of the environmental fluid by the thermal appears to be increased, and the enhancement of volumetric growth may be due in part to entrainment of turbulent fluid from the friction layer. An attempt is made numerically to evaluate this effect. Vortexes formed in the friction layer are found to be fatter and less intense than in the low-friction environment, and they also break contact with the ground.

The results of the laboratory experiments are found to agree very well with those of a numerical simulation in which the initial conditions were similar.

Full access
L. Ronald Johnson, Paul L. Smith, Thomas H. Vonder, and Donald Reinke

Abstract

The relationship of the rainfall from convective clouds to area-time integrals determined from satellite infrared data using a fixed infrared-temperature threshold is investigated. Concurrent radar and rapid-scan satellite data obtained during field projects in the northern High Plains and the southeastern United States were used in this study. The fixed IR threshold appropriate for each region was determined by an optimization procedure that identified the brightness threshold that yields the strongest relationship between estimated rainfall from a cloud cluster and its satellite area-time integral (ATI) for each dataset. For the North Dakota–Montana area the optimization procedure indicated that the area enclosed by the −22.5°C isotherm provides satellite ATI values most closely related to the estimated rainfalls. For the southeastern United States project, the optimized temperature threshold was 8.5°C. The difference between the thresholds determined for the two geographic areas suggests that a different “calibration” for each distinct area may be needed to make use of this relationship. Slopes of the two log–log rainfall-ATI regressions are less than unity, indicating that a relative horizontal expansion and/or increase in persistence of a cloud cluster exceeds the associated increase in precipitation. Implications for the Geostationary Operational Environmental Satellite precipitation index are discussed. New results concerning the rain volume-radar ATI relationship for the southeastern United States are also appended to the paper.

Full access