LLM – a nonhydrostatic model applied to high-resolving simulations of turbulent fluxes over heterogeneous terrain

Summary ?At the Deutscher Wetterdienst (DWD) an internal project named LITFASS was running to determine the representative turbulent fluxes of heat and momentum over heterogeneous land surfaces by observation and simulation. The project took advantage of the infrastructure of the Research Division at the DWD, where model research capacity is combined with the measurements made at and around the Meteorological Observatory Lindenberg. The paper describes the simulation component of the LITFASS-project. It consists of a high-resolving model, derived from the new operational non-hydrostatic, compressible Lokal-Modell (LM), which is denoted LLM (LITFASS-Lokal-Modell). The integration area covers the lower atmosphere in the vertical up to 3000 m with 39 model layers. The horizontal size of the integration area with 145 × 145 grid points (horizontal mesh width Δs = 96.5 m) corresponds to a typical grid box of a meso-scale model. The LLM has to operate under real meteorological conditions. Therefore, the LLM is driven by time-dependent measured vertical profiles of wind, temperature and humidity and surface-based measurements (of radiation, precipitation, soil properties) supported by satellite information. The profiles are available for a great variety of weather situations occurring during the simulation period (1–20 June 1998). First model results from extended 24 hour-integrations against different kinds of measurements are discussed. They reveal the LLM to become a promising validation instrument, from which a systematic, sustainable validation system can be established beyond LITFASS for improving parameterization schemes in the NWP models of the DWD. Received July 18, 2001; revised March 15, 2002; accepted May 30, 2002     

Latent heat fluxes simulated with a non-hydrostatic weather forecast model using actual surface properties from measurements and remote sensing

In this study the influence of land-surface parameters on latent heat fluxes simulated with the numerical weather prediction model Lokalmodell (LM) of the German Meteorological Service is investigated. The area of interest is the LITFASS area during the LITFASS-2003 campaign. Based on simulations with varying soil and vegetation properties, we confirm that simulated latent heat fluxes strongly depend on soil moisture and leaf area index. Both parameters are difficult to obtain from in situ measurements with sufficient spatial resolution over heterogeneous land surfaces. Therefore, a procedure is proposed to determine area average values of soil moisture from time domain reflectometer measurements performed at a limited number of sites. The area averages cover the 7 × 7 km² grid cells of the LM around Lindenberg (south-east of Berlin). Furthermore, satellite inferred plant parameters from NOAA–AVHRR are used to initialise model runs; the derived vegetation parameters show notable differences with those in the standard input of LM. The latent heat fluxes from the LM are compared with the aggregated eddy-covariance-measurements, and while the operational LM shows a strong overestimation of latent heat fluxes, it is demonstrated that the application of land-surface parameters derived from measurements can significantly reduce the deviation between the simulated and measured latent heat fluxes.     

Area-Averaged Surface Fluxes Over the Litfass Region Based on Eddy-Covariance Measurements

Micrometeorological measurements (including eddy-covariance measurements of the surface fluxes of sensible and latent heat) were performed during the LITFASS-2003 experiment at 13 field sites over different types of land use (forest, lake, grassland, various agricultural crops) in a 20 × 20 km² area around the Meteorological Observatory Lindenberg (MOL) of the German Meteorological Service (Deutscher Wetterdienst, DWD). Significant differences in the energy fluxes could be found between the major land surface types (forest, farmland, water), but also between the different agricultural crops (cereals, rape, maize). Flux ratios between the different surfaces changed during the course of the experiment as a result of increased water temperature of the lake, changing soil moisture, and of the vegetation development at the farmland sites. The measurements over grass performed at the boundary-layer field site Falkenberg of the MOL were shown to be quite representative for the farmland part of the area. Measurements from the 13 sites were composed into a time series of the area-averaged surface flux by taking into account the data quality of the single flux values from the different sites and the relative occurrence of each surface type in the area. Such composite fluxes could be determined for about 80% of the whole measurement time during the LITFASS-2003 experiment. Comparison of these aggregated surface fluxes with area-averaged fluxes from long-range scintillometer measurements and from airborne measurements showed good agreement.     

Evaporation over a Heterogeneous Land Surface: EVA_GRIPS and the LITFASS-2003 Experiment—An Overview

The Evaporation at Grid/Pixel Scale (EVA_GRIPS) project was realised in order to determine the area-averaged evaporation over a heterogeneous land surface at the scale of a grid box of a regional numerical weather prediction or climate model, and at the scale of a pixel of a satellite image. EVA_GRIPS combined surface-based and airborne measurements, satellite data analysis, and numerical modelling activities. A mesoscale field experiment, LITFASS-2003, was carried out in the heterogeneous landscape around the Meteorological Observatory Lindenberg (MOL) of the German Meteorological Service in May and June, 2003. The experiment was embedded in the comprehensive, operational measurement program of the MOL. Experimental determination of surface fluxes on a variety of spatial scales was achieved by employing micrometeorological flux stations, scintillometers, a combination of ground-based remote sensing instruments, and the Helipod, a turbulence probe carried by a helicopter. Surface energy fluxes were also derived from satellite data. Modelling work included the use of different Soil–Vegetation–Atmosphere Transfer schemes, a large-eddy simulation model and three mesoscale atmospheric models. The paper gives an overview on the background of EVA_GRIPS, and on the measurements and meteorological conditions during LITFASS-2003. A few general results are discussed.     

Insolation estimates for the LITFASS area derived from high resolution satellite data

Summary ?For the LITFASS-98 experiment, from June 1 until June 30, 1998, the spatially resolved insolation at surface could be computed from NOAA-14 AVHRR data applying the modular analysis scheme SESAT (Strahlungs- und Energiebilanzen aus Satellitendaten). The satellite inferred insolation for this period shows for clear-sky regions a good agreement with surface based observations with a rms error of 76 Wm⁻². For cloudy conditions the insolation is overestimated with respect to ground based observations, with a rms error between 83 and 118 Wm⁻², depending on the cloud optical thickness. This overestimation can be explained by the surface heterogeneity, leading to underestimated cloud optical thickness, and also by a fixed relative humidity below clouds (55%, dry atmosphere) and a fixed horizontal visibility (50 km, clear atmosphere). A detailed study of comparable scales in space and time, considering the different observation geometries and sampling intervals, shows that a 30 min ground based observation can be compared with a 8 × 8 km² mean by the satellite data. Received July 12, 2001; revised April 29, 2002; accepted June 7, 2002     

Monitoring of Mesoscale Precipitation Systems in the Alps and the Northern Alpine Foreland by Radar and Rain Gauges

Summary.  Mesoscale convective precipitation systems in the Alpine region are studied by analyzing radar and rain gauge data. The data from weather radars in Austria, France, Germany, and Switzerland are combined into a composite. Availability of radar data restricts the study mainly to the northern part of the Alpine region. Mesoscale convective systems (MCS) occur often in this region and are comparable to large systems observed in the USA. Seven precipitation events lasting one to six days from the years 1992–1996 are examined in detail. They all moved west to east and showed no diurnal preference in formation or dissipation. They reach sizes of 2 − 6 · 10⁴ km². MCS with leading-line trailing-stratiform structure tended to be larger and more intense. A 25-year set of rain gauge data indicates that a giant MCS (covering more than 4 · 10⁴ km² with more than 30 mm/day) occurs every 6 years in the northern Alpine region. MCS occur more frequently in the southern Alpine region. Received February 25, 1999/Revised June 29, 1999     

Editorial: Scientific results of the European NEWBALTIC project

1 (¹ Numerical Studies of the Energy and Water Cycle of the BALTIC Region) stands for two international projects funded by the European Community in the years 1996–2000. The campaign was embedded into the continental-scale experiment BALTEX²(² Baltic Sea Experiment), a subproject of GEWEX³(³ Global Energy and Water Cycle Experiment) (Raschke, 1994; Bengtsson, 1995). The present special issue emerged from two NEWBALTIC progress meetings held in Sweden: One in June 1999 in Abisko and one in January 2000 at Chalmers University of Technology in Gothenburg. The NEWBALTIC community welcomed the suggestion of Professor Bengtsson, Chairman of the BALTEX Science Steering Group, to present selected contributions of the considerable body of NEWBALTIC results in a special MAP⁴(⁴ Meterology and Atmospheric Physics) issue. This collection, it was agreed, might serve as an example for the current state of modelling hydro-meteorological key processes on the continental scale of the Baltic Sea and its environment (see also Raschke et al., 1998, 2001). All papers submitted went through the review process regularly applied by MAP. The present special issue is opened by a review paper of Lennart Bengtsson on the objectives of BALTEX and on the central goal of the NEWBALTIC project: The energy and water cycle of the Baltic Sea. BALTEX in general was planned as a decade-long endeavour with a preparatory phase 1992–1993, a build-up phase 1994–1996 and a major research phase 1997–2001. Most of the work within the two NEWBALTIC projects have been carried out during the major research phase. In the words of the project folder distributed early in the 90ies: BALTEX, as a research programme integrating three disciplines into one single project, will be mutually stimulating and is expected to overcome traditional embarrassing barriers between meteorological, hydrological and oceanographic research. NEWBALTIC, as an important component of BALTEX, has done a visible step into this direction. Concerning the specific time periods chosen, most of the NEWBALTIC studies of this issue concentrate upon a three months period August-September-October 1995 referred to as PIDCAP⁵.(⁵ Pilot Study of Intense Data Collection and Analysis of Precipitation) The total of 13 papers eventually selected for publication in the present special issue are devoted to the two central aspects of the NEWBALTIC campaign: To understand the processes as represented in modern high-resolution coupled models, and to evaluate and digest observed data. So the papers of the present issue have been grouped into these two categories.     

Simulation of surface energy fluxes using high-resolution non-hydrostatic simulations and comparisons with measurements for the LITFASS-2003 experiment

Land-surface heterogeneity effects on the subgrid scale of regional climate and numerical weather prediction models are of vital interest for the energy and mass exchange between the surface and the atmospheric boundary layer. High-resolution numerical model simulations can be used to quantify these effects, and are a tool used to obtain area-averaged surface fluxes over heterogeneous land surfaces. We present high-resolution model simulations for the LITFASS area near Berlin during the LITFASS-2003 experiment, which were carried out using the non-hydrostatic model FOOT3DK of the University of Köln with horizontal resolutions of 1 km and 250 m. The LITFASS-2003 experimental dataset is used for comparison. The screen level quantities show good quality for the simulated pressure, temperature, humidity and wind speed and direction. Averaged over the four week experimental period, simulated surface energy fluxes at land stations show a small bias for the turbulent heat fluxes and an underestimation of the net radiation caused by excessive cloudiness in the simulations. For eight selected days with low cloud amounts, the net radiation bias is close to zero, but the sensible heat flux shows a strong positive bias. Large differences are found for latent heat fluxes over a lake, which are partly due to local effects on the measurements, but an additional problem seems to be the overestimation of the turbulent exchange under stable conditions in the daytime internal boundary layer over the lake. In the area average over the LITFASS area of 20 ×  20 km², again a strong positive bias of 70 W m^?2 for the sensible heat is present. For the low soil moisture conditions during June 2003, the simulation of the turbulent heat fluxes is sensitive to variations in the soil type and its hydrological properties. Under these conditions, the supply of ground water to the lowest soil layer should be accounted for. Different area-averaging methods are tested. The experimental set-up of the LITFASS-2003 experiment is found to be well suited for the computation of area-averaged turbulent heat fluxes.     

First Results of Measurements with a Newly-Designed Phased-Array Sodar with RASS

Summary At a special measuring site for boundary-layer studies as well as land-surface processes the Meteorological Observatory Lindenberg of the German Weather Service (DWD) has recently put into operation a newly-desi gned phased-array SODAR/RASS, which has been developed by METEK on behalf of the DWD. This system provides the vertical profiles of the three-di mensional wind vector in the boundary layer on an operational basis and is furthermore suitable for getting information on the profile of virtual temperature up to about 400 m in height based on the addition of RASS components. The following paper describes both the technique of this SODAR/RASS and the various modes of operation as well as the different options in managing the system. First evaluations on the data availability concerning the maximum height coverage will give an impression on the system’s capabilities. Finally, the accuracy of the derived profiles of winds and temperature will be investigated by means of comparisons of the SODAR/RASS data with measurements of a six-sonde tethered-balloon system as well as meteorological data of a 99 m tower in the vicinity of the system. Received November 27, 1998 Revised April 9, 1999     

Processing and quality control of flux data during LITFASS-2003

Different aspects of the quality assurance and quality control (QA/QC) of micrometeorological measurements were combined to create a comprehensive algorithm which was then applied to experimental data from LITFASS-2003 (Lindenberg Inhomogeneous Terrain—Fluxes between Atmosphere and Surface: a long term Study). Eddy-covariance measurements of the latent heat flux were the main focus of the QA/QC efforts. The results of a turbulence sensor intercomparison experiment showed deviations between the different eddy-covariance systems on the order of 15%, or less than 30 W m⁻², for the latent heat flux and 5%, or less than 10 W m⁻², for the sensible heat flux. In order to avoid uncertainties due to the post-processing of turbulence data, a comprehensive software package was used for the analysis of experimental data from LITFASS-2003, including all necessary procedures for corrections and quality control. An overview of the quality test results shows that for most of the days more than 80% of the available latent heat flux data are of high quality so long as there are no instrumental problems. The representativeness of a flux value for the target land-use type was analysed using a stochastic footprint model. Different methods to calculate soil heat fluxes at the surface are discussed and a sensitivity analysis is conducted to select the most robust method for LITFASS-2003. The lack of energy balance closure, which was found for LITFASS-2003, can probably be attributed to the presence of low-frequency flux contributions that cannot be resolved with an averaging time of 30 min. Though the QA/QC system has been developed for the requirements of LITFASS-2003, it can also be applied to other experiments dealing with similar objectives.     

Influence of vegetation changes during the Last Glacial Maximum using the BMRC atmospheric general circulation model

 The influence of different vegetation distributions on the atmospheric circulation during the Last Glacial Maximum (LGM, 21 000 years before present) is investigated. The atmospheric general circulation model of the Bureau of Meteorology Research Center was run using a modern vegetation and in a second experiment with a vegetation reconstruction for the LGM. It is found that a change from conifer to desert and tundra causes an additional LGM cooling of 1–2 °C in Western Europe, up to −4 °C in North America and −6 °C in Siberia. An expansion of dryland vegetation causes an additional annual cooling of 1–2 °C for Australia and northern Africa. On the other hand, an increase of temperature (2 °C) is found in Alaska due to changes in circulation. In the equatorial region the LGM vegetation leads to an increased modelled temperature of 0.5–1.5 °C and decreased precipitation (30%) over land due to a reduction of the tropical rainforest, mainly in Indonesia, where the reduction of precipitation over land is associated with an increase of precipitation of 30% over the western Pacific. Received: 15 December 1999 / Accepted: 10 January 2001     

Evaluating spatial scales of climate variability in sub-Saharan Africa

Summary Spatial scales of variability in seasonal rainfall over Africa are investigated by means of statistical and numerical techniques. In the statistical analysis spatial structure is studied using gridded 0.5° resolution monthly data in the period 1948–1998. The de-seasonalized time series are subjected to successive principal component (PC) analysis, allowing the number of modes to vary from 10 to 24, producing cells of varying dimension. Then the original rainfall data within each cell are cross-correlated (internal), then averaged and compared with the adjacent cells (external) for each PC solution. By considering the ratio of internal to external correlation, the spatial scales of rainfall variability are evaluated and an optimum solution is found whose cell dimensions are approximately 10⁶ km². The aspect of scale is further studied for southern Africa by consideration of numerical model ensemble simulations over the period 1985–1999 forced with observed sea surface temperatures (SSTs). The hindcast products are compared with observed January to March (JFM) rainfall, based on a station-satellite merged analysis of precipitation (CMAP) data at 2.5° resolution. Validations for different sized areas indicate that cumulative standardized errors are greatest at the scale of a single grid cell (10⁴ km²) and decrease 20–30% by averaging over successively larger areas (10⁶ km²).     

Estimation of Probable Maximum Precipitation for Dams in the Hongru River Catchment, China

Summary During the summer season, typhoons form in the western north Pacific Ocean and travel westward towards China. Some recurve northward off the coast, whereas others continue in over land. These typhoons bring heavy rainfall to the Huai river basin in eastern central China. In August 1975, the remnant of typhoon Nina caused exceptionally heavy rainfall in the Hongru river basin, in the mountainous upper reaches of the Huai river. The rainfall lasted five days from 4 to 8 August. This type of nearly stationary typhoon can cause rainfall of large intensity for a long duration, and is suitable for maximization to derive probable maximum precipitation (PMP) estimates. The PMP is transformed into a probable maximum flood hydrograph that is subsequently used to design spillways etc. In this study the PMP values have been estimated using a hydrometeorological method involving depth-area-duration analysis, moisture maximization, and altitude adjustment for typhoon Nina, for 1, 2, and 3 days duration. Areal PMP values were obtained for the entire Hongru river catchment, as well as for the subcatchments upstream the dams at Banqiao (762 km²), Shimantan (230 km²), Boshan (580 km²), and Suyahu (4 498 km²). For point values, the PMP was estimated to 1 200 mm/day, 1 460 mm/2 days, and 1 910 mm/3 days at altitudes about 100 m, which agrees well with previous studies. Received February 21, 1997 Revised May 27, 1997     

Testing a GCM land surface scheme against catchment-scale runoff data

A GCM land surface scheme was used, in off-line mode, to simulate the runoff, latent and sensible heat fluxes for two distinct Australian catchments using observed atmospheric forcing. The tropical Jardine River catchment is 2500 km² and has an annual rainfall of 1700 mm y^–1 while the Canning River catchment is 540 km², has a Mediterranean climate (annual rainfall of 800 mm y^–1) and is ephemeral for half the year. It was found that the standard version of a land surface scheme developed for a GCM, and initialised as for incorporation into a GCM, simulated similar latent and sensible heat fluxes compared to a basin-scale hydrological model (MODHYDROLOG) which was calibrated for each catchment. However, the standard version of the land surface scheme grossly overestimated the observed peak runoff in the wet Jardine River catchment at the expense of runoff later in the season. Increasing the soil water storage permitted the land surface scheme to simulate observed runoff quite well, but led to a different simulation of latent and sensible heat compared to MODHYDROLOG. It is concluded that this 2-layer land surface scheme was unable to simulate both catchments realistically. The land surface scheme was then extended to a three-layer model. In terms of runoff, the resulting control simulations with soil depths chosen as for the GCM were better than the best simulations obtained with the two-layer model. The three-layer model simulated similar latent and sensible heat for both catchments compared to MODHYDROLOG. Unfortunately, for the ephemeral Canning River catchment, the land surface scheme was unable to time the observed runoff peak correctly. A tentative conclusion would be that this GCM land surface scheme may be able to simulate the present day state of some larger and wetter catchments but not catchments with peaky hydrographs and zero flows for part of the year. This conclusion requires examination with a range of GCM land surface schemes against a range of catchments. Received: 9 June 1995 / Accepted: 4 April 1996     

The 2001 Mesoscale Convective Systems over Iberia and the Balearic Islands

Summary This paper characterizes Mesoscale Convective Systems (MCSs) during 2001 over Iberia and the Balearic Islands and their meteorological settings. Enhanced infrared Meteosat imagery has been used to detect their occurrence over the Western Mediterranean region between June and December 2001 according to satellite-defined criteria based on the MCS physical characteristics. Twelve MCSs have been identified. The results show that the occurrence of 2001 MCSs is limited to the August–October period, with September being the most active period. They tend to develop during the late afternoon or early night, with preferred eastern Iberian coast locations and eastward migrations. A cloud shield area of 50.000 km² is rarely exceeded. When our results are compared with previous studies, it is possible to assert that though 2001 MCS activity was moderate, the convective season was substantially less prolonged than usual, with shorter MCS life cycles and higher average speeds. The average MCS precipitation rate was 3.3 mm·h⁻¹ but a wide range of values varying from scarce precipitation to intense events of 130 mm·24 h⁻¹ (6 September) were collected. The results suggest that, during 2001, MCS rainfall was the principal source of precipitation in the Mediterranean region during the convective season, but its impact varied according to the location. Synoptic analysis based on NCEP/NCAR reanalysis show that several common precursors could be identified over the Western Mediterranean Sea when the 2001 MCSs occurred: a low-level tongue of moist air and precipitable water (PW) exceeding 25 mm through the southern portion of the Western Mediterranean area, low-level zonal warm advection over 2 °C·24 h⁻¹ towards eastern Iberia, a modest 1000–850 hPa equivalent potential temperature (θ_e) difference over 20 °C located close to the eastern Iberian coast, a mid level trough (sometimes a cut-off low) over Northern Africa or Southern Spain and high levels geostrophic vorticity advection exceeding 12·10⁻¹⁰ s⁻² over eastern Iberia and Northern Africa. Finally, the results suggest that synoptic, orographic and a warm-air advection were the most relevant forcing mechanisms during 2001.     

Composite wind and temperature profiles obtained from a complex of in-situ and remote sensing measurement systems for the forcing of a boundary layer model

Summary ?This paper describes the configuration of measurement systems operated continuously at the Meteorological Observatory Lindenberg with the aim of constructing combined profiles of wind and temperature – so-called composite profiles – covering the boundary layer with high temporal and vertical resolution. This is required for the forcing of a micro-α-scale model in order to simulate the atmospheric boundary layer structure over a heterogeneous landscape during the LITFASS-98 experiment. The problems of combining measurements of different remote sensing and in-situ systems are briefly discussed. Although the measuring range of individual remote sensing systems is variable, the height coverage of wind and temperature profile measurements by sodar/RASS and two wind profiler radar/RASS complement each other very well. Using a simple merging procedure composite wind and temperature profiles have been synthesized based on radiosonde, windprofiler/RASS, sodar/RASS and tower measurements. Time-height cross sections of hourly composite profiles show considerably more details of the boundary layer structure than simple radiosonde interpolation due to the higher sampling frequency, higher vertical resolution and increased accuracy at the lower levels. Finally some qualifications of the formulated algorithm are suggested for future application. Received June 18, 2001; revised May 30, 2002; accepted June 6, 2002     

On summing the components of radiative forcing of climate change

 Radiative forcing is a useful concept in determining the potential influence of a particular mechanism of climate change. However, due to the increased number of forcing agents identified over the past decade, the total radiative forcing is difficult to assess. By assigning a range of probability distribution functions to the individual radiative forcings and using a Monte-Carlo approach, we estimate the total radiative forcing since pre-industrial times including all quantitative radiative forcing estimates to date. The resulting total radiative forcing has a 75–97% probability of being positive (or similarly a 3–25% probability of being negative), with mean radiative forcing ranging from +0.68 to +1.34 W m⁻², and median radiative forcing ranging from +0.94 to +1.39 W m⁻². Received: 14 March 2001 / Accepted: 1 June 2001     

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     

Rain-aerosol coupling in the tropical atmosphere of Southeast Asia: distribution and scavenging ratios of major ionic species

Both aerosol and rainwater samples were collected and analyzed for ionic species at a coastal site in Southeast Asia over a period of 9 months (January–September 2006) covering different monsoons. In general, the occurrence and distribution of ionic species showed a distinct seasonal variation in response to changes in air mass origins. Real-time physical characterization of aerosol particles during rain events showed changes in particle number distributions which were used to assess particle removal processes associated with precipitation, or scavenging. The mean scavenging coefficients for particles in the range 10–500 nm and 500–10 μm were 7.0 × 10⁻⁵ ± 2.8 × 10⁻⁵ s⁻¹ and 1.9 × 10⁻⁴ ± 1.6 × 10⁻⁵ s⁻¹, respectively. A critical analysis of the scavenging coefficients obtained from this study suggested that the wet removal of aerosol particles was greatly influenced by rain intensity, and was particle size-dependent as well. The scavenging ratios, another parameter used to characterize particle removal processes by precipitation, for NH₄ ⁺, Cl⁻, SO₄ ²⁻, and NO₃ ⁻ were found to be higher than those of Na⁺, K⁺, and Ca²⁺ of oceanic and crustal origins. This enrichment implied that gaseous species NH₃, HCl, and HNO₃ could also be washed out readily. These additional sources of ions in precipitation presumably counter-balanced the dilution effect caused by high total precipitation volume in the marine and tropical area.     

Soil occupation and atmospheric variations over Sobradinho Lake area. Part one: an observational analysis

Summary Sobradinho Lake lies in the S?o Francisco River Basin, in one of the most arid regions in Northeastern Brazil, within a land stretch categorized as the Lower-middle S?o Francisco, situated at about 40 km away from the municipality of Petrolina (09°23′S–40°30′W) in the state of Pernambuco. The dam, in its full capacity, consists of a lake of approximately 4,214 km²; 280 km in length, the width of which varies from 5 to 50 km. The dam storage capacity is that of 34.1 billion m³ of water. Being situated in a semi-arid region, the dam brought about significant development to local irrigated agriculture. The caatinga ecosystem has, for that matter, undergone considerable changes. Statistical analysis techniques applied to data collected before and after the filling of the lake, made it possible both to make an assessment of the impact of the dam construction on the region meteorology and to diagnose the variability of such an impact on environmental conditions. Results showed that the dam has brought about considerable changes to regional meteorology. The alterations were observed to be more significant as regards atmospheric humidity and wind speed.