Showing papers in "Journal of Applied Meteorology and Climatology in 1984"

On the Average Value of Correlated Time Series, with Applications in Dendroclimatology and Hydrometeorology

Abstract: In a number of areas of applied climatology, time series are either averaged to enhance a common underlying signal or combined to produce area averages. How well, then, does the average of a finite number (N) of time series represent the population average, and how well will a subset of series represent the N-series average? We have answered these questions by deriving formulas for 1) the correlation coefficient between the average of N time series and the average of n such series (where n is an arbitrary subset of N) and 2) the correlation between the N-series average and the population. We refer to these mean correlations as the subsample signal strength (SSS) and the expressed population signal (EPS). They may be expressed in terms of the mean inter-series correlation coefficient r as SSS ≡ (Rn,N)2 ≈ n(1 + (N − 1)r)/ N(1 + (N − 1)r), EPS ≡ RN)2 ≈ Nr/1 + (N − 1)r.Similar formulas are given relating these mean correlations to the fractional common variance which arises as a parameter in a...

The Palmer Drought Severity Index: Limitations and Assumptions

Abstract: The structure of the Palmer Drought Severity Index (PDSI), which is perhaps the most widely used regional index of drought, is examined. The PDSI addresses two of the most elusive properties of droughts: their intensity and their beginning and ending times. Unfortunately, the index uses rather arbitrary rules in quantifying these properties. In addition, the methodology used to standardize the values of the PDSI for different locations and months is based on very limited comparisons and is only weakly justified on physical or statistical grounds. Under certain conditions, the PDSI values are very sensitive to the criteria for ending an “established” drought and precipitation during a month can have a very large effect on the PDSI values for several previous months. The distribution of the PDSI conditioned on the value for the previous month may often be bimodal. Thus, conventional time series models may be quite limited in their ability to capture the stochastic properties of the index.

A Universal Scale of Apparent Temperature

Abstract: Based on the total thermal resistance required by a human model to effect equilibrium, a scale is prepared showing apparent temperature for any combination of dry-bulb temperature, vapor pressure, wind speed and extra radiation likely to be encountered meteorologically. Application to normal midday climates of the United States shows that dry-bulb temperature is modified by the three other variables by from −5 to +7 K. However, multiple linear regression indicates that dry-bulb temperature correlates most strongly with apparent temperature, and provides simple computing formulas.

Extreme High-Temperature Events: Changes in their probabilities with Changes in Mean Temperature

Abstract: Most climate impact studies rely on changes in means of meteorological variables, such as temperature, to estimate potential climate impacts, including effects on agricultural production. However, extreme meteorological events, say, a short period of abnormally high temperatures, can have a significant harmful effect on crop growth and final yield. The characteristics of daily temperature time series, specifically mean, variance and autocorrelation, are analyzed to determine possible ranges of probabilities of certain extreme temperature events [e.g., runs of consecutive daily maximum temperatures of at least 95°F (35°C)] with changes in mean temperature of the time series. The extreme temperature events considered are motivated primarily by agricultural concerns, particularly, the effects of high temperatures on corn yields in the U.S. Corn Belt. However, runs of high temperatures can also affect, for example, energy demand or morbidity and mortality of animals and humans. The relationships betw...

Time Series Models to Simulate and Forecast Wind Speed and Wind Power

Abstract: A general approach for modeling wind speed and wind power is described. Because wind power is a function of wind speed, the methodology is based on the development of a model of wind speed. Values of wind power are estimated by applying the appropriate transformations to values of wind speed. The wind speed modeling approach takes into account several basic features of wind speed data, including autocorrelation, non-Gaussian distribution, and diurnal nonstationarity. The positive correlation between consecutive wind speed observations is taken into account by fitting an autoregressive process to wind speed data transformed to make their distribution approximately Gaussian and standardized to remove diurnal nonstationarity. As an example, the modeling approach is applied to a small set of hourly wind speed data from the Pacific Northwest. Use of the methodology for simulating and forecasting wind speed and wind power is discussed and an illustration of each of these types of applications is presen...

The Influence of Atmospheric Stability on Potential Evaporation

Abstract: The Penman relationship for potential evaporation is modified to simply include the influence of atmospheric stability on turbulent transport of water vapor. Explicit expressions for the stability-dependent, surface exchange coefficient developed by Louis are used. The diurnal variation of potential evaporation is computed for the stability-dependent and original Penman relationships using Wangara data. The influence of afternoon instability increases the aerodynamic term of the modified Penman relationship by 50% or more on days with moderate instability. However, the unmodified Penman relationship predicts values of daily potential evaporation close to that of the stability-dependent relationship. This agreement is partly due to compensating overestimation during nighttime hours. Errors due to use of daily-averaged variables are examined in detail by evaluating the nonlinear interactions between the diurnal variation of the variables in the Penman relationship. A simpler method for estimating t...

Complex Principal Component Analysis: Theory and Examples

Abstract: Complex principal component (CPC) analysis is shown to be a useful method for identifying traveling and standing waves in geophysical data sets. Combinations of simple progressive and standing oscillations are used to examine the properties of this technique. These examples illustrate that although CPC analysis allows for the identification of traveling waves, many of the drawbacks associated with conventional principal component analysis remain, and sometimes become worse; e.g. the interpretation of CPC solutions is more difficult since both amplitude and phase relationships must be considered. A method for linearly transforming complex principal components was devised in order to identify regional relationships within large geophysical data sets. The errors in CPC analysis resulting from limited sample sizes are discussed.

Global vegetation indices from the NOAA-7 meteorological satellite

Abstract: Northern and Southern Hemisphere polar stereographic maps of “vegetation index” are now being produced by the National Oceanic and Atmospheric Administration. The maps are derived from visible and near-infrared data from NOAA's operational polar orbiting satellites. The data are composited over a weekly period to minimize cloud and scan angle effects. The mapped images are being made available to the public in both image and tape format, on a regular schedule.

Enhancement of Convective Precipitation by Mesoscale Variations in Vegetative Covering in Semiarid Regions

Abstract: It is hypothesized that planting bands of vegetation with widths of the order of 50–100 km in semiarid regions could, under favorable large-scale atmospheric conditions, result in increases of convective precipitation. These increases, which could be greater than those associated with the uniform vegetating of large areas, would occur through three major mechanisms. The first would be the modification of the environment to a state more conducive to the formation of moist convection through an increase of low-level moist static energy. This increase would be associated with a decrease in albedo, an increase in net radiation, and an increase in evapotranspiration. The second important mechanism would be the generation of mesoscale (horizontal scale of 20–200 km) circulations associated with the surface inhomogeneities created on this scale by the vegetation. The third mechanism would be the increase of atmospheric water vapor through decreased runoff and increased evaporation. A number of observati...

The determination of view-factors in urban canyons

Abstract: An important aspect of longwave radiation exchange within urban canopies is the degree to which the sky is obscured by surrounding buildings, a concept which may be described by the sky view-factor (ψs). Hitherto, expressions for sky view-factors in the urban context have been derived for symmetrical urban canyons of infinite length. In this paper an analytical expression is presented which permits the determination of sky view-factors for the more realistic case of urban canyons in which buildings are irregularly aligned and of finite length.

The Effect of Irrigation on Warm Season Precipitation in the Southern Great Plains.

Abstract: The synoptic and subsynoptic atmospheric processes that accompany statistically determined periods of irrigation-induced rainfall increases during the warm season in the Texas Panhandle are examined. Major results are as follows. Irrigation appears to increase precipitation only when the synoptic condition provides low-level convergence and uplift, such that the additional moisture produced by irrigation (normally confined to the lowest 10–20 m of the atmosphere) is allowed to ascend to cloud base. Stationary fronts are the most favorable such synoptic condition because they fulfill the requirement for longer time durations than moving fronts or surface low pressure centers. The effect of irrigation is more noticeable during generally rainy periods because such periods often contain the types of significant rainfall events that provide sustained low-level convergence over the irrigated region. Because the mean storm track is closer to north Texas in June than in July and August, the irrigation-pr...

Atmospheric Dispersion from Elevated Sources in an Urban Area: Comparison between Tracer Experiments and Model Calculations

Abstract: Atmospheric dispersion experiments were carried out in the Copenhagen area under neutral and unstable conditions. The tracer sulphurhexafluoride was released without buoyancy from a tower at a height of 115 m and then collected at ground-level positions in up to three crosswind series of tracer sampling units, positioned 2–6 km from the point of release. The site was mainly residential having a roughness length of 0.6 m. The meteorological measurements performed during the experiments included the three-dimensional wind velocity fluctuations at the height of release. Dispersion parameters estimated from the measured tracer concentrations were compared with dispersion parameters calculated by various standard methods. These included methods based on the measured wind variances at the experiments and methods based on a stability classification of the atmospheric conditions. The wind variance-based methods are seen to be better than the stability-based methods in predicting the variation of σy. In a...

Review of Weibull Statistics for Estimation of Wind Speed Distributions

Abstract: A statistical distribution commonly used for describing measured wind speed data is the Weibull distribution. A review of the methods found in the statistical literature for the purpose of estimation of the parameters in Weibull distributions is given, with a special emphasis on the efficiency of the different methods. From this review, the most appropriate method for a given application can be chosen. The general conclusion is that maximum likelihood estimators should be used due to their large sample efficiency. However, they require an iterative minimization. The recommended closed form estimators when there are few observations (say, less than 25) are the least-squares estimators. If wind speed data with a good speed resolution are available, the closed form ten fractile estimators are very good. The simplest are the two fractile estimators. They, however, require around three times as many observations as the maximum likelihood estimators in order to be of the same accuracy. If mean and stan...

Equivalent Radar Reflectivity Factors for Snow and Ice Particles

Abstract: Expression analytique reliant le coefficient de reflectivite radar au coefficient equivalent exprime en fonction de la taille des particules

Modeling of Ice Accretion on Wires

Abstract: A time-dependent numerical model of ice accretion on wires, such as overhead conductors, is presented. Simulations of atmospheric icing are made with the model in order to examine the dependence of the accreted ice amount on atmospheric conditions. The results show that in wet growth (glaze formation) under constant atmospheric conditions, the growth rate increases with time until the process changes to dry growth. In dry growth (rime formation) the growth rate typically increases with time at the beginning of the icing process, but later decreases with time when the ice deposit has grown bigger. The effect of air temperature on the ice load turns out to be rather small for the first 24 hours of icing in typical dry growth conditions, but it is important for long-term icing. The ultimate ice load may either increase or decrease with decreasing air temperature, depending on the other atmospheric conditions and on the duration of icing. These results largely explain the difficulties encountered in ...

Eurasian Snow Cover versus Indian Monsoon Rainfall—An Extension of the Hahn-Shukla Results

Abstract: The apparent inverse relationship between Eurasian mean winter snow cover extent and the following warm season Indian monsoon rainfall, described by Hahn and Shukla for the 1967–75 period, is substantiated by the addition of five subsequent years of data if known deficiencies in satellite snow observations are accommodated. In this respect, elimination of a bias due to under-observation of snow cover in the Himalayan region during 1967–74 was crucial for the attainment of statistically significant correlations. Nonsignificant correlations for a shorter period (1971–80) suggest that Eurasian and Himalayan region winter snow cover extent, as well as that of Eurasia less the Himalayan region, are all about equally well related to the subsequent Indian monsoon rainfall. Furthermore, Eurasian and Himalayan snow cover extent derived from satellite observations are found to be highly correlated.

Microburst Wind Structure and Evaluation of Doppler Radar for Airport Wind Shear Detection

Abstract: The horizontal and vertical structure of airflow within microbursts has been determined using Doppler weather radar data from the Joint Airport Weather Studies (JAWS) Project. It is shown that the downdraft typically associated with microbursts is about 1 km wide and begins to spread horizontally at a height below 1 km. The median time from initial divergence at the surface to maximum differential wind velocity across the microburst is five minutes. The height of maximum differential velocity is about 75 m, and the median velocity differential is 22 m/s over an average distance of 3.1 km. The outflow of the air is asymmetric, averaging twice as strong along the maximum axis compared to the mininum axis. Some technical requirements for a radar system to detect microbursts and to provide aircraft with early warnings of the onset of windshear are identified.

Parameterization of Droplet Terminal Velocity and Extinction Coefficient in Fog Models

Abstract: Fog droplet spectra data were obtained in several advection fogs at the AFGL Weather Test Facility at Otis Air National Guard Base, Massachusetts, using two PMS forward-scatter spectrometer probes (FSSP-100). Approximately 90 h of droplet data were recorded simultaneously at 5 m and 30 m above the ground throughout the life cycle of 11 fogs during 1980 and 1981. This large data set was used to develop new parameterizations of the extinction coefficient and the mean droplet terminal velocity in terms of liquid water content and droplet concentration for fogs with liquid water contents greater than 0.018 g m−3. The advection fog data show an excellent correlation of 95% between the extinction coefficient and liquid water content. The regression line for these two quantities is more nearly linear than found by other investigators. The correlation between the mean droplet terminal velocity and various microphysical parameters is not as high as that for the extinction coefficient and liquid water cont...

Diurnal Variability of Regional Cloud and Clear-Sky Radiative Parameters Derived from GOES Data. Part I: Analysis Method

Abstract: A hybrid bispectral threshold method (HBTM) is developed for hourly regional cloud and radiative parameters from geostationary satellite visible and infrared radiance data. The quantities derived with the HBTM include equivalent blackbody temperatures for clear skies, for the total cloud cover and for the cloud cover at three levels in the atmosphere; the total fractional cloud cover and the fractional cloud amounts at three altitudes; and the clear-sky and total cloud reflectance characteristics. Geostationary satellite data taken during November 1978 are analyzed. A minimum reflectance technique is used to determine clear-sky brightness. A visible bidirectional reflectance model is derived for clear ocean areas. Clear-sky radiative temperature is found with a bispectral clear radiance technique during daylight hours. An empirical model is derived to predict clear-sky temperature at night. A combination of previously published infrared threshold and bispectral techniques is used to determine the...

The Interpretation and Estimation of Effective Sample Size

Abstract: Statistical and dynamical relationships between observed values of a geophysical system or model effectively reduce the number of independent data. This reduction is expressible in terms of the covariance structure of the process and, in some instances, it is reasonable to devise a measure of the “effective sample size” in terms of sample statistics. Here we discuss the concept of “effective sample size,” and, having settled upon one of several possible definitions, examine various methods of estimating this quantity. It is found that “effective sample size” is quite difficult to estimate reliably. However, a procedure is described which we feel could be used successfully; it is noted that the concept could be extended to spatial arrays of data, in some circumstances.

The Characteristics of Wind Velocity that Favor the Fitting of a Weibull Distribution in Wind Speed Analysis

Abstract: The derivation of the Weibull distribution from the bivariate normal distribution provides theoretical justification for its use in wind speed analysis if four conditions are met. These conditions are that the orthogonal components of horizontal wind velocity transformed by raising them to the power k/2 are normally distributed, have zero means, have equal variances, and are uncorrelated. These four conditions specify a circular normal distribution for the transformed wind velocity components. Real world wind velocity patterns, however, are seldom, if ever, circular normal. Instead, the effects of topography and frontal systems produce a different wind speed distribution from each direction. This helps explain why the Weibull distribution gives only an approximate fit to the observed wind speed frequency distribution. The fit, at seven British Columbia coastal stations, was best at those stations with the most nearly circular wind velocity patterns and the lowest proportions of calms. It is sugge...

A Comparison of Several `Single-Pass' Estimators of the Standard Deviation of Wind Direction.

Abstract: Computation of the standard deviation of wind direction σθ generally requires repeated consideration of the individual measurements of wind direction. This need for multiple passes through the data sample can create a storage problem for small or remote data acquisition systems. Statistical quantities that can be computed during a single pass through incoming wind speed and direction data are first reviewed, and relationships between these quantities are discussed. A single-pass estimator for σθ is then developed and found to correspond to within ±2% of conventional, two-pass σθ computations for unimodal angular distributions spanning the full σθ range of from 0 to 103.9°. Similar comparisons are presented for other, recently proposed estimators of σθ.

Decreasing Diurnal Temperature Range in the United States and Canada from 1941 through 1980.

Abstract: An appreciable number of nonurban stations in the United States and Canada have been identified with statistically significant (at the 90% level) decreasing trends in the monthly mean diurnal temperature range between 1941–80. The percentage of stations in the network showing the decrease is higher than expected due to chance throughout the year, with a maximum reached during late summer and early autumn and a minimum in December. Monte Carlo tests indicate that during five months the field significance of the decreasing range is above the 99% level, and in 12 months above the 95% level. There is a negligible probability that such a result is due to chance. In contrast, trends of increasing or decreasing monthly mean maximum or minimum temperatures have at most only two months with field significance at or above the 90% level. This is related to the tendency toward increasing temperature in the western portions of North America and decreasing temperature in the east. The physical mechanism respon...

The Area-Time Integral as an Indicator for Convective Rain Volumes

Abstract: Digital radar data are used to investigate further a simple technique for estimating rainfall amounts on the basis of area coverage information. The basis of the technique is the existence of a strong correlation between a measure of the rain area coverage and duration called the Area-Time Integral (ATI) and the rain volume. This strong correlation is again demonstrated using echo cluster data from the North Dakota Cloud Modification Project 5 cm radars. Integration on a scan-by-scan basis proved to be superior for determining ATI values to the hour-by-hour integration used previously. A 25 dB(z) reflectivity threshold was found suitable for the ATI calculation. The correlation coefficient on log-log plots of cluster rain volume versus ATI is approximately 0.98, indicating a power-law relationship between the variables. The exponent of that relationship is just a little higher than one, which indicates that the cluster average rainfall rate is almost independent of the storm size and duration. A ...

Diurnal variability of regional cloud and clear-sky radiative parameters derived from GOES data. Part II : November 1978 cloud distributions.

Abstract: Regional (250 × 250 km2) diurnal cloud variability is examined using mean hourly cloud amounts derived from November 1978 GOES-East visible and infrared data with a hybrid bispectral threshold technique. A wide variety of diurnal variations in cloud cover is presented. A morning maximum in low cloudiness is found over much of the eastern Pacific. Many regions in the western Atlantic have peak low-cloud cover near noon. Low clouds reach a maximum most often near noon over most of South America and in the morning over North America. Midlevel clouds are most frequent in the evening over oceans and in the early morning over land. High-cloud maxima are found mainly in the late afternoon over land and in the midafternoon over the oceans. An early morning minimum in high-cloud-top temperature is observed over marine areas. The amplitude of the semidiurnal component of cloudiness is generally much less than that of the diurnal component. The largest diurnal cloud variations occur over the southeastern Pa...

Icing Conditions Encountered by a Research Aircraft

Abstract: The characteristics of clouds which have led to airframe icing on an instrumented Beechcraft Super King Air are summarized. The icing encounters occurred at altitudes from 0–8000 m MSL, in summer and winter, in stratiform and cumuliform clouds, and at temperatures from 0 to −30°C. The characteristics of icing encounters in different areas and in different seasons are compared. The fraction of measurements exceeding various threshold values of liquid water content, average liquid water content over a given distance, volume-median droplet diameter, droplet concentration, ice crystal concentration, and potential ice accumulation are given. The effects of these cloud characteristics on aircraft performance were measured by comparing the rate of climb of the aircraft with ice to the rate of climb for the clean aircraft under the same conditions. Most icing encounters led to a reduction in the rate of climb that increased linearly with the path integral of the supercooled liquid water content. The volu...

Lidar Observations of Mixed Layer Dynamics: Tests of Parameterized Entrainment Models of Mixed Layer Growth Rate

Abstract: Ground based lidar measurements of the atmospheric mixed layer depth, the entrainment zone depth and the wind speed and wind direction were used to test various parameterized entrainment models of mixed layer growth rate. Six case studies under clear air convective conditions over flat terrain in central Illinois are presented. It is shown that surface heating alone accounts for a major portion of the rise of the mixed layer on all days. A new set of entrainment model constants was determined which optimized height predictions for the dataset. Under convective conditions, the shape of the mixed layer height prediction curves closely resembled the observed shapes. Under conditions when significant wind shear was present, the shape of the height prediction curve departed from the data suggesting deficiencies in the parameterization of shear production. Development of small cumulus clouds on top of the layer is shown to affect mixed layer depths in the afternoon growth phase.

Diurnal Variability of Regional Cloud and Clear-Sky Radiative Parameters Derived from GOES Data. Part III: November 1978 Radiative Parameters

Abstract: The diurnal variability of the radiation emitted and reflected from the earth-atmosphere is investigated at the regional scale using November 1978 GOES-East visible and infrared data and GOES-derived cloud information. Narrowband GOES data are converted to broadband radiances using spectral calibration functions determined empirically from colocated Nimbus-7 ERB and GOES-East measurements over ocean, land and cloud surfaces. Shortwave radiances are used to estimate radiant exitances with bidirectional reflectance models derived from GOES and aircraft data for ocean, land and clouds. Average albedo over clear ocean and land changed by factors of 4.2 and 2.2, respectively, for a solar zenith angle range of 0 to 80°. Average cloud albedo changed by a factor of 1.8 for the same range of solar zenith angles, but varied considerably from region to region. Mean clear-sky longwave radiant exitance varied diurnally from 2 W m−2 over some ocean areas to 100 W m−2 in one high elevation desert area in the An...

Long-range tropospheric transport of pollution aerosols into the Alaskan Arctic

Abstract: Noncrustal vanadium and manganese are used as chemical tracers for pollution-derived aerosols (collected over a period of four years in the near-surface air at Barrow, Alaska), in order to investigate tropospheric long-range transport of anthropogenic pollution from midlatitudes to the Alaskan Arctic. The analysis is based upon subjectively identifying characteristic transport pathway types using daily circumpolar weather maps. The transport occurs when the midlatitudinal and Arctic atmospheric circulations manifest quasi-persistent circulation patterns. Rapid transport of aerosols, on the order of 7–10 days, is dominated by quasi-stationary anticyclones and takes place along their peripheries where pressure gradients are relatively strong. The seasonal variation in concentration of the Arctic pollution-derived aerosol is related to the seasonal variation in the occurrence and position of midlatitude blocking anticyclones, of the Arctic anticyclone and of the Asiatic anticyclone. The positions of...

The Identification of 10- to 20-Year Temperature and Precipitation Fluctuations in the Contiguous United States.

Abstract: A potentially fruitful approach to assessing society's sensitivity to climate change is to study the impacts, perceptions and adjustments of recent climate fluctuations. We set out to determine if the recent (1931–82) United States climate record exhibits fluctuations of sufficient scope and magnitude to be useful in a complement of retrospective, empirical studies of climate impacts. The search for fluctuations was designed specifically to identify areas and periods in which the climate within an epoch was terminated by a rather sharp transition to another epoch with a climate unlike the previous epoch. The largest 10- to 20-year temperature and precipitation climate fluctuations were identified across the contiguous United States, along with various scenarios of simultaneous change of temperature and precipitation for the four seasons and annually. All possible 10- to 20-year nonoverlapping “consecutive epochs” within 344 state climatic divisions (as defined by the National Climatic Data Center...