Aerosol load study in urban area by Lidar and numerical model

Vertical profiles of particle mass concentration in the urban canopy above the city of Lyon have been obtained from Lidar measurements of atmospheric backscattering, over a period of three days. The concentrations measured at 50 m above the ground have been compared with the mass concentration of PM10 measured by a ground-based sampler located near the Lidar site. At certain times during the measurement campaign, the Lidar concentration measurements at 50 m agree reasonably well with the concentrations at ground level but at other times the differences between the two sets of measurements are so great that they cannot be explained by possible uncertainties in the data processing. Even when the Lidar and ground-based measurements coincide, there are significant differences between the two signals. To explain these differences we have computed the trajectories of the air parcels that pass over the Lidar, using a numerical model for the wind field that takes into account surface features such as relief and changes in roughness. This analysis showed that the differences can be explained by the meteorological conditions (wind speed and direction, vertical profiles of temperature) and the positions of the different sources of particulate matter relative to the measurement site. The combination of Lidar, ground-based sampler and air mass trajectory calculations is shown to be a powerful tool for discriminating between different sources of pollution, which could be useful in enforcing an urban air quality policy.     

3D-air quality model evaluation using the Lidar technique

This paper reports on a model investigation of a particular episode of tropospheric ozone formation in the city of Lyon, France. A large-scale measurement campaign involving ground-based analyzers, sampling, Sodars and Lidars has been used to validate the model results. Based on validated meteorological data and primary pollutant concentrations, the numerical model has been run to obtain 3D ozone concentration profiles during the whole campaign (22–25 June 1999). The results are compared to the ozone Lidar vertical profiles. Good agreement between Lidar data and model predictions is first obtained on 22 June (but not on the following days). On 23 and 24 June, ozone concentrations are significantly underestimated by the model. The ozone Lidar measurements allowed identifying large import processes from high altitudes that explain the difference. In a second model simulation, these imports are taken into account as new boundary conditions. This yielded good agreement between the experimental data and the predicted ozone concentrations over the whole period. The evidence of high altitude ozone intrusion is confirmed by back-trajectories calculations.     

Use of passive ambient ozone (O3) samplers in vegetation effects assessment

A stochastistic, Weibull probability model was developed and verified to simulate the underlying frequency distributions of hourly ozone (O3) concentrations (exposure dynamics) using the single, weekly mean values obtained from a passive (sodium nitrite absorbent) sampler. The simulation was based on the data derived from a co-located continuous monitor. Although at the moment the model output may be considered as being specific to the elevation and location of the study site, the results were extremely good. This effort for the approximation of the O3 exposure dynamics can be extended to other sites with similar data sets and in developing a generalized understanding of the stochastic O3 exposure-plant response relationships, conferring measurable benefits to the future use of passive O3 samplers, in the absence of continuous monitoring.     

Atmospheric distribution of particulate- and gas-phase phthalic esters (PAEs) in a Metropolitan City, Nanjing, East China

Phthalic acid esters (PAEs) are used in many branches of industry and are produced in huge amounts throughout the world. An investigation on particulate- and gas-phase distribution of PAEs has been conducted in Nanjing (China). The 12-h daily sampling program (from 8:00 am to 8:00 pm) for ten consecutive days was conducted in April, July and October 2005, and in January 2006 at about 1.5m above the ground level. For comparative purposes, sampling events were simultaneously conducted at two stations, one at the urban center and the other about 12 km from city center for suburban background monitoring. It was observed that the most abundant members of the PAE group were dimethyl phthalate (DMP) (10.1 ng m(-3), average), diethyl phthalate (DEP) (3.4 ng m(-3)), dibutyl phthalate (DBP) (58.8 ng m(-3)), butylbenzyl phthalate (BBP) (3.2 ng m(-3)), di-2-ethylhexyl phthalate (DEHP) (20.3 ng m(-3)) and di-n-octyl phthalate (DOP) (1.2 ng m(-3)). The average contribution of PAEs in the gas phase to the total PAE concentration (Sigma(6)PAE, sum of six PAE congeners) ranged from 75.0% to 89.2%. Both particulate- and gas-phase Sigma(6)PAE concentrations decreased with increasing temperature. Experimentally determined gas-particle partitioning (K(p)) of PAEs is well-correlated with their vapor pressure. The Sigma(6)PAE levels in the urban area are approximately 3.5 times as high as the levels found at the suburban station. The vertical profiles from 1.5 to 30.0m above the ground display slight height dependence.     

Combined effect of boundary layer recirculation factor and stable energy on local air quality in the Pearl River Delta over southern China

Atmospheric boundary layer (ABL) has a significant impact on the spatial and temporal distribution of air pollutants. In order to gain a better understanding of how ABL affects the variation of air pollutants, atmospheric boundary layer observations were performed at Sanshui in the Pearl River Delta (PRD) region over southern China during the winter of 2013. Two types of typical ABL status that could lead to air pollution were analyzed comparatively: weak vertical diffusion ability type (WVDAT) and weak horizontal transportation ability type (WHTAT). Results show that (1) WVDAT was featured by moderate wind speed, consistent wind direction, and thick inversion layer at 600~1000 m above ground level (AGL), and air pollutants were restricted in the low altitudes due to the stable atmospheric structure; (2) WHTAT was characterized by calm wind, varied wind direction, and shallow intense ground inversion layer, and air pollutants accumulated in locally because of strong recirculation in the low ABL; (3) recirculation factor (RF) and stable energy (SE) were proved to be good indicators for horizontal transportation ability and vertical diffusion ability of the atmosphere, respectively. Combined utilization of RF and SE can be very helpful in the evaluation of air pollution potential of the ABL.

Implications: Air quality data from ground and meteorological data collected from radio sounding in Sanshui in the Pearl River Delta showed that local air quality was poor when wind reversal was pronounced or temperature stratification state was stable. The combination of horizontal and vertical transportation ability of the local atmosphere should be taken into consideration when evaluating local environmental bearing capacity for air pollution.     

Nocturnal boundary layer characteristics and land breeze development in Houston,Texas during TexAQS II

The nocturnal boundary layer in Houston, Texas was studied using a high temporal and vertical resolution tethersonde system on four nights during the Texas Air Quality Study II (TexAQS II) in August and September 2006. The launch site was on the University of Houston campus located approximately 4 km from downtown Houston. Of particular interest was the evolution of the nocturnal surface inversion and the wind flows within the boundary layer. The land–sea breeze oscillation in Houston has important implications for air quality as the cycle can impact ozone concentrations through pollutant advection and recirculation. The results showed that a weakly stable surface inversion averaging in depth between 145 and 200 m AGL formed on each of the experiment nights, typically within 2–3 h after sunset. Tethersonde vertical winds were compared with two other Houston data sets (High Resolution Doppler Lidar and radar wind profiler) from locations near the coastline and good agreement was found, albeit with a temporal lag at the tethersonde site. This comparison revealed development of a land breeze on three nights which began near the coastline and propagated inland both horizontally and vertically with time. The vertical temperature structure was significantly modified on one night at the tethersonde site after the land breeze wind shift, exhibiting near-adiabatic profiles below 100 m AGL.     

Ambient ozone (O3) and adverse crop response: a unified view of cause and effect

This paper presents a cohesive view of the dynamics of ambient O(3) exposure and adverse crop response relationships, coupling the properties of photochemical O(3) production, flux of O(3) from the atmosphere into crop canopies and the crop response per se. The results from two independent approaches ((a) statistical and (b) micrometeorological) were analyzed for understanding cause-effect relationships of the foliar injury responses of tobacco cv Bel-W3 to the exposure dynamics of ambient O(3) concentrations. Similarly, other results from two independent approaches were analyzed in: (1) establishing a micrometeorological relationship between hourly ambient O(3) concentrations and their vertical flux from the air into a natural grassland canopy; and (2) establishing a statistical relationship between hourly ambient O(3) concentrations in long-term, chronic exposures and crop yield reductions. Independent of the approach used, atmospheric conditions appeared to be most conducive and the crop response appeared to be best explained statistically by the cumulative frequency of hourly ambient O(3) concentrations between 50 ppb and 90 ppb (100 and 180 microg m(-3)). In general, this concentration range represents intermediate or moderately enhanced hourly O(3) values in a polluted environment. Further, the diurnal occurrence of this concentration range (often approximately between 0900 and 1600 h in a polluted, agricultural environment) coincided with the optimal CO(2) flux from the atmosphere into the crop canopy, thus high uptake. The frequency of occurrence of hourly O(3) concentrations > 90 ppb (180 microg m(-3)) appeared to be of little importance and such concentrations in general appeared to occur during atmospheric conditions which did not facilitate optimal vertical flux into the crop canopy, thus low uptake. Alternatively, when > 90 ppb (180 microg m(-3)) O(3) concentrations occurred during the 0900-1600 h window, their frequency of occurrence was low in comparison to the 50-90 ppb (100-180 microg m(-3)) range. Based on the overall results, we conclude that if the cumulative frequency of hourly ambient O(3) concentrations between 50-62 ppb (100-124 microg m(-3)) occurred during 53% of the growing season and the corresponding cumulative frequency of hourly O(3) concentrations between 50-74 ppb (100-148 microg m(-3)) occurred during 71% of the growing season, then yield reductions in sensitive crops could be expected, if other factors supporting growth, such as adequate soil moisture are not limiting.     

干旱区地表石油污染物风蚀作用大气传输量估算研究

通过焉耆盆地石油开发区石油污染物水环境污染途径特点的分析 ,认为落地油将通过风蚀作用由大气传输以降尘的形式落入水体 ,对水环境产生影响。本文以土壤侵蚀的风洞实验求得污染源源强 ,采用高斯模式对地表风蚀作用的传输量进行估算 ,进而对石油开发区落地油通过大气传输途径对区域水环境 (博斯腾湖 )的影响进行了预测 ,建立了相应的模型。为人们了解干旱区石油污染物风蚀作用水环境污染影响提供了初步依据 ,对干旱区湖泊石油污染的全面防治提供了科学范式     

Assessing the meteorological conditions of a deep Italian Alpine valley system by means of a measuring campaign and simulations with two models during a summer smog episode

The typical features of a summer smog episode in the highly complex terrain of the Province of Bolzano (Northern Italy) were investigated by numerical modelling with two non-hydrostatic models, ground-based monitoring stations, and vertical profiling with two sodars and an ultra-light aircraft. High ozone concentrations are most likely to appear in situations where the local circulation in the valleys is decoupled from the synoptic flow above during anticyclonic weather regimes. Both models (MM5 and TVM) produced similar results and were able to simulate the local winds in the valleys. A comparison of the measured data to the model simulation results made an evaluation of the performance of the two models in such complex terrain possible. Both models proved to be suitable for the simulation of the meteorological conditions during such summer smog episodes in complex terrain.     

Photochemical formation and transport of ozone in Athens,Greece

An evaluation of the diurnal variation of the hourly ozone concentrations measured at five sites in Greater Athens from June until early September 1984 indicates that photosmog episodes in Greater Athens are associated with the sea breeze circulation. Due to local air circulation in the Athens basin, precursors of O₃ are transported to and accumulated in the Saronikos Bay during the morning hours while the land breeze is blowing. At noon, when the sea breeze sets in, the O₃ formed over the sea is brought back to the coast and to central Athens where it increases the local O₃ concentration by a factor of 3–5 within a few hours. The O₃ levels often remain high throughout the night. During the photochemical smog episodes, all of them accompanied by well-developed sea breezes, the U.S. Air Quality Standard of 120 ppb O₃ was exceeded for 4–7 h day⁻¹. Peak O₃ concentrations up to nearly 200 ppb were recorded in the smog episodes.Relatively high O₃ concentrations were measured on the island of Aegina. They tend to remain high during the night and can be attributed only to primary pollutant transport from Greater Athens advected by the land breeze. The O₃ values obtained at Mount Immitos (1000 m above MSL) suggest that, first, the sea breeze inhibits the influence of vertical thermal convection up to heights above 600 m, and second, no O₃ is noticeable from above the tropopause to ground level or from long-range transport.     

Comparison of concentration/flux ratios measured with a backscatter Lidar and with a chemical tracer

A series of backscatter Lidar measurements were made around a bio-waste power station at Eye in Suffolk over a period of 10 days in May 1999. These measurements were supplemented with bag samples of SF₆ tracer, analysed on site using gas chromatography with an electron capture detector. Despite problems with contamination, a detection limit of 20 ppt was eventually achieved and this permitted useful plume measurements from a release rate of 1–2 l min⁻¹. Concentration/flux ratios were estimated from the Lidar measurements using an integral technique. Of the Lidar runs obtained, 24 were coincident with a tracer release. After allowing for the background of both aerosol and tracer, it was apparent that the independent calibrations of concentration/flux ratio from Lidar or tracer agreed with each other to within 20–60%. This permits the Lidar scans to be used to estimate peak near-ground concentrations, though because of various technical difficulties (poor alignment, background sources of aerosol, or an inconvenient wind direction) this cannot always be achieved. Besides giving confidence in the Lidar calibration, the tracer measurements were valuable in permitting aerosol from the stack to be distinguished from aerosol from fugitive sources. Meteorological parameters were logged simultaneously with the dispersion measurements. These parameters included conventional means of wind speed and direction, temperature, humidity and insolation, and also micrometeorological measurements of turbulence and of turbulent fluxes. The Lidar was used to estimate wind speed and direction at plume height and the boundary layer depth and cloud-base where possible. Source emission characteristics were also logged.     

Arctic haze and clouds observed by lidar during four winter seasons of 1993–1997, at Eureka,Canada

A Mie-scattering-polarized Haze Lidar was installed at Eureka, Canada (80°N, 86°W), in February 1993 and has been used to observe the arctic haze during winter seasons from 1993–94 to 1996–97. Although it is difficult to distinguish between the arctic haze and other scattering objects (mainly clouds) under the specific conditions, two methods were used to classify those objects into two groups. First the vertical profiles of the arctic haze and clouds were compared with meteorological data observed by a radiosonde, and the relations between the scattering and depolarization ratios and the meteorological data were investigated. Then, since the arctic haze had very stable layers, the time dependency of the correlation coefficient between the different vertical profiles for the arctic haze and clouds was investigated. After the scattering objects were classified into two groups (the arctic haze and the clouds), the scattering ratio, the depolarization ratio, and the occurrence probabilities of them were investigated statistically. The statistical results, from the observations over the last four winter seasons, indicate that the average values of the scattering and depolarization ratios of the arctic haze were respectively 1.27 and 1.34%. The average humidity over ice of the layer containing the arctic haze was 42±19% (cloud: 92±13%). The arctic haze was observed at altitudes less than 3 km frequently and at altitudes of 3–5 km occasionally.     

Spatial variations of ground level ozone concentrations in areas of different scales

The spatial variation of ground level ozone concentrations was investigated for areas of three different scales: (1) an air quality management district (a region about 100×70 km²) in northern Taiwan, (2) the neighborhood (about 2 km in radius) of an air quality monitoring station, and (3) an open field (about 400×600 m²) surrounded by 3- and 4-story buildings in an elementary school. Analysis of data on hourly ozone concentration, obtained at 13 m above the ground at 21 monitoring stations in the air quality management district, showed that the stations downwind of the urban center in the district had significantly higher ozone concentrations. Measurements for 8-h average ozone concentrations at 1.5 m above the ground by passive samplers showed that, in a flat area about 2 km in radius, the ratios of the ozone concentration at open areas to that at the monitoring station (0.86–0.93) were significantly higher than those obtained at areas with higher traffic flow and density of buildings (0.60–0.68). For the open field in an elementary school, the 8-h average ozone concentrations at 1.5 m above the ground at sites less than 10 m from the nearest building were considerably lower than those at sites farther away from buildings. The results indicated that, in areas of small scales, the spatial distributions of ozone concentration were highly non-uniform and there were appreciable day-to-day variability in spatial distribution. Such variability should be taken into account in determining the extent to which an individual is exposed to ozone.     

Ground-based remote sensing measurements of aerosol and ozone in an urban area: A case study of mixing height evolution and its effect on ground-level ozone concentrations

We have estimated the mixing height (MH) and investigated the relationship between vertical mixing and ground-level ozone concentrations in Seoul, Korea, by using three ground-based active remote sensing instruments operating side by side: micro-pulse lidar (MPL), differential absorption lidar (DIAL), and differential optical absorption spectroscopy (DOAS). The MH is estimated from MPL measurements of aerosol extinction profiles by the gradient method under convective conditions. Comparisons of the MHs estimated from MPL and radiosonde measurements show a good agreement (r²=0.99). Continuous MPL measurements with high temporal and vertical resolution reveal the diurnal variations of the MH under convective conditions and the presence of a residual layer during the nighttime. Comprehensive measurements of ozone and aerosol by MPL, DIAL and DOAS during an high ozone episode (24–26 May 2000) in Seoul, Korea, reveal that (1) photochemical ozone production and advection from upwind regions (the western part of Seoul) contribute two peaks of ozone concentrations at the ground around 14:00 and 18:00 local time on 25 May 2000, respectively, and (2) the entrainment and the fumigation processes of ozone aloft in the nighttime residual layer into the ground is a major contributor of high concentrations of ground-level ozone observed on the following day (26 May 2000).     

Development of a regional hydrologic soil model and application to the Beerze--Reusel drainage basin

The soil compartment is an important interface between the atmosphere and the subsurface hydrosphere. In this paper a conceptual approach for regional hydrologic soil modelling (RHSM) is presented, which provides two important qualities for modelling. First, the soil compartment is directly coupled to the atmosphere via the land surface and to the aquifers. Second, extremely fine (5cm vertical) resolutions of the soil system can be realized at regional scales (several hundreds of km(2)). This high-resolution modelling could be achieved by parallel computation techniques. The RHSM approach is applied to the Beerze-Reusel drainage basin, which belongs to the Meuse River basin. Moisture transport in the soil system was calculated with extremely high vertical resolution at a regional scale based on rainfall-evaporation data for the year 2000. As a result, highly resolved regional groundwater recharge pattern addressing the heterogeneity of soil systems could be determined.     

Prediction of the vertical profile of ozone based on ground-level ozone observations and cloud cover

A number of statistical techniques have been used to develop models to predict high-elevation ozone (O3) concentrations for each discrete hour of day as a function of elevation based on ground-level O3 observations. The analyses evaluated the effect of exclusion/inclusion of cloud cover as a variable. It was found that a simple model, using the current maximum ground-level O3 concentration and no effect of cloud cover provided a reasonable prediction of the vertical profile of O3, based on data analyzed from O3 sites located in North Carolina and Tennessee. The simple model provided an approach that estimates the concentration of O3 as a function of elevation (up to 1800 m) based on the statistical results with a +/- 13.5 ppb prediction error, an R2 of 0.56, and an index of agreement, d1, of 0.66. The inclusion of cloud cover resulted in a slight improvement in the model over the simple regression model. The developed models, which consist of two matrices of 24 equations (one for each hour of day for clear to partly cloudy conditions and one for cloudy conditions), can be used to estimate the vertical O3 profile based on the inputs of the current day's 1-hr maximum ground-level O3 concentration and the level of cloud cover.     

Evolution of the magnitude and spatial extent of the weekend ozone effect in California's South Coast Air Basin, 1981-2000

Since the mid-1970s, ozone (O3) levels in portions of California's South Coast Air Basin (SoCAB) on weekends have been as high as or higher than levels on weekdays, even though emissions of O3 precursors are lower on weekends. Analysis of the ambient data indicates that the intensity and spatial extent of the weekend O3 effect are correlated with-day-of-week variations in the extent of O3 inhibition caused by titration with nitric oxide (NO), reaction of hydroxyl radical (OH) with nitrogen dioxide (NO2), and rates of O3 accumulation. Lower NO mixing ratios and higher NO2/oxides of nitrogen (NOx) ratios on weekend mornings allow O3 to begin accumulating approximately an hour earlier on weekends. The weekday/weekend differences in the duration of O3 accumulation remained relatively constant from 1981 to 2000. In contrast, the rate of O3 accumulation decreased by one-third to one-half over the same period; the largest reductions occurred in the central basin on weekdays. Trends in mixing ratios of O3 precursors show a transition to lower volatile organic compound (VOC)/NOx ratios caused by greater reductions in VOC emissions. Reductions in VOC/NOx ratios were greater on weekdays, resulting in higher VOC/NOx ratios on weekends relative to weekdays. Trends in VOC/NOx ratios parallel the downward trend in peak O3 levels, a shift in the location of peak O3 from the central to the eastern portion of the basin, and an increase in the magnitude and spatial extent of the weekend O3 effect.     

Consequences of elevated carbon dioxide and ozone for foliar chemical composition and dynamics in trembling aspen (Populus tremuloides) and paper birch (Betula papyrifera).

Atmospheric chemical composition affects foliar chemical composition, which in turn influences the dynamics of both herbivory and decomposition in ecosystems. We assessed the independent and interactive effects of CO2 and O3 fumigation on foliar chemistry of quaking aspen (Populus tremuloides) and paper birch (Betula papyrifera) at a Free-Air CO2 Enrichment (FACE) facility in northern Wisconsin. Leaf samples were collected at five time periods during a single growing season, and analyzed for nitrogen. starch and condensed tannin concentrations, nitrogen resorption efficiencies (NREs), and C:N ratios. Enriched CO2 reduced foliar nitrogen concentrations in aspen and birch; O3 only marginally reduced nitrogen concentrations. NREs were unaffected by pollution treatment in aspen, declined with 03 exposure in birch, and this decline was ameliorated by enriched CO2. C:N ratios of abscised leaves increased in response to enriched CO2 in both tree species. O3 did not significantly alter C:N ratios in aspen, although values tended to be higher in + CO2 + O3 leaves. For birch, O3 decreased C:N ratios under ambient CO2 and increased C:N ratios under elevated CO2. Thus, under the combined pollutants, the C:N ratios of both aspen and birch leaves were elevated above the averaged responses to the individual and independent trace gas treatments. Starch concentrations were largely unresponsive to CO2 and O3 treatments in aspen. but increased in response to elevated CO2 in birch. Levels of condensed tannins were negligibly affected by CO2 and O3 treatments in aspen, but increased in response to enriched CO2 in birch. Results from this work suggest that changes in foliar chemical composition elicited by enriched CO2 are likely to impact herbivory and decomposition, whereas the effects of O3 are likely to be minor, except in cases where they influence plant response to CO2.     

Evidence for cloud venting of mixed layer ozone and aerosols

Observations are presented which substantiate the hypothesis that significant vertical exchange of ozone (O₃) and aerosol pollutants occurs between the mixed layer and the free troposphere during cumulus cloud convective activity. Flight experiments conducted in July 1981 utilized the airborne UV-DIAL (Ultra-Violet Differential Absorption Lidar) system developed by NASA. This system provides simultaneous range resolved O₃ concentration and aerosol backscatter profiles with high spatial resolution. Data were obtained during the afternoon along east-west and south-north intersecting transects over North Carolina in the presence of active, non-precipitating cumulus clouds. Evening transects were obtained in the area indicated by trajectory calculations to be the current position of the air mass sampled earlier in the day. Space-height cross-section analyses for the evening flight show the cloud ‘debris’ as patterns of aerosol and O₃ in excess of the ambient free tropospheric background. The O₃ excess was approximately the value of the concentration difference between the afternoon mixed layer and free troposphere measured in the afternoon from independent in-situ vertical soundings made by another aircraft.     

Comparative statistical study of hourly precipitation determined by radar-based Stage IV and ground-based methods in the North Central United States

Detailed hourly precipitation data are required for long-range modeling of dispersion and wet deposition of particulate matter and water-soluble pollutants using the CALPUFF model. In sparsely populated areas such as the north central United States, ground-based precipitation measurement stations may be too widely spaced to offer a complete and accurate spatial representation of hourly precipitation within a modeling domain. The availability of remotely sensed precipitation data by satellite and the National Weather Service array of next-generation radars (NEXRAD) deployed nationally provide an opportunity to improve on the paucity of data for these areas. Before adopting a new method of precipitation estimation in a modeling protocol, it should be compared with the ground-based precipitation measurements, which are currently relied upon for modeling purposes. This paper presents a statistical comparison between hourly precipitation measurements for the years 2006 through 2008 at 25 ground-based stations in the north central United States and radar-based precipitation measurements available from the National Center for Environmental Predictions (NCEP) as Stage IV data at the nearest grid cell to each selected precipitation station. It was found that the statistical agreement between the two methods depends strongly on whether the ground-based hourly precipitation is measured to within 0.1 in/hr or to within 0.01 in/hr. The results of the statistical comparison indicate that it would be more accurate to use gridded Stage IV precipitation data in a gridded dispersion model for a long-range simulation, than to rely on precipitation data interpolated between widely scattered rain gauges.

Implications:

The current reliance on ground-based rain gauges for precipitation events and hourly data for modeling of dispersion and wet deposition of particulate matter and water-soluble pollutants results in potentially large discontinuity in data coverage and the need to extrapolate data between monitoring stations. The use of radar-based precipitation data, which is available for the entire continental United States and nearby areas, would resolve these data gaps and provide a complete and accurate spatial representation of hourly precipitation within a large modeling domain.