Potential Temperature Difference Research Articles

Inversion effects on mountain lee waves

AbstractThe effect of a sharp low‐level temperature inversion on flow over a mountain is investigated via a series of two‐dimensional idealized numerical model simulations. The main focus of the study is the effect of the inversion on the formation of lee waves, lee‐wave rotors, low‐level hydraulic jumps and the occurrence of wave breaking aloft. The idealized problem considered consists of an upwind velocity profile that is independent of height (above the boundary layer) and directed normal to an isolated two‐dimensional ridge. The upstream stratification consists of a neutral layer immediately above the ground capped by a sharp temperature inversion. Above this, the atmosphere is stably stratified and the Brunt–Väisälä frequency is independent of height. Simulations were conducted for a range of inversion strengths (measured by the difference in potential temperature across the inversion) and inversion heights. The effect of both a free‐slip and a no‐slip lower boundary condition is investigated. Results show that, when the upwind Froude number (defined in the usual way for two‐layer shallow‐water flow) falls below a critical value, a short‐wavelength resonant lee wave forms downwind of the mountain on the inversion. It is shown that both the critical Froude‐number value and the wavelength of the lee wave are accurately predicted by linear theory. The lee‐wave amplitude, however, can be significantly underestimated by linear theory if the wavelength is less than the hill length scale. In the case of a no‐slip boundary condition, if the wave amplitude is sufficiently large, boundary‐layer separation occurs underneath the wave crests and closed rotor circulations occur. In general, flow separation (and rotors) do not occur in the free‐slip case. In both the free‐slip and no‐slip flows, as the Froude number decreases the lee wave is eventually replaced by a stationary hydraulic jump above the lee slope of the mountain. Copyright © 2004 Royal Meteorological Society.

Relationship between the Monin–Obukhov Stability Parameter and the Bulk Richardson Number at Sea under Unstable Conditions, Derived From a Turbulence-Closure Model

Abstract Relationships between the Monin–Obukhov stability parameter and the bulk Richardson number are useful for explicit calculation of fluxes through the air–sea interface. The most straightforward method assumes simple proportionality and is supported well by experimental evidence in a recent work by Grachev and Fairall. On the other hand, the reference iterative method is regarded as more accurate. To recognize possible differences, calculated values of the proportionality factor as a function of wind speed and air–sea virtual potential temperature difference are shown. The calculation is based on commonly used sea roughness specifications and the Mellor–Yamada turbulence-closure model, which has been shown in previous studies to reproduce the three-sublayer structure of the atmospheric surface layer under convective conditions and to predict mean flow profiles that are consistent with empirical data. The results show that the proportionality factor varies with the wind speed and virtual potential t...

The distortion of weak fronts by mesoscale orography

We examine the influence of mesoscale orography or mountains on fronts when the background flow is sufficiently slow and strongly stratified such that the air flows around the sides, rather than over the top, of the orography and when the fronts are sufficiently weak that the potential temperature differences across the frontal zones are much less than the potential temperature differences within each air mass over the height of the orography. These conditions are common for fronts interacting with the European Alps. By drawing together Drazin's (Tellus 13 (1961) 239) classic analysis of low Froude number flows past isolated orography, where the flow is layerwise irrotational, and a material surface description of the weak front, the salient features of the front blocking and deformation by mesoscale orography are captured and studied. When cold fronts are advected past an isolated orographic feature, the distortion of the front is so large near the surface of the orography, that a narrow portion of the front eventually overturns, leading to enhanced precipitation. The general influence of the shape and dimensions of the orography on front deformation and the localised overturning process is examined within an analytical framework.

Tracer‐based determination of vortex descent in the 1999/2000 Arctic winter

A detailed analysis of available in situ and remotely sensed N2O and CH4 data measured in the 1999/2000 winter Arctic vortex has been performed in order to quantify the temporal evolution of vortex descent. Differences in potential temperature (θ) among balloon and aircraft vertical profiles (an average of 19–23 K on a given N2O or CH4 isopleth) indicated significant vortex inhomogeneity in late fall as compared with late winter profiles. A composite fall vortex profile was constructed for 26 November 1999, whose error bars encompassed the observed variability. High‐latitude extravortex profiles measured in different years and seasons revealed substantial variability in N2O and CH4 on θ surfaces, but all were clearly distinguishable from the first vortex profiles measured in late fall 1999. From these extravortex‐vortex differences we inferred descent prior to 26 November: as much as 397 ± 15 K (1σ) at 30 ppbv N2O and 640 ppbv CH4, and falling to 28 ± 13 K above 200 ppbv N2O and 1280 ppbv CH4. Changes in θ were determined on five N2O and CH4 isopleths from 26 November through 12 March, and descent rates were calculated on each N2O isopleth for several time intervals. The maximum descent rates were seen between 26 November and 27 January: 0.82 ± 0.20 K/day averaged over 50–250 ppbv N2O. By late winter (26 February to 12 March), the average rate had decreased to 0.10 ± 0.25 K/day. Descent rates also decreased with increasing N2O; the winter average (26 November to 5 March) descent rate varied from 0.75 ± 0.10 K/day at 50 ppbv to 0.40 ± 0.11 K/day at 250 ppbv. Comparison of these results with observations and models of descent in prior years showed very good overall agreement. Two models of the 1999/2000 vortex descent, SLIMCAT and REPROBUS, despite θ offsets with respect to observed profiles of up to 20 K on most tracer isopleths, produced descent rates that agreed very favorably with the inferred rates from observation.

Climatology of ozone transport from the free troposphere into the boundary layer south of the Alps during North Foehn

In the Swiss Alpine foothills south of the Alps, on the northern edge of the Po Basin of Italy, 10 years of ozone data were analyzed to deduce the climatological influence of North Foehn on the boundary layer concentrations of ozone. It was found that northerly winds are connected with low relative humidity and small differences in potential temperature between the Jungfraujoch (3600 m above sea level (asl)) and Mendrisio (350 m asl) and therefore are primarily caused by North Foehn. This confirms that North Foehn transports air masses from high altitudes above the Alpine crest down into the lower boundary layer. On days with North Foehn the yearly cycles of ozone are very similar at Jungfraujoch and Mendrisio, around 40 ppb in winter and 60 ppb in summer. Compared to the whole data set in Mendrisio, these values are, at comparable temperatures, high in winter and very low in summer, when ozone concentrations often exceed 120 ppb during daytime. In contrast, during nighttime the North Foehn causes an enhancement of the ozone concentration near the ground during the whole year. The frequencies of the North Foehn events are 4–6% from May to October and 6–10% from November to April. Thus North Foehn is an important process for the ozone concentrations in the Po Basin.

Microrockets for Smart Dust

This paper details the design and fabrication ofmillimeter-scale solid propellant rockets for one-timedeployment of wireless sensor platforms, known as Smart Dust.Each microrocket assembly is an integrated system, incorporatinga combustion chamber, composite propellant grain, nozzle,igniter, and thermoelectric power converter. Solid propellant isadvantageous for a millimeter-scale single-use device because ofits simple implementation, unlike liquid propellants, whichrequire a more elaborate system of pumps and valves. Thereforethe total system volume and complexity are minimized.One type of combustion chamber was fabricated in silicon;however, thermal losses to the silicon sidewalls during combustionthrough a 1.5 mm2 cross section of fuel were too high toreliably maintain a burn. Successful combustion was demonstratedin cylindrical alumina ceramic combustion chambers with thermalconductivities five times lower than silicon and cross sectionsof 1-8 mm2. Thrusts of 10-15 mN were measured forceramic rockets weighing under l g, with specific impulses up to15 s.Silicon nozzles integrated with polysilicon microheaters andthermopiles for thermal power conversion were microfabricated ina single process. Fuel ignition by polysilicon microheaterssuspended on a low-stress nitride (LSN) membrane wasdemonstrated. Microheaters require less than 0.5 W of power toignite a propellant composed primarily of hydroxyl-terminatedpolybutadiene (HTPB) with ammonium perchlorate (AP) oxidizer.They are suspended for thermal isolation through bulkpost-processing by a backside deep reactive ion etch (DRIE). Theetched hole beneath the igniter area also serves as a nozzlethrough which high-velocity combustion gases exit the rocket.Thermopiles, which generate voltages proportional to hot andcold junction temperature differentials, have been fabricated inthe same process as igniters, and span backside DRIE thermalisolation cavities. Ten-junction thermopiles produced a maximumpower of 20 µW. With potential temperature differences ofhundreds of degrees and a total of 120 thermocouple junctionsfabricated on the silicon nozzle chip, hundreds of milliwatts ofpower could feasibly be produced during the microrocket's flight andused to augment the Smart Dust power supply.

Urban street-canyon flows with bottom heating

Urban street-canyon flows in the presence of street-bottom heating are investigated using a two-dimensional numerical model with a k– ε turbulence closure scheme. The street aspect ratio ( H/ D, where H and D are the building height and the width between two buildings, respectively) varies from 0.6 to 3.6 (in 0.2 interval) and the initial potential temperature difference between the street-canyon bottom and the air (Δ Θ) ranges from 0 to 16 K (in 2 K interval). Five flow regimes are identified. Regime I is observed when the aspect ratio is very small but the bottom heating is very strong ( H/ D=0.6 and Δ Θ⩾10 K). In regime I, as the heating intensity increases, the thermally induced vortex expands but the mechanically induced vortex contracts. Regime II is mainly observed when the aspect ratio is relatively small or the bottom heating is weak. In regime II, the vortex intensity increases with increasing heating intensity. Regime III is observed when the bottom heating is relatively significant (Δ Θ⩾4 K) and the aspect ratio lies in the range of 1.2–2. This regime differs from regime II in that the vortex induced by temperature gradient on the upwind side of the upper layer has meaningful intensity and size and the maximum horizontal velocity decreases with increasing heating intensity. When the bottom heating is relatively significant, regime IV is most commonly observed. This consists of two corotating vortices in the vertical within combined streamlines. Regime V exhibits two counterrotating vortices induced on both sides of the maximum temperature axis in the lower layer. The upper vortex is little influenced by the bottom heating. The numerical model result is shown to be reasonably in good agreement with the wind-tunnel data.

Experimental GOES Sounder Products for the Assessment of Downburst Potential

Abstract Several experimental products derived from Geostationary Operational Environmental Satellite (GOES) Sounder retrievals (vertical profiles of temperature and moisture) have been developed to assist weather forecasters in assessing the potential for convective downbursts. The product suite currently includes the wind index (WINDEX), a dry microburst index, and the maximum difference in equivalent potential temperature (θe) from the surface to 300 hPa. The products are displayed as color-coded boxes or numerical values, superimposed on GOES visible, infrared, or water vapor imagery, and are available hourly, day and night, via the Internet. After two full summers of evaluation, the products have been shown to be useful in the assessment of atmospheric conditions that may lead to strong, gusty surface winds from thunderstorms. Two case studies are presented: 1) a severe downburst storm in southern Arizona that produced historic surface wind speeds and damage, and 2) multiple dry and wet downbursts in...

What causes long‐term temporal changes in the south Atlantic?

Two zonal sections at 11°S in the South Atlantic, separated in time by 11 years, provide temperature differences in the deep ocean. The aim of this case study is to check whether intrinsic temperature changes are sufficiently large to identify long‐term water mass property variations which could be related to climate change. Potential temperature differences on isobaric surfaces in the deep ocean here reach several tenths of °C. They can be caused by vertical (cross‐isopycnal) or horizontal (isopycnal) advection and mixing, or by intrinsic water mass changes. The effect of vertical transport is removed by using neutral (density) surfaces. The effect of horizontal transport is determined by using a mixing parameterization for temperature and silica on neutral surfaces. The residual intrinsic temperature changes are, with a few local exceptions, within the range of the ±0.05°C uncertainty, and the temperature changes can thus be explained by advection and mixing alone.

Characteristics of marine boundary layers during two Lagrangian measurement periods: 1. General conditions and mean characteristics

Two sets of Lagrangian measurements were made during the southern Aerosol Characterization Experiment (ACE1) south of Tasmania, Australia, in December 1995. This paper intends to provide an overview of the general conditions encountered during the two intensive observational periods. The measurements by the NCAR C‐130 provide the main data set for this study. We also use the sea surface temperature obtained from the R/V Discoverer and the European Centre for Medium‐Range Weather Forecasts (ECMWF) analyses field for the large‐scale divergence field. Emphases of the paper are on the atmospheric and oceanic environment and the boundary layer mean structure during the six flights in the two Lagrangian measurement periods. The large scale features, such as variations of sea surface temperature, synoptic conditions, and large‐scale velocity fields, are discussed. These large‐scale environments had significant influences on boundary layer turbulence and the inversion structure. The boundary layer mean structure and its evolution along the Lagrangian trajectory are also studied using two‐dimensional cross‐section plots of vertical and horizontal (along the flight track) variation of potential temperature, water vapor, wind components, and ozone concentration. The most prominent feature of the boundary layer is the two‐layered structure observed throughout Lagrangian B and during the last flight of Lagrangian A. The two layers have detectable differences in potential temperature, water vapor, and, to a lesser extent, ozone concentration. These differences make it necessary to study the exchange between the two layers. Low‐level cloud structure and cloud microphysics are also discussed. We emphasize, though, that the results on cloud fractions should be used with caution due to the variable nature of the cloud bands observed during ACE1.

Interpretation of Crown Radiation Temperatures of a Dense Douglas fir Forest with Similarity Theory

Infrared crown radiation temperatures as observed over a dense Douglas fir forest are analyzed in the context of similarity theory and the concept of transport resistances. As such we obtain a rather high value of the roughness length for heat, which is about equal to the roughness length for momentum. This value can be explained by the more efficient transport of heat relative to momentum in the roughness sublayer of the forest. Correcting for this effect we arrive at the classic value for homogeneous terrain of about 0.1 times the roughness length for momentum. For unstable cases the presence of enhanced mixing of heat in the roughness sublayer leads to a modified integral stability function for the dimensionless potential temperature difference between the surface and the top of the roughness sublayer. The observations give some evidence for this different stability behaviour. The analysis suggests that during daytime the radiative surface temperature and the aerodynamic surface temperature are not significantly different when used to estimate fluxes. Daytime trunk space air temperature is satisfactory parameterized with the concept of gusts and with surface renewal analysis. As such it is related to the sensible heat flux and the storage heat flux. Night time radiation temperatures at times strongly deviate from the expected behaviour based on similarity theory and the roughness length for heat, suggesting that the concept of a single surface temperature is too simple for such cases.

夏の関東平野における積雲対流の日々変化

Cumulus convection occurs over the Kanto plain during the afternoons of fair summer days. By investigating cloud amount data, it was revealed that the afternoon cloud amount on a given day, and that on the following day are dependent on each other. If the cloud amount is small on a given day, it lends to be greater on the following day, and vice versa. It can be concluded that fair weather does not tend to persist from day to day, when the Kanto plain is covered by the subtropical anticyclone and the weather conditions are stable, The interdiurnal variation was then simulated in two-dimensional numerical experiments. Days with strong and weak convection appeared quasi-periodically, even though the complete diurnal boundary conditions were given corresponding to the sea and land breezes. By examining the results of the numerica experiments, it was shown that the interdiurnal variation resulted from differences in vertical profiles of potential temperature and water vapor mixing ratio in the lower troposphere. The over-adjustment' effect that the condition will become less favorable for cumulus convection on the following day as a result o strong convection, is essentially important for the interdiurnal variation. Furthermore, the differences in the vertical profiles were compared with those from aerological data, in order to examine the above mentioned results of the interdiurnal variation. The differences simulated in the numerical experiments were qualitatively similar to those found in the aerological data, although the differences in potential temperature were not statistically significant.

The depth of the stably stratified internal boundary layer over the sea

As an air mass is advected from a heated land surface over a cooler sea, it is extensively modified by the underlying surface with the subsequent formation of a stably stratified internal boundary layer (SIBL). Due to weak turbulence, changes in surface forcing are communicated very slowly across the depth of the SIBL and its growth rate cannot be described in terms of local parameters. Therefore, it has been proposed that the growth of the SIBL is controlled by the fetch integrated value of surface heat flux. This approach is adopted in the present paper in order to develop a simple equation capable to describe the development of the SIBL as a function of downwind distance and external forcings, such as pressure gradient force, surface roughness and potential temperature difference between sea and land surface. The derived relationship is tested against a limited data set from the Öresund experiment.

Tropical Thermostats and Low Cloud Cover

The ability of subtropical stratus low cloud cover to moderate or amplify the tropical response to climate forcing such as increased CO2 is considered. Cloud radiative forcing over the subtropics is parameterized using an empirical relation between stratus cloud cover and the difference in potential temperature between 700 mb (a level that is above the trade inversion) and the surface. This relation includes the empirical negative correlation between SST and low cloud cover and is potentially a positive feedback to climate forcing. Since potential temperature above the trade inversion varies in unison across the Tropics as a result of the large-scale circulation and because moist convection relates tropospheric temperature within the convecting region to variations in surface temperature and moisture, the subtropical potential temperature at 700 mb depends upon surface conditions within the convecting region. As a result, subtropical stratus cloud cover and the associated feedback depend upon the entire tropical climate and not just the underlying SST. A simple tropical model is constructed, consisting of separate budgets of dry static energy and moisture for the convecting region (referred to as the “warm” pool) and the subtropical descending region (the “cold” pool). The cold pool is the location of stratus low clouds in the model. Dynamics is implicitly included through the assumption that temperature above the boundary layer is horizontally uniform as a result of the large-scale circulation. The tropopause and warm pool surface are shown to be connected by a moist adiabat in the limit of vanishingly narrow convective updrafts. Stratus low cloud cover is found to be a negative feedback, increasing in response to doubled CO2 and reducing the tropically averaged warming in comparison to the warming with low cloud cover held fixed. Increased low cloud cover is shown to result from the increased difference in surface temperature between the warm and cold pools, and the increased low-level static stability over the warm pool, equal to the increase in potential temperature along the moist adiabat originating in the warm pool mixed layer.

The Stability of Cloud Top with Regard to Entrainment: Amendment of the Theory of Cloud-Top Entrainment Instability

The stability of a uniformly saturated cloud layer separated from an overlying nonturbulent unsaturated layer by a thin inversion is considered. The stability of the interface can be described by entraining a parcel of air from above the inversion into the cloud layer below and by subsequently studying the effect of the mixing on the buoyancy of the parcel. From the relevant momentum equation for the parcel it is shown that the important quantity to consider is the total buoyancy (total mass of the parcel times the virtual potential temperature difference between parcel and environment) of the parcel per unit mass of entrained air. The total buoyancy is a more general and useful concept than all other parameters discussed in the literature.

A System for the Hourly Assimilation of Surface Observations in Mountainous and Flat Terrain

Abstract An assimilation system is presented that was designed to provide timely, detailed, and coherent analyses of surface data, even when the data are collected in rough terrain where station elevations differ widely and observations are often subject to local effects. Analyses with improved spatial continuity are obtained from these data through careful choice of analysis method and variables. The analysis method has the ability to handle varying data density, and the analysis variables, when possible, were chosen in such a way as to cancel out the effects of elevation differences. In addition, the method accounts for physical blocking and channeling by mountainous terrain by incorporating elevation and potential temperature differences in its horizontal correlation functions. The correlation functions also enable the method to move accurately represent surface gradients. An hourly analysis cycle is used in which each analysis uses as a background the previous hourly analysis (a 1-h persistence foreca...

A Radar and Electrical Study of Tropical “Hot Towers”

Abstract Radar and electrical measurements for deep tropical convection are examined for both “break period” and “monsoonal” regimes in the vicinity of Darwin, Australia. Break period convection consists primarily of deep continental convection, whereas oceanic-based convection dominates during monsoonal periods, associated with the monsoon trough over Darwin. Order-of-magnitude enhancements in lightning flash rates for the “break period” regime are associated with 10–20-dB enhancements in radar reflectivity in the mixed-phase region of the convection compared with the monsoonal regime. The latter differences are attributed to the effect of convective available potential energy (CAPE) and its nonlinear influence on the growth and accumulation of ice particles aloft, which are believed to promote charge separation by differential particle motions. CAPE, in turn, is largely determined by the boundary-layer wet-bulb temperature. Modest differences (1°–3°C) in wet-bulb potential temperature between land and s...

Determination of similarity functions of the resistance laws for the planetary boundary layer using surface-layer similarity functions

Functional forms of the universal similarity functions A, B (for wind components parallel and normal to the surface stress), and C (for potential temperature difference) are determined based on the generalized theory of the resistance laws for the Planetary Boundary Layer (PBL). The similarity-profile functions for the surface layer are matched with the velocity and temperature-defect profiles that are assumed to have shapes modified by certain powers of nondimensional height z/h, where h is the PBL height. The powers of the outer-layer profile functions are determined, so that the functions become negligible in the surface layer. To close the temperature defect law, an assumption that the temperature gradient across the top of the PBL is continuous with the stratification of the overlying atmosphere is used. The result of this assumption is that nondimensional momentum and temperature profiles in the PBL can be described in terms of four basic ratios: (1) roughness ratio η= ζ/h (2) scale-height ratio λ=|f|h/u*, (3) ambient stratification parameter κ=γh/θ*, and (4) stability parameter μ=h/L, where L is the Monin-Obukhov length, z0 is the surface roughness, γ is the upper-air stratification, u* is the friction velocity, and θ* is the temperature scale at the surface. For stable conditions, the scale-height ratio can be related to the atmospheric stability and the upperair stratification, and the generalized similarity and Rossby number similarity theories become identical. Under appropriate boundary conditions, function A is explicitly dependent on the stability parameter γ, while B is a function of scale-height ratio λ, which in turn depends on the stability. Function C is shown to be dependent on the stability and the upper-air stratification, due to the closure assumption used for the temperature profile.

South foehn studies since the ALPEX experiment

After the ALPEX Special Observing Period in 1982, further field measurement campaigns have been carried out to study the south foehn in the Eastern Alps. In addition, foehn situations were investigated statistically on the basis of a 4-years-record of upper-air and synoptic data combined with data from a special regional network. In this paper, distribution of air masses around the Alps, flow patterns and the typical evolution of a foehn situation are illustrated by the case 8 November 1982. Several possible definitions of “shallow foehn” are given, yielding a share of approximately 10% of shallow foehn observations. Mainly statistical data are used to falsify the wide-spread version of the “thermodynamical foehn theory” attributing the warmth of the foehn to release of latent heat and postulating foehn air rising from the ground of the Po Valley. Instead of this, the foehn air originates from 2000–2500 m asl over the Po Valley, while the air near ground remains blocked. The potential temperature difference between southern and northern valleys during foehn is to be explained by the stable stratification in the south, not by the effects of precipitation. These findings are in agreement with Hann's original theory.

The structure of the stably stratified internal boundary layer in offshore flow over the sea

Observations obtained mainly from a research aircraft are presented of the mean and turbulent structure of the stably stratified internal boundary layer (IBL) over the sea formed by warm air advection from land to sea. The potential temperature and humidity fields reveal the vertical extent of the IBL, for fetches out to several hundred of kilometres, geostrophic winds of 20–25 m s−1, and potential temperature differences between undisturbed continental air and the sea surface of 7 to 17 K. The dependence of IBL depth on these external parameters is discussed in the context of the numerical results of Garratt (1987), and some discrepancies are noted.