| 1. |
- Bengtsson, Lars
(author)
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Effective concentration time for design storms in complex urban basins
- 1991
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In: Atmospheric Research. - 0169-8095. ; 27, s. 137-150
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Journal article (peer-reviewed)abstract
- Analytical solutions for runoff caused by a moving rain storm of time-varying rain intensity from a complex drainage basin are derived assuming constant concentration times for the different systems within the basin. It is found that the runoff caused by a peaked rain intensity distribution is higher than that from a uniform distribution of the same mean intensity. It is also shown that the storm movement influences the runoff peak, especially in elongated basins. Correction factors for the non-uniform rain intensity distribution and for the storm movement are introduced allowing the rational formula to be used in its traditional way for the first design of a storm drainage system. The concept of effective kinematic time of concentration is introduced to be used in the rational formula.
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| 2. |
- Bower, K. N., et al.
(author)
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The Great Dun Fell experiment 1995 : An overview
- 1999
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In: Atmospheric Research. - 0169-8095. ; 50:3-4, s. 151-184
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Journal article (peer-reviewed)abstract
- During March and April of 1995 a major international field project was conducted at the UMIST field station site on Great Dun Fell in Cumbria, Northern England. The hill cap cloud which frequently envelopes this site was used as a natural flow through reactor to examine the sensitivity of the cloud microphysics to the aerosol entering the cloud and also to investigate the effects of the cloud in changing the aerosol size distribution, chemical composition and associated optical properties. To investigate these processes, detailed measurements of the cloud water chemistry (including the chemistry of sulphur compounds, organic and inorganic oxidised nitrogen and ammonia), cloud microphysics and properties of the aerosol and trace gas concentrations upwind and downwind of the cap cloud were undertaken. It was found that the cloud droplet number was generally strongly correlated to aerosol number concentration, with up to 2000 activated droplets cm-3 being observed in the most polluted conditions. In such conditions it was inferred that hygroscopic organic compounds were important in the activation process. Often, the size distribution of the aerosol was substantially modified by the cloud processing, largely due to the aqueous phase oxidation of S(IV) to sulphate by hydrogen peroxide, but also through the uptake and fixing of gas phase nitric acid as nitrate, increasing the calculated optical scattering of the aerosol substantially (by up to 24%). New particle formation was also observed in the ultrafine aerosol mode (at about 5 nm) downwind of the cap cloud, particularly in conditions of low total aerosol surface area and in the presence of ammonia and HCl gases. This was seen to occur at night as well as during the day via a mechanism which is not yet understood. The implications of these results for parameterising aerosol growth in Global Climate Models are explored.
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| 3. |
- Kawamura, Akira, et al.
(author)
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Parameterization of rain cell properties using an advection-diffusion model and rain gage data
- 1996
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In: Atmospheric Research. - : Elsevier BV. - 0169-8095. ; 42:1-4, s. 67-73
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Journal article (peer-reviewed)abstract
- To reduce flooding risks and improve urban drainage management, there is a need to increase the forecasting accuracy for rainfall models on small typical urban time and space scales. Increased rainfall forecasting accuracy will in turn improve runoff prediction and thus, prevent flooding hazards, decrease pollution discharge through combined sewers, increase waste water treatment efficiency, etc. For this purpose, we analyzed the parameters of a two-dimensional stochastic advection-diffusion model including a Fourier domain method and an extended Kalman filter algorithm for investigation of motion, shape, size, and intensity distribution of convective rainfall. The resulting set of model parameters (advective velocity, apparent turbulent diffusion, and development/decay of rainfall rate) is used to study convective rainfall variability. It appears that the speed at which the rainfall cell is advected is not dependent on the cell development stage or apparent diffusion. Instead, there is a dependence between the source/sink term and apparent diffusion. This can be explained by the turbulent updraft of warm air which results in large rainfall intensity increase. This strong turbulence results in larger diffusion (and vice versa). The behavior of the model parameters is therefore physically explainable and relevant. The results can be used as first choice of parameter values when modeling convective rainfall over ungaged areas.
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| 4. |
- Martinsson, Bengt G., et al.
(author)
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Droplet nucleation and growth in orographic clouds in relation to the aerosol population
- 1999
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In: Atmospheric Research. - 0169-8095. ; 50:3-4, s. 289-315
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Journal article (peer-reviewed)abstract
- The formation and development of orographic clouds was studied in a field experiment comprising several measurement sites at a mountain ridge. The influence of the aerosol population present on the cloud microstructure was studied in relation to the dynamics in the cloud formation. Droplet nucleation scavenging was investigated by the introduction of a non-dimensional particle diameter related to the process, and it was found that the scavenging rose rapidly in a relatively narrow particle size interval. The size dependency of the scavenging could partly be explained by external mixture of the aerosol. The large particles in the cloud interstitial aerosol was found to be of a chemical nature which allows for only a very weak uptake of water, implying that the chemical composition of these particles rather than entrainment of dry air prevented the droplet nucleation. The aerosol particle number concentration was found to strongly influence the cloud microstructure. Droplet number concentrations up to approximately 2000 cm-3 were observed together with a substantially reduced effective droplet diameter. The observed effect of elevated particle number concentrations in orographic clouds was generalised to the climatologically more important stratiform clouds by the use of a cloud model. It was found that the microstructure of stratiform clouds was strongly dependent on the aerosol population present as well on the dynamics in the cloud formation.
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| 5. |
- Niemczynowicz, Janusz, et al.
(author)
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What practioners need from theoreticans
- 1996
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In: Advances in Atmospheric Sciences. - : Elsevier BV. ; 42, s. 5-18
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Journal article (peer-reviewed)abstract
- The results of any hydrological calculation are highly dependent on rainfall data used. The spatial variability of precipitation, that is very substantial even on very small catchments, can never be completely revealed by a network of gauges. Equally, the time variability and extremes, sampled in a set of data with limited duration, are seldom adequately revealed. Thus, we deal with rainfall data which are inadequate for the problem in question. Applied practice in rainfall-runoff calculations and modelling sometimes contradict with known physical phenomena. The dynamics of a rainfall field are not considered in most practical applications. Recent significant progress in theoretical understanding and description of rainfall process has only limited influence on common practice. Fractal theory and related different methods of data processing seems to have a great potential to provide such an increase of knowledge
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| 6. |
- Swietlicki, Erik, et al.
(author)
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A closure study of sub-micrometer aerosol particle hygroscopic behaviour
- 1999
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In: Atmospheric Research. - 0169-8095. ; 50:3-4, s. 205-240
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Journal article (peer-reviewed)abstract
- The hygroscopic properties of sub-micrometer aerosol particles were studied in connection with a ground-based cloud experiment at Great Dun Fell, in northern England in 1995. Hygroscopic diameter growth factors were measured with a Tandem Differential Mobility Analyser (TDMA) for dry particle diameters between 35 and 265 nm at one of the sites upwind of the orographic cloud. An external mixture consisting of three groups of particles, each with different hygroscopic properties, was observed. These particle groups were denoted less-hygroscopic, more-hygroscopic and sea spray particles and had average diameter growth factors of 1.11-1.15, 1.38-1.69 and 2.08-2.21 respectively when taken from a dry state to a relative humidity of 90%. Average growth factors increased with dry particle size. A bimodal hygroscopic behaviour was observed for 74-87% of the cases depending on particle size. Parallel measurements of dry sub-micrometer particle number size distributions were performed with a Differential Mobility Particle Sizer (DMPS). The inorganic ion aerosol composition was determined by means of ion chromatography analysis of samples collected with Berner-type low pressure cascade impactors at ambient conditions. The number of ions collected on each impactor stage was predicted from the size distribution and hygroscopic growth data by means of a model of hygroscopic behaviour assuming that only the inorganic substances interacted with the ambient water vapour. The predicted ion number concentration was compared with the actual number of all positive and negative ions collected on the various impactor stages. For the impactor stage which collected particles with aerodynamic diameters between 0.17-0.53 μm at ambient relative humidity, and for which all pertinent data was available for the hygroscopic closure study, the predicted ion concentrations agreed with the measured values within the combined measurement and model uncertainties for all cases but one. For this impactor sampling occasion, the predicted ion concentration was significantly higher than the measured. The air mass in which this sample was taken had undergone extensive photochemical activity which had probably produced hygroscopically active material other than inorganic ions, such as organic oxygenated substances.
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| 7. |
- Blas, M., et al.
(author)
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Temporal and spatial variations of fog in the Western Sudety Mts., Poland
- 2002
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In: Atmospheric research. - 0169-8095 .- 1873-2895. ; 64:04-jan, s. 19-28
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Journal article (peer-reviewed)abstract
- The ridges of the Western Sudety are well exposed to the humid maritime air masses that are mainly associated with westerly atmospheric circulation. Fog is the most frequently observed atmospheric phenomenon, being present on average 45% of the time, with 250-300 days with fog per year. This study on temporal variation of fog in both daily and annual cycles is based on 30 years of measurements (1961 - 1990) made on Mt. Szrcnica at 1362 m a.s.l. Based on the data from 51 meteorological stations and the results of fog deposition field measurements, a digital model of fog frequency in the Western Sudety was prepared using a Geographic Information System. The model shows that altitude is not the only important factor controlling fog frequency. Such parameters as slope aspect and position of a particular site in relation to local and regional morphology are also of crucial importance.
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| 8. |
- Ahmad, Noor Azlinda, 1978-, et al.
(author)
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Some features of electric field waveform of Narrow Bipolar Pulses
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In: Atmospheric research. - 0169-8095 .- 1873-2895.
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Journal article (peer-reviewed)abstract
- Narrow Bipolar Pulses (NBPs) are generated by intra-cloud discharge processes and they are of interest due to their strong broadband and high frequency (HF) emissions. In this study, we present some features of electric field waveform of NBPs which have not been reported in the literature. The HF emission was observed to begin simultaneously with the onset of NBPs indicating no streamers or stepped-leader process was taking place before the initiation of NBPs. The electric field waveforms of NBPs were characterized by many fine peaks embedded intermittently on the rising and decaying edge of NBPs suggesting that some small scale electrical discharges were involved during the formation of NBPs.
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| 9. |
- Azorin-Molina, Cesar, et al.
(author)
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Biases in wind speed measurements due to anemometer changes
- 2023
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In: Atmospheric Research. - 0169-8095. ; 289
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Journal article (peer-reviewed)abstract
- This research presents a case study of the biases and discontinuities that were introduced in observed long-term mean wind-speed and gust data-series due to anemometer changes in a meteorological station in northern Spain, operated by the Spanish State Meteorological Agency: San Sebastian-Igueldo. Field and wind-tunnel experiments with predefined conditions have been presented in the literature, however this research uses a real case study to assess the impact of anemometer changes on wind speed measurements due to three factors being: (i) the 3-cup anemometer model (SEAC vs. THIES companies); (ii) sensor height (∼19.95 m vs. ∼20.45 m) and (iii) sensor age (20-years old vs. new). Our results show (a) substantial biases in the measured wind speed and daily peak wind gusts, with the new THIES anemometer reporting stronger surface winds than the old SEAC anemometer; (b) opposing biases under weak (negative) and moderate-strong (positive) winds; and (c) significant breakpoints in the long-term wind data-series, which highlight the importance of data homogenization. National Weather Services and climate assessment groups will benefit from these findings since errors in wind speed and gust measurements can be minimized by implementing systematic observation protocols. Robust anemometer observations provide a basis for accurate quantification of the magnitude of changes and the variability of surface winds.
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| 10. |
- Azorin-Molina, Cesar, et al.
(author)
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Evaluating anemometer drift: A statistical approach to correct biases in wind speed measurement
- 2018
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In: Atmospheric research. - : Elsevier BV. - 0169-8095. ; 203, s. 175-188
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Journal article (peer-reviewed)abstract
- Recent studies on observed wind variability have revealed a decline (termed “stilling”) of near-surface wind speed during the last 30–50 years over many mid-latitude terrestrial regions, particularly in the Northern Hemisphere. The well-known impact of cup anemometer drift (i.e., wear on the bearings) on the observed weakening of wind speed has been mentioned as a potential contributor to the declining trend. However, to date, no research has quantified its contribution to stilling based on measurements, which is most likely due to lack of quantification of the ageing effect. In this study, a 3-year field experiment (2014–2016) with 10-minute paired wind speed measurements from one new and one malfunctioned (i.e., old bearings) SEAC SV5 cup anemometer which has been used by the Spanish Meteorological Agency in automatic weather stations since mid-1980s, was developed for assessing for the first time the role of anemometer drift on wind speed measurement. The results showed a statistical significant impact of anemometer drift on wind speed measurements, with the old anemometer measuring lower wind speeds than the new one. Biases show a marked temporal pattern and clear dependency on wind speed, with both weak and strong winds causing significant biases. This pioneering quantification of biases has allowed us to define two regression models that correct up to 37% of the artificial bias in wind speed due to measurement with an old anemometer.
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