Studies of the atmospheric stability characteristics during the solar eclipse of February 16, 1980

Observations made with a monostatic sodar and from a 120 m instrumented tower have been used to study the variations in the atmospheric boundary layer at Tarapur (19° 50 N, 72° 41 E) during the solar eclipse of February 16, 1980. Atmospheric instability was reduced below normal values during the eclipse but the atmosphere at no time became stable.     

Occurrence of roll circulations in a shallow boundary layer

Meteorological measurements taken at the Näsudden wind turbine site during slightly unstable conditions have been analyzed. The height of the convective boundary layer (CBL) was rather low, varying between 60 and 300 m. Turbulence statistics near the ground followed Monin-Obukhov similarity, whereas the remaining part of the boundary layer can be regarded as a near neutral upper layer. In 55% of the runs, horizontal roll vortices were found. Those were the most unstable runs, with -z _i/L > 5. Spectra and co-spectra are used to identify the structures. Three roll indicators were identified: (i) a low frequency peak in the spectrum of the lateral component at low level; (ii) a corresponding increase in the vertical component at mid-CBL; (iii) a positive covariance {ovvw} together with positive wind shear in the lateral direction (V/z) in the CBL. By applying these indicators, it is possible to show that horizontal roll circulations are likely to be a common phenomenon over the Baltic during late summer and early winter.     

Nitrate in the atmospheric boundary layer of the tropical South Pacific: Implications regarding sources and transport

Weekly bulk aerosol samples collected at Funafuti, Tuvalu (8°30S, 179°12E), American Samoa (14°15S, 170°35W), and Rarotonga (21°15S, 159°45W), from 1983 through most of 1987 have been analyzed for nitrate and other constituents. The mean nitrate concentration is about 0.11 g m^–3 at each of these stations: 0.107±0.011 g m^–3 at Funafuti; 0.116±0.008 at American Samoa; and 0.117±0.010 at Rarotonga. Previous measurements of mineral aerosol and trace metal concentrations at American Samoa are among the lowest ever recorded for the near-surface troposphere and indicate that this region is minimally affected by transport of soil material and pollutants from the continents. Consequently, the nitrate concentration of 0.11 g m^–3 can be regarded as the natural level for the remote marine boundary layer of the tropical South Pacific Ocean. In contrast, over the tropical North Pacific which is significantly impacted by the transport of material from Asia and North America, the mean nitrate concentrations are about three times higher, 0.29 and 0.36 g m^–3 at Midway and Oahu, respectively. The major sources of the nitrate over the tropical South Pacific are still very uncertain. A very significant correlation between the nitrate concentrations at American Samoa and the concentrations of ²¹⁰Pb suggests that transport from continental sources might be important. This continental source could be lightning, which occurs most frequently over the tropical continents. A near-zero correlation with ⁷Be indicates that the stratosphere and upper troposphere are probably not the major sources. A significant biogenic source would be consistent with the higher mean nitrate concentrations, 0.16 to 0.17 g m^–3, found over the equatorial Pacific at Fanning Island (3°55N, 159°20W) and Nauru (0°32S, 166°57E). The lack of correlation between nitrate and nss sulfate at American Samoa does not necessarily preclude an important role for marine biogenic sources.     

High-Latitude Springtime Photochemistry. Part I: NOx, PAN and Ozone Relationships

Springtime measurements of NO_x, ozone, PAN,J(NO₂), and other compounds were made near Ny-Ålesund,Svalbard (78°54N, 11°53E), in 1994 and Poker Flat,Alaska (65°08N, 147°29W), in 1995. At Svalbard medianmixing ratios for PAN and NO_x of 237 and 23.7 pptv,respectively, were observed. The median mixing ratios at Poker Flat for PANand NO_x were 79.5 and 85.9 pptv, respectively. These data areused to estimate thermal PAN decomposition using several differentapproaches. At Svalbard PAN decomposition was very small, while at PokerFlat up to 30 pptv/h PAN decomposed. At both sites the NO_x/PANratio increased with temperature between –10 and 20°C implyingthat PAN decomposition is an important NO_x source. In-situozone production was calculated from the measured NO, NO₂,O₃, J(NO₂), and temperature data, using thesteady state assumption Median ozone production was 605 pptv/h at PokerFlat, and one order of magnitude smaller at Svalbard during the daytime.Only at Poker Flat could a direct influence on the diurnal ozone cycle beobserved from in-situ production. These results imply that PAN decompositionis a major source of NO_x in the high latitude troposphere, andthat this contributes to the observed spring maximum in surface ozone.     

Effects of heat and water vapor transport on eddy covariance measurement of CO2 fluxes

Flux densities of carbon dioxide were measured over an arid, vegetation-free surface by eddy covariance techniques and by a heat budget-profile method, in which CO₂ concentration gradients were specified in terms of mixing ratios. This method showed negligible fluxes of CO₂, consistent with the bareness of the experimental site, whereas the eddy covariance measurements indicated large downward fluxes of CO₂. These apparently conflicting observations are in quantitative agreement with the results of a recent theory which predicts that whenever there are vertical fluxes of sensible or latent heat, a mean vertical velocity is developed. This velocity causes a mean vertical convective mass flux (= _cw for CO₂, in standard notation). The eddy covariance technique neglects this mean convective flux and measures only the turbulent flux _c w. Thus, when the net flux of CO₂ is zero, the eddy covariance method indicates an apparent flux which is equal and opposite to the mean convective flux, i.e., _c w = – _c w. Corrections for the mean convective flux are particularly significant for CO₂ because _cw and _c w are often of similar magnitude. The correct measurement of the net CO₂ flux by eddy covariance techniques requires that the fluxes of sensible and latent heat be measured as well.     

Wind characteristics in the lowest 340 m of the atmospheric boundary layer at Pune,India

Wind characteristics in the lowest 340 m (agl) of the atmospheric boundary layer at Pune (18°32N, 73°51E, 559 m ASL) have been investigated using the pilot balloon wind observations obtained during the summer monsoon seasons of 1976, 1979 and 1980. Variations in the zonal and meridional components of wind at the surface, 40, 150 and 340 m (agl) have been described. Frequency distribution of the zonal component showed an unimodal character at the surface, which gradually approached a multimodal character at 340 m. The distribution pattern of the wind components was, by and large, normal. Spectral analysis of the wind components showed that the spectral energy was predominantly shared by 5–7 and 2–3 day periods.     

Time series analyses of concentration and wind fluctuations

Analyses of concentration fluctuation (C) spectra from boundary-layer smoke plume experiments at six separate locations show that the spectra from these experiments generally exhibit an inertial subrange at high frequencies with a slope of -5/3 and indicate peak energy at a time period of about 50 to 100 s. These periods of peak energy are a factor of two to five less than those for the peak of the wind speed fluctuation (u or v) spectra. A general spectral formula fits normalized spectra from the U.S. and Australia, where the frequency, n, is made dimensionless by multiplying by the plume dispersion parameter, _y , and dividing by the wind speed, u. Peak energy occurs at a dimensionless frequency of n _y/u equal to about 0.15. The Kolmogorov constant in the inertial subrange is estimated from a set of averaged spectra. Cross-spectra indicate little relation between concentration and wind fluctuations. However, most of the correlation that exists is due to periods larger than about 10 or 20 s.     

Recurrence statistics of concentration fluctuations in plumes within a near-neutral atmospheric surface layer

This study examines the statistical properties of the concentration derivative, , for a dispersing plume in a near-neutrally stratified atmospheric surface layer. Towards this goal, the probability density function (pdf) of , and the conditional pdf of given a fixed concentration level, , have been measured. These pdfs are found to be modeled well by a generalizedq-Gaussian (gqG) distribution with intermittency exponent,q, equal to 0.3 and 3/4, respectively. These results highlight the strong intermittency effect (patchiness) of the small-scale concentration eddy structures in the plume. The distribution of time intervals between successive high peaks in the squared derivative process, x², is found to be well approximated by a power-law distribution, implying that occurrences of these high peaks are much more clustered than would be predicted by a Poisson or shot-noise process. The results are used to improve models for the joint pdf of and , and for the expected number of upcrossings per unit time interval of a fixed concentration level that have been proposed by Kristensenet al. (1989). The predictions of the improved models are in accord with observations, and suggest that the intercorrelation between and must be explicitly incorporated if good estimates of the upcrossing intensity are to be obtained.     

Equatorial disturbances,monsoons, and 1982–1983 ENSO

Summary Interannual modes are described in terms of three-month running mean anomaly winds (u,v), outgoing longwave radiation (OLR), and sea surface temperature (T _* ). Normal atmospheric monsoon circulations are defined by long-term average winds (u _n,v _n) computed every month from January to December. Daily winds are grouped into three frequency bands, i.e., 30–60 day filtered winds (u _L,v _L); 7–20 day filtered winds (u _M,v _M); and 2–6 day filtered winds (u _S,v _S). Three-month running mean anomaly kinetic energy (signified asK _L , K _M , andK _S , respectively) is then introduced as a measure of interannual variation of equatorial disturbance activity. Interestingly, all of theseK _L , K _M , andK _S perturbations propagate slowly eastward with same phase speed (0.3 ms^–1) as ENSO modes. Associated with this eastward propagation is a positive (negative) correlation between interannual disturbance activity (K _L , K _M , K _S ) and interannualu (OLR) modes. Namely, (K _L , K _M , K _S ) becomes more pronounced than usual nearly simultaneously with the arrival of westerlyu and negativeOLR (above normal convection) perturbutions. In these disturbed areas with (K _L , K _M , K _S >0), upper ocean mixing tends to increase, resulting in decreased sea surface temperature, i.e.T _* 0. Thus, groups (not individual) of equatorial disturbances appear to play an important role in determiningT _* variations on interannual time scales. HighestT _* occurs about 3 months prior to the lowestOLR (convection) due primarily to radiational effects. This favors the eastward propagation of ENSO modes. The interannualT _* variations are also controlled by the prevailing monsoonal zonal windsu _n, as well as the zonal advection of sea surface temperature on interannual time scales. Over the central Pacific, all of the above mentioned physical processes contribute to the intensification of eastward propagating ENSO modes. Over the Indian Ocean, on the other hand, some of the physical processes become insignificant, or even compensated for by other processes. This results in less pronounced ENSO modes over the Indian Ocean.With 10 FiguresContribution No. 89-6, Department of Meteorology, University of Hawaii, Honolulu, Hawaii.     

The Value Of The β Coefficient In The Relaxed Eddy Accumulation Method In Terms Of Fourth-Order Moments

The commonly measured value of in the relaxed eddy accumulationmethod of about 0.56is shown to arise from the non-Gaussiannature of turbulence. Fourth-orderGram–Charlier functions forthe two-dimensional probability distributionsof variation in the horizontal component of wind velocityand concentrations of water vapour, carbondioxide and methane with respect to thevertical component of wind velocity are used to examinethe value of .An analytical solution for ispresented in terms of fourth-order moments.Under mean conditions, this solution givesa value for of0.557. Variation of is shown to be controlledprimarily by the ratio of the mean ofc'w3 (where c'is relevant to the entity of interest andw' is vertical component of windvelocity) to the correlationcoefficient between the entity concentrationand vertical component of wind velocity.     

On the enhanced smoothing over topography in some mesometeorological models

An equation is derived for the components of the horizontal (turbulent) frictional force in the -coordinate system with special attention to mesometeorological flow models. The starting point is the horizontal equation of motion in its flux-form in the -system in which we replace (following Reynolds' procedure) the velocity components u,v and % MathType!MTEF!2!1!+- % feaafeart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGafq4WdmNbai % aaaaa!37B8! \[ \dot \sigma \] aswell as other relevant quantities by terms of the form u = + u,..., = ± + % MathType!MTEF!2!1!+- % feaafeart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGafq4WdmNbai % Gbauaaaaa!37C3! \[ \dot \sigma ' \] , etc. ( = time average of u; u = fluctuating part of u.) Next, the equation is averaged with respect to time and terms which we believe are small in mesometeorological flows, are neglected. On expressing by an appropriate expression that involves w, the result shows the appearance of two new terms which, have not been considered previously in the published literature. While the expression earlier used in the literature involved the -derivative of % MathType!MTEF!2!1!+-% feaafeart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaWaa0aaaeaace% WG1bGbauaaceWG3bGbauaaaaaaaa!380B!\[\overline {u'w'} \] alone, the new terms add the -derivatives of % MathType!MTEF!2!1!+-% feaafeart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaWaa0aaaeaace% WG1bGbauaadaahaaWcbeqaaiaaikdaaaaaaaaa!37EC!\[\overline {u'^2 } \] and % MathType!MTEF!2!1!+-% feaafeart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaWaa0aaaeaace% WG1bGbauaaceWG2bGbauaaaaaaaa!380A!\[\overline {u'v'} \] for the x-component of the force, and the -derivatives of % MathType!MTEF!2!1!+-% feaafeart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaWaa0aaaeaace% WG2bGbauaadaahaaWcbeqaaiaaikdaaaaaaaaa!37ED!\[\overline {v'^2 } \]} and % MathType!MTEF!2!1!+-% feaafeart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaWaa0aaaeaace% WG1bGbauaaceWG2bGbauaaaaaaaa!380A!\[\overline {u'v'} \] for the y-component, where and are the slopes of the -surfaces in the x- and y-directions, respectively. Further, a few numerical simulations of the sea-breeze over topography are carried out with and without the correction terms. It is shown that when corrections terms are not included the effective smoothing is stronger above the sloping regions and may amount to as high as 50 percent of the convergence with slopes of ~.04. The ìnclusìon of the new terms does not lead to any special computational difficulties and for that reason there is no compelling reason to neglect them, all the more so because, as is shown, the addition of the new terms results in a consistent apportioning of the degree of horizontal diffusion.On leave from CIMMS, Norman, OK.Now visiting Dept. of Met., Helsinki, Finland.     

On the turbulence structure in the stable boundary layer over the Greenland ice sheet

Turbulence measurements from a 30 m tower in the stably stratifiedboundary layer over the Greenland ice sheet are analyzed. The observationsinclude profile and eddy-correlation measurements at various levels. Atfirst, the analysis of the turbulence data from the lowest level (2 m aboveground) shows that the linear form of the non-dimensional wind profile(m) is in good agreement with the observations for z/L <0.4, whereL represents the Obukhov length. A linear regression yieldsm=1+5.8z/L. The non-dimensional temperature profile (h) at the2m level shows no tendency to increase with increasing stability. The datafrom the upper levels of the tower are analyzed in terms of both localscaling and surface-layer scaling. The m and the h values show atendency to level off at large stability (z/>0.4) where represents the local Obukhov length. Hence, the linear form of the functions is no longer appropriate under such conditions. The bestcorrespondence to the data can be achieved when using the expression ofBeljaars and Holtslag for m and h. The vertical profiles of theturbulent fluxes, the wind velocity variances and temperature variance arealso determined. The momentum flux profile and the profiles of wind speedvariances are in general agreement with other observations if a welldeveloped low-level wind maximum occurs, and the height of this maximum isused as a height scale.     

High resolution spectral features of a series of aromatic hydrocarbons and BrO: Potential interferences in atmospheric OH-measurements

Naphthalene (C₁₀H₈), several other hydrocarbons, mostly derivates of naphthalene, and bromine oxide (BrO) were analyzed for narrow band (0.01 nm) absorption lines in the wavelength range between 307.7 and 308.3 nm to study their potential impact on OH radical measurements by differential absorption spectroscopy.Only naphthalene showed narrow band absorption lines in this wavelength region. From nine naphthalene lines the differential absorption cross-section was determined.The strongest naphthalene line at 308.002 nm is close to the Q ₁(2) OH line, but about a factor of 200 weaker (=(65.2±15.3)×10^-20 cm²/molec). The corresponding detection limit for naphthalene is about 15 ppt. We re-evaluated some spectra of our OH measurement campaign in July 1987 with respect to naphthalene and obtained an upper limit of 30 ppt for its concentration.BrO was recorded in the larger wavelength interval between 307.7 and 308.7 nm. Structured absorptions were only observed at wavelengths above 308.2 nm and no significant structures were found in the vicinity of the Q ₁(2) and Q ₁(3) OH lines.     

Analysis of the Saharan dust regional transport

Summary The Balkan Peninsula is situated in the impact zone of Saharan dust storms. The case of Saharan dust transport to Belgrade in the period of 14–17 April 1994 is analyzed using the Eta model for synoptic and meso scale processes. Air back trajectories are calculated at six model levels from 434 up to 5129m with horizontal grid resolutions of 1°×1° and 10×10. Following cyclonic circulation the dust was picked up from North Africa, and transported over Mediterranean. Simultaneously, according to the analysis of the three lowest trajectories, transport of trace metals from Macedonia and southern Serbia by the Koshava wind might be dominant in the observed episode. Turbulent flow enhanced the coagulation process of initially clean dust particles with particles containing Pb and Cd. The coagulation and scavenging processes below and in clouds increased deposition rates of Pb and Cd in Belgrade in the course of wet removal, and consequently trough resuspension processes. Dry deposition samples contained characteristic particles up to 30µm in diameter with Fe content of 11 to 15 atomic% and significant ratio Si/Fe of 3 to 5, determined for selected single particles by the SEM/EDX method. Following dry and wet deposition of Cd and Pb, a residual effect of dustfall is noticed throughout the vegetational period.     

Calculation of velocity skewness in real and artificial plant canopies

Wind velocities within a plant canopy are much more strongly skewed than those of the air flow above. We have examined the governing Eulerian equations for the velocity products u _i, u _j u_k using data from a wind tunnel study with an artificial canopy consisting of an array of 5 cm lengths of monofilament fishing line, and from measurements in corn (Zea mays L).Simple parameterizations for pressure-velocity correlations, and for the quadruple velocity products allowed reasonably accurate calculations of the third moments using measured profiles of the mean velocity, variance and covariance fields. Comparisons of individual terms in the rate equations for ovu _i, u _j u _krevealed that diffusion (from above) and mean shear were most important in creating large skewness in the canopy. A drag term also contributed but was of lesser importance. These terms were balanced by return-to-isotropy and a turbulence interaction term. A quasi-Gaussian approximation considerably underestimated the magnitude of the fourth moments within the canopy.     

Cold surges along the front range of the Rocky mountains: Development of a classification scheme

Summary A classification scheme for cold surges occurring along the Front Range of the Rocky Mountains has been developed using NMC surface and upper air data. A separation is seen in the data between cases with parent anticyclones that moved from Canada south into the United States (Northern type) and those which moved from the Eastern Pacific Ocean east into the central US (Western type). The Northern surges are further broken down into three types, A, B and B, when the degree of southward motion of the anticyclone and the leading edge of the cold air is considered. An anticyclone movement south of 40°N distinguishes type A, while anticyclone tracks remaining north of 40°N characterize types B and B. The leading edge of the surge (the surge line) associated with the type A anticyclone travels south of 25N, while the type B anticyclones produce two distinct surge line movements, one remaining north of 25°N and the other moving south of 25°N. It is not possible to break down the Western surges into distinct types. As a rule, the anticyclones associated with Western surges remain between 50°N and 30°N, with the surge line moving south to 18°N on average.Though the classification scheme is developed in a relatively subjective manner, the statistical robustness of the scheme is verified through application of the nonparametric Mann-Whitney test. Results of the test clearly provide support for the division of the surges that was obtained.With 16 Figures     

Maximum precipitation for different short-term intervals

Summary A time series and extreme value analysis of maximum precipitation for distinct time intervals from 10 minutes to 1 day have been deduced for the observatory Zagreb-Gri ( = 45°49 and = 15°59,H _s = 157 m) for the period between 1908 and 1985. The Spearman rank correlation test revealed that the short-term precipitation maxima series for the time intervals of 10 to 30 minutes and 8 to 24 hours indicate no significant trend for the 95 percent probability level. The 40-minute to 4-hour precipitation maxima exhibited an increaese in recent time. Annual maximumt-minute precipitation was estimated using the Gumbel distribution for sets of data gradually prolonged with 10-year steps towards the past. The ratios (q) of maximum precipitation estimates for the shorter periods and of estimates for the reference period (1908–1985) have been calculated. Their positions according to the confidence interval on the reference estimates were determined. Confident extreme value estimates could be obtained using at least the 50–60 year data series.With 3 Figures     

Clear sky ultraviolet-B radiation in the English Midlands: A comparison of measured and modelled values

Summary Spectral measurements were made of solar ultraviolet-B (UVB) irradiance at Sutton Bonington (52° 50N, 1° 15W) under cloudless skies using a Licor LI 1800 scanning spectroradiometer. The finite bandpass of the instrument and the steep shape of the UVB spectra caused overestimation of irradiance at short wavelengths. Spectra were corrected mathematically for these effects.The corrected spectra were compared to estimates of global UVB irradiance as a function of zenith angle and amount of ozone. Comparisons were made at 300 nm, 310 nm and 320 nm. Estimates were significantly greater (p = 0.05) than the measurements except at 320 nm where differences were not significant. The differences may have been the result of overestimation of UVB at short wavelengths, since some of the assumptions on which the estimates were based may not be valid for Sutton Bonington conditions.

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Zusammenfassung Unter wolkenlosem Himmel wurden in Sutton Bonington (52° 50N, 1° 15W) mittels eines Licor LI 1800 Spektroradiometer-Scanners Spektralmessungen von Ultraviolett-B (UVB)-Strahlung durchgeführt. Die endliche Bandbreite des Instruments und der steile Anstieg der UVB-Spektren führten zu einer Überschätzung der Strahlung im kurzwelligen Bereich. Die Spektren wurden in dieser Hinsicht mathematisch korrigiert.Die korrigierten Spektren wurden mit den Schätzwerten der globalen UVB-Strahlung — als Funktion von Zenithwinkel und Ozonmenge — bei 300nm, 310nm und 320nm verglichen. Hiebei erwiesen sich die Schätzwerte wesentlich höher (p = 0,05) als die Meßwerte, nur bei 320nm waren die Unterschiede nicht signifikant. Die Unterschiede könnten aus der Überschätzung von UVB im kurzwelligen Bereich resultieren, da einige der Schätzungsvoraussetzungen auf die Bedingungen in Sutton Bonington nicht zutreffen dürften.

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On leave from Department of Agronomy and Soils, Washington State University, Pullman, WA, U.S.A.

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With 4 Figures     

On the stable stratification of the nocturnal lower troposphere inferred from lidar observations over Pune,India

The nocturnal structure of the lower troposphere is studied using aerosol profile data (50–2800 m AGL) obtained with a bistatic, continuous wave, Argon ion lidar system during October 1986–September 1989 at Pune (18°32 N, 73°51 E, 559m AMSL), India. The top of the nocturnal groundbased inversion is taken as the height above ground where the negative vertical gradient in aerosol concentration first reaches a maximum. During the post-sunset period over this station, this height is as low as 160m and frequently lies around 550m. Greater heights are observed in pre-monsoon months and smaller ones during the southwest monsoon season. Positive vertical gradients in aerosol concentration, indicative of stable/elevated layers, appear frequently around 750m. Temporal variations of aerosol concentration gradients in two adjacent air layers, 920–1000m and 100–1100m, provide evidence that stability increases downward in the early night hours.     

An observational study of heat fluxes and their relationships with net radiation

Experimental evidence indicates that the diurnal behaviour of the fluxes of heat into the ground and into the atmosphere versus the net flux of radiation can be modelled by closed curves, the hourly values folowing one another in either a clockwise or counter clockfashion. A general formulation to express the different heat fluxes as a function of net radiation is proposed. This relationship between the different heat fluxes and can be expressed as a sum of three terms: the first indicates a direct proportionality to , the second gives the deviation from linearity and depends on /t, and the third gives the value of the flux when = 0. The formulae are then expressed as a function of time and the ratios between the heat fluxes and are evaluated. A comparison with the approximations generally used shows that the latter may be considered as particular cases of the more general equations proposed here.