The study of the atmospheric boundary layer flow over two-dimensional low-sloped hills under a neutral atmosphere finds numerous applications in meteorology and engineering, such as the development of large-scale atmospheric models, the siting of wind turbines, and the estimation of wind loads on transmission towers and antennas. In this paper, the intermediate variable technique is applied to the momentum equations in streamline coordinates to divide the flow into regions, with each characterized by the dominance of different terms. Using a simple mixing-length turbulence closure, a simplified form of the x momentum equation is solved for the fully turbulent region, resulting in a modified logarithmic law. The solution is expressed as a power series correction to the classical logarithmic law that is valid for flat terrain. A new parameter appears: the effective radius of curvature of the hill. The modified logarithmic law is used to obtain new equations for the speedup, the relative speedup, the maximum speedup, and the height at which it occurs. A new speedup ratio is proposed to calculate the relative speedup at specific heights. The results are in very good agreement with the Askervein and Black Mountain field data.
A variational method is employed to compute surface sensible heat fluxes over a deciduous forest using observed temperature, temperature variance, and wind. Because the variational approach is able to take into account comprehensive observational meteorological conditions over a heterogeneous surface, it is applicable to the computations of sensible heat flux over a forest canopy in which the conventional flux-variance method is difficult to use. Verifications using the direct eddy-correlation measurements over a deciduous forest during the fully leafed summer of 1988 and the leafless winter of 1990 show that the variational method yields very good agreements between the computed and the measured sensible heat fluxes. It is also shown that the variational method is much more accurate than the flux-variance method in computations of sensible heat flux over a forest canopy.
Measurements from passive microwave satellite instruments such as the Advanced Microwave Sounding Unit B (AMSU-B) are sensitive to both liquid and ice cloud particles. Radiative transfer modeling is exploited to simulate the response of the AMSU-B instrument to mixed-phase clouds over land. The plane-parallel radiative transfer model employed for the study accounts for scattering and absorption from cloud ice as well as absorption and emission from trace gases and cloud liquid. The radiative effects of mixed-phase clouds on AMSU-B window channels (i.e., 89 and 150 GHz) and water vapor line channels (i.e., 183 +/- 1.3, and 7 GHz) are studied. Sensitivities to noncloud parameters, including surface temperature, surface emissivity, and atmospheric temperature and water vapor profiles, are also quantified. Modeling results indicate that both cloud phases generally have significant radiative effects and that the 150- and 183 +/- 7-GHz channels are typically the most sensitive channels to integrated cloud properties (i.e., liquid water path and ice water path). However, results also indicate that AMSU-B measurements alone are probably insufficient for retrieving all mixed-phase cloud properties of interest. These results are supported by comparisons of AMSU-B observations of a mixed-phase cloud over the Atmospheric Radiation Measurement (ARM) Program's Southern Great Plains (SGP) site with corresponding calculated clear-sky values.
In this paper an integrated assessment of the vertically integrated water vapor (IWV) measured by radiosonde, microwave radiometer (MWR), and GPS and modeled by the limited-area mesoscale model of MeteoSwiss is presented. The different IWV measurement techniques are evaluated through intercomparisons of GPS to radiosonde in Payerne, Switzerland, and to the MWR operated at the Institute of Applied Physics at the University of Bern in Switzerland. The validation of the IWV field of the nonhydrostatic mesoscale Alpine Model (aLMo) of MeteoSwiss is performed against 14 GPS sites from the Automated GPS Network of Switzerland (AGNES) in the period of 2001-03. The model forecast and the nudging analysis are evaluated, with special attention paid to the diurnal cycle. The results from the GPS-radiosonde intercomparison are in agreement, but with a bimodal distribution of the day-to-night basis. At 0000 UTC, the bias is negative (-0.4 kg m(-2)); at 1200 UTC, it is positive (0.9 kg m(-2)) and the variability increases. The intercomparison of GPS to MWR shows better agreement (0.4 kg m(-2)), with a small increase of the daytime bias with 0.3 kg m(-2). The intercomparison of MWR to the radiosonde gives a bimodal distribution of the bias, with an increase in the standard deviation at the daytime measurement. The relative bias is negative (-3%) at 0000 UTC and is positive (3%) at 1200 UTC. Based on this cross correlation, it can be concluded that the bimodal distribution is a result of radiosonde humidity measurements. Possible reasons are the solar-heating correction or sensor errors. The monthly bias and standard deviation of aLMo exhibit a strong seasonal dependence with a pronounced dry bias during the warm months of May-October 2002. The diurnal IWV cycle in 2001 shows good model performance between 0000 and 0900 UTC but IWV underestimation by up to 1.5 kg m(-2) for the rest of the day. In 2002 the diurnal cycle shows a systematic dry bias in both the analysis and forecast that is more pronounced in the analysis. This substantial underestimation of IWV was found to correlate with overestimation of aLMo precipitation, especially light precipitation up to 0.1 mm (6 h)(-1) in 2002. There is strong evidence that this underestimation can be related to the dry radiosonde bias in midday summer observations. The aLMo dry bias is about 1.0-1.5 kg m(-2) greater in the nudging analysis as compared with the forecast initialized at 0000 UTC.
An Atmospheric Radiation and Cloud Station (ARCS) was established on the island of Nauru by the Atmospheric Radiation Measurement (ARM) Program. Analysis of the Nauru99 field experiment data indicated that measurements at the ARCS were affected by a cloud plume that was induced by diurnal heating of the island. During the Nauru Island Effects Study, instrumentation was installed at a second site to develop criteria for identifying when the cloud plume occurs and to quantify its effect on ARCS measurements. The plume directional heading and frequency of occurrence are affected by the large-scale tropical circulation. During the present study, in which an El Nino was developing, Nauru was in a region of active convection, and easterly trade winds were not dominant; plumes were observed in 25% of satellite images. and only one-half of the observed plumes were downwind of the ARCS site. Surface wind direction, surface air temperature, and downwelling solar radiation at the two sites were used to identify periods when the cloud plume affected surface measurements. Differences in low-cloud frequency and surface radiation between plume-affected and non-plume-affected periods were examined. Existence of the cloud plume increased the average low-cloud frequency of occurrence from 20%, to 35%, decreased the average down-welling, shortwave radiation by 50-60 W m(-2), and increased the average downwelling longwave radiation by 5-10 W m(-2). Installing a suite of surface meteorological instruments and a global shortwave radiometer at a second site will allow for the long-term quantification of the cloud plume effect on the radiation field at the ARCS site.
This paper investigates wintertime cold-air pools in a basinlike part of the Danube Valley, located in the German state of Bavaria. Specifically, the focus is on cold-pool events restricted to the basin area, that is, not extending to the more elevated parts of the Alpine foreland. An analysis of observational data indicates that the delay of warm-air advection in the basin area relative to the Alpine foreland plays a major role in these events. However, the relationship between warming in the Alpine foreland and a temperature deficit in the northeast-southwest-oriented basin appears to depend sensitively on the ambient wind direction. A statistically significant correlation is found only for westerly and southerly wind directions but not for easterly directions. To examine the dynamical reasons for this phenomenon, idealized numerical simulations have been conducted. They are initialized with a pronounced cold pool in the basin area and examine the response of the cold pool to the dynamical forcing imposed by a geostrophically balanced large-scale wind field of various directions and strengths. Sensitivity tests consider the effects of the surrounding mountain ranges and of turbulent vertical mixing. The model results indicate that the most important dynamical processes capable of dissolving cold-air pools in a large basin are (i) ageostrophic advection of the cold air toward lower ambient pressure and (ii) downstream advection by the ambient flow. The former might also be interpreted as an adjustment of the cold air to the external pressure gradient, which can be balanced by the development of a spatial gradient in cold-pool depth. In principle, both advection processes are most effective in the along-basin direction because the advected air does not have to surmount significant topographic obstacles. However, a combination of several effects induced by the surrounding mountain ranges-for example, upstream flow deceleration and wake formation-modifies the dependence of the cold-pool persistence on the ambient wind direction. In agreement with observational data, the simulations with full topography predict a higher tendency for cold-pool persistence in the Danube basin for westerly and southerly flow than for easterly flow. Turbulent vertical mixing is found to make a significant contribution to the erosion of cold pools, but its effect is smaller than the sensitivity to the ambient wind direction.
This paper seeks to understand the role that climate variability has on annual yield of sugarcane in Louisiana. Unique features of sugarcane growth in Louisiana and nonclimatic, yield-influencing factors make this goal an interesting and challenging one. Several methods of seeking and establishing the relations between yield and climate variables are employed. First, yield-climate relations were investigated at a single research station where crop variety and growing conditions could be held constant and yield relations could be established between a predominant older crop variety and a newer one. Interviews with crop experts and a literature survey were used to identify potential climatic factors that control yield. A statistical analysis was performed using statewide yield data from the American Sugar Cane League from 1963 to 2002 and a climate database. Yield values for later years were adjusted downward to form an adjusted yield dataset. The climate database was principally constructed from daily and monthly values of maximum and minimum temperature and daily and monthly total precipitation for six cooperative weather-reporting stations representative of the area of sugarcane production. The influence of 74 different, though not independent, climate-related variables on sugarcane yield was investigated. The fact that a climate signal exists is demonstrated by comparing mean values of the climate variables corresponding to the upper and lower third of adjusted yield values. Most of these mean-value differences show an intuitively plausible difference between the high-and low-yield years. The difference between means of the climate variables for years corresponding to the upper and lower third of annual yield values for 13 of the variables is statistically significant at or above the 90% level. A correlation matrix was used to identify the variables that had the largest influence on annual yield. Four variables [ called here critical climatic variables (CCV)], mean maximum August temperature, mean minimum February temperature, soil water surplus between April and September, and occurrence of autumn ( fall) hurricanes, were built into a model to simulate adjusted yield values. The CCV model simulates the yield value with an rmse of 5.1 t ha(-1). The mean of the adjusted yield data over the study period was 60.4 t ha(-1), with values for the highest and lowest years being 73.1 and 50.6 t ha(-1), respectively, and a standard deviation of 5.9 t ha(-1). Presumably because of the almost constant high water table and soil water availability, higher precipitation totals, which are inversely related to radiation and temperature, tend to have a negative effect on the yields. Past trends in the values of critical climatic variables and general projections of future climate suggest that, with respect to the climatic environment and as long as land drainage is continued and maintained, future levels of sugarcane yield will rise in Louisiana.
An empirical model for predicting the maximum surface wind speed associated with a tropical cyclone after crossing the east coast of India is described. The model parameters are determined from the database of 19 recent cyclones. The model is based upon the assumption that tropical cyclone winds decay exponentially after landfall. A method for correcting the forecast during subsequent observation hours is also presented. Results show that without the correction factor the absolute mean error ranges from 6.1 to 4.9 kt (1 kt = 0.5144 m s(-1)) and the root-mean-square error ranges from 7.9 to 5.6 kt, with both decreasing over time. With the incorporation of the correction procedure, a significant improvement in the forecast skill is noticed for the case in which it is tested using the dependent sample. The model is expected to be very useful to operational forecasters.
Determination of the scale of the interaction between precipitation and topography is important for the accurate interpolation of rainfall in mountainous areas and also provides insight into the physical processes involved. In this paper, trivariate thin-plate smoothing splines are used to investigate the scale of interaction between monthly precipitation and topography by interpolating monthly rainfall over three subregions of the Australian continent, incorporating different climatic conditions and rainfall types. The interpolations are based upon elevations derived from digital elevation models (DEMs) of various resolutions. All of the DEMs are local averages of version 2.0 of the 9-s-resolution DEM of Australia. The results suggest that the optimal scale of the interaction between precipitation and topography, as it pertains to the elevation-dependent interpolation of monthly precipitation in Australia, is between 5 and 10 km. This is in agreement with results of similar studies that addressed daily precipitation over Switzerland.
Abstract Climate studies of precipitation have generally focused on daily or longer time scales of precipitation accumulation. The main objective of this work was to identify the precipitation characteristics of storms based on 15-min precipitation data, including storm total precipitation, storm duration, mean storm intensity, and maximum 15-min intensity. A group of precipitation characteristics was subjected to a cluster analysis that identified nine regions of the conterminous United States with homogeneous seasonal cycles of mean storm precipitation characteristics. Both mean and extreme statistics were derived for each characteristic and season for each zone. Continuous probability density functions were generated that appropriately fit the empirical distributions of storm total precipitation and maximum 15-min intensity. The storm characteristics, in turn, were a function of seasonal water availability from source regions, atmospheric water vapor capacity, and storm precipitation mechanism. This is...
A catalog containing an unprecedented amount of historical data in central Mexico, covering almost six centuries (1450-1900), is used. This is a catalog of agricultural disasters that includes events associated with hydrometeorological phenomena, or hazards, whose effects were mainly felt in the agricultural sector, such as droughts. An analysis of the historical series of droughts in central Mexico for the period of 1450-1900 is performed. Periods of frequent drought centered at the years 1483, 1533, 1571, 1601, 1650, 1691, 1730, 1783, 1818, and 1860 have been identified. In particular, droughts in Mexico City and northwest Mexico that were identified through poor tree-ring growth are included in the frequent drought periods obtained in this work. Moreover, it was found that droughts occurred in El Nino years mainly for events of very strong and strong strengths, at a significant level. Also, most droughts lasted for 1 or 2 yr. Last, by analyzing the periodicities of the drought time series it was found that those that are the most conspicuous are the quasi-bidecadal frequencies of 18.9 and 21 yr.
One of the goals of the Global Precipitation Measurement project, the successor to the Tropical Rainfall Measuring Mission (TRMM), is to produce a 3-hourly global rainfall map using several spaceborne microwave radiometers. It is important, although often difficult, to classify radiometer observations over land as either "rain" or "no rain" because background land surface conditions change significantly with time and location. In this study, a no-rain brightness temperature database was created to infer land surface conditions using simultaneous observations by TRMM Microwave Imager (TMI) and precipitation radar (PR) with a resolution of 1 month and 1 degrees latitude x 1 degrees longitude. This paper proposes new rain/no-rain classification (RNC) methods that use the database to determine the background brightness temperature. The proposed RNC methods and the RNC method developed for the Goddard profiling algorithm (GPROF; the standard rain-rate retrieval algorithm for TMI) are applied to all TMI observations for the entire year of 2000, and the results are evaluated against the RNC made by PR as the "truth." The first method (M1) simply uses the average brightness temperature at 85-GHz vertical polarization [denoted as TB (85 V)] under no-rain conditions as the background brightness temperature at 85-GHz vertical polarization [denoted as TB, (85 V)]. The second method (M2) uses a regression equation between TB (85 V) and TB (22 V) under no-rain conditions from the database. Here, TB, (85 V) is calculated by substituting the observed TB (22 V) into the regression equation. The ratio of accurate rain detection by GPROF to all rain occurrences detected by PR was 59%. This ratio was 57% for M1 and 63% for M2. The ratio with the weight of the rain rate was 81% for M1 and 86% for M2; it was 80% for GPROF. These comparisons were made by setting a threshold using a constant coefficient k(0) to make the ratio of false rain detection to all no-rain occurrences detected by PR almost the same (approximately 0.85%) for all three methods. Further comparisons among the methods are made, and the reasons for the differences are investigated herein.
This paper analyzes the long-term (1901-2002) temporal trends in the agroclimate of Alberta, Canada, and explores the spatial variations of the agroclimatic resources and the potential crop-growing area in Alberta. Nine agroclimatic parameters are investigated: May-August precipitation (PCPN), the start of growing season (SGS), the end of the growing season (EGS), the length of the growing season (LGS), the date of the last spring frost (LSF), the date of the first fall frost (FEE), the length of the frost-free period (FFP), growing degree-days (GDDs), and corn heat units (CHUs). The temporal trends in the agroclimatic parameters are analyzed by using linear regression. The significance tests of the trends are made by using Kendall's tau method. The results support the following conclusions. 1) The Alberta PCPN has increased 14% from 1901 to 2002, and the increment is the largest in the north and the northwest of Alberta, then diminishes (or even becomes negative over two small areas) in central and southern Alberta, and finally becomes large again in the southeast corner of the province. 2) No significant long-term trends are found for the SGS, EGS, and LGS. 3) An earlier LSF, a later FEE, and a longer FFP are obvious all over the province. 4) The area with sufficient CHU for corn production, calculated according to the 1973-2002 normal, has extended to the north by about 200-300 km, when compared with the 1913-32 normal, and by about 50-100 km, when compared with the 1943-72 normal; this expansion implies that the potential exists to grow crops and raise livestock in more regions of Alberta than was possible in the past. The annual total precipitation follows a similar increasing trend to that of the May-August precipitation, and the percentile analysis of precipitation attributes the increase to low-intensity events. The changes of the agroclimatic parameters imply that Alberta agriculture has benefited from the last century's climate change.
Daily rainfall totals are a key input for hydrological models that are designed to simulate water and pollutant flow through both soil and waterways. Within New Zealand there are large areas and many river catchments where no long-term rainfall observations exist. A method for estimating daily rainfall over the whole of New Zealand on a 5-km grid is described and tested over a period from January 1985 to April 2002. Improvement over a spatial interpolation method was gained by scaling high-elevation rainfall estimates using simulated mesoscale model rainfall surfaces that are generated for short periods in 1994 and 1996. This method is judged to produce reasonable and useful estimates of daily rainfall.
Source-receptor relationships for mercury (Hg) and other trace elements wet deposited in southeastern Florida were investigated using daily event precipitation samples collected over a 1-yr period in 1995-96. Data collected in Davie, Florida, showed significantly higher (2.2 times) volume-weighted mean (VWM) Hg concentrations during the spring and summer seasons relative to winter. A meteorological tracer analysis, utilizing the ratio of trace elements lanthanum (La) to cerium (Ce), determined that "feed" air to precipitation cells arriving at the site incorporated local urban emissions more frequently during the spring and summer seasons. Using Weather Surveillance Radar-1988 Doppler (WSR-88D) data, analysis of precipitation-cell path histories for events collected at 17 sites in southeastern Florida over I month in the summer of 1995 showed significantly higher (2.0 times) VWM Hg concentrations in events with easterly cell paths than in those with westerly paths for storms arriving at Everglades sites. Significantly higher VWM concentrations for known anthropogenic elements vanadium (V) and nickel (Ni) (oil combustion) and lead (Pb) and antimony (Sb) (waste incineration) were also observed in events with easterly paths. In addition, events arriving from the cast (offshore) that impacted the coastal sites were significantly lower in Hg (a factor of 2.6) than were events from the east that impacted the urban sites (after events moved inland over emission sources). This study demonstrates that source-receptor relationships can be established by analyzing the meteorological conditions corresponding to each precipitation event, and it supports the previous finding that local anthropogenic sources played the dominant role in the wet deposition of Hg to southern Florida and the Everglades during the study period.
Dual-Doppler lidar observations are used to assess the accuracy of single-Doppler retrievals of microscale wind and temperature fields in a shear-driven convective boundary layer. The retrieval algorithm, which is based on four-dimensional variational data assimilation, is applied by using dual- and single-Doppler lidar data that are acquired during the Joint Urban 2003 field experiment. The velocity field that was retrieved using single-Doppler data is compared directly with radial velocities that were measured by a second noncollocated lidar. Dual-Doppler retrievals are also performed and then compared with the single-Doppler retrieval. The linear correlation coefficient and rms deviation between the single-Doppler retrieval and the observations from the second lidar are found to be 0.94 and 1.2 m s(-1), respectively. The high correlation is mainly the result of good agreement in the mean vertical structure as observed by the two lidars. Comparisons between the single- and dual-Doppler retrieval indicate that the single-Doppler retrieval underestimates the magnitude of fluctuations in the crossbeam direction. Vertical profiles of horizontally averaged correlations between the single- and dual-Doppler retrievals also show a marginal correlation (0.4-0.8) between one of the horizontal velocity components. Again, this suggests that the retrieval algorithm has difficulty estimating the crossbeam component from single-Doppler data.
Relationships among convective planetary boundary layer (PBL) evolution and land surface properties are explored using data from the Atmospheric Radiation Measurement Program Cloud and Radiation Test Bed in the southern Great Plains. Previous attempts to infer surface fluxes from observations of the PBL have been constrained by difficulties in accurately estimating and parameterizing the conservation equation and have been limited to multiday averages or small samples of daily case studies. Using radiosonde and surface flux data for June, July, and August of 1997, 1999, and 2001, a conservation approach was applied to 132 sets of daily observations. Results highlight the limitations of using this method on daily time scales caused by the diurnal variability and complexity of entrainment. A statistical investigation of the relationship among PBL and both land surface and near-surface properties that are not explicitly included in conservation methods indicates that atmospheric stability in the layer of PBL growth is the most influential variable controlling PBL development. Significant relationships between PBL height and soil moisture, 2-m potential temperature, and 2-m specific humidity are also identified through this analysis, and it is found that 76% of the variance in PBL height can be explained by observations of stability and soil water content. Using this approach, it is also possible to use limited observations of the PBL to estimate soil moisture on daily time scales without the need for detailed land surface parameterizations. In the future, the general framework that is presented may provide a means for robust estimation of near-surface soil moisture and land surface energy balance over regional scales.
This study provides an intensive evaluation of the Global Precipitation Climatology Project (GPCP) 1 degrees daily (1DD) rainfall products over the Mississippi River basin, which covers 435 1 degrees latitude x 1 degrees longitude grids for the period of January 1997-December 2000 using radar-based precipitation estimates. The authors' evaluation criteria include unconditional continuous, conditional (quasi) continuous, and categorical statistics, and their analyses cover annual and seasonal time periods. The authors present spatial maps that reflect the results for the 1 degrees grids and a summary of the results for three selected regions. They also develop a statistical framework that partitions the GPCP-radar difference statistics into GPCP error and radar error statistics. They further partition the GPCP error statistics into sampling error and retrieval error statistics and estimate the sampling error statistics using a data-based resampling experiment. Highlights of the results include the following: 1) the GPCP 1DD product captures the spatial and temporal variability of rainfall to a high degree, with more than 80% of the variance explained, 2) the GPCP 1DD product proficiently detects rainy days at a large range of rainfall thresholds, and 3) in comparison with radar-based estimates the GPCP 1DD product overestimates rainfall.
The relationship between hemispherical sky cover and nadir-view cloud fraction is examined by using both model simulations and surface observations. Monte Carlo simulations of ground-based hemispherical measurements are based on four-dimensional cloud fields produced by a large-eddy simulation model. Surface hemispherical observations are performed during the Atmospheric Radiation Measurement Program's Cloudiness Intercomparison Intensive Operational Period. It is shown that (i) 15-min averages of frequently sampled (30 s) sky cover provide a reasonable estimation of the cloud fraction for limited fields of view and that (ii) this estimation can be substantially improved (for cumulus clouds) if additional information about the cloud aspect ratio is incorporated into the retrieval process.
Momentum and heat budget equations for katabatic flows on sloping surfaces are revisited. Terms in these equations are evaluated using wind and potential temperature data from four tethered-balloon data collection systems on a 3-km line running down a 1.6° slope at the foot of the Oquirrh Mountains in Utah’s Great Salt Lake valley. The analyses focus on the development with downslope distance of the katabatic flow and the associated negatively buoyant layer under synoptically undisturbed conditions. With strong ambient stratification, the katabatic flow shows little variation between sites, suggesting a state close to local equilibrium. When the ambient stratification is weaker, the acceleration of the katabatic flow between two tethersonde sites is systematically larger than what would be predicted based on observed buoyancy. Comparison of observed flow direction with the local topographic gradient indicates that slope curvature, associated with small deviations from the basically planar slope, may be responsible for the anomalous increase. It is concluded that the cross-slope homogeneity of the flow, which is assumed in simplified katabatic flow models, may be significantly disturbed even on slopes that appear to be planar to the observer.
