bluuebunny/crossref_metadata_2025 huggingface dataset:
Radiative heating in the lower stratosphere and the distribution of ozone in a two‐dimensional model
Authors: Haigh J. D.
Published: February 2007

The sensitivity of a two‐dimensional model to the diabatic heating rates used in the lower stratosphere is discussed. Different radiation schemes are described; in particular a longwave scheme which allows for interaction between surface temperature, ozone amounts and the heating rates in the ozone 9·6 μm band. Near‐infrared heating is also included.

A model run including the new radiation schemes is compared with a control run in which radiative equilibrium is assumed in the 150–30 mb region (as in previous versions of this model) and with a run in which fixed heating rates are adopted in the same region. The temperature and ozone distributions of each run are discussed. The ozone structure is very sensitive to the scheme used and the inclusion of the new schemes improves the model's ozone distribution. The importance of the diabatic heating in determining the mean meridional circulation and the transport of trace constituents is stressed.

Perturbation experiments of Haigh and Pyle are repeated in the model including the new schemes. The effect on ozone of increasing CO2 is found to have a stronger latitudinal and seasonal variation. The effect of including chlorofluorocarbons is a larger ozone depletion at all latitudes. The problems associated with assessing the uncertainties in model predictions are discussed.

A simple scheme for representing cloud cover in the longwave scheme is described. The effect of inclusion of clouds on heating in the ozone 9·6 μm band, and thus on the ozone distribution, is discussed. The results of changing the effective albedo in the solar heating scheme are considered briefly.
Baroclinic instability in a long wave environment. Part I: Review
Authors: Grotjahn Richard
Published: February 2007

A brief survey of past theoretical studies of the instability of curving flows is presented. Most of the studies consider linear instability. Finite amplitude, nonlinear studies are not considered here. Most of the early papers were concerned with finding instabilities; later papers include more discussion of the structural and other properties of the unstable modes. It is found that nearly all these studies lack a proper discussion of the energetics of the unstable modes. Proper energy conversions are derived and examined in part II.
Parametrization of the shortwave flux over high albedo surfaces as a function of cloud thickness and surface albedo
Authors: Shine K. P.
Published: February 2007

A simple method for the calculation of the net shortwave flux at the surface for overcast situations is presented. Explicit account is taken of the effect of cloud optical thickness and multiple reflections between the surface and cloud base. Using simple two‐stream theory, a theoretical form for the flux is proposed, and then results from a detailed radiative transfer scheme are used to determine the values of a number of empirical coefficients. Over a range of cosines of the solar zenith angle from 0.1 to 0.7, of cloud optical thickness from 1.8 to 20 and of surface albedo from 0.0 to 0.9, the empirical equation reproduces results from the radiative transfer scheme to within 2.7%, and is shown to perform well outside the range of fit. The method was derived with high latitudes in mind but is applicable generally. It is shown that the spectral properties of snow and ice surfaces play an important role in surface net flux prediction. For totally overcast skies the influence of cloud height and atmospheric water vapour content are shown to be minor. Finally, for clear skies, an empirical equation due to J. W. Zillman is investigated and by adjustments to his coefficients, is brought into good agreement with the detailed calculations described here.
A study of the structure of mid‐latitude depressions in a numerical model using trajectory techniques. I: Development of ideal baroclinic waves in dry and moist atmospheres
Authors: Golding B.
Published: February 2007

Trajectory‐tracing techniques are used to investigate the structure and air flow of developing baroclinic waves simulated by a numerical atmospheric forecast model. Three stages are described. The structure of the linear stage is explained with reference to the normal mode theory of baroclinic instability. In the mature stage, flow separation zones form which are identified as warm and cold fronts. At the occluded stage, circulation of cold air round the centre of the depression leads to changes in the structure of the cold front. When the initial state is nearly saturated, the overall development is speeded up, and at the mature stage the rate of deepening of the depression is doubled at the surface. In the occluded stage formation of a warm air conveyor belt is observed. Comparisons are made with previously published integrations by specially constructed models in order to establish a basis for comparison of forecasts from real data which will appear in a future paper.
The formation of multiple fronts
Authors: Hoskins B. J. , Neto E. Caetano , Cho H.‐R.
Published: February 2007

Using two‐dimensional semi‐geostrophic frontogenesis models and three‐dimensional semi‐geostrophic baroclinic wave models, it is shown that atmospheric development in the presence of non‐uniformities in pre‐existing temperature distributions may be expected to lead to the frequent occurrence of multiple frontal structures. In general, such non‐uniformities in the temperature distribution will be important if they significantly change the ambient temperature gradient. The frontal structures associated with the irregularities tend to be very shallow. They are possible candidates for the triggering mechanism for frontal troughs well behind a cold front and for shallow rainbands ahead of such a front. However, it seems that a separate mechanism is still necessary for the production of the middle‐level banded structures that are frequently observed.
The seasonal mean flow and distribution of large‐scale weather systems in the southern hemisphere: The effects of moisture transports
Authors: James I. N. , Anderson D. L. T.
Published: February 2007

Circulation statistics, based on seasonal mean fields calculated from archived analyses at ECMWF, are presented for the southern hemisphere. Particular emphasis is placed on the large‐scale distribution of the high frequency (periods 2–6 days) transient eddy activity during the period July–August 1982. This was found to be a maximum over the Atlantic and Indian Oceans, and a minimum over the Pacific. The relative distribution of the temperature and momentum fluxes suggests that the high frequency transients are baroclinic waves, which reach large amplitudes and then decay, mainly as a result of Rossby wave radiation.

The location at which the transients began to grow cannot be related to the regional variations of the mean flow. In particular, it is poorly related to the distribution of low‐ and middle‐level baroclinicity defined on the basis of dry, linear theory. It is suggested that the entrainment of moisture into the low‐level westerlies over the Atlantic could account for the large increase of transient activity there. It is shown that the major low‐level flux of moisture from the tropics into the southern hemisphere mid‐latitudes occurred over South America.

Since moisture data cannot be extracted reliably from the data archives, moisture fluxes are deduced indirectly. Use is made of a general circulation model of the southern hemisphere March flow to illustrate the entrainment of moisture into the baroclinic zone over the mid‐Atlantic.
Eddy correlation measurements of energy partition for Amazonian forest
Authors: Shuttleworth W. James , Gash John H. C. , Lloyd Colin R. , Moore Christopher J. , Roberts John , Filho Ari De O. Marques , Fisch Gilberto , De Paula Silva Filho Vicente , Ribeiro Maria De Nazaré Góes , Molion Luiz C. B. , De Abreu Sá Leonardo D. , Nobre J. Carlos A. , Cabral Osvaldo M. R. , Patel Sukaran R. , Carvalho De Moraes J.
Published: February 2007

Measurements of energy partition for Amazonian forest made with novel eddy correlation equipment are presented for eight dry days in September 1983. These are interpreted to provide estimates of the aerodynamic and surface resistance for this vegetation type. Daily total evaporation for a transpiring canopy accounts for 70% of the available radiant energy, and is two thirds of conventional estimates of potential evaporation. The results are used to provide an initial calibration of a simple, physically based model of daily evaporation for Amazonian rain forest.
Observations of radiation exchange above and below Amazonian forest
Authors: Shuttleworth W. James
Published: February 2007

Measurements of shortwave and longwave components of the radiation budget were made above and below tropical forest in the Amazon Basin for a total duration of 203 hours in September 1983. Albedo was (12.25±0.2)%, and showed a small variation with solar altitude. The net outward flux of longwave radiation was around 30 W m−2, and fairly constant over the day. The relationship between net radiation, R, and solar radiation, S, was adequately described by the expression R = (0.858±0.006)S ‐ (35.0±1.9) (W m−2). The fraction of solar radiation reaching the bottom of the canopy was low, about one per cent: net radiation in this position was consistent with low soil heat flux and low soil evaporation.
The influence of the spectral response of satellite sensors on estimates of broadband albedo
Authors: Shine K. P. , Henderson‐Sellers A. , Slingo A.
Published: February 2007

The radiative impact of cloudiness variations on climate depends on the relative magnitudes of changes in the planetary albedo and the outgoing longwave radiation. R. D. Cess and co‐workers showed in 1982 that the calculated direction of the cloud‐climate feedback (i.e. whether the earth‐atmosphere system warms or cools with changing cloud amount) depended on the satellite data sets used in their analysis. It was suggested that the different spectral resolutions of the satellite sensors might be responsible. This note examines that suggestion. It is shown that the change in planetary albedo measured by the narrow band scanning radiometers on board NOAAs 2 to 5 as a result of cloudiness variation can be an overestimate of the actual change by a factor of up to 1.38 at low latitudes. The overestimate is not sufficient to explain completely the differences amongst the satellite data. Nevertheless, the results indicate that satellite spectral response must be carefully considered if the strength (and possibly even the sign) of the cloud‐climate feedback is to be correctly assessed from narrow band satellite data. Over snow‐covered surfaces it is shown that the NOAA data may underestimate the effect of cloud cover changes by over 70%, and overestimate the planetary albedo by over 0.2.
An experimental study of the remote location of lightning flashes using a VLF arrival time difference technique
Authors: Lee A. C. L.
Published: February 2007

The operational ‘cathode ray direction finding’ (CRDF) network for the location of lightning flashes at ranges up to thousands of kilometres can be replaced by a system using the ‘arrival time difference’ (ATD) technique. The ATD technique has theoretical advantages over CRDF, which could lead to substantially improved flash location, but the extent to which it suffers from certain propagation effects in practice must be quantified.

In an experiment using the ATD technique, equipments were deployed in the U.K. and Gibraltar. These captured the vertical electric field of VLF electromagnetic radiation from individual lightning flashes, synchronized to CRDF‐selected flashes and recorded against an accurate timebase. By analysing the data using techniques analogous to those of hyperbolic navigation, the flash location was inferred, together with a measure of the data consistency.

ATD location of independent flashes observed in time proximity frequently gave the tight grouping expected from an isolated active thunderstorm cell. Groups of diameter 2–10 km were observed at megametre ranges.

ATD flash locations were compared with CRDF and other meteorological data. Positioning agreed within the limited accuracy of current techniques, but the CRDF system's potential for poor accuracy was highlighted.

An analysis method was developed relating the internal consistency of the ATD flash data to geographical location accuracy, and ATD accuracy of less than 5 μs was measured experimentally. On this basis, an ATD system can locate flashes over a synoptic‐scale area with errors comparable with a storm cell diameter.
The semi‐annual oscillation and equatorial tracer distributions
Authors: Gray L. J. , Pyle J. A.
Published: February 2007

Some interesting low latitude features of tracer distributions revealed by recent satellite experiments have been studied using a two‐dimensional atmospheric model. It is found that the equatorial semi‐annual oscillation, forced in the model by simply prescribing a suitable momentum convergence, is important in determining the tracer behaviour. In particular, the meridional circulation associated with the semi‐annual oscillation leads to a successful model simulation of the ‘double peak’ in N2O and CH4 mixing ratios, observations of which show, at certain times of the year, maxima on a fixed pressure surface in the subtropics with a local minimum at the equator.
Metastable stratus tops
Authors: Rogers David P. , Telford James W.
Published: February 2007

Previously, it has been shown that a saturated or near‐saturated mixture of cloud top air with cloud‐free air which has a somewhat lower wet‐bulb potential temperature, θw, will be denser than the surrounding cloud air, and will descend through the cloud layer. This has been referred to as entrainment instability. It is often presumed that this mechanism results in the rapid entrainment of dry air, which may lead to the breakup of a continuous stratocumulus deck.

Airborne observations of the marine planetary boundary layer are presented. These, along with previously published observations, indicate that θw at cloud top may exceed that of the air above by as much as 5°C without strong entrainment being active. The fact that potential instability is not always accompanied by rapid entrainment suggests that the process is in a metastable state until sufficiently disturbed. These observations show the vertical transfer mechanisms in stratus cloud and reveal the presence of a case with a drop‐free region between cloud top and the base of the inversion. Theoretical arguments are proffered to explain these phenomena and to account for the fact that entrainment episodes are delayed and intermittent.
Simulations of the Saharan atmosphere—dependence on moisture and albedo
Authors: Cunnington W. M. , Rowntree P. R.
Published: February 2007

The sensitivity of an atmospheric general circulation model to changes in atmospheric water content, soil moisture content, radiation scheme and albedo have been investigated using the Meteorological Office global 11‐layer model. Previous integrations had produced excessive rainfall in the Saharan region. This paper reports experiments which show the importance for a good Saharan simulation of the initial atmosphere and soil moisture specifications, of the radiation scheme and of the surface albedo specification. It is shown that a dry Sahara with realistic circulation patterns is simulated only if the surface and atmosphere are initially realistically dry and the model's longwave radiation is dependent on atmospheric water vapour content. With a moist atmosphere, introduced either initially or by rapid evaporation from a wet soil, or by using a radiation scheme without water vapour dependence, there is increased net downward radiation which enhances ascent and convective activity and leads eventually to excessive rainfall in the desert region with a reversal of the usual circulation pattern to give 700 mb westerly winds at 5°–15°N associated with a cyclonic circulation over the Saharan region which draws in moist air from the south. As expected from Charney's hypothesis on the maintenance of deserts, the use of more realistic albedos tends to transfer rainfall from regions of higher albedo (e.g. deserts) to regions of lower albedo (e.g. forests).

An extension of Charney's hypothesis to include moisture is proposed in which any increase in diabatic heating, whether due to reduced albedo or increased moisture availability, must be compensated by increased cold advection; in the tropics this typically occurs through ascent of potentially colder air leading to rainfall if the air is moist enough or the ascending layer sufficiently deep. Some implications of the possibility of two distinct summer circulation patterns for the theory of past North African climate are discussed.
Ice accretions on fixed cylinders
Authors: Prodi F. , Levi L. , Levizzani V.
Published: February 2007

A comparison of ice accretions obtained on fixed and on rotating deposits was expected to provide a better understanding of the basic accretion mechanisms as well as being useful in many practical applications. Therefore, the morphological and physical properties of ice deposits accreted in a wind tunnel on fixed cylinders have been studied at temperatures, liquid water contents and air speed ranges previously tested for rotating cylinders.

The deposits obtained are studied as to their external shape and internal morphology, crystallographic properties and local density distributions.

The external shape is mainly dependent on the growth regime (dry or wet‐spongy) and the wind speed. In dry growth and high wind speed the experimental profile is lenticular in agreement with prediction; however, at lower wind speeds, the growth rate at the stagnation line is reduced with respect to the flanks, and the deposit develops a central depression. In wet or spongy growth, run back is effective at high wind speeds, while shedding becomes important at lower speeds.

Observation of the crystal structure of fixed deposits shows that grain size and orientation depend on the growth regime and temperature in a similar way as with the rotating cylinders. However, the grain elongation is more pronounced in the fixed deposits, especially at lower temperatures.

The local density distribution of the deposits is measured from X‐ray micrographs. The stagnation line density is maximum or close to the maximum at impact speeds of 26 and 10 ms−1, while in deposits obtained at 5 and 2.5 ms−1 it drops to remarkably lower values.

The deposit profiles are compared with previous model investigations, and information is provided on the behaviour of some growth parameters which should be introduced into the model in order to simulate the experimental process better.

The angular profile of the local density is also compared with predictions by previous investigators. It is shown that the best representation of the experimental points is achieved by calculating the mean volume radius of droplets at the collection angle θ and the corresponding impact speed, and using a new relationship between ice density and the Macklin parameter.
Solar radiation anomalies caused by the El Chichon volcanic cloud: Measurements and model comparisons
Authors: Rao C. R. Nagaraja , Takashima T.
Published: February 2007

The eruption of the volcano El Chichon (17.33°N 93.20°W) in southern Mexico over the period 28 March to 4 April 1982 introduced massive amounts of sulphur dioxide into the stratosphere. The resulting sulphuric acid particulate cloud has affected appreciably the received solar radiation at the surface; measurements made at several locations in the continental United States and Hawaii have indicated that the directly transmitted solar radiation was depleted by up to 25%; diffuse sky radiation was enhanced by factors of up to 3; and global radiation was lowered by up to 5%. These reported anomalies are compared with the results of radiative transfer computations in a realistic model of a cloud‐free, vertically inhomogeneous, plane‐parallel turbid atmosphere. The model yields the observed trends in the direct, diffusely transmitted and global solar radiation under normal and anomalous conditions.
Toward atmospheres without tops: Absorbing upper boundary conditions for numerical models
Authors: Rasch Philip J.
Published: February 2007

The appropriate upper boundary condition (UBC) formulation for the dynamical equations used in atmospheric physics is discussed in terms of both theoretical and computational aspects. The previous work on the UBC formulation is reviewed in the context of a linear mid‐latitude primitive equation (PE) model. A new technique for constructing the UBC is introduced. The technique depends upon the existence of analytic solutions to simplifications to the equations of motion. These analytic solutions are used to construct the exact radiation UBCs (often used in tidal theory and studies of the upper atmosphere) which are non‐local in time and space. Approximate UBCs, which are local in time, are formed through rational approximations to the exact radiation UBCs. The technique is demonstrated to be effective for both Rossby and gravity modes. The UBC is tested for computational problems initially in the linear PE model, and subsequently in a forced, damped nonlinear quasi‐geostrophic model.
Non‐stationary Markov chains for modelling daily radiation data
Authors: Spirkl Wolfgang , Ries Harald
Published: February 2007

Continuous seasonal variations are described by expressing each transition probability of the Markov chain as the normalized exponential of a truncated Fourier polynomial. The Fourier coefficients are found by maximum likelihood estimation. The method does not make use of the orthogonality of the Fourier functions and therefore is applicable to data sets with gaps. The appropriate number of Fourier coefficients as well as the appropriate order of the Markov chain is determined by means of Akaike's information criterion. The method was applied to 19 years of data concerning the duration of sunshine at Weihenstephan (48.4°N 11.7°E). Five categories were identified according to the daily relative duration of sunshine. It was found that an order‐one chain with five Fourier coefficients provides an adequate model. Significant seasonal variations were detected especially for the extreme categories of overcast and clear days.
The effect of a sea surface temperature anomaly on a prediction of the onset of the south‐west monsoon over India
Authors: Kershaw R.
Published: February 2007

At the time of the onset of the monsoon in June 1979 the sea surface temperatures in the eastern Arabian Sea were higher than normal. A prediction experiment designed to reveal the effects of this anomaly on the onset is described. This involves two 8‐day forecasts made using the same model and the same initial conditions. They differ only in using different sea surface temperatures in the eastern Arabian Sea. A better prediction of the onset is obtained by using the anomalously high temperatures rather than the normal climatological values. In particular, the development of a tropical storm over the Arabian Sea, the strengthening of the Somali jet, and the northward movement of the rainfall over India are all better predicted. The mechanism of the onset in the model is discussed and two important feedback loops are identified, one involving moisture‐flux convergence and the other involving the surface fluxes of sensible and latent heat. The most important factor in both loops is the release of latent heat over the Arabian Sea. A linear model is used to show that the onset may be thought of as a response to this heating. The roles of barotropic and baroclinic processes in the onset are also discussed.
A two‐dimensional model of mesoscale frontogenesis in the ocean
Authors: Bleck R. , Onken R. , Woods J. D.
Published: February 2007

A two‐dimensional primitive equation model conserving potential vorticity is used to simulate the effect of a horizontal barotropic deformation field on a pre‐existing isopycnic potential vorticity gradient. The novel feature of the prognostic model is that it accurately describes the development of the density and velocity distributions up to local Rossby number 0.5 and beyond. The model coordinates are based in the vertical on quasi‐isopycnic vertical coordinates and in the horizontal on geostrophic coordinates. The model is integrated forward in time for three days creating a density front and a jet of 0.4 m s−1in the surface layers. Cross‐jet velocities are one order of magnitude smaller. Conservation of potential vorticity leads to a modulation of spacing between isopycnals as both the cyclonic and anticyclonic vorticity associated with the horizontal shear of the accelerating jet increase. Vortex stretching and compression sustain a strong vertical circulation. The paper shows how the jet kinetic energy is limited by the amount of available potential energy in a catchment area defined by the extent of the region of confluence. The jet becomes shallower as frontogenesis approaches this limit.

The model is also applicable to the development of permanent streams in the large‐scale circulation of the upper ocean, by slow (order one month) frontogenesis due to gyre‐scale deformation acting on an isopycnic potential vorticity gradient in the seasonal thermocline.

The vigorous upwelling on the anticyclonic side of the jet raises nutrients through the euphotic zone, increasing primary productivity. This explains observed meoscale modulations of biomass.
A four‐layer model for the heat budget of homogeneous land surfaces
Authors: Choudhury B. J. , Monteith J. L.
Published: February 2007

The model is an explicit solution of the equations which define the conservation of heat and water vapour in the system consisting of uniform vegetation and soil. The vegetation has two layers, the first extending from a reference height in the atmosphere to the virtual sink for momentum and the second from the virtual sink to the soil surface. Soil between the surface and the damping depth is divided into an upper, completely dry layer and a lower wet layer. Throughout the system, differences of potential per unit flux are specified by resistances, notably the conventional surface resistance governing the loss of vapour from foliage and a new soil resistance, assumed proportional to the accumulated loss of water by evaporation from the soil surface, which is proportional to the square root of elapsed time. Since the total energy available to the system is limited by absorption of radiation, the increase of transpiration from foliage as it expands decreases evaporation from the soil. Conversely, as soil dries, transpiration rate per unit of foliage area increases. This interaction of vapour fluxes is governed by the behaviour of the saturation vapour pressure deficit within the vegetation, identified as a major variable.

General implications of the model are consistent with observations and estimates of evaporation rate from a stand of wheat grown in Arizona agree well with measurements from lysimeters. The model is applicable to a wide range of problems in agricultural meteorology and hydrology. With simplification, it could be used to obtain surface exchanges of sensible and latent heat in atmospheric general circulation models.
Modelling tropical deforestation: A study of GCM land‐surface parametrizations
Authors: Dickinson Robert E. , Henderson‐Sellers Ann
Published: February 2007

Tropical deforestation, by changing land surfaces, may have important consequences for the climate system. Predicting even the local, immediate effects of replacing tropical broadleaf forest with impoverished grassland has been difficult, because the land‐surface parametrization schemes used previously in climate models have been inadequate. The forest canopy is particularly important for the surface‐energy budget in tropical regions, and models neglecting the occurrence of such a canopy may give an unrealistic partitioning between various surface‐energy fluxes. Inclusion of a land‐surface scheme with a vegetation canopy into a version of the National Center for Atmospheric Research (NCAR) Community Climate Model (CCM) with a diurnal as well as a seasonal cycle permits an exploratory study of the possible effects of tropical deforestation. In a 13‐month integration that assumes that all of the Amazon tropical forest in South America is replaced by impoverished grassland, surface hydrological and temperature effects dominate the response. Reduced mixing and less interception and evaporation from the canopy cause runoff to increase and surface temperatures to rise by 3–5 K. The period of driest soil is increased in the model from one month to several, but the possibility that this change is random cannot be excluded. Increased temperatures and drier soil could have a detrimental impact on survival of the remaining forest and on attempts at cultivation in deforested areas. The land‐surface model, driven in a stand‐alone mode by prescribed atmospheric conditions and with an imposed seasonal cycle of rainfall, mimics the seasonal cycle of soil moisture and runoff found in the CCM. Hence, it is used to estimate the relative contribution of the various changes imposed to simulate deforestation in the CCM with respect to the model's response at the surface. The change in surface roughness interacting with the canopy hydrology is evidently a major factor in determining the surface response to deforestation. However, the response to change in roughness is less pronounced for simpler models.
Stably stratified shear flow over low hills
Authors: Hunt J. C. R. , Richards K. J. , Brighton P. W. M.
Published: February 2007

An analysis is presented for the mean flow over hills with low slopes, for various kinds of stable stratification in the approach flow. (The Froude numberFbased on the height is always greater than unity.) The analysis is based on dividing up the flow into different regions with different dynamics.

We concentrate on the flow in the inviscid outer region of the flow. It is shown that in the lower part of the outer region, the middle layer, where the shear is large, the upwind velocity profile and the shape of the hill, rather than the upwind density profile, largely determine the vertical perturbation velocity. In the ‘upper layer’ the structure of the flow depends largely on theformof the upwind density profile. For example, this largely determines the location of maximum surface wind. New solutions forstableconditions are obtained when there is a constant density gradient, when (N/UB)2∞ z−2and when there is an elevated inversion layer. In the first and third of these cases, we compare the results with the hydrostatic approximation, and also obtain new general formulae for the near‐surface winds. An approximate formula is developed for surface winds at arbitrary values ofF(>1) for uniform stable stratification.
Surface airflow around Windless Bight, Ross Island, Antarctica
Authors: O'Connor William P. , Bromwich David H.
Published: February 2007

Ross Island, Antarctica is located along the Transantarctic Mountains and is subjected to a strong southerly mountain‐parallel wind regime. Despite this, Windless Bight on the island's southern coast is a region of anomalous calm. The atmospheric boundary layer dynamics that gives rise to this phenomenon is analysed both theoretically and observationally. It is the strong static stability of the boundary layer air encountering the high steep topography of Ross Island, that causes a stagnation zone resulting in the calm conditions of Windless Bight.

Direct and proxy observations of boundary layer winds provide a detailed description of airflow around the island. The anomalous (with respect to the synoptic pressure field) but persistent north‐easterly winds at Scott Base are due to the deflection of highly stable, low‐level air around Hut Point Peninsula. By contrast, the less frequent, strong southerly winds which override the peninsula are associated with the influx of warm maritime air from cyclonic systems to the east. It is inferred that flow of air around the terrain of Ross Island gives rise to locally strong winds; these are responsible for the ice breakout and polynya occurrences in McMurdo Sound.

The surface airflow past Ross Island can be modelled by a two‐dimensional, steady, frictionless, irrotational, incompressible flow past an obstacle, with a shape based on an island height contour. The flow is assumed to separate from the eastern and western sides of the island and form a wake downstream. A solution for this flow is presented, based on potential theory for streaming motion past an obstacle. The streamline and isobar patterns clearly depict the stagnation region in Windless Bight. For approaching winds of 20 ms−1 the local pressure field is perturbed by several millibars.
On the modelling of warm orographic rain by the seeder‐feeder mechanism
Authors: Robichaud Alain J. , Austin Geoffrey L.
Published: February 2007

A two‐dimensional model of the seeder–feeder mechanism for the orographic enhancement of rain is presented. It satisfactorily reproduces observed rainfall rate distributions and is helpful in studying some aspects of the physics of orographic rain. The modelling approach is based on the principle of continuity of the water substance and on the two‐dimensional theory of small perturbations of airflow over topographical ridges of modest dimensions. The model is significantly different from those found in the literature and is used to clarify two major points. Firstly, it emphasizes that there is a range of hill dimensions for which the seederfeeder mechanism plays a major role. That is, the model predicts that a change of regime occurs for larger hills (half length > 20 km) where the seeder‐feeder mechanism is no longer dominant because of the possibility of rain development within the feeder cloud and also because the horizontal scale of rain drift becomes small compared with that of the topography. Secondly, it is also suggested that the difference of wind curvature below and above a low‐level jet may be important in determining the intensity of orographic rain. The physical process behind the latter finding is closely linked to partial reflection of upward propagating gravity waves which may lead to ‘resonance’ of the orographically induced vertical velocity.
Selection of baroclinic waves
Authors: Appleby J. C.
Published: February 2007

A two‐layer baroclinic flow simultaneously unstable to two discrete wavelengths is altered by the self‐interaction of the waves so that either may attain an equilibrium at finite amplitude. The stability of the two possible states, and hence the ‘selection’ by the flow of one predominant wavelength, is determined, in the absence of hysteresis, by a principle of minimum available potential energy. In the limit of weak nonlinearity this is equivalent to maximum barotropic amplitude, but not in general to maximum wave amplitude within each layer as previously suggested.
The effects of entrainment on the growth of droplets in continental cumulus clouds
Authors: Bower K. N. , Choularton T. W.
Published: February 2007

In this paper data are presented from aircraft passes through continental cumulus clouds obtained during the Cooperative Convective Precipitation Experiment. The effect of evaporation of liquid water from the cloud following the entrainment of free tropospheric air is to totally remove a fraction of the droplets of all sizes rather than partially evaporate most of the droplets. Supersaturations in the cloud are found on average to be considerably higher than those predicted for an adiabatic cloud and the supersaturation is not closely tied to the vertical wind.

These observations are found to have a number of important consequences for cloud droplet growth: 1. The average lifetime of individual droplets is considerably less than the age of the cloud and is not expected to exceed about five minutes. This has important consequences for the oxidation of SO2 to sulphate. 2. Statistically favoured droplets that avoid evaporation are observed to grow much larger than average droplets and to sizes larger than would occur at the same altitude in an adiabatic cloud.
Spin vectors and rates of change of wind direction
Authors: White A. A.
Published: February 2007

Established formulae for various rates of change of horizontal wind direction may be unified by a treatment in terms of the spin vector, S, of a vector field M. The spin vector measures the rate of change of direction of M, in the appropriate osculating plane, and may be defined with respect to any infinitesimal displacement in space and time. When M is a horizontal vector field, the osculating plane is horizontal, and S immediately gives the rate of change of direction relative to a fixed horizontal axis. The formulae involved are those of Hide and of Bryden for the rate of change of geostrophic wind direction with height, and of Burk and Staley and of Lecluyse and Neumann for the Lagrangian rate of change of wind direction. When M is a 3‐dimensional vector field, the orientation of the osculating plane is not in general constant and the Cartesian components of S do not represent the rate of change of direction in the most concise terms. The Lagrangian spin vector of the 3‐dimensional velocity field may nevertheless be considered as a basic kinematic quantity, on a par with the rates of change of kinetic energy, vorticity and divergence.
The slow equations
Authors: Lynch Peter
Published: February 2007

A filtered system of equations is derived, using the ideas of normal mode initialization. This slow equation system models the low frequency rotational atmospheric motions; there are no solutions corresponding to gravity waves. The prognostic element of the system is an equation expressing the conservation of potential vorticity. The slow equations differ from the general balance system at second order in the Rossby number, and are free from the spurious solutions found in that system. Integration of a barotropic model with the slow equations shows them to be highly accurate when compared with the primitive equations. Since the slow equation model has no high frequency solutions and is free from data shock, it may be useful for continuous assimilation of observational data.
Sources and sinks of zonal momentum in the middle atmosphere diagnosed using the diabatic circulation
Authors: Shine Keith
Published: February 2007

Radiative heating rates calculated using monthly mean climatological fields of temperature and ozone are used to diagnose the diabatic circulation in the middle atmosphere. This circulation is then used to infer the zonal force per unit mass.

Previous diagnostic studies of the zonal force per unit mass have considered only the northern hemisphere outside the summer period and for heights up to about 60 km. The present study allows calculations for both hemispheres in all seasons at heights up to about 75 km. Strong inter‐hemispheric differences in the diagnosed fields are found in the winter and spring hemispheres, whilst the summer and autumn hemispheres are quite similar. The drag on the mean wind in the southern winter mesosphere is found to be up to double that found in the northern winter mesosphere. It is argued that this feature may be due to an enhanced transmission of gravity waves through the southern winter stratosphere; this may, in turn, explain observed inter‐hemispheric differences in temperature for the winter months. Differences between the diagnosed fields and those inferred in modelling studies are also discussed.

Some attention is also paid to a problem that pervades this and similar studies. The calculated radiative heating rates do not allow a unique diabatic circulation to be diagnosed because of unexplained departures from the global mean mass balance. The diagnosed circulation is thus subject to some uncertainty which puts definite limits on the confidence that can be placed on diagnosed quantities, especially in the stratosphere. It must be recognized that derived quantities are admixtures of the true quantity and imperfections in the data and radiative transfer scheme.
Atmospheric loadings of marine aerosol during a Hebridean cyclone
Authors: Smith M. H. , Consterdine I. E. , Park P. M.
Published: February 2007

During a marine aerosol field trial at a coastal site in the Outer Hebrides, a depression formed in the North Atlantic, and moved directly over the site. Wind speeds on site gusted past 100m.p.h., and some data were gathered at very high wind speeds.

We found that the volume of sea salt aerosol at 30 ms−1 was more than an order of magnitude higher than that at 15 ms−1. This is contrary to previous, rather speculative, findings that the aerosol loadings of the atmosphere became less sensitive to wind speed as the wind speed increases beyond 15 ms−1.
Inversion of cloudy satellite sounding radiances by nonlinear optimal estimation. II: Application to TOVS data
Authors: Eyre J. R.
Published: February 2007

A new approach to inverting satellite sounding data for use in numerical weather prediction (NWP) was proposed in a previous paper, where it was applied to simulated data from the TIROS Operational Vertical Sounder (TOVS). In this paper the method has been applied successfully to real TOVS data from the NOAA‐9 satellite, in a case study taken from the period of the Generation of Atlantic Lows Experiment (GALE). Temperature and humidity profiles, surface temperature and microwave emissivity, and cloud‐top pressure and fractional coverage have been retrieved simultaneously. The retrieved profiles have been compared statistically with co‐located radiosonde measurements. Also, fields of retrieved values have been compared qualitatively with corresponding NWP background fields (which provided the ‘first guess’ for the inversion) and with image data from TOVS itself and from the Advanced Very High Resolution Radiometer on the same satellite.

Impact on the temperature field was found to be small, as expected over an area for which the conventional observing network is relatively dense. However, changes to the background field were generally found to be beneficial. At mid to upper tropospheric levels, the retrievals modified the background humidity field considerably. Retrieved values were both horizontally coherent and consistent with corresponding images and the expected dynamics. Retrieved cloud parameters have not been studied in great detail but have been found to be reasonable when compared with image data.
On the zonal‐scale selection and vertical structure of equatorial intraseasonal waves
Authors: Wang Bin , Chen Jiukang
Published: February 2007

Observed tropical intraseasonal oscillations exhibit a preferential zonal scale of wavenumber one or two and a baroclinic structure with largest flow field amplitude in the upper troposphere. Unstable interaction of boundary layer friction‐induced moisture convergence with condensational heating could cause the scale selection and result in the asymmetric baroclinic structure by means of coupling of vertical normal modes. A vertically continuous linear model describing two‐dimensional equatorial motion demonstrates this feedback mechanism and permits detailed study of vertical structure of unstable moist Kelvin waves.
Analytical model for the growth of the coastal internal boundary layer during onshore flow
Authors: Gryning Sven‐Erik , Batchvarova Ekaterina
Published: February 2007

A model of near‐neutral and convective steady‐state internal boundary layer evolution is presented. the model deals with the internal boundary layer that forms over land in coastal and lake areas during onshore winds. Near the ground, the growth of the internal boundary layer is controlled by friction velocity in accordance with surface layer theory. Farther downwind, the growth is determined by the atmospheric stability and friction velocity within the internal boundary layer, and the temperature gradient in the air above. the wind profile inside the internal boundary layer is assumed to follow Obukhov similarity theory. an expression for the strength of the inversion that caps the layer is derived and used in the model. A comparison is carried out with independent experimental observations of internal boundary layer growth in the sea‐land transition.

Kinematic heat flux through the top of the internal boundary layer is described by the formulation − \documentclass{article}\pagestyle{empty}\begin{document}$ \overline {(w'\theta')} $\end{document}h = 0.2\documentclass{article}\pagestyle{empty}\begin{document}$ \overline {(w'\theta')} $\end{document}s + 2.5 uT/gh + the Zilitinkevich correction. the terms 0.2\documentclass{article}\pagestyle{empty}\begin{document}$ \overline {(w'\theta')} $\end{document}s and 2.5u T/gh account for convective and mechanical turbulence, respectively, and the Zilitinkevich correction is a turbulent kinetic‐energy storage term that ensures finite growth rate of the internal boundary layer near the ground. the relative importance of mechanical and convective turbulence as well as the Zilitinkevich correction is discussed. the Zilitinkevich correction dominates the growth process of the internal boundary layer when it is lower than roughly 50m. As the layer grows, the importance of the Zilitinkevich correction diminishes. Then mechanical turbulence dominates the growth process until the internal boundary layer has reached a height of approximately −1.4L. Further growth is controlled mainly by convective turbulence. Conditions of high wind speed and large values of the potential temperature gradient over water result in a deep zone where the growth of the internal boundary layer is controlled by mechanical turbulence. With near‐zero potential temperature gradient over water, the zone becomes shallow and may vanish.
Onset of sahelian drought viewed as a fluctuation‐induced transition
Authors: Demaree G. R. , Nicolis C.
Published: February 2007

A statistical analysis of representative sets of rainfall data in the west Sahel region is performed and a jump transition in the late sixties is detected. It is suggested that the main features associated with the record can be accounted for by a fluctuation‐induced transition between two stable precipitation regimes: a ‘humid’ and a ‘dry’ state. A minimal model involving the dominant nonlinearities is constructed and its parameters determined from the data. an attempt is made to arrive at a statistical prediction of the duration of the alternate regimes through the relative stability of the two stable states as deduced from mean values and higher moments of residence times. the behaviour predicted by the model is in good qualitative agreement with the record.
On the cloud absorption anomaly
Authors: Stephens Graeme L. , Tsay Si‐Chee
Published: February 2007

This paper provides an overview of the subject of absorption of solar radiation by water clouds in the earth's atmosphere. the paper summarizes the available evidence which points to disagreements between theoretical and observed values of both cloud absorption and reflection. the importance of these discrepancies, particularly to remote sensing of clouds as well as to studies of cloud physics and earth radiation budgets, is emphasized. Existing cloud absorption and reflection measurements are reviewed and the persistent differences that exist between calculated and measured near‐infrared cloud albedos are highlighted. Various explanations for these reflection and absorption discrepancies are discussed and a simple outline of the theory of cloud absorption is provided. This outline is used to examine the large‐droplet hypothesis as well as the effects of absorbing aerosol and enhanced water vapour continuum absorption. A further hypothesis regarding the effects of cloud inhomogeneities is also examined. While the theory of cloud absorption is not completely understood, especially with regard to inhomogeneous clouds, the underlying conclusion of this paper points to the need for better measurements of solar radiation in clouds, water vapour absorption and microphysics properties of clouds.