Warm Air Flow Research Articles

A Numerical Modeling Study of Warm Offshore Flow over Cool Water

Abstract Numerical simulations of boundary layer evolution in offshore flow of warm air over cool water are conducted and compared with aircraft observations of mean and turbulent fields made at Duck, North Carolina. Two models are used: a two-dimensional, high-resolution mesoscale model with a turbulent kinetic energy closure scheme, and a three-dimensional large-eddy simulation (LES) model that explicitly resolves the largest turbulent scales. Both models simulate general aspects of the decoupling of the weakly convective boundary layer from the surface, as it is advected offshore, and the formation of an internal boundary layer over the cool water. Two sets of experiments are performed, which indicate that complexities in upstream surface conditions play an important role in controlling the observed structure. The first (land–sea) experiments examine the transition from a rough surface having the same temperature as the ambient lower atmosphere, to a smooth ocean surface that is 5°C cooler. In the second (barrier island) experiment, a 4-km strip along the coastline having surface temperature 5°C warmer than the ambient atmosphere is introduced, to represent a narrow, heated barrier island present at the Duck site. In the land–sea case, it is found that the mesoscale model overpredicts turbulent intensity in the upper half of the boundary layer, forcing a deeper boundary layer. Both the mesoscale and LES models produce only a small change in the boundary layer shear and tend to decrease the momentum flux near the surface much more rapidly than the observations. Results from the barrier-island case are more in line with the observed momentum and turbulence structure, but still have a reduced momentum flux in the lower boundary layer in comparison with the observations. The authors find that turbulence in the LES model generated by convection over the heated land surface is stronger than in the mesoscale model, and tends to persist offshore for greater distances because of greater shear in the upper boundary layer winds. Analysis of the mesoscale model results suggests that better estimation of the mixing length could improve the turbulence closure in regions where the surface fluxes are changing rapidly.

Diagnostic study on the structural characteristics of a typical mei-yu front system and its maintenance mechanism

In this paper, a typical mei-yu front process with heavy rainfall from June 12 to 15 in 1998 is analyzed. The results show that the mei-yu front is a front system which consists of an iso-θe dense area with strong horizontal gradient, a deep-convective cloud tower band, a passageway transporting warm and moist air flow from the summer monsoon surge in the mid and low levels to the south of the mei-yu front, and a migrating synoptic scale trough to the north of the mei-yu front, which transports cold and dry air southward in the mid and upper levels. The maintenance of the mei-yu front is realized by: (1) is a positive feedback between the moist physical process enhancing frontogenesis and the development of the strong convective system in front of the mei-yu front; (2) the sustaining system to the north of the mei-yu front which is a migrating synoptic scale trough transporting cold and dry air to the mei-yu front and positive vorticity to the mesoscale system in front of the mei-yu front.

On the dynamics of the splitting process of cyclones near southern Greenland

Cyclones approaching Greenland from south-west often experience a splitting near the southern tip of the island. With the aid of numerical analyses and derived diagnostic fields of such a case the dynamics of this process is investigated. It turns out that the warm air flow ahead of the cyclones is fully blocked by the mountains of southern Greenland and split into one part running along the western coast northwards and the other part going round the southern tip in north-easterly direction. This is connected with pressure fall, low-level convergence and ascending motion in the layers up to 700 hPa. Through that the movement of the original cyclone is influenced, but also a new vorticity maximum forms in the eastern part of the warm air flow. With the aid of a modified Q vector diagnosis it can be shown that the ascent of the warm air is caused partly by the frontogenesis at the fore part of the frontal zone and partly by the different turning of the isotherms in the orographically disturbed wind field around southern Greenland. When an additional upper forcing comes into play, e.g. ahead of an upper trough, a new cyclogenesis takes place in the frontal trough east of the island.

Study of the thermal behavior of a latent heat cold storage unit operating under frosting conditions

A study is performed of the thermal behavior of a latent heat cold storage unit operating under frosting conditions. This unit is employed to maintain the temperature inside the refrigerated compartment of a truck below 265 K. The system consists of parallel plates filled with a phase change material (PCM) that absorbs heat from the flow of warm moist air. A mathematical model for the system is first presented and, next, validated with numerical and experimental data. It is then exploited to assess the effects of design parameters and operating conditions on the performance of the system. The recommended thickness and distance separating the PCM plates are found to be 50×10 −3 and 30×10 −3 m, respectively. The results indicate that the performance of the unit is enhanced by turbulent air flow in spite of the increased pressure loss and accentuated frost growth. The unit also performs well even when the surrounding relative humidity is 100%.

Falsely Increased Bispectral Index Values in a Series of Patients Undergoing Cardiac Surgery Using Forced-Air-Warming Therapy of the Head

Falsely Increased Bispectral Index Values in a Series of Patients Undergoing Cardiac Surgery Using Forced-Air-Warming Therapy of the Head

The Atmospheric Boundary Layer In An Arctic Wintertime On-Ice Air Flow

A warm on-ice air flow from the open water over the Arctic sea ice in the Fram Straitwas, for the first time, systematically measured on 12 March 1998 by aircraft in thelowest 3 km over a 300-km long distance. The air mass modification and the processesinvolved are discussed. Over the water, air temperature was lower than water temperature so that a convectiveboundary layer (CBL) was present as initial condition. As soon as the CBL passed theice edge, a shallow stable internal boundary layer (IBL) was formed. In the residual CBL, turbulence and pre-existing convective clouds dissolved within about 20 km. Within about the same distance, due to the transition from unstable to stable stratification, the influence of surface friction increased in the IBL and decreased above the IBL with consequent generation of a low-level jet at IBL top. The IBL was strongly stratified with respect to both temperature and wind. The wind shear was around 0.1 s-1 so that the Richardson number in the IBL was subcritical and turbulence was generated. The IBL top grew to about 145 m over 230 km distance. The growth of the IBL was not monotonic and was influenced by (a) inhomogeneous ice surface temperatures causedby both different ice thickness and changes in the cloud conditions, and (b) leads in theice deck. At the front side of the on-ice flow, the air mass boundary between the warmair and the cold Arctic air was sharp (12 K over 10 km) at low levels and tilted withheight. Observations suggest that the stratified IBL was lifted as a slab on top of thecold air.

AN INTERFEROMETRIC INVESTIGATION OF NATURAL CONVECTION IN A PARTIALLY OPENED ENCLOSURE WITH A DISCRETE HEAT SOURCE

Natural convection in an enclosure with an opening in the right vertical wall and a heat source on the bottom surface was investigated using a holographic interferometric technique. In particular, emphasis was placed on the effects caused by changing the opening length, divider height, and heat source temperature. When the enclosure was partially opened, warm air from inside escaped from the upper part of the opening and was replaced by surrounding air, which flowed into the enclosure from the lower part of the opening. The flow rates of inflow and outflow through this opening increased with larger opening length, smaller divider height, and higher heater temperature. When the opening length was small, the opening did not significantly affect the upward flow of warm air from the heater, and resulted in a symmetrical temperature distribution. The divider prevented the development of upward flow from the heat source such that the temperature in the absence of a divider was generally higher than that for the longest divider. For cases with a large opening length, the upward flow was forced to move into the enclosure's left-hand side by the increased inflow. The effect of the divider height was not significant due to the increased flow rates through the increased opening length. The temperatures achieved with the longest divider were a little higher than those for the other cases due to the lower cold flow rate and the blocking of the cold air inflow by the longest divider.

Surface stress in offshore flow and quasi‐frictional decoupling

Aircraft data collected at approximately 15 m above the sea surface in the coastal zone are analyzed to examine the spatial distribution of surface stress. Advection of stronger turbulence from land dominates the near‐surface turbulence for the first few kilometers offshore. With offshore flow of warm air over cold water, strong stratification leads to very small surface stress. Because the stability restricts the momentum transfer to the waves, the aerodynamic surface roughness decreases to very small values, which in turn decreases atmospheric mixing. The redevelopment of the boundary layer farther downstream is examined. Computation of fluxes from observations for stable cases is difficult due to a variety of errors including large random flux errors, possible instrumental loss of small‐scale flux, difference between the surface flux and that at the observational level, and inadvertent capture of mesoscale motions in the computed turbulent fluctuations. Although the errors appear to be substantial, the aircraft momentum fluxes compare favorably with those from sonic anemometers on two buoys and a tower at the end of a 570‐m pier, even with near collapse of the turbulence.

Vertical Structure Of Turbulence In Offshore Flow During Rasex

The adjustment of the boundary layer immediately downstream froma coastline is examined based on two levels of eddy correlation data collected on a mast at the shore and six levels of eddy correlation data and profiles of mean variables collected from a mast 2 km offshore during the Riso Air-Sea Experiment. The characteristics of offshore flow are studied in terms of case studies and inter-variable relationships for the entire one-month data set. A turbulent kinetic energy budget is constructed for each case study. The buoyancy generation of turbulence is small compared to shear generation and dissipation. However, weakly stable and weakly unstable cases exhibit completely different vertical structure. With flow of warm air from land over cooler water, modest buoyancy destruction of turbulence and reduced shear generation of turbulence over the less rough sea surface cause the turbulence to rapidly weaken downstream from the coast. The reduction of downward mixing of momentum by the stratification leads to smaller roughness lengths compared to the unstable case. Shear generation at higher levels and advection of stronger turbulence from land often lead to an increase of stress and turbulence energy with height and downward transport of turbulence energy toward the surface. With flow of cool air over a warmer sea surface, a convective internal boundary layer develops downstream from the coast. An overlying relatively thick layer of downward buoyancy flux (virtual temperature flux) is sometimes maintained by shear generation in the accelerating offshore flow.

Structure of Offshore Flow

Abstract The horizontal and vertical structure of the mean flow and turbulent fluxes are examined using aircraft observations taken near a barrier island on the east coast of the United States during offshore flow periods. The spatial structure is strongly influenced by the surface roughness and surface temperature discontinuities at the coast. With offshore flow of warm air over cool water, the sea surface momentum flux is large near the coast and decreases rapidly with increasing offshore distance or travel time. The decrease is attributed to advection and decay of turbulence from land. The rate of decrease is dependent on the characteristic timescale of the eddies in the upstream land-based boundary layer that are advected over the ocean. As a consequence, the air–sea momentum exchange near the coast is influenced by upstream conditions and similarity theory is not adequate to predict the flux. The vertical structure reveals an elevated layer of downward momentum flux and turbulence energy maxima over ...

A New Way to Mitigate Frost Heave Around Manholes

ABSTRACT Frost heave of pavement around manholes in northern cities all over the world has been a costly problem ever since sanitary and storm sewers were installed. Freezing of frost-susceptible soil under the road structure can lead to substantial uplift, deformation and cracking of the pavement surface. This new technology is based on utilizing the heat in flowing water in sanitary and storm sewers, as well as the existing heat in the soil located deep around the manholes. Inside the manhole during winter, the air temperature is cold at the top and warm at the bottom generating air convection. The warm air flows from the bottom to the top of the manhole where it cools; this cold air then flows down to the bottom of the manhole where it warms up again. The heat energy in manholes with a perforated steel cover is simply lost. The innovation described calls for a closed system, where the heat energy is transferred to the area under the pavement structure, thus preventing the 0°C isotherm from reaching the frost-susceptible soil below. In collaboration with the City of Nepean, part of Greater Ottawa, Ontario, four manholes were reconstructed using the new frost-heave mitigation principle. A third manhole was instrumented as a control. The author, who is also the applicant for a patent on this new technology, provided the design for the reconstruction. The results of the trials during the winter of 1998–99 demonstrated that the innovation “works,” as the frost did not reach the frost susceptible soil (Leda clay) surrounding the manhole and did not cause any pavement deformation. No reference are provided, because no similar technology exists.

Surface Heterogeneity and Vertical Structure of the Boundary Layer

The concepts of vertical structure of the boundary layer over homogeneous surfaces are discussed, including the roughness sublayer, surface layer (inertial layer) and outer layer. As an interpretive literature survey, the vertical depth of the influence of surface heterogeneity, relative to such vertical structure, is examined in terms of blending height theory, convective boundary-layer scaling and internal boundary-layer theory. These concepts are examined with data over different surface types. The rich variety of types of surface heterogeneity and background flow preclude description of their influence on the boundary layer by one single approach. Nonetheless, new scaling arguments offer promise for convective conditions. Internal boundary layers are found to form only in certain situations and even then may exhibit more diffuse vertical structure compared to the textbook internal boundary layer. New scaling arguments are developed to describe the potential for internal boundary-layer development in flow of warm air over a cooler surface. These arguments explain some aspects of the observations but remain incomplete.

Warm air sensation for assessment of block after spinal anaesthesia

We have evaluated a new method of assessing dermatomal sensory levels after regional anaesthesia based on warm sensation. Sensory levels were assessed in 30 patients after spinal anaesthesia using a respiratory gas humidifier, adapted to deliver a constant flow of warm air at 40 +/- 0.2 degrees C. This was compared with the cold sensation from ethyl chloride spray. The frequency distribution of the dermatomal differences showed 96.6% of the comparisons were between +1 and -1 dermatomes. The median difference in dermatomal levels between the two methods of assessment was 0 (interquartile range 0-1) (P = 0.65). We conclude that the warm air method compares favourably with ethyl chloride spray and both can be used interchangeably.

Synoptic climatology of Northern Hemisphere available potential energy collapses

Three recurring regional patterns of extratropical baroclinic development associated with synoptic-scale collapses of Northern Hemisphere available potential energy (APE) are identified using a 1979–95 time series derived from the National Centers for Environmental Prediction (NCEP) reanalysis. A time series of the intraseasonal signal (from 1.6 to 180 days) of APE is used to discern an average cycle of approximately 3 days in the APE generation rate dA/dt (referred to as APE depletion rate if negative). An APE depletion event is defined as a fall and subsequent rise in the time series of dA/dt associated with this cycle.We define synoptic-scale APE collapses as APE depletion events with maximum depletion rates (dA/dtmin) and maximum APE falls (ΔAmin) of less than — 0.145×106 J m-2 day-1 and —0.280×106 J m-2, respectively. All are cold season (15 October–15 April) events. APE collapses were classified based on the evolution of regional synoptic patterns during the 2 days centered at the time of dA/dtmin. All are accompanied by deep tropospheric warming. The west Pacific warm surge (Type A) is driven by cyclogenesis over Japan and anticyclogenesis over the west-central North Pacific. The Bering warm surge (Type B) is associated with an intense southerly flow across the Bering Strait brought on by cyclogenesis near the Kamchatka Peninsula and an intense anticyclone over Alaska. The Atlantic Canada warm surge (Type C) is characterized by an onshore flow of warm air ahead of a continental storm track over eastern North America.

Measuring Bulk-Specific Gravity of Fine Aggregates: Development of New Test Method

Bulk specific gravity of the fine aggregate is used in hot-mix asphalt volumetric-mix design (including Superpave) to determine the amount of asphalt binder absorbed by the aggregate and the percentage of voids in the mineral aggregate. The current test method (AASHTO T84) uses a cone method to establish the saturated surface dry (SSD) condition of the sample, which is necessary to conduct the test. This method does not work satisfactorily for fine aggregates that are very angular and have rough surface texture and, therefore, do not slump readily when in SSD condition. A research project was undertaken to develop automated equipment and a method of establishing the SSD condition of the fine aggregate. The wet sample of the fine aggregate is placed in a rotating drum and subjected to a steady flow of warm air. The temperature gradient of the incoming and outgoing air and the relative humidity of the outgoing air are monitored to establish the SSD condition. Two prototype devices were constructed. The test results obtained with the second prototype device are encouraging and are reported. Further improvements to be made to the second prototype device to improve the repeatability and reproducibility of the test have been identified.

A Study on the Characteristics of Natural Convection in a Partially Opened Enclosure with a Heat Source

Natural convection heat transfer in an enclosure with an opening in the right veritcal wall and a heat source at the bottom surface is investigated using a holographic interferometric technique. The effects of the opening length, divider length attached to the top wall, and heater temperature on the temperature distribution are examined. The opening length as well as the divider length greatly affects the degree of inflow and outflow of air. In the case of small opening length, the opening doesnt affect much the upward warm air flow resulting in the symmetric temperature distribution .The upward flow in hindered by the divider resulting in the decrease of heat transfer from the heater region to the upper region. The longest divider shows the highest temperature in the lower region of enclosure. In the case of large opening length, the inward cold flow moves the upward flow to the left direction. Among the cases of different divider length, the case of H(sub)d=0.25H shows the highest temperature in the lower region of enclosure.

Pacific and atlantic ‘bomb‐like’ deepenings in mature phase: A comparative study

AbstractThe mesoscale dynamics of a secondary cyclone sampled during the field phase of the Fronts and Atlantic Storm‐Track Experiment (FASTEX) are documented using airborne Doppler radar data. the high quality of the mesoscale airborne Doppler dataset collected within this secondary cyclone is highlighted. This study demonstrates, however, the need to combine in the near future airborne Doppler radar and dropsonde measurements, in order to access multiscale processes involved in the mature stage of this FASTEX cyclone both in clear‐air and in precipitating regions. the studied secondary cyclone developed on the trailing cold front of a parent low situated over Greenland, and underwent the most explosive deepening (roughly −54 mb in 24 hours) sampled in the multiscale sampling area during FASTEX. the main flows responsible for the mesoscale organization of the cyclone are: a warm airflow emerging from the base of the synoptic warm conveyor belt that generates the most active part of the cloud head; a dry intrusion approaching in the dry slot the cyclone centre; a cold conveyor belt that contributes to the upward mass transport towards the upper part of the polar‐front cloud band; and a cold northerly flow in the western part of the cloud head, which tends to wrap around the low centre and is likely to release convective instability through air‐sea interactions. This study also documents a slight shift in position between the high vertical‐vorticity core associated with the active part of the cloud head and the location of the pressure minimum.The detailed three‐dimensional structure of this Atlantic secondary cyclone is also compared with that gathered within another ‘bomb‐like’ deepening (defined as more than a 24 mb surface pressure drop in 24 hours) observed over the Pacific ocean during the Coastal Observation and Simulation with Topography (COAST) experiment. Some similarities are found. the main differences appear to be linked to the warm seclusion that is exclusive to the studied Pacific cyclone. More pronounced ascending motions are found within the cloud head of the Pacific ‘reference’ case, and the deepening results from a more constructive effect of geostrophic and non‐geostrophic processes than for the Atlantic ‘reference’ case.

Observations of Strong Surface Radar Ducts over the Persian Gulf

Abstract Ducting of microwave radiation is a common phenomenon over the oceans. The height and strength of the duct are controlling factors for radar propagation and must be determined accurately to assess propagation ranges. A surface evaporation duct commonly forms due to the large gradient in specific humidity just above the sea surface; a deeper surface-based or elevated duct frequently is associated with the sudden change in temperature and humidity across the boundary layer inversion. In April 1996 the U.K. Meteorological Office C-130 Hercules research aircraft took part in the U.S. Navy Ship Antisubmarine Warfare Readiness/Effectiveness Measuring exercise (SHAREM-115) in the Persian Gulf by providing meteorological support and making measurements for the study of electromagnetic and electro-optical propagation. The boundary layer structure over the Gulf is influenced strongly by the surrounding desert landmass. Warm dry air flows from the desert over the cooler waters of the Gulf. Heat loss to the ...

Relationships between winter atmospheric circulation patterns and extreme tree growth anomalies in the Sierra Nevada

Tree-ring data from mid-elevation (∼2000 m) giant sequoia (Sequoiadendron giganteum) and high elevation (∼3500 m) pines (Pinus balfouriana, Pinus albicaulis) were used to select extreme growth years from which temperature, precipitation and large-scale winter (November–March, NM) 500 mb circulation patterns associated with the extreme tree growth anomalies were examined. Winters preceding extreme high growth in both giant sequoia and pines are warm and wet and are characterized by anomalous low pressure in the northeastern Pacific Ocean and a tendency for southwesterly flow and advection of warm maritime air into California. For the pines, such winters exhibit a pattern of anomalous low pressure in the northern Pacific, anomalous high pressure over northwestern Canada and anomalous low pressure across the southern US. NM 500 mb heights suggest more meridional circulation during the warm and dry winters preceding extreme low growth in giant sequoia. Atmospheric circulation during these winters exhibits a persistent trough/ridge pattern between the central Pacific and the western US. Storms are deflected away from California during these winters. NM atmospheric circulation patterns associated with extreme low growth in the pines exhibit maximum westerlies north of their mean position and the tendency for enhanced ridging in the northeast Pacific, which advects cool dry air into the Sierra Nevada. As dendroclimatic reconstructions are more frequently employed in order to better understand past variability of temperature and precipitation, synoptic dendroclimatological studies such as this one provide useful insights about atmospheric circulation. © 1998 Royal Meteorological Society

Stagnation-line melting of ice cylinders transverse to warm air flow

A model for the melting of ice cylinders transverse to a warm air stream is presented, in which the ice layer is assumed to break at the stagnation line. This work has been conducted in an attempt to model an earlier experimental study and as a first step in modelling warm-air defrosting of heat pump evaporators. It was recognized that the critical circular ice cylinder cross-section would change, as the melting progressed, to an ellipse, resulting in increased heat transfer such that the melting rate remained approximately constant with decrease in cylinder size. The results compared favourably with the experimental values.