Search Results

You are looking at 1 - 10 of 24 items for

  • Author or Editor: Richard Clark x
  • All content x
Clear All Modify Search
Andrea Smith, Richard Clark, and Richard Jeffries
Open access
Richard P. James and John H. E. Clark

Abstract

Dry intrusions play an important role in modulating precipitation patterns both in the midlatitudes and in the Tropics. The lifting of unsaturated air aloft often leads to destabilization and the enhancement of precipitation rates, and may occasionally contribute to the occurrence of severe weather. A method for qualitatively diagnosing vertical motion in a region of elevated dry advection is presented. The procedure measures the rate of propagation of relative humidity isopleths relative to the flow and deduces the sign of the vertical velocity. Changes in static stability are inferred, leading to the possibility of improved short-term forecasting of precipitation associated with dry intrusions.

The procedure is illustrated with a case study involving heavy snowfall associated with a dry intrusion in the mid-Atlantic region. A diagnosis of ascent within the dry intrusion is obtained from satellite imagery and confirmed using numerical model output.

Full access
Thomas R. Parish and Richard D. Clark

Abstract

Extensive measurements were made of the summertime Great Plains low-level jet (LLJ) in central Kansas during June and July 2015 as a component of the Plains Elevated Convection at Night (PECAN) field study. Here, the authors describe the early phase of the LLJ development on 20 June 2015. Half-hourly soundings were launched to monitor the progress of the jet. An airborne mission was also conducted using the University of Wyoming King Air research aircraft. Vertical sawtooth patterns were flown along a fixed track at 38.7°N between longitudes 98.9° and 100.3°W to document changes in the potential temperature and wind profiles. Ageostrophic winds during the LLJ formation were also assessed. In addition, a high-resolution numerical simulation of the 20 June 2015 LLJ case was conducted using the Weather Research and Forecasting Model. Observations and model results show that the early stage of development consisted of a rapid increase in wind speed in the hours just after sunset with less pronounced directional change. The LLJ evolution is similar to that expected from an inertial oscillation of the ageostrophic wind following the stabilization of the near-surface layer.

Full access
John H. E. Clark, Richard P. James, and Richard H. Grumm

Abstract

The processes responsible for a banded snowfall region during a December 1997 East Coast storm are examined. Conventional data plus a numerical simulation with the fifth-generation Pennsylvania State University–NCAR Mesoscale Model (MM5) are used. Calculations of slantwise potential area near the bands suggest that the release of conditional symmetric instability played a role in their formation. The location and timing for the appearance of negative moist potential vorticity (MPV) cannot, however, be reconciled with band formation. A balanced MPV model based on the geostrophic momentum approximation is developed. It provided new insights into the mechanisms of MPV generation. A swath of negative balanced MPV now coincides with the snowbands. MPV sources are proposed that are linked to a vigorous mesoscale updraft near the bands. The updraft occurred on the warm, moist side of a zone of midtroposphere frontogenesis. Negative MPV develops through differential ageostrophic transports of geostrophic momentum and equivalent potential temperature. Of these, differential vertical equivalent potential temperature transport was the most efficient and accounted for the largest fraction of model-produced negative MPV tendencies near the bands. This mechanism was particularly strong in the lower troposphere near the mesoscale updraft.

Full access
Thomas R. Parish, Alfred R. Rodi, and Richard D. Clark

Abstract

A case study of the kinematical and dynamical evolution of the summertime Great Plains low level jet (LLJ) is presented. Airborne radar altimetry was used to discern the x and y components of the geostrophic wind at three levels in the lower atmosphere throughout the LLJ episode. Results appear to confirm previous theoretical and numerical studies regarding the importance of the diurnal cycle of heating over sloping terrain in producing an oscillating horizontal pressure gradient force. Inertial turning of the LLJ as a result of frictional decoupling was also documented. It is concluded that the inertial oscillation resulting from the sudden decrease in friction in the lower atmosphere during the early evening is the dominant mechanism in forcing this example of a summertime Great Plains LLJ.

Full access
Sepideh Yalda, Gary Zoppetti, Richard Clark, and Kathleen Mackin
Full access
S. Clark Rowland, Richard G. Layton, and David R. Smith

Abstract

Clean, single crystals of silver iodide have been found to serve as a very poor nucleating agent for ice. The controlled introduction of impurities on the crystal surface by means of photolytic decomposition has been shown to greatly increase its nucleating ability.

Full access
Thomas R. Parish, Richard D. Clark, and Todd D. Sikora

Abstract

The Great Plains low-level jet (LLJ) has long been associated with summertime nocturnal convection over the central Great Plains of the United States. Destabilization effects of the LLJ are examined using composite fields assembled from the North American Mesoscale Forecast System for June and July 2008–12. Of critical importance are the large isobaric temperature gradients that become established throughout the lowest 3 km of the atmosphere in response to the seasonal heating of the sloping Great Plains. Such temperature gradients provide thermal wind forcing throughout the lower atmosphere, resulting in the establishment of a background horizontal pressure gradient force at the level of the LLJ. The attendant background geostrophic wind is an essential ingredient for the development of a pronounced summertime LLJ. Inertial turning of the ageostrophic wind associated with LLJ provides a westerly wind component directed normal to the terrain-induced orientation of the isotherms. Hence, significant nocturnal low-level warm-air advection occurs, which promotes differential temperature advection within a vertical column of atmosphere between the level just above the LLJ and 500 hPa. Such differential temperature advection destabilizes the nighttime troposphere above the radiatively cooled near-surface layer on a recurring basis during warm weather months over much of the Great Plains and adjacent states to the east. This destabilization process reduces the convective inhibition of air parcels near the level of the LLJ and may be of significance in the development of elevated nocturnal convection. The 5 July 2015 case from the Plains Elevated Convection at Night field program is used to demonstrate this destabilization process.

Restricted access
Clark J. Weaver, Anne R. Douglass, and Richard B. Rood

Abstract

The NASA/Goddard three-dimensional chemistry and transport model is driven by winds from a stratospheric data assimilation system. Synoptic- and planetary-scale patterns, apparent in satellite observations of trace constituents, are successfully reproduced for seasonal integrations. As model integrations proceed, however, the quality of simulations decreases, and systematic differences between calculation and measurement appear. The differences are explained by examining the zonal-mean residual circulation. The vertical residual velocity * is calculated two ways: (i) from the diabatic heating rates and temperature tendency and (ii) from the Eulerian vertical velocity and the horizontal eddy heat flux convergence. The results from these calculations differ substantially. Periodic insertion of observational data during the assimilation process continually shocks the general circulation model and produces these differences, which leads to an overestimate of the mean vertical heat and constituent transport. Such differences are expected to be general to all data assimilation products. This interpretation is corroborated by two-dimensional (2D) model calculations. When * is calculated from (ii), the 2D ozone evolution is unrealistic and qualitatively similar to the 3D model simulation. The 2D ozone evolution is reasonable when * is calculated from (i).

Full access
Clark J. Weaver, Anne R. Douglass, and Richard B. Rood

Abstract

Ozone simulations are performed in an attempt to simulate laminar events with the frequency observed in balloon ozone sondes. The winds are taken from the Goddard Earth Observing System Data Assimilation System (GEOS DAS); the importance of horizontal and vertical resolution to production of lamina are investigated. A simulation with a high horizontal reolution (grid spacing 1° latitude by 1.25° longitude) and high vertical resolution (∼300 m grid spacing) isentropic model produces lamination frequencies close to the balloon sonde climatology near the polar vortex edge but exhibits too much lamination in the subtropics. This indicates that the GEOS DAS winds contain the information to produce laminar events, although such small-scale features are not manifest in the more commonly used 2° latitude by 2.5° longitude transport model, which uses the hybrid sigma-pressure vertical coordinate. The zonal average ozone tendencies due to horizontal mixing in the lamina-producing models are similar to the tendencies in coarser resolution models that show no lamination, suggesting that it is not necessary to resolve laminar events to maintain a realistic ozone budget. The comparison of the modeled lamination frequency with the balloon sonde climatology indicates that the model horizontal mixing at the vortex edge is accurate but in the subtropics the mixing is excessive.

Full access