Variation on PAR to global solar radiation ratio along altitude gradient in Naeba Mountain

Summary We investigated the ratio of photosynthetically active photon flux (Q _p ) to global solar radiation (R _s ) at three sites along different altitudes in Naeba Mountain, Japan at various temporal scales based on 3 years measurement data (1999–2001). The lowest values of the ratio ever reported were found for all sites on both an hourly and a daily scale. A similar slight diurnal pattern was found for all sites based on the monthly mean hourly values of the ratio. However, different sites exhibited different seasonal courses. Statistically significant altitude dependencies were found for the hourly Q _p /R _s under both clear and cloudy weather conditions (t-test, P < 0.001). For clear weather conditions, the hourly Q _p /R _s exhibited an increasing trend with altitude at an average rate of 3.6% per km. The increasing rate was more noticeable below 900 m (8.8% per km) than above (0.7% per km). The inverse trend was found for hourly Q _p /R _s with the altitude under cloudy weather conditions. The hourly Q _p /R _s decreased from 550 m to 1500 m at a rate of 1.8% per km. Again, a major decrease occurred below 900 m, which had the rate of 4.2% per km, compared with 0.2% per km over 900 m. Although the same tendencies were noted for daily Q _p /R _s , under clear sky conditions, they were not as statistically significant as the hourly counterpart (t-test, P < 0.021). The increasing rate of Q _p /R _s at this scale under clear weather conditions was near that of the hourly rate, but below the 900 m rate was reduced to near half of the hourly rate (4.7% per km). And the rate over 900 m increased to 2.7% per km. On the other hand, statistically significant altitude effect was noted for the daily under cloudy weather conditions (t-test, P < 0.002). A rapidly decreasing rate was found for it along the altitude gradient. The reason was due to the large decreasing rate below 900 m (12.4% per km). But again similar to the hourly Q _p /R _s , a very small decreasing rate of daily Q _p /R _s was found over 900 m under cloudy weather conditions. These results suggest the necessity of considering the altitude dependency of Q _p /R _s in future studies.     

The impact of the PBL scheme and the vertical distribution of model layers on simulations of Alpine foehn

Summary This paper investigates the influence of the planetary boundary-layer (PBL) parameterization and the vertical distribution of model layers on simulations of an Alpine foehn case that was observed during the Mesoscale Alpine Programme (MAP) in autumn 1999. The study is based on the PSU/NCAR MM5 modelling system and combines five different PBL schemes with three model layer settings, which mainly differ in the height above ground of the lowest model level (z ₁). Specifically, z ₁ takes values of about 7 m, 22 m and 36 m, and the experiments with z ₁ = 7 m are set up such that the second model level is located at z = 36 m. To assess if the different model setups have a systematic impact on the model performance, the simulation results are compared against wind lidar, radiosonde and surface measurements gathered along the Austrian Wipp Valley. Moreover, the dependence of the simulated wind and temperature fields at a given height (36 m above ground) on z ₁ is examined for several different regions. Our validation results show that at least over the Wipp Valley, the dependence of the model skill on z ₁ tends to be larger and more systematic than the impact of the PBL scheme. The agreement of the simulated wind field with observations tends to benefit from moving the lowest model layer closer to the ground, which appears to be related to the dependence of lee-side flow separation on z ₁. However, the simulated 2 m-temperatures are closest to observations for the intermediate z ₁ of 22 m. This is mainly related to the fact that the simulated low-level temperatures decrease systematically with decreasing z ₁ for all PBL schemes, turning a positive bias at z ₁ = 36 m into a negative bias at z ₁ = 7 m. The systematic z ₁-dependence is also observed for the temperatures at a fixed height of 36 m, indicating a deficiency in the self-consistency of the model results that is not related to a specific PBL formulation. Possible reasons for this deficiency are discussed in the paper. On the other hand, a systematic z ₁-dependence of the 36-m wind speed is encountered only for one out of the five PBL schemes. This turns out to be related to an unrealistic profile of the vertical mixing coefficient. Correspondence: Günther Z?ngl, Meteorologisches Institut der Universitat München, 80333 München, Germany     

Sampling of Chemical Constituents in Electrically Active Convective Systems: Results and Cautions

On flights of a P3-B turboprop and a WB-57F jet aircraft within thunderstorm systems, short term spikes (1–2 sec or less in duration) in NO and O₃ were recorded and are attributed to hot or cold discharges occurring on the aircraft fuselage or air sampling inlets. One such spike of only ∼300 msec duration reached 325 ppbv of NO on the flight of the jet aircraft while at 13.8 km altitude. In a lightning flash to the P3-B aircraft, production of NO (expected) and NO₂ (unexpected) were directly observed. The NO production was ∼ 1.7 x 10²² molecules/m of flash length. In the P3-B flight at 5.5 km altitude and over a distance of ∼ 275 km within a highly electrically active thunderstorm complex in the equatorial central Pacific Ocean, there was no evidence of production of O₃ or CO by lightning flashes or by any type of hot or cold discharge involved in the development of free-air lightning flashes.     

Airborne measurements of turbulent fluxes during LITFASS-98: Comparison with ground measurements and remote sensing in a case study

Summary ?Simultaneous flight measurements with the research aircraft Do 128 and the helicopter-borne turbulence probe Helipod were performed on 18 June 1998 during the LITFASS-98 field experiment. The area-averaged turbulent vertical fluxes of momentum, sensible, and latent heat were determined on a 15 km × 15 km and a 10 km × 10 km flight pattern, respectively. The flights were carried out over heterogeneous terrain at different altitudes within a moderately convective boundary layer with Cumulus clouds. Co-spectra-analysis demonstrated that the small scale turbulent transport was completely sampled, while the comparatively small flight patterns were possibly of critical size regarding the large-scale turbulence. The phygoide of the airplane was identified as a significant peak in some co-spectra. The turbulent fluxes of momentum and sensible heat at 80 m above the ground showed systematic dependence on the location of the flight legs above the heterogeneous terrain. This was not observed for the latent heat flux, probably due to the vertical distribution of humidity in the boundary layer. Statistical error analysis of the fluxes F showed that the systematic statistical error ΔF was one order of magnitude smaller than the standard deviation σ _F . The difference between area-averaged fluxes derived from simultaneous Helipod and Do 128 measurements was much smaller than σ _F , indicating that the systematic statistical error was possibly over-estimated by the usual method. In the upper half of the boundary layer the airborne-measured sensible heat flux agreed well with windprofiler/RASS data. A linear fit was the best approximation for the height dependence of all three fluxes. The linear extrapolations of the latent and sensible heat fluxes to the ground were in good agreement with tower, scintillometer, and averaged ground-station measurements on various surface types. Systematic discrepancies between airborne and ground-based measurements were not found. Received June 18, 2001; revised December 21, 2001; accepted June 3, 2002     

Microclimatological characteristics of a miscanthus (Miscanthus cv. giganteus) stand during stable conditionsat night in the nonvegetative winter period

Summary ?Microclimatological data obtained during a field experiment in the nongrowing winter period were used to study the microclimatologically stable night conditions of a 200 × 150 m miscanthus (Miscanthus cv. giganteus) stand and compared to open field conditions. The microclimatological pattern within the miscanthus canopy was characterized by long-wave radiative cooling of the plant stand and by an established temperature inversion within the canopy at calm nights. The results show that there are significant differences in air temperature and energy balance components between the open field and the miscanthus field during calm and clear nights. In general, net radiation difference during the cold and calm nights was relatively constant and about 20 W m⁻² less negative in miscanthus (because of lower surface temperatures) than at the open field. Air temperature differences also remained fairly constant and were up to 3 °C lower than at the open field (at the height of 1 m). Through thermal inversion cold air accumulated in the lower parts of the canopy as shown by the vertical air temperature profiles. They showed a greater amplitude within the diurnal cycle in the miscanthus stand than in the open field. Through the onset of wind, temperature profiles changed rapidly and differences diminished. Vertical katabatic air drainage into the canopy layers was estimated indirectly by using the energy balance approach. It was calculated from the significant energy balance closure gap and showed a mean air exchange rate of up to 22 m³ m⁻² h⁻¹, related to a stand volume of 1 m² area and 4 m height, during the mostly calm and clear nights, depending on the canopy net radiation and turbulent heat exchange forced by slight wind spells. Quantitative uncertainties in calculated cold air drainage which are introduced by the measurement method and certain assumptions in the calculations, were considered in a sensitivity analysis. In spite of these uncertainties evidence of katabatic cold air flow is given. Received July 29, 1999; revised June 11, 2001; accepted March 14, 2002     

A Study of the Internal Boundary Layer due to a Roughness Change in Neutral Conditions Observed During the LINEX Field Campaigns

Summary As an aspect of the LINEX field studies (1996–1997; Lindenberg near Beeskow, Germany), the characteristics of the internal boundary layer (IBL) that is associated with a step change of the surface roughnesses in neutral constant stress layers was investigated and is reported in this paper. Both smooth to rough (in 1996) and rough to smooth (in 1997) types of flow, have been studied based upon the profiles of mean wind and temperature realised from a 10-m mast and eddy correlation measurements taken at two levels (2 m and 5 m). Depending upon wind direction, the fetch at the site varied between 140 m and 315 m within the wind sector (200° to 340°) used for the field investigations. The height of the IBL, δ, had been determined from the intersect of the logarithmic wind-profiles below (< 2 m) and above (> 6 ) the interface. Values of δ obtained at the experimental site compared fairly well to the existing theoretical/empirical fetch-height relationships of the form: δ=aċx ^b , where a, b, are empirical constants. The ratio for the friction velocities below and above the IBL as measured directly by the eddy correlation techniques showed that for fetches less than 250 m there was an increase (decrease) of about 20% of the momentum flux arising from the smooth to rough (rough to smooth) transitions. Influences of distant obstructions (e.g., bushes, pockets of trees) on the surface flow were markedly important on the examined wind profiles and such can be indicative as multiple IBLs. Received September 1, 1997 Revised August 5, 1998     

Regional effects of large-scale extreme wind events over orographically structured terrain

Summary ?Above orographically structured terrain considerable differences of the regional wind field may be identified during large-scale extreme wind events. So far, these regional differences could not be resolved by climate models. To determine the relationships between large-scale atmospheric conditions, the influence of orography, and the regional wind field, data measured in the upper Rhine valley within the framework of the REKLIP Regional Climate Project were analyzed and calculations were made using the KAMM mesoscale model. In the area of the upper Rhine valley, ratios of the wind velocity in the Rhine valley at 10 m above ground level, ν_val, and the large-scale flow velocity, ν_lar, are between ν_val/ν_lar ≈ 0.1 and ν_val/ν_lar ≈ 1. The ν_val/ν_lar ratio exhibits a strong dependence on thermal stratification, δ, and decreases from ν_val/ν_lar ≈ 1 at δ = 0 K m⁻¹ to ν_val/ν_lar ≈ 0.2 at δ = 0.0075 K m⁻¹. In areas, where the lateral mountainous border of the Rhine valley is interrupted, the ν_val/ν_lar ratio increases again with increasing stability or decreasing Froude number. This is obviously due to flow around the Black Forest under stable stratification. It is demonstrated by model calculations that a complex wind field develops in the Rhine valley at small Froude numbers (Fr < 1) irrespective of the direction of large-scale flow. The ν_val/ν_lar ratio is characterized by small values in the direct lee side (ν_val/ν_lar ≈ 0.2) and high values on the windward side of the lateral mountainous border of the Rhine valley (ν_val/ν_lar ≈ 0.8). Received October 22, 2001; revised June 18, 2002; accepted June 23, 2002     

Sea-breeze Front Variations in Space and Time

Summary  This paper is a contribution to experimental meteorology: A sea-breeze front was investigated by aircraft observations and thorough numerical analysis using an unprecedented number of runs crossing the same front within a timespan of . The 33 runs were flown in a situation of offshore geostrophic wind of 5 m/s in 1000 hPa and with the strategy of obtaining information on the four-dimensional field (t=time, x=cross-coastal coordinate, y=coast-parallel coordinate, z=height): 9 runs in x-direction (and reverse) at different heights to yield x,z-cross-sections of the observed meteorological quantities (specific humidity q, potential temperature Θ and the components u, v and w of the wind velocity), assuming a frozen structure in time; the next 7 runs again in x-direction but all at the same level and on the same track to yield x,t-diagrams of the same quantities in order to study the temporal changes compared to those with x and z; the next 10 runs as a zig-zagging flight track crossing the front but drifting in y-direction, all at the same height, in order to obtain the y-dependency; andfinally 7 runs for another x,z-cross-sectional analysis, which can be compared to that evaluated from the runs at the beginning of the mission. The paper describes the 4-dimensional dependencies in detail. Pure x-variations at constant z are expressed by VCM low-pass filtered space series (VCM=variance conserving multiresolution, according to Howell and Mahrt, 1994). The x,z-analyses are similar to those in Kraus et al. (1990) and Finkele et al. (1995) verifying these results. The comparison of the x,z-studies gained from the data at the beginning and at the end of the mission show how the sea-breeze frontal area changes its structure. The fluctuations (in time) revealed by the low-pass filtered x,t-runs (same track and same height) are smaller than the contour intervals chosen in the x,z-cross-sections. This shows, that the single runs, from which the x,z-cross-sections are constructed, reliably and significantly contribute to the interpolated structure. The paper also demonstrates the overall development of the front within the 3^1/2 h of continuous observation. The x,y-fields demonstrate that the y-dependency of the various quantities is generally one order of magnitude smaller than the x-dependency and that the assumption of negligible y-dependency holds in the first order of approximation for a fairly homogeneous coast. Convective disturbances of a horizontal scale of 1 to 4 km at the landward side of the front, embedded in the offshore flow and bouncing against the landward propagating sea-breeze front, considerably contribute to variations of the frontal propagation speed and of the frontal shape and also to changes of the parameters with the along-frontal coordinate y. Received April 24, 1998 Revised November 3, 1998     

Local foehn effects in the upper Isar Valley, Part 2: numerical simulations

Summary The local wind system in the upper Isar Valley (Bavarian Alps) near Mittenwald has the peculiarity that regularly strong foehn-like nocturnal flows occur, mainly during clear nights in autumn and winter. We will refer to this phenomenon as “Minifoehn”, as its properties are similar to the classical deep foehn in the sense that its breakthrough into the Isar Valley usually brings a striking increase in temperature and a concomitant decrease in relative humidity. Numerical simulations with the MM5 model reveal that this phenomenon is related to a nocturnal drainage flow originating from a plateau south of Mittenwald. This flow is driven by the temperature difference between this plateau (1180 m) and the free atmosphere above Mittenwald (920 m, 15 km north of the plateau) at the same level. The air masses flow through two different valleys that merge again further downstream. The upper part of one of the two drainage currents goes over a small mountain ridge (1180 m) south-west of Mittenwald and then descends into the Isar Valley, leading to an advection of potentially warm air towards Mittenwald. This branch of the drainage current constitutes the Minifoehn. The remaining part of the drainage current flows through a narrow gap towards the Isar Valley and then joins the drainage flow of this valley. As these air masses are significantly cooler than the Minifoehn branch, large horizontal temperature gradients can be found around Mittenwald. The dynamical behaviour of the cold air flow turns out to be qualitatively consistent with shallow-water theory only in the absence of a forcing by large-scale winds. Otherwise, gravity-wave induced pressure perturbations interact with the drainage flow and modify the low-level flow field. The simulations show that the gravity waves are excited by the mountain range that separates the two valleys mentioned above. Moreover, the simulations indicate that the structure of this nocturnal wind system is not very sensitive to the direction of synoptic-scale winds as long as they come from the southern sector. On the other hand, ambient northerly winds are able to prevent the drainage flow and therefore the local foehn effects in the Isar Valley provided that synoptic winds are strong enough. The results of the MM5 simulations are in good agreement with the measurements and observations described in part 1 of this study.     

Vertical Structure of Selected Turbulence Characteristics above an Urban Canopy

Summary In this paper the results of an urban measurement campaign are presented. The experiment took place from July 1995 to February 1996 in Basel, Switzerland. A total of more than 2000 undisturbed 30-minute runs of simultaneous measurements of the fluctuations of the wind vector u′, v′, w′ and the sonic temperature θ _s ′ at three different heights (z=36, 50 and 76 m a.g.l.) are analysed with respect to the integral statistics and their spectral behaviour. Estimates of the zero plane displacement height d calculated by the temperature variance method yield a value of 22 m for the two lower levels, which corresponds to 0.92 h (the mean height of the roughness elements). At all three measurement heights the dimensionless standard deviation σ _w /u _* is systematically smaller than the Monin-Obukhov similarity function for the inertial sublayer, however, deviations are smaller compared to other urban turbulence studies. The σ_θ/θ_* values follow the inertial sublayer prediction very close for the two lowest levels, while at the uppermost level significant deviations are observed. Profiles of normalized velocity and temperature variances show a clear dependence on stability. The profile of friction velocity u _* is similar to the profiles reported in other urban studies with a maximum around z/h=2.1. Spectral characteristics of the wind components in general show a clear dependence on stability and dimensionless measurement height z/h with a shift of the spectral peak to lower frequencies as thermal stability changes from stable to unstable conditions and as z/h decreases. Velocity spectra follow the −2/3 slope in the inertial subrange region and the ratios of spectral energy densities S _w (f)/S _u (f) approach the value of 4/3 required for local isotropy in the inertial subrange. Velocity spectra and spectral peaks fit best to the well established surface layer spectra from Kaimal et al. (1972) at the uppermost level at z/h=3.2. Received September 26, 1997 Revised February 15, 1998     

Mesoscale convective systems along the Meiyu frontin a numerical model

Summary  Two organized mesoscale convective systems (MCSs) developed sequentially along the Meiyu front over the Yangzi-Huai River basin and caused severe flooding over eastern China during 12–13 June 1991. In this paper, the structure and evolution of these MCSs are studied with a high-resolution (18 km) numerical simulation using the Fifth Generation Penn-State/NCAR Mesocale Model (MM5). The model reproduced the successive development of these two MCSs along the Meiyu front. The evolution of these MCSs was recorded clearly on satellite-derived cloud-top black body temperature (T _bb ) maps. A mesoscale low-level jet (mLLJ) and a mesoscale upper-level jet (mULJ) were simulated, respectively, to the south and east of each of these two MCSs. Our analyses shows that the mLLJ and mULJ were formed as a responses to the intense convection associated with the MCS. The mLLJs transported warm, moist air with equivalent potential temperature greater than 352 K into the MCSs, and strong low-level convergence can be identified on the left-front end of the mLLJ. This strong convergence was associated with intense upward motion in the MCS with speed up to 80 cm s⁻¹. Much of inflow into the MCSs extends up to the middle and upper troposphere, and ventilated through the mULJ. The development of the MCSs was also associated with substantial increase in potential vorticity (PV). The build up of PV in the lower-level along the Meiyu front was in turn related to a local intensification of the frontal equivalent potential temperature gradient, suggesting a relationship between the MCSs and the local enhancement and cyclogenesis of the front. In a sensitivity experiment without the effect of latent heating, a series of ascent centers with average separation of about 300 km were simulated. This result suggests that the initial formation of the MCSs along the Meiyu front could occur in absence of moist-diabatic process. Since the horizontal velocity gradient across the Meiyu front near the synoptic-scale low-level jet (LLJ) was quite large while the corresponding temperature gradient across the frontal zone was rather weak, we speculate that barotropic process may be responsible for triggering these MCSs along the Meiyu front. Received December 28, 1999 Revised May 11, 2000     

Mesoscale modeling of katabatic winds over Greenland and comparisons with AWS and aircraft data

Summary Simulations of the katabatic wind system over the Greenland ice sheet for the two months April and May 1997 were performed using the Norwegian Limited Area Model (NORLAM) with a horizontal resolution of 25 km. The model results are intercompared and validated against observational data from automatic weather stations (AWS), global atmospheric analyses and instrumented aircraft observations of individual cases during that period. The NORLAM is able to simulate the synoptic developments and daily cycle of the katabatic wind system realistically. For most of the cases covered by aircraft observations, the model results agree very well with the measured developments and structures of the katabatic wind system in the lowest 400 m. Despite NORLAM’s general ability of reproducing the four-dimensional structure of the katabatic wind, problems occur in cases, when the synoptic background is not well captured by the analyses used as initial and boundary conditions for the model runs or where NORLAM fails to correctly predict the synoptic development. The katabatic wind intensity in the stable boundary layer is underestimated by the model in cases when the simulated synoptic forcing is too weak. An additional problem becomes obvious in cases when the model simulates clouds in contrast to the observations or when the simulated clouds are too thick compared to the observed cloud cover. In these cases, the excessive cloud amount prevents development of the katabatic wind in the model. Received September 22, 2000/Revised March 16, 2001     

A Measurement Based Sensitivity Analysis of the Penman-Monteith Actual Evapotranspiration Model for Crops of Different Height and in Contrasting Water Status

Summary This paper presents a study of the sensibility of the Penman-Monteith evapotranspiration model to climatic (available energy and vapour pressure deficit) and parametric (aerodynamic and canopy resistances, r _a and r _c respectively) factors in a semi-arid climate, for crops in contrasting water status (well irrigated and under water stress) and of different heights. Three experiments were carried out in southern Italy on reference grass (≈ 0.1 m), grain sorghum (≈ 1 m) and sweet sorghum (≈ 3 m). For this analysis the sensitivity coefficients, taken as hourly means, were evaluated during the growth season when the crops completely covered the soil. The relative errors on evapotranspiration were also evaluated for r _a and r _c . The results showed that, for reference grass, available energy and aerodynamic resistance play a major role. For crops under water stress the most important term to evaluate is canopy resistance. For a tall crop, as sweet sorghum, the role of the vapour pressure deficit is fundamental, both when the crop is in good water status and under water stress. Received July 14, 1997 Revised February 5, 1998     

Seasonal variations of gravity waves revealed in rawinsonde data at Pohang,Korea

Summary Seasonal variations of gravity wave characteristics are investigated using rawinsonde data observed at Pohang observatory, Korea (36°2′N, 129°23′E) during the one-year period of 1998. Analysis is carried out for two atmospheric layers representing the troposphere (2–9 km) and stratosphere (17–30 km). There exist clear seasonal variations in amplitudes of temperature and wind perturbations and wave energy in the stratosphere, with their maxima in wintertime and minima in summertime. A strong correlation is found between the wave activity and the strength of the jet stream, but there is no clear correlation between the wave activity and the vertical gradient of static stability. The intrinsic frequency and vertical and horizontal wavelengths of gravity waves in the stratosphere are 2f–3f, where f is the Coriolis parameter, and 2–3 km and 300–500 km, respectively. The intrinsic phase velocity directs westward in January and northeastward in July. The vertical flux of the stratospheric zonal momentum is mostly negative except in summertime with a maximum magnitude in January. Topography seems to be a major source of stratospheric gravity waves in wintertime. Convection can be a source of gravity waves in summertime, but it is required to know convective sources at nearby stations, due to their intermittency and locations relative to floating balloons.     

Mean Characteristics of the Katabatic Flow of a 1024 m High Knife Edge Mountain

Summary In this study an attempt is made to examine and analyse the mean characteristics of the katabatic flows at the western slope foot of a 1024 m high knife edge mountain using a meteorological tower and three surface meteorological stations. In addition, the frequency distribution of the occurrence of the katabatic flow over one year period is studied along the characteristics of the flow arriving in the neighbouring urban area at a distance of 1.5 km. It was found that the katabatic flow occurs mainly in autumn and spring with the highest frequency in April. The flow is generally characterised by small depth as it is affected substantially by the background flow. The expected direction of the katabatic wind dominates mainly at the level of 7 m, where the influence of the background flow is minimised. At the level of 18 m the wind direction shifts, due to the interaction of the katabatic wind with the background flow. The katabatic flow can penetrate at a distance of 1.5 km being substantially weakened. Received September 18, 1996 Revised August 4, 1997     

Measurements of surface ozone at semi-arid site Anantapur (14.62°N, 77.65°E, 331 m asl) in India

In the present study, an attempt has been made to examine the governing photochemical processes of surface ozone (O₃) formation in rural site. For this purpose, measurements of surface ozone and selected meteorological parameters have been made at Anantapur (14.62°N, 77.65°E, 331 m asl), a semi-arid zone in India from January 2002 to December 2003. The annual average diurnal variation of O₃ shows maximum concentration 46 ppbv at noon and minimum 25 ppbv in the morning with 1σ standard deviation. The average seasonal variation of ozone mixing ratios are observed to be maximum (about 60 ppbv) during summer and minimum (about 22 ppbv) in the monsoon period. The monthly daytime and nighttime average surface ozone concentration shows a maximum (55 ± 7 ppbv; 37 ± 7.3 ppbv) in March and minimum (28 ± 3.4 ppbv; 22 ± 2.3 ppbv) in August during the study period. The monthly average high (low) O₃ 48.9 ± 7.7 ppbv (26.2 ± 3.5 ppbv) observed at noon in March (August) is due to the possible increase in precursor gas concentration by anthropogenic activity and the influence of meteorological parameters. The rate of increase of surface ozone is high (1.52 ppbv/h) in March and lower (0.40 ppbv/h) in July. The average rate of increase of O₃ from midnight to midday is 1 ppbv/h. Surface temperature is highest (43–44°C) during March and April months leading to higher photochemical production. On the other hand, relative humidity, which is higher during the rainy season, shows negative correlation with temperature and ozone mixing ratio. It can be seen that among the two parameters are measured, correlation of surface ozone with wind speed is better (R ²=0.84) in compare with relative humidity (R ²=0.66).     

Absorption and fluorescence properties of rainwater during the cold season at a town in Western Portugal

This study aims at evaluating the variability of the optical properties of chromophoric dissolved organic matter (CDOM) of rainwater during the cold season, specifically between Autumn and Winter periods. The spectroscopic characteristics of rainwater samples collected at a town (Aveiro) in western Portugal were assessed by UV-Vis absorbance and three-dimensional excitation-emission matrix (EEM) fluorescence spectroscopies. Rainwater samples showed similar characteristics to those of natural humic substances when analysed by UV-Vis absorbance spectroscopy, but a significant difference was observed in the volume weight average (VWA) of absorbances between Autumn and Winter. In general, the EEM fluorescence spectra of the Autumn and Winter samples disclosed the presence of six fluorophores with different VWA specific fluorescence intensities: three humic-like (λ _excitation/λ _emission ≈ 230/415 nm; 290/415 nm; and 340/415 nm) and three protein-like (λ _excitation/λ _emission ≈ 230/350 nm; 280/340 nm; and 225/300 nm), but one of the humic-like peaks (≈340/415 nm) does not always appear in the EEM fluorescence spectra of the Winter samples. During the cold season, chromophoric compounds are important constituents of rainwater dissolved organic matter and the presence of these highly absorbing and fluorescing compounds may exert a determining effect in atmospheric absorption of solar radiation.     

Vertical Interactions in a Nocturnal Multi-Scale Wind System Influenced by Atmospheric Stability in a Coastal Area

Summary  The winter wind regime of G?teborg, located on the West coast of Sweden, is composed of three different wind systems besides the ambient wind; a nocturnal low level jet (NLLJ), a winter land breeze (WLB) and an urban heat island circulation (UHIC). An inversion divides the air column into two layers, one between 10 – 50 m and one between 50 – 100 m. The UHIC is located in the lower layer, the WLB in the top layer and the NLLJ above the top layer. The intensity of the interacting processes depends on the stability of each layer as calculated from the bulk Richardson number (BRi_low and BRi_high) using continuous data collected during four years (1991 – 94) from two sites (one within and one outside the urban area) and sampled at three levels. In the evening the WLB develops from the ground level and increases in height until after midnight. At about the same time an UHIC develops in the urban area, below the WLB and causing an uplift of the latter. However, at both sites the WLB does not exceed the 100 m level. At this time BRi in both layers are below one resulting in continuous coupling between the WLB, the UHIC layers and the regional wind. Consequently, the exchange of momentum is still effective between all layers and this is highlighted by a change in the wind direction and a regulation of wind-speed to more constant levels. When BRi_high≥1, the layers become frictionally decoupled, as indicated by a return in the wind direction in the top level to the regional wind, and an acceleration of the top wind. The top level then becomes incorporated in to a nocturnal low-level jet (NLLJ) system. The normally acknowledged development of the NLLJ, with a start around sunset, is in this case delayed for several hours at the top level. The reason for this is that there are meso-scale/local wind systems present in layers beneath the jet causing an interaction between the layers. In the morning, when the layers are again coupled the top layer wind is once more influenced by the WLB and therefore changes direction and speed. The local and meso-scale wind systems thus delay the current nocturnal wind development. Received August 24, 1998 Revised March 17, 1999     

Dynamical effects of environmental vertical wind shear on tropical cyclone motion, structure, and intensity

Summary A series of numerical experiments on an f plane are conducted using the fifth-generation Pennsylvania State University-National Center for Atmospheric Research Mesoscale Model, version 3 (MM5) to investigate how environmental vertical wind shear affects the motion, structure, and intensity of a tropical cyclone. The results show that a tropical cyclone has a motion component perpendicular to the vertical shear vector, first to the right of the shear and then to the left. An initially axisymmetric, upright tropical cyclone vortex develops a downshear tilt and wavenumber-one asymmetry when embedded in environmental vertical wind shear. In both small-moderate shears, a storm weakens slightly compared to that in a quiescent environment. The circulation centers between 300 hPa and the surface varies from 20 km to over 80 km. The secondary circulation becomes quite asymmetric about the surface cyclone center. As a result, convection on the upshear-right quadrant diminishes, limiting the upward heat transport in the eyewall and thus lowering the warm core and leading to a weakening of the storm. In strong vertical shear (above 12 m s⁻¹), the vertical tilt exceeds 160 km in 48 h of simulation and the secondary circulation on the upshear side is completely destroyed with low-level outflow. The axisymmetric component of eyewall convection weakens remarkably and becomes much less penetrative. As a result, the warm core becomes weak and appears at lower levels and the storm weakens rapidly accordingly. This up-down weakening mechanism discussed in this study is different from those previously discussed. It emphasizes the penetrative role of eyewall convection in transporting heat from the ocean to the mid-upper troposphere, maintaining the warm core structure of the tropical cyclone. The vertical shear is found negative to eyewall penetrative convection.     

Flux Footprints in the Convective Boundary Layer: Large-Eddy Simulation and Lagrangian Stochastic Modelling

We investigated the flux footprints of receptors at different heights in the convective boundary layer (CBL). The footprints were derived using a forward Lagrangian stochastic (LS) method coupled with the turbulent fields from a large-eddy simulation model. Crosswind-integrated flux footprints shown as a function of upstream distances and sensor heights in the CBL were derived and compared using two LS particle simulation methods: an instantaneous area release and a crosswind linear continuous release. We found that for almost all sensor heights in the CBL, a major positive flux footprint zone was located close to the sensor upstream, while a weak negative footprint zone was located further upstream, with the transition band in non-dimensional upwind distances −X between approximately 1.5 and 2.0. Two-dimensional (2D) flux footprints for a point sensor were also simulated. For a sensor height of 0.158 z _i, where z _i is the CBL depth, we found that a major positive flux footprint zone followed a weak negative zone in the upstream direction. Two even weaker positive zones were also present on either side of the footprint axis, where the latter was rotated slightly from the geostrophic wind direction. Using CBL scaling, the 2D footprint result was normalized to show the source areas and was applied to real parameters obtained using aircraft-based measurements. With a mean wind speed in the CBL of U = 5.1 m s⁻¹, convective velocity of w _* = 1.37 m s⁻¹, CBL depth of z _i = 1,000 m, and flight track height of 159 m above the surface, the total flux footprint contribution zone was estimated to range from about 0.1 to 4.5 km upstream, in the case where the wind was perpendicular to the flight track. When the wind was parallel to the flight track, the total footprint contribution zone covered approximately 0.5 km on one side and 0.8 km on the other side of the flight track.