Showing papers in "Quarterly Journal of the Royal Meteorological Society in 1980"

Correction of flux measurements for density effects due to heat and water vapour transfer

Abstract: When the atmospheric turbulent flux of a minor constituent such as CO2 (or of water vapour as a special case) is measured by either the eddy covariance or the mean gradient technique, account may need to be taken of variations of the constituent's density due to the presence of a flux of heat and/or water vapour. In this paper the basic relationships are discussed in the context of vertical transfer in the lower atmosphere, and the required corrections to the measured flux are derived. If the measurement involves sensing of the fluctuations or mean gradient of the constituent's mixing ratio relative to the dry air component, then no correction is required; while with sensing of the constituent's specific mass content relative to the total moist air, a correction arising from the water vapour flux only is required. Correspondingly, if in mean gradient measurements the constituent's density is measured in air from different heights which has been pre-dried and brought to a common temperature, then again no correction is required; while if the original (moist) air itself is brought to a common temperature, then only a correction arising from the water vapour flux is required. If the constituent's density fluctuations or mean gradients are measured directly in the air in situ, then corrections arising from both heat and water vapour fluxes are required. These corrections will often be very important. That due to the heat flux is about five times as great as that due to an equal latent heat (water vapour) flux. In CO2 flux measurements the magnitude of the correction will commonly exceed that of the flux itself. The correction to measurements of water vapour flux will often be only a few per cent but will sometimes exceed 10 per cent.

Some simple solutions for heat-induced tropical circulation.

Abstract: A simple analytic model is constructed to elucidate some basic features of the response of the tropical atmosphere to diabatic heating. In particular, there is considerable east-west asymmetry which can be illustrated by solutions for heating concentrated in an area of finite extent. This is of more than academic interest because heating in practice tends to be concentrated in specific areas. For instance, a model with heating symmetric about the equator at Indonesian longitudes produces low-level easterly flow over the Pacific through propagation of Kelvin waves into the region. It also produces low-level westerly inflow over the Indian Ocean (but in a smaller region) because planetary waves propagate there. In the heating region itself the low-level flow is away from the equator as required by the vorticity equation. The return flow toward the equator is farther west because of planetary wave propagation, and so cyclonic flow is obtained around lows which form on the western margins of the heating zone. Another model solution with the heating displaced north of the equator provides a flow similar to the monsoon circulation of July and a simple model solution can also be found for heating concentrated along an inter-tropical convergence line.

Destruction of ozone at the earth's surface

Abstract: Several hundred new measurements of the surface resistance to ozone uptake of grass, soil, sand, fresh water, sea water and snow are presented. Grass and soil show, in agreement with previous work, a daytime surface resistance of median value 100 sm−1. The first direct evidence is presented of a nocturnal increase in surface resistance for grass and soil, with night-time values of surface resistance of about 300 sm−1. The surface resistances of sea water and snow are considerably larger, about 1000 to 2000 sm−1. A representative global ozone destruction rate for the earth's surface is derived using the above information and other published ozone data. Allowance is made for the latitudinal variation of the different types of surface, their various ozone destruction constants, the latitudinal and diurnal variation of ozone in the surface air and the diurnal variation of eddy transfer near the earth's surface. The global average ozone destruction rate is estimated to be in the range 2 to 6 × 1029 molecules s−1 (0.5 to 1.5 × 1012kg yr−1). This range extends to slightly higher values than previous estimates. The ratio of ozone destruction rate between the southern and northern hemispheres is about 2:3. The implications for the tropospheric ozone cycle are discussed.

Surface influence upon vertical profiles in the atmospheric near-surface layer

Abstract: Observations from two towers situated in flat, tree-covered terrain (z0 lying between 0.4 and 0.9 m) have been used to investigate the flux-profile relations in the height range z/z0 from 5 to 85, where z is the height above the zero-plane displacement. The analysis confirms a lower height limit (at z = z*) to the validity of the Monin and Obukhov functions ϕM, H(z/L) in unstable conditions and, by implication, of the logarithmic wind law in neutral conditions. We find z*/z0 ≃ 35 and 150 for wind at the denser and less dense (lower z0) surfaces, whilst for temperature z*/z0 ≃ 100. The level z* corresponds with the top of the transition layer, within which it is assumed the profiles depend additionally upon a length scale zs, related to surface wake generation. On the assumption that z* α zs, modification of the profiles in the transition layer is then described through a function o(z/z*) whose explicit form is derived from length-scale considerations in a region of wake-shear interaction. The observed non-dimensional profiles ϕ° are well represented by ϕ° ≃ 0.5ϕ(z/L)exp(0.7z/z*) Both for wind and temperature. For wind at both surfaces, the depth z* is approximately constant in unstable conditions and equal to 3δ, δ being the tree spacing. We tentatively conclude that δ is the relevant surface length scale zs characterizing the wake field and depth of penetration z*.

The influence of entrainment on the evolution of cloud droplet spectra: I. A model of inhomogeneous mixing

Abstract: In this, the first of two related papers, we present calculations of the growth of a population of condensate droplets rising above cloud base within small cumuli which are entraining undersaturated environmental air. It is assumed, on the basis of dimensional arguments and laboratory experiments on entrainment, conducted within a cloud droplet evolution tunnel, that this mixing process is inhomogeneous. In the extreme situation to which the calculations apply undersaturated air is entrained in a stream, or in blobs, and some droplets of all sizes are completely removed from the condensate spectrum by evaporation, while others do not change in size. This is equivalent to assuming that the time constant for turbulent mixing (ττ) is large relative to that for droplet evaporation (τr), and is thus the antithesis to the homogeneous model utilized by other workers, which assumes implicitly that ττ/τr = 0. The calculations based on the extreme inhomogeneous model produce spectral shapes which agree well with those reported in cumulus by Warner (1969a) and indicate that a small proportion of the droplets can grow several times faster through the condensate spectrum than classical theory predicts.

A laboratory model of an atmospheric mesofront

Abstract: Consideration is given to the leading edge of a well-defined flow of dense air without significant condensation, such as occurs at a thunderstorm outflow or a sea-breeze front. Explanation of the front is proposed in terms of a gravity (density) current formed by the denser air. Observations of such flows are related both to laboratory gravity current heads in which head, tail and calm wind conditions are simulated and to a simple, theoretical gravity current model based primarily on a momentum balance. Good agreement is found between the atmosphere, laboratory and analytical results. The mean flow in the gravity current behind the head is greater than the velocity of the head, a result of mixing between the two fluids. This mixing nearly all occurs at the leading edge of the current. The excess height of the head of the gravity current is interpreted as the mixing region at the leading edge, and this mixed fluid lays down a stable layer above the following gravity current. In the laboratory the effect of a headwind is to flatten the profile of the raised head at the front and to reduce its rate of advance by about three-fifths of the value of the opposing flow.

A cloud parametrization scheme derived from GATE data for use with a numerical model

Abstract: A cloud parametrization scheme which allows for low, medium, high and convective clouds has been developed from GATE data for use in the Meteorological Office 11-layer tropical model. The problems involved in using synoptic observations to derive methods of predicting clouds are discussed. Only limited success was obtained in relating observed cloud amounts to relative humidity and atmospheric temperature structure. The restrictions imposed on the cloud scheme by the model's resolution and by its inability to produce a perfect simulation are considered. In the light of these difficulties a simple approach was adopted based on the assumption that condensation on the smallest scales is part of a larger-scale condensation regime related to the synoptic scale situation. The scheme has been designed to reproduce the main features of a cloud field by relating the large-scale meteorological features associated with a cloud distribution to model variables. Low, medium and high cloud amounts are determined from a quadratic relationship with relative humidity. Low cloud has also been related to the temperature lapse rate in an attempt to model the persistent areas of sub-tropical stratocumulus occurring under inversions. A relative humidity relationship is inappropriate for convective cloud which has, therefore, been related to the convective mass flux calculated in the convection scheme of the model. The scheme has been reasonably successful in predicting the cloudiness associated with the ITCZ and the NE. and SE. trades. The cloud fields showed a good degree of coherence from day to day and there were no signs of unrealistic feedbacks between radiation, cloud and dynamics.

An experimental study of the profiles of wind speed, shearing stress and turbulence at the crest of a large hill

Abstract: Measurements have been made, under neutral conditions, of the turbulent wind structure on a 100m tower at the crest of a hill of height h = 170m. A local velocity maximum, or ‘jet’, was observed at about h/5, below which the ratio of increase in wind speed to the upwind value at the same height was 1.07, and almost independent of height near the surface. The r.m.s. turbulence components σw σv and σw were also approximately doubled near the surface, but whereas σu and σv both decreased with height, the surface increase in σw extended over the entire height of measurement. Reynolds stress was almost uniform above the ‘jet’, but increased sharply towards the surface, to 3–5 times the upwind value. Departures from neutrality affected the profile shape and the magnitude of turbulence fluctuations very markedly. Although the size of hill lay outside the range of validity of an analytical theory of Jackson and Hunt, their expressions for velocity and horizontal pressure gradient were in good agreement with the observations. Numerical theories by Frost, Harper and Fichtl and by Taylor, for hills of different shape, but similar slope to the experimental hill, also produced values of surface stress and velocity increase of the magnitude observed.

The diagnosis of middle latitude synoptic development

Abstract: The use of diagnostics based on different forms for the forcing term in the omega equation is explored. These forms are the two-level and continuous versions of the approximation used by Sutcliffe (1947) in his development theory, the usual dynamical meteorology version involving vorticity and thermal advection, and that involving the so-called Q-vectors which was introduced by Hoskins et al. (1978). The diagnostics are applied to a model baroclinic wave and to a subjectively analysed real data case. The Sutcliffe form is simplest and gives a global view of the system movement and development, but details such as active frontal regions are missed. The vorticity and thermal advection form has few advantages. It is demonstrated that the Q-vector analysis can provide more information than the Sutcliffe form in describing details of system development, particularly with respect to (a) a vectorial view of the horizontal ageostrophic motion field, and (b) some indication of the intensity of frontal circulations. A case is presented for including Q-vector fields in low- and mid-tropospheric forecast charts.

Microphysics and dynamics of clouds associated with mesoscale rainbands in extratropical cyclones

Abstract: Measurements obtained with research aircraft in clouds associated with various types of mesoscale rainbands in extratropical cyclones have been examined together with radar data and other detailed measurements. The analyses have yielded information on the structure and microphysical characteristics of the clouds and the nature of the vertical air motions in the clouds associated with each type of mesoscale rainband, and schematic models of the different mesoscale rainbands have been constructed. In clouds accompanying warm frontal rainbands, ice particles formed above the warm front, probably in shallow convective cells, and fell into the stable air mass below, where they grew by aggregation and helped glaciate stratiform clouds. Warm sector rainbands resembled squall lines. A younger, convectively active rainband occurred just ahead of an older, less convectively active one. The younger rainband contained both cloud liquid water and ice particles, and ice particles were growing by riming. The older rainband was nearly glaciated, with ice particles growing by aggregation. Narrow cold frontal rainbands were located along the convergence lines at the advancing edges of cold frontal zones. Each was associated with a convective updraught, 1 to 5 km wide and 1 to 3ms−1 in magnitude, and a similarly narrow downdraught. The core of the updraught contained young, developing cloud, with much higher cloud liquid water content and lower ice particle concentration than in its immediate surroundings. Ice particles grew by aggregation and were heavily rimed. Wide cold frontal rainbands (several tens of kilometres in width) occurred when lifting above a cold front was enhanced to several tens of centimetres per second. Clouds in this region consisted of supercooled water and ice and contained embedded convective elements. Below the cold front, high concentrations of ice particles grew by aggregation. Prefrontal surges of cool and dry air occurred above the warm front and ahead of the cold front in occluded cyclones. A surge rainband, resembling a wide cold frontal rainband, occurred at the leading edge of the surge and was followed by a field of convective elements. These elements, sometimes arranged in lines, were in various stages of development. Postfrontal rainbands were lines of convective clouds located well behind a cold front. They may have been associated with secondary fronts and sometimes appeared to behave as organized convective systems.

The angular distribution and interception of diffuse solar radiation below overcast skies

Abstract: The angular distribution of diffuse radiation was measured from May 1976 to May 1977 with a set of purpose-built actinometers. Radiance distributions for 98 overcast hours were fitted well by an expression similar to the conventional function for a ‘standard overcast sky’ (SOC). However, values of the coefficient b averaged 1.23, significantly smaller than for the SOC (b = 2). Expressions for the interception of diffuse irradiance by sloping planes are discussed.

The blocking of middle latitude westerly winds by planetary waves

Abstract: A study of hemispheric data suggests that the splitting of westerly winds by blocking anticyclones is initially due to simple interference between stationary planetary waves with very large amplitudes but normal phases. A simple model is then used to investigate Green's hypothesis that the blocking anticyclone, once formed, can be maintained by baroclinic waves travelling in the split jet. The results are in good agreement with observations, particularly the vertical structure. A resonance seems likely at zonal wavenumber four.

The albedo of a tropical evergreen forest

Abstract: The diurnal variation of the average albedo, during 1970, over a tropical dry evergreen forest and a nearby clearing was investigated. The study is based on a most comprehensive set of solar radiation data in Southeast Asia. A strong dependence of the albedo on the zenith angle is evident. This dependence is subdued during cloudy days. The average midday albedo for the winter monsoon season is 10.6 for the forest and 13.4 for the clearing; for the summer monsoon it is 12.0 for the forest and 14.6 for the clearing. Generally, results show great uniformity. The largest standard deviations are 0.05 with a more representative value in the range 0.01–0.02.

The numerical modelling of storm surges in the Bay of Bengal

Abstract: A numerical model is developed for the simulation of surges generated by a tropical cyclone in the Bay of Bengal. The analysis area extends from approximately 11°N to 22°N and, in the northeastern sector of the Bay, includes a representation of the Ganges-Brahmaputra-Meghna river system in Bangladesh. The extent of the analysis area allows three days of the surge-generating capacity of a cyclone originating in the southern Indian Ocean to be recorded before landfall at the Bangladesh coast. The incorporation of the river system permits a potentially deep inland penetration of surges originating in the Bay. The model is non-linear and this allows a determination of the interactive effect between surge and the astronomical tide. Numerical experiments are described that relate to the change in surge response resulting from a change in the cyclone track during the 24 hours preceding landfall. An account is given of the interaction between surge and tide in the Bay and the river system.

Charge transfer accompanying individual collisions between ice particles and its role in thunderstorm electrification

Abstract: Experiments were conducted with a wind tunnel in a cold-room in order to investigate the individual charges transferred when ice spheres collided with an artificial hailstone. The charge transferred to a sublimating hailstone was negative and had a magnitude proportional to the velocity of impact and to the diameter of the ice sphere to the power 1.7. The charge transferred when 100μm diameter ice spheres collided with the hailstone at a velocity of 8ms−1 was typically –15 fC. No variation of charging could be detected over the temperature range –5 to –25 °C, or when the ice was doped with impurities, or when hailstones of different surface roughness were prepared. Hailstones which were cooled below the ambient cold-room temperature and subsequently grew by deposition, charged positively by typically 100 fC per collision. However, a hailstone which was cooled by the same amount but was maintained in a sublimating condition charged negatively. These experiments show that thermal effects did not play a direct role in the charge transfer process. Experiments were performed with a riming hailstone in order to simulate the conditions found in a natural cloud. The hailstone charged positively at temperatures of –5°C and –10°C and negatively at –15°C and –20°C with liquid water contents in the range 0.05 gm−3 to 0.85gm−3, the magnitudes of the charges being typically 30 fC. It is suggested that the charge transfer occurs from one ice surface to the other at the contact interface and that the driving force for the charge transfer at the contact interfaces is the different surface potentials and/or different charge carrier densities of ice surfaces formed in different ways. It is possible that the charge carriers may be surface ions of a liquid like layer on the ice surface. The results with the riming hailstone are consistent with thunderstorm observations that the negative charge centre is above the –10°C isotherm and that, although precipitation particles are predominantly negatively charged, a mixture of signs is usually present.

Airborne and ground‐based studies of thunderstorms in the vicinity of Langmuir Laboratory

Abstract: Flights through the central regions of thunderstorms were made over New Mexico on 6 and 15 August 1977 with the ONR/NMIMT Schweitzer aeroplane which carried equipment designed to measure all three components of the electric field, and the charge, Q, and diameter, d, of individual precipitation elements. On the earlier day, information was also obtained with: a rain-gauge network surrounding Langmuir Laboratory; a 3 cm radar; an acoustic system for locating lightning channels; a ground-based field-change meter. The first cell on 6 August produced precipitation at the ground but no lightning. Vertical fields, Ex, of up to about 50kVm−1 and precipitation charge densities ρ of up to −0.5 C km−3 were recorded within the cloud. The second cell, which grew as the first one decayed, produced 7 lightning strokes in 9 minutes during which time the radar revealed vigorous vertical growth in a narrow zone containing precipitation. Thunder reconstructions showed the acoustic sources for the first flash of this cell to be very near the top of the cloud at an altitude of 10 km a.s.1. The subsequent flashes produced acoustic signals from progressively lower in the cloud. When the radar echo reached its maximum height lightning activity ceased. Ex values of up to about 50kVm−1 and pp values of down to −1 Ckm−3 were measured. ρp was consistently negative, individual charges being less than ±40 pC. Q values were within the inductive limit for a thundercloud at breakdown but no systematic relation between Q and d was found. Six penetrations were made through the thundercloud of 15 August, which produced only two lightning strokes. The Ex records were indicative of a (±) dipole located near the cloud top, at around –13°C. Fields of up to about 100kVm−1 and ρp values (positive and negative) of around 5Ckm−3 were measured. Q values of up to ±250 pC were recorded, with charges around ±50 pC being commonly found. No systematic Q-d relation was revealed, and smaller precipitation particles frequently carried charges (positive or negative) in excess of the inductive limit. On both days estimated precipitation rates were of order 10mmh−1 and on most occasions the pilot reported precipitation particles to be either ‘ice’ or ‘mixed liquid water and ice’.

Numerical experiments on the inland penetration of the sea breeze

Abstract: A two-dimensional primitive-equation model is used to investigate various aspects of the sea-breeze phenomenon. The influence of soil moisture content is examined by varying the Bowen ratio of the underlying land surface. This is equivalent to controlling the magnitude of the sensible heat flux at the ground since the diurnal variation of net radiation minus ground heat flux is approximately the same in all cases. It is found that the inland penetration rate of the sea breeze varies significantly with the degree of soil moisture. The model predicts a significant slowing of the frontal advance around midday for the case of ‘relatively weak’ sensible heating over the land, as well as reproducing the frontal acceleration observed in late afternoon for all cases. The sea-breeze circulation detaches itself from the coastline around sunset and moves rapidly inland with diminishing magnitude and this aspect is discussed in relation to observations in the south of England. An attempt is made to model the southern England sea breeze of 14 June 1973. Various experiments are discussed, including one in which different values of Bowen ratio are specified ahead of and behind the front. Fairly good agreement is found, although it is not possible to reproduce all features of the inland penetration curve for that day. Some implications of these experiments for the production of daily forecasts by mesoscale models are briefly discussed.

Redistribution of scalars during upper ocean frontogenesis:A numerical model

Abstract: A two-dimensional frontogenesis model forced by a barotropic horizontal deformation field has been developed. Turbulent mixing is neglected and the long-front flow is assumed geostrophic. The horizontal gradients at the surface become infinite after 3–4 days, assuming a deformation rate of 10−5s−1. In reality we expect turbulent mixing to prevent the final collapse to a singularity. The inclination of cross-front streamlines to isopycnals decreases rapidly below the surface; the horizontal density gradients do not therefore increase rapidly there. However, such flow can effectively increase the horizontal gradients of passive scalars (we consider temperature here) whose isopleths are inclined to the isopycnals. In a typical example the isotherm pattern makes a front appear to be an order of magnitude thinner and to penetrate much deeper than does the isopycnal pattern. The isotherm patterns produced vary markedly with the initial inclination of isopycnals and isotherms.

The transport of minor atmospheric constituents between troposphere and stratosphere

Abstract: Measurements of the water vapour content of air in the upper troposphere and lower stratosphere are reviewed. They are interpreted to indicate that significant upward movement of air through a pressure level of about 100 mb, by processes of any scale, occurs only in equatorial latitudes, but they leave uncertain the mechanism of the upward transfer. This implies that contaminants emitted into the atmosphere below this level can reach the upper stratosphere only by way of the equatorial tropopause and after passage through precipitating cloud. The minimum hemispheric exchange of mass through this level is about 1017kg per year. The mass balance of several minor atmospheric constituents, naturally or artificially introduced in the surface layers or in the high stratosphere, is shown to be consistent with this minimum mass exchange.

Dynamical models of two‐dimensional downdraughts

Abstract: Observational and numerical model data have indicated the crucial role of the downdraught in the motion, structure and regeneration of severe storms. This paper considers downdraught dynamics in two dimensions, using both a numerical and an analytical model. In the numerical model the downdraught is produced and maintained by a constant heat sink. The adjustment of an ambient inflow, and the resultant downdraught and potential regeneration of storm cells, is simulated. To elucidate physical principles, steady analytical models are also described in which a simple parametrization of downdraught thermodynamics is used. Two distinct types of individual draught dynamics, labelled symmetric and jump, have been identified allowing a qualitative description of six dynamically consistent updraughts and downdraughts. In particular, a cold symmetric downdraught and a cold jumptype updraught are analysed, the former having negative, the latter positive outflow shear. These analytical solutions agree well with steady states developed by the numerical model. The numerical model is also used to describe the propagation of downstream and upstream gust fronts, and these results are in good agreement with atmospheric and tank data.

Volcanic eruptions and long-term temperature records: An empirical search for cause and effect

Abstract: The ‘superposed epoch’ analysis method of compositing temperatures is employed to aid in the search for evidence of a drop in surface temperature due to large volcanic dust veils. The temperature records examined have been chosen from the archives at the National Center for Atmospheric Research according to the length and completeness of the record. Various groupings of the data stations are used as data bases and the strength of the ‘volcanic signal’ determined in the different cases. A seasonal (winter-summer) investigation of the signal is also conducted. A significant dip in temperature can be found within a few years after the major eruption dates in most of the dust veil temperature superpositions, whereas the noise appears to vary randomly from superposition to superposition. The evidence points to a stronger, but less statistically significant, response to the effects of large volcanic dust veils in higher latitudes, although a definite conconclusion requires, among other things, a larger and more representative data base. The seasonal investigation reveals a stronger summer signal in the majority of cases.

Stratospheric transport by stationary planetary waves ‐ the importance of chemical processes

Abstract: The transport of reactive trace gases by stationary planetary scale waves has been investigated with the aid of Matsuno's stationary planetary wave model (1970) and the linear eddy equation derived from the tracer continuity equation. It is shown that, for stationary fluxes, K-theory fields, as introduced by Reed and German (1965), may be defined. These become matrix fields when photochemical coupling effects are considered. The eddy flux of a particular trace gas is related to the gradients of the mean concentrations of the other chemically coupled trace gases, as well as to the gradients of its own mean concentration. In the special case of a chemically inert tracer our treatment indicates Kyy = Kxx = 0 and Kyx = – Kxy in contradiction to the results obtained by Reed and German (1965) but in agreement with recent results of Clark and Rogers (1978) and Matsuno (1980). The consequences of photochemical coupling effects for eddy fluxes are investigated quantitatively for two simple chemical schemes involving the pairs O3 and NOx, and NO and NO2. These effects are shown to be significant especially for NO and NO2 whose photochemical time-constants are short. Furthermore, even in regions of the atmosphere where coupling effects become small, such as the lower stratosphere for O3 and NOx, the uncoupled K-fields are quite different for various species indicating a source of error in current 1- and 2-dimensional photochemical models. The relative importance of dynamical and chemical contributions to the uncoupled K-fields is dependent on the characteristic time-scales for photochemical and zonal advection processes. The relation of our work to a Lagrangian description of the transport process is discussed.

A numerical study of radiation fog with an explicit formulation of the microphysics

Abstract: A model of radiation fog is described containing an explicit formulation of the microphysics of the condensation process. The coupling between the radiative and microphysical processes within the model means that apart from the specification of the turbulence and CCN spectrum it is deterministic. Results are presented which indicated that: (i) The drop-size distribution is sensitive to the CCN concentration but not to the presence of large CCN. (ii) Radiative loss from the drops can have a significant effect upon their growth. (iii) The drop-size distribution is very sensitive to reductions in the condensation coefficient to a value below 0.033. Differences between observed and model drop-size distributions are attributed to an underestimate in the model of the level of mixing within a mature fog.

Mass divergence in tropical weather systems Paper I: Diurnal variation

Abstract: The diurnal variation of mass divergence and vertical velocity is documented for tropical summertime oceanic weather systems in the Western Pacific, Western Atlantic and the GATE region. It is shown that it is large and has the same basic character in all regions. Gray and Jacobson (1977) proposed that the diurnal variation of mass convergence is a result of diurnal differences in the net radiative and convective heating rates in the thick cirrus-shield covered weather systems and their surrounding clear areas. Fingerhut (1978) has developed a numerical model which appears to substantiate this hypothesis. A comparison of observations with results from his model reveals that a simple radiation-condensation model simulates most of the observed diurnal variations of convergence. The hypothesis is that radiational forcing is one of the major contributors to the maintenance and modulation of tropical weather systems.

Cluster analysis of rainfall stations of the Indian peninsula

Abstract: Principal component analysis is applied to derive patterns of temporal variation of the rainfall at fifty-three stations in peninsular India. The location of the stations in the coordinate space determined by the amplitudes of the two leading eigenvectors is used to delineate them into eight clusters. The clusters obtained seem to be stable with respect to variations in the grid of stations used. Stations within any cluster occur in geographically contiguous areas.

Diurnal and seasonal variation of convection in the wind‐mixed layer of the ocean

Abstract: Buoyant convection in the wind-mixed layer of the ocean provides a source of turbulent kinetic energy that is restricted to a layer whose depth, C, is equal to, or less than, H, the depth of the mixed layer. At night C = H, and convection contributes significantly to deepening the mixed layer by turbulent entrainment. During most of the day, when the rate of heat input by the Sun exceeds the rate of surface heat loss by evaporation, conduction and long wave radiation, C < H and convection does not contribute to entrainment. When C < H, C = D, the thermal compensation depth, whose magnitude is independent of any consequences of turbulent mixing or entrainment. D can be calculated from a model of solar energy absorption in the ocean. Such a model is used to determine the diurnal variation of D at selected locations. The shape of this variation (a ‘top hat’ distribution) suggests the use of two parameters Dmin (the noon value) and P10 (the proportion of each day for which D < 10m). The seasonal and meridional variations of these parameters reveal the corresponding variations in the contribution of convectively generated turbulent kinetic energy to mixed layer deepening by turbulent entrainment. The parameter P10 can be used to take account of the daytime quenching of convection by the Sun in climate models that do not resolve the diurnal cycle.

A two-dimensional numerical study of horizontal roll vortices in the neutral atmospheric boundary layer

Abstract: The dynamics of large-scale, horizontal roll vortices in the neutral planetary boundary layer are investigated by means of a two-dimensional numerical model. The rolls are assumed to be two-dimensional and are calculated explicitly, while small-scale turbulence is parametrized by a mixing-length hypothesis. Although buoyancy effects are never negligible in the atmosphere, the assumptions of the turbulence modelling are supported by atmospheric observations of large eddies which are highly elongated in a direction close to that of the geostrophic wind, together with the observed partitioning of turbulence energy between the large eddies and a distinctly smaller scale. The results indicate a strong sensitivity to the roll orientation, and also the presence of a slow variation on the Coriolis timescale.

On the entropy balance of the earth‐atmosphere system

Abstract: The entropy balance associated with a Budyko-Sellers climatic model is developed. It is shown that different regimes, associated with decreasing, as well as increasing values of entropy production (which measures the rate of dissipation in the system) in the course of time are possible. This immediately poses the problem of stability of steady states of the climatic system. An explicit criterion of climatic stability is thus derived, which is expressed in terms of thermodynamic quantities related to excess entropy production. The results are illustrated on simple cases involving diffusive energy transport. A comparison with Paltridge's minimum entropy exchange principle is also attempted.

The dynamical behaviour of a two‐dimensional model of the stratosphere

Abstract: For the atmosphere below approximately 60 km the dynamical behaviour of the zonal mean circulation model described by Harwood and Pyle (1975, 1977) is discussed in detail and compared with observations. The major features of the observed extra-tropical circulation are reproduced by the model, although the principal jets are overestimated. Equatorial temperatures are representative but horizontal temperature gradients are too large, in keeping with the excess wind speed. The roles played in forcing the mean meridional circulation by the diabatic heating and the eddy fluxes of angular momentum and heat are considered. The temperature structure of the summer stratopause is discussed in detail; the model results indicate that this region is in radiative equilibrium to a good approximation. The model has been used to investigate stratospheric ozone and in this paper the sensitivity of the ozone distribution to a number of the model variables is discussed. Reasonable changes in the specified eddy diffusion coefficients and momentum fluxes can alter the total ozone column by around 10%. For small changes (∼ a few tenths K d−1) in the radiative heating of the lower stratosphere even larger changes in the ozone amount are found in the model. The net radiative heating in the lower stratosphere is difficult to calculate. The implication of these sensitivity experiments for pollution perturbation calculations is stressed. Unless all feedback processes are included calculations of pollution-induced changes in ozone of around 10% must be regarded with caution.