Cold-air Pool Research Articles

Numerical Prediction of a Cold-Air Outbreak: A Case Study with Ensemble Forecasts

Abstract The forecastability of a cold-air outbreak over eastern North America during January 1985 has been studied with ensemble forecasts from the NCAR Community Climate Model version 2 run at T42 horizontal resolution. The cold-air outbreak case was characterized by a pool of very cold air (T < −35°C at 850 mb) that moved southward into the central United States and intensified. The ensemble’s 10 member forecasts were initialized at 0000 UTC 15 January 85, a few days before the cold-air pool began its southward movement and reached its peak intensity. The ensemble members predicted the southward passage of the cold air but faster and weaker than analyzed. The predicted weakening of the cold-air pool was consistent with the model’s systematic error. Quasi-Lagrangian diagnosis of the 850-mb temperature tendency budget revealed that the analyzed intensification of the cold-air pool was due to residual rather than adiabatic effects. These residual effects could have been diabatic in origin but also attribu...

Surface Mesoscale Processes during the 1994 Palm Sunday Tornado Outbreak

The tornadic storms that developed in the 27 March 1994 Palm Sunday outbreak were confined to a narrow zone extending from central and northern Alabama to western North Carolina. Analysis of surface observations and soundings is used to examine the mesoscale environment of the region starting 14 h prior to storm development. The evolution of a shallow front that formed the northern boundary of the outbreak region is tied to several diabatic processes including evaporation of precipitation and differential solar heating. The resulting front was found to both limit severe convection and focus supercell development later in the day. During the night before the outbreak, as copious widespread precipitation fell into dry air, evaporation maintained a cold air pool north of the front. By contrast, moderate southerly flow provided warm, moist conditions to the south. Precipitation-enhanced cold air damming along the eastern slopes of the Appalachians also may have provided a source of cold air for subsequent frontogenesis over areas farther west. During the daylight hours, differential solar heating across the front further enhanced frontogenesis. Intensification of convection just prior to the first tornadoes was found to be associated with areas of breaks in the overcast near and upstream of tornadogenesis. Similarly, cells that intensified were moving over a surface that had been thoroughly moistened by previous rainfall. Supercells that intersected and moved along the frontal boundary maintained their tornadic strength for many hours, whereas storms that crossed the boundary disintegrated. Blockage of inflow by upstream storm cells may also have contributed to the rapid reduction of intensity of one of the tornadic cells.

Dynamical Constraints on Katabatic Wind Cessation in Adélie Land, Antarctica

The katabatic wind events observed in the coastal zone of Adelie Land, Antarctica, on 27 November and 3 December 1985 are simulated with a hydrostatic mesoscale atmospheric model coupled to a snow moder. The diurnal cycle of insolation is strong. The main difference in the forcing between the two events is the large-scale wind, which is weak on 27 November and moderate on 3 December. In both cases temperature and wind are characterized by well-marked diurnal cycles. In particular, katabatic winds blow during nighttime and upslope winds during daytime. In both cases the katabatic airstream slows down progressively over the ocean. Consequently, continental air piles up and this generates a pool of cold air responsible for a pressure gradient force opposing the katabatic wind. An amplification of the slowing down results. When, in the morning, insolation increases, the surface inversion weakens but the influence of the cold air pool increases. The katabatic how starts to decay over the coastal zone and then retreats progressively toward the ice sheet interior. When the large-scale wind is weak, the surface warming is sufficient for generating an additional upslope bouyancy force, and anabatic flow develops over the ice sheet in the afternoon. When the large-scare wind is moderate and downslope, the pilling up of cold air is important and this has a dramatic impact on the flow. A sharp spatial transition is generated between downslope and upslope winds over the ocean. This discontinuity moves toward the ice sheet interior in the morning and is responsible for the sudden cessation of the katabatic flow seen by static observers. The results of the simulations are used in order to refine a simple parameterization of Antarctic katabatic winds.

Toxic gas exposures in ice arenas.

To determine the number of states with laws monitoring toxic gases in ice arenas. To inform physicians who care for ice skaters of the dangers of carbon monoxide and nitrogen dioxide exposure in ice arenas. Survey, literature review. Health Departments of the 50 states and Washington D.C. At risk are users of ice arenas. Vigorous exercise (hockey or competitive figure skating) and underlying pulmonary or cardiovascular disease increase the risk toxicity. Toxic gas concentration is determined by the amount of production from internal combustion engines, effectiveness and use of the ventilation system, and the "cold air pool" over the ice. Number of states with laws regulating toxic gases in ice arenas. Only two states (Minnesota and Rhode Island) have laws regulating toxic gases in indoor ice areas. Physicians and the public are generally unaware of this problem. Toxic gas exposure in ice arenas is under recognized and underreported. The risks are not documented in journals generally seen by physicians who care for skaters. Conversion to electric ice resurfacing machines--the best solution--is not economically practical; legislation on the monitoring of toxic gas levels offers a reasonable alternative.

The Temporal Behavior of Numerically Simulated Multicell-Type Storms. Part I. Modes of Behavior

Abstract The temporal behavior of mature multicellular model storms, created in an experiment that varied the vertical wind shear layer depth, is examined herein. These storms form new cells at low levels on the storm's forward side, in or near the forced lifting zone at the edge of the evaporationally chilled subcloud cold-air pool. Each moves upward and rearward within the storm as it intensifies, matures, and decays and becomes replaced by a new cell development. As a result, the storms oscillate in time with respect to updraft intensity and the generation of condensation and surface rainfall. A few model storms oscillate in a simply periodic fashion during maturity, generating a series of nearly identical cells separated by a nearly constant period. Other storms are still periodic but in a more complex fashion, manifesting repeat cycles consisting of two or more cells. Several simulations appear quite aperiodic. Spectral analyses of temporal statistics reveal the existence of a fundamental period of o...

Sensitivity studies of TOVS retrievals with 3I and ITPP retrieval algorithms: Application to the resolution of meso-scale phenomena in the Antarctic

Retrievals from TIROS-N Operational Vertical Sounder (TOVS) data with the International TOVS Processing Package (ITPP version 3.3) and the Improved Initialisation Inversion (3I version 2) algorithms are investigated. The comparative study comprises the retrieval mechanism from the first guess to the final result for collocated radiosonde/satellite data as well as the problem of horizontal resolution for simulated Antarctic cold air pools. For the latter problem, TOVS brightness temperatures are computed for simulated NOAA-10 overpasses in the Weddell Sea region of Antarctica by the ITPP and the 3R (Rapid Radiance Recognition) forward models for retrievals with ITPP and 3I, respectively. The main results of this study are:

Wave trains over the sea due to sea breezes

Meteorological observation by a helicopter was carried out to investigate the structure of sea breezes over the coastal area of Tosa Bay in Shikoku island, Japan. Several groups of wave trains were found over the sea during a flight made on 25 November 1992. Not only the terrain barrier but the remaining cold air pools formed in valleys hindered further advance of the sea breeze inland, so that the presence of such wave trains may appear to be due to the effect of the secondary flow which supplies moister and cooler air from behind the sea breeze front.

Wintertime Dispersion in a Mountainous Basin at Roanoke, Virginia: Tracer Study

Abstract During January 1989, five nighttime SF6 tracer experiments were conducted in Roanoke, Virginia. The experiments were designed to help identify and understand the dispersion characteristics of a basin atmosphere during winter stagnation conditions. The basin studied was the Roanoke basin located on the eastern slopes of the Appalachian Mountains. This paper documents this tracer study and gives results from the experiment conducted on the night of 16–17 January 1989. A cold-air pool formed in the basin beginning after the evening transition period and filled to near the elevation of the lowest mountain barrier. A simple model of the ascent rate of the top of this cold-air pool is proposed. A sharp potential temperature jump was present at the top of this fully developed cold-air pool. Vertical measurements of tracer showed the initial ground-level plume to become elevated and ride over the top of the cold-air pool. Horizontal plume spread was enhanced over that expected from turbulent diffusion al...

Climatology of Mesoscale Warm and Cold Fronts in the Kanto Plain.

A statistical analysis is made of mesoscale warm fronts (MWFs) and mesoscale cold fronts (MCFs) in the Kanto plain by using hourly surface data covering eleven years. A series of objective criteria are used to select 190 MWFs and 177 MCFs, and composite analyses are made of their local structure, seasonal and diurnal variabilities, and the larger-scale environment. MWFs are associated with southeast-southwest winds or northwest winds blowing against a shallow cold-air pool having a depth of a few hundred meters. The MWF associated with southeast-southwest winds corresponds to the warm-air intrusion during the passage of a cyclone, while the MWF associated with northwest winds corresponds to a strong northwesterly surge. The frequency of MWFs is highest between 06 and 12 JST. The MCF generally occurs during a cold-air surge. From late autumn to winter, the MCF is mainly associated with northwest winds accompanied by shallow cold air in its leading edge, and is more frequent in the nighttime than in the daytime. From spring to summer, MCFs associated with northeast or east winds are more frequent than those associated with northwest winds, and have a strong tendency to occur in the afternoon.

A Numerical Study of a Squall Line over the Taiwan Strait during TAMEX IOP 2

A two-dimensional numerical cloud model was used to investigate a squall line that occurred over the Taiwan Strait on 16 May 1987 during TAMEX (Taiwan Area Mesoscale Experiment). This squall line illustrated multicellular behavior as revealed by many Doppler analyses. The simulated squall line produced 13 new cells successively in a 6-h simulation, thus maintaining itself in a long-lasting state. The simulation result recaptured many of the interesting features also observed. The air in the front-to-rear inflow was ingested into the squall-line system primarily from the low levels in front of the system and accelerated upward and backward over a surface pool of cold air in the system-relative frame. Beneath this front-to-rear flow, a rear-to-front current developed between 2 and 4 km in altitude. It descended into the cold pool near the surface. Part of this descending air moved forward toward the gust-front region and part of it moved backward. The relative pressure maximum coincided with the cold pool near the surface and was overlaid by the relative pressure minimum in the midlevels where the convective region corresponded to the decrease of pressure deviation. When the aged cells moved back relative to the system, the decreasing pressure deviation moved back as well. The vertical pressure-gradient force associated with the dynamic part contributed to the formation of a new cell below the level of free convection, while the sum of the vertical pressure-gradient force associated with buoyancy and the buoyancy itself dominated the development of a cell once this cell had ascended above the level of free convection. The vertical transport of horizontal momentum normal to the squall line was found to he countergradient in a layer between 2.5 and 6.5 km, which could not be explained in terms of diffusion. In addition, this vertical flux of horizontal momentum normal to the squall line was independent of the mean vertical shear below 8.25 km, except at the 1.75-km height where the vertical shear changed signs very rapidly. An experiment was made to investigate the role of the ice-phase microphysics on the formation and structure of the simulated squall line. The comparison between our simulation results and other model studies will be presented here.

Nocturnal air and road surface temperature variations in complex terrain

AbstractThis paper is part of an ongoing project dealing with the modelling of local climate for predicting temperature variations and risks of road slipperiness under various synoptic conditions. Temperature recordings from mobile measurements taken along road stretches have been analysed to determine the influence of valleys on the variation in air and road surface temperatures. During clear, calm nights, the variation in air temperature between valley bottoms and surrounding areas can be related to geometric properties of valleys, such as depth and width. It is seen that temperature differences between valleys and surrounding areas increase with increasing depth and width. The prevailing wind speed has been found to be a major factor controlling the variation in temperature. The variation in air temperature is totally smoothed out in situations in which there is an ambient wind speed exceeding 3 m s−1. In the interval 2–3 m s−1, the degree of wind exposure in valleys is of importance for the magnitude of the air temperature differences. Counter‐radiation from clouds has been found to significantly reduce the intensity of cold air pools. Under partly cloudy situations (3–5 octas) with no wind, the temperature difference between valley bottoms and surrounding areas differs by approximately 6°C in comparison with clear, calm situations. The relationship between differences in air and road surface temperature has also been studied. In general, an increase in cold air pool intensity of 1°C lowers the road surface temperature by 0·4°C.

Case Study of an Orographically Induced Mesoscale Vortex (Denver Cyclone)

Abstract Observations taken during the Convection Initiation and Downburst Experiment (CINDE) are used to describe the formation and structure of an orographically induced mesoscale vortex that frequently occurs in northeastern Colorado. This vortex, known locally as the Denver Cyclone due to its proximity to the Denver metropolitan area, is frequently associated with severe weather. We present a case study of the Denver Cyclone of 25 June 1987, that formed during the late morning hours and remained nearly stationary for over 24 hours. Interesting features of the case study vortex are: low-level convergence into the center of the cyclone during nighttime hours but divergence at the center when daytime heating becomes significant; a very shallow initial vertical extent at night, growing to nearly 1500 m during the daytime hours; a cold pool of air on the west side of the vortex, with highest surface potential temperatures present in a warm plume on the east side; a perturbation low pressure of ∼150 Pa in t...

Mesoscale Vortices in the Weddell Sea Region (Antarctica)

Preliminary results of investigations of Antarctic mesocyclones in the Weddell Sea region are presented for the Antarctic summer periods 1983–88. Based on NOAA and METEOR satellite images, a total of 195 mesoscale vortices (scale less than 1000 km) were observed and classified using a simple classification scheme, depending on lifetime and cortex diameter. The highest frequency of vortices was found over the ice free part of the eastern Weddell Sea. Mean synoptic conditions for vortices with scales less than 400 km near Halley Station indicate an offshore flow being most favorable for their development. A case study with satellite images, surface measurements, synoptic analyses derived from TOVS data shows the evolution of several mesoscale vortices in a pool of cold air in the eastern Weddell Sea.

An Examination of Extreme Cold Air Outbreaks over Eastern North America

The sequence of development and thermodynamic aspects of two strong cold air outbreaks over eastern North America during January 1977 are described. In the first outbreak, surface cyclogenesis occurs prior to the outbreak onset (local 850 mb temperature decreases). Regions of strong cold air advection and adiabatic warming are found immediately upstream of the cyclone over the cold air outbreak area. Since the two regions are nearly superimposed, the effect of advective cooling is partially opposed by adiabatic warming. In the second outbreak, surface cyclogenesis follows the outbreak onset. In this case, local and advective cooling is observed over a larger region as a cold air pool over central Canada is transported southeastward. Initially, adiabatic warming is weak or replaced by adiabatic cooling over eastern North America as cold air advection dominates the thermodynamic energy balance. As downstream cyclogenesis proceeds during the latter stages of the outbreak, adiabatic warming intensifies over the cold air advection region, reducing the cooling effect. An inspection of 15 other strong cold air outbreaks in a ten-winter sample reveals a qualitatively similar relationship among the size of the cold air advection region, timing of the surface cyclogenesis and areal averaged 850 mg temperatures. It is hypothesized that the intensity of cold air outbreaks over eastern North America is proportional to the areal coverage of the cold air advection region and timing, relative to cold air advection onset, of surface cyclogenesis.

MODELLING OF LOCAL CLIMATE FOR PREDICTION OF ROAD SLIPPERINESS

Recent research concerning the Swedish Road Weather Information System (VVIS) is discussed along with a method for determining temperature variations attributable to topography during different weather situations. Topographical areas are classified according to factors determining temperature differences. The investigation concentrates on areas where great temperature differences occur during different weather situations, producing a risk of local road slipperiness. Geographical factors analyzed and discussed include valleys, height differences, shadow areas, bridges, and regional climate. Cold air pool intensities are shown to be influenced by weather but a variation between different valleys also occurs owing to valley width area of cold air production, wind shelter, and ratio between height and width of valley slopes. During neutral or unstable lapse rate conditions, the lowest road surface temperature is closely related to the air temperature and decreases with increasing altitude above sea level. Screened areas have a lower surface temperature than areas exposed to sun. Factors such as bridge size and type also affect road surface temperature on bridges. These different factors are integrated into a local climatological model. (Author/TRRL)

Regional boundary layer airflow patterns derived from digital NOAA-AVHRR data

Abstract NOAA AVHRR (Advanced Very High Resolution Radiometer) data make a significant contribution to information about boundary layer air flow patterns, especially over complex terrain, where wind fields are very often totally disconnected from upper air flows. An evaluation procedure which is based on different approaches, depending on the regional meteorological situation, is presented in this paper. It-includes (1) mapping of local cold air pools, (2) mapping of the vertical extent of temperature inversions and (3) mapping the extent and dynamics of fog layers. All the examples refer to the Swiss Alps and the surrounding basins.

Numerical Simulation of a Midlatitude Squall Line in Two Dimensions

A two-dimensional, anelastic cloud model was used in attempts to numerically replicate the observed structure of a midlatitude squall line. Initial conditions were adapted from observations of the 22 May 1976 Oklahoma line. Model simulations were made with and without considering the ice phase of water. These model storms have, within the constraints of the model's geometry, replicated the basic multicellular character and general line-normal airflow typical of these lines. In addition, the structure and intensity of the subcloud cold air pool and the propagation speeds developed by the storms appear to be reasonable. Further, it was found that the initial conditions chosen resulted in model storms which were not only long-lasting but also essentially repetitive, indicating that a state of quasi-equilibrium had been attained. The storms did not decay because the environmental conditions ahead of the storms were favorable and essentially unchanged during the course of the simulations. The inclusion of ice phase processes resulted in the production of more realistic appearing features in the trailing portion of the storm as well as more widespread precipitation. These were for the most part due to the enhanced rearward transport of precipitation particles from the convective cells which resulted from including ice, particularly low density snow. The underlying structures of these model storms were investigated by averaging model fields across time, smoothing out the transient components. These analyses indicated that the addition of ice had its greatest impact on the scale of the storm's circulation features.

Observations of complex-terrain flows using acoustic sounders: Experiments, topography, and winds

Acoustic sounders have now been used extensively in a series of noctural drainage flow experiments carried out by the U.S. Department of Energy's Atmospheric Studies in Complex Terrain (ASCOT) program. Doppler acoustic sounders, located in three different valleys during the sequence of experiments, reveal drainage-wind profiles that depend strongly on the ambient meteorological conditions and the elevation of each observing site relative to surrounding terrain. In elevated sites that drain easily, Doppler-sounder derived wind profiles show a simply-structured flow; in lower lying areas, subject to topographic constriction and cold-air pooling, and where Archimedean forces are comparable to those due to synoptic and mesoscale pressure gradients, the wind profiles show considerable vertical and temporal variation. In particular, in the Geysers area of northern California, the seabreeze and the depth of the Pacific Coast marine inversion affect not only the initiation of drainage winds but also their subsequent evolution.

Numerical modelling of Bora winds

The Bora winds are produced by cold stable air which pours over the Dinaric Alps, often producing intense winds along the Adriatic Coast. Although the flow appears qualitatively similar to the hydraulic flow described by the shallow-water equations, there are certain significant differences: the cold low-level air is continuously stratified and a critical layer in the winds typically occurs near the inversion which caps the cold pool of air. Through two-dimensional numerical mountain wave simulations, we investigate the extent to which hydraulic theory can be used to describe the Bora winds. We analyze the structure of the Bora flow derived from aircraft observations collected during the ALPEX field phase on 15 April 1982 and compare it with a numerical simulation initialized from upstream sounding data. By varying the environmental sounding in our simulations, we find that for this case, neither the critical layer nor the inversion layer play a fundamental dynamical role in generating the strong winds along the lee slope. Instead, the wave overturning which occurs beneath the inversion appears to be the most important factor in producing the strong response. This overturning produces shooting flow over the lee slope and strongly resembles the hydraulic flow which occurs both in shallow water theory and in simulations in which over-turning is suppressed. We believe the hydraulic jump-like mechanism producing the strong Bora slope winds is fundamentally similar to the underlying mechanism which produces the intense winds along the lee slope of the Rocky Mountains. This occurs despite significant differences in the character of the larger scale flow in these two situations.

Mesoscale Features of the Michigan Land Breeze Using PAM II Temperature Data

During the period 16–20 December 1983, the northern United States was enduring a record-breaking cold air outbreak. The cold air brought temperatures as low as −40°C to some areas while bringing considerable lake-effect snows to locations in the Ice of the Great Lakes. In western Lower Michigan much of the snow fell from the wind-parallel snowbands that resulted as the cold, strong northwesterly winds crossed the relatively warm waters of Lake Michigan. Analysis of surface weather data from PAM II stations in the region showed that during daylight hours of 18 December, when the winds were fairly strong, temperatures across Lower Michigan were relatively uniform. By 19 December, the strong winds subsided and strong radiational cooling after sunset resulted in the formation of a very cold pool of air over western Lower Michigan. Separating this cold air from the warm areas near the shore was the land-breeze front. Along the south end of the lake, radiational cooling served to intensify an existing south-to-north temperature gradient that resulted as cold air sweeping around the south end of the lake stood in contrast to lake-warmed air over southwest Michigan. As a result of this intensification, the cold air pushed northward as a density current or land breeze. Eventually, radiationally cooled air would cover all of western Lower Michigan.