Rain Variability Research Articles

DEVEX-disdrometer evaluation experiment: Basic results and implications for hydrologic studies

Increasing our understanding of the small scale variability of drop size distributions (DSD), and therefore of several bulk characteristics of rainfall processes, has major implications for our interpretation of the remote sensing based estimates of precipitation and its uncertainty. During the spring and summer of 2002 the authors conducted the DEVEX experiment (disdrometer evaluation experiment) to compare measurements of natural rain made with three different types of disdrometers collocated at the Iowa City Municipal Airport in Iowa City, Iowa in the Midwestern United States. This paper focuses on the evaluation of the instruments rather than analysis of the hydrometeorological aspects of the observed events. The comparison demonstrates discrepancies between instruments. The authors discuss the systematic and random effects in terms of rainfall quantities, drop size distribution properties, and the observed drop size vs. velocity relationships. Since the instruments were collocated, the effects of the natural variability of rain are reduced some with time integration, isolating the instrumental differences. The authors discuss the status of DSD measurement technologies and the implications for a range of hydrologic applications from remote sensing of rainfall to atmospheric deposition to soil erosion and sediment transport in the environment. The data set collected during the DEVEX experiment is made available to the research community.

Modern pollen-rain characteristics of tall terra firme moist evergreen forest, southern Amazonia

The paucity of modern pollen-rain data from Amazonia constitutes a significant barrier to understanding the Late Quaternary vegetation history of this globally important tropical forest region. Here, we present the first modern pollen-rain data for tall terra firme moist evergreen Amazon forest, collected between 1999 and 2001 from artificial pollen traps within a 500 × 20 m permanent study plot (14°34′50″S, 60°49′48″W) in Noel Kempff Mercado National Park (NE Bolivia). Spearman's rank correlations were performed to assess the extent of spatial and inter-annual variability in the pollen rain, whilst statistically distinctive taxa were identified using Principal Components Analysis (PCA). Comparisons with the floristic and basal area data of the plot (stems ≥10 cm d.b.h.) enabled the degree to which taxa are over/under-represented in the pollen rain to be assessed (using R-rel values). Moraceae/Urticaceae dominates the pollen rain (64% median abundance) and is also an important constituent of the vegetation, accounting for 16% of stems ≥10 cm d.b.h. and ca. 11% of the total basal area. Other important pollen taxa are Arecaceae (cf. Euterpe), Melastomataceae/Combretaceae, Cecropia, Didymopanax, Celtis, and Alchornea. However, 75% of stems and 67% of the total basal area of the plot ≥10 cm d.b.h. belong to species which are unidentified in the pollen rain, the most important of which are Phenakospermum guianensis (a banana-like herb) and the key canopy-emergent trees, Erisma uncinatum and Qualea paraensis.

The relationship between tibetan snow depth, ENSO, river discharge and the monsoons of Bangladesh

Using satellite estimates of snow depth, we examine the interannual variability of the monsoon rains of Bangladesh, an area greatly affected by land surface hydrological processes including Himalayan snowpack size, snowmelt river flooding, and Bay of Bengal storm surge. For the twentieth century, we found Bangladesh monsoon rainfall (BMR) to be uncorrelated with the All‐Indian Monsoon Index. This result is consistent with previous findings for shorter time records. We next used a short 9‐year record of satellite estimates of April snow depth for the Himalayan region and concurrent seasonal El Niño–Southern Oscillation (ENSO) conditions in the equatorial Pacific to develop an empirical model that explains a high percentage of BMR interannual variability. Inclusion of late spring river discharge levels further improves the empirical model representation of BMR for June–September. These results, though with a limited length satellite record, suggest that BMR interannual variability is constrained by concurrent ENSO conditions, spring Himalayan snowpack size and land surface flooding. The same results could not be obtained from analyses using satellite estimates of snow cover. These findings stress the need for development of a quality longer record of satellite estimated snow depth. The twentieth‐century analysis also indicates that BMR should be considered independently of Indian monsoon rainfall.

Mesoscale convective systems

Mesoscale convective systems (MCSs) have regions of both convective and stratiform precipitation, and they develop mesoscale circulations as they mature. The upward motion takes the form of a deep‐layer ascent drawn into the MCS in response to the latent heating and cooling in the convective region. The ascending layer overturns as it rises but overall retains a coherent layer structure. A middle level layer of inflow enters the stratiform region of the MCS from a direction determined by the large‐scale flow and descends in response to diabatic cooling at middle‐to‐low levels. A middle level mesoscale convective vortex (MCV) develops in the stratiform region, prolongs the MCS, and may contribute to tropical cyclone development. The propagation of an MCS may have a discrete component but may further be influenced by waves and disturbances generated both in response to the MCS and external to the MCS. Waves of a larger scale may affect the propagation velocity by phase locking with the MCS in a cooperative mode. The horizontal scale of an MCS may be limited either by a balance between the formation rate of convective precipitation and dissipation of stratiform precipitation or by the Rossby radius of the MCV. The vertical redistribution of momentum by an MCS depends on the size of the stratiform region, while the net vertical profile of heating of the large‐scale environment depends on the amount of stratiform rain. Regional variability of the stratiform rain from MCSs affects the large‐scale circulation's response to MCS heating.

Distribution and intraseasonal variability of rain over Indian Oceanic region from Indian Remote Sensing Satellite IRS‐P4 Multifrequency Scanning Microwave Radiometer

Multifrequency Scanning Microwave Radiometer (MSMR) onboard Indian Remote Sensing Satellite (IRS‐P4) has been utilized for measurement of precipitation over Indian Oceanic region. MSMR‐derived monthly rain rates for the month of August for three consecutive years are compared with that from TMI and PR. A reasonably good agreement is found. Rain distributions are presented for three seasons, namely, premonsoon (February–May), monsoon (June–September), and postmonsoon (October–January) for the entire period of MSMR from June 1999 to September 2001. The year‐to‐year difference in the three seasons is found in agreement with the weather reports. Latitudinal distribution of the MSMR‐derived rain rate for the Arabian Sea and the Bay of Bengal and for the entire study area are analyzed and are found to be in good agreement with climatology. Furthermore, starting from 60°E longitude up to 95°E, at every 5° interval, latitudinal distribution of rain rate is plotted for the three monsoon seasons. It is found that the vigor of monsoon rain begins building up between 65° and 70°E and goes beyond 95°E. A time series and wavelet analysis of daily rain rate is presented for monsoon seasons of three consecutive years. Wavelet analysis clearly shows the presence of long waves of 30–60 and 10–20 day oscillations and also of synoptic scale of 3–6 day oscillations in the monsoon rain. Furthermore, Hovmoller diagrams of daily rain over the northern Bay of Bengal are analyzed for monsoon seasons of three years, and some northward propagation of rain is reported.

State of the Science: Radar View of Tropical Cyclones

Radar played an important role in studies of tropical cyclones since it was developed in the 1940s. In the last 15 years, technological improvements such as the U.S. National Oceanic and Atmospheric Administration (NOAA) WP-3D tail airborne Doppler radar, the operational Weather Service Radar 1988-Doppler (WSR-88D) radar network, portable Doppler radars, and the first spaceborne radar system on the National Aeronautics and Space Administration Tropical Rainfall Measuring Mission (NASA TRMM) satellite have produced a new generation of tropical cyclone data whose analysis has given scientists an unprecedented opportunity to document the dynamics and rainfall of tropical cyclones, and has led to improved understanding of these devastating storms. The NOAA WP-3D airborne Doppler datasets led to improved understanding of the symmetric vortex and the major asymmetries. The addition of a second airborne Doppler radar on the other WP-3D enabled true dual-Doppler analyses and the ability to study the temporal evolution of the Kinematic structure over 3–6 h. The advent of the WSR-88D Doppler radar network, and the construction of portable Doppler radars that can be moved to a location near tropical cyclone landfall, has also generated new and unique datasets enabling improved understanding of 1) severe weather events associated with landfalling tropical cyclones, 2) boundary layer wind structure as the storm moves from over the sea to over land, and 3) spatial and temporal changes in the storm rain distribution. The WP-3D airborne Doppler and WSR-88D data have also been instrumental in developing a suite of operational single Doppler radar algorithms to objectively analyze a tropical cyclone's wind field by determining the storm location and defining the primary, secondary, and major asymmetric circulations. These algorithms are used operationally on the WP-3D aircraft and on the ground at NOAA's Tropical Prediction Center/National Hurricane Center. The WSR-88D rainfall data, together with new satellite microwave passive and active sensors on the NASA TRMM satellite, are proving useful in studies of the temporal and spatial variability of rain in tropical cyclones. The instantaneous satellite snapshots provide rain estimates to improve our understanding of tropical cyclone rain distributions globally, providing estimates from one instrument and common algorithms in each basin, while the WSR-88D provides high-temporal-resolution rain estimates (1 h), to improve our understanding of the temporal variability of the rain as the storm makes landfall. While these new datasets have led to improved understanding, they have also led to a number of new challenges that the radar meteorology community must face by transferring the understanding gained into new applications and improved numerical weather prediction. These challenges will drive our science well into the next century.

Determination of cloud and precipitation characteristics in the monsoon region using satellite microwave and infrared observations

We have studied the cloud and precipitation features in the Indian summer monsoon region based on satellite microwave and infrared observations. Emphases are particularly given to the differences between stratiform and convective rains and between rains over land and over ocean. In the studied region, average rainfall rate of convective clouds is about 6 times higher than for stratiform clouds while the latter covers 4 to 5 times more area than the former. As a result, convective rains produce higher rain total than stratiform rains with a ratio of about 3 : 2. The difference between convective and stratiform rains is evident virtually by any satellite signatures – cloud top temperature, and microwave emission, scattering and combined signatures. Cloud top temperature appears no skill to reflect surface rainfall rates for stratiform rains while colder cloud top temperatures correspond to higher rainfall rates for convective clouds. As rainfall rate increases, microwave emission signature reaches saturation much quicker for stratiform rains than for convective rains. For convective rains, the rainfall rate – scattering signature relation shows a distinct difference between rains over land and over ocean. Corresponding to the same rainfall rate, the scattering signature over land is about twice as high as that over ocean. The mean vertical precipitation profiles showed that stratiform rains have a constant rainfall rate below freezing level and a sharp drop-off above, regardless over land or ocean. Convective rain profiles, on the other hand, often have the maximum rainfall rate below the freezing level, implying a significant growth of raindrops by warm microphysical processes, such as coalescence. It is also found that given the same surface rainfall rate, the amount of ice particles above freezing level is substantially greater for convective rains over land than over ocean, which explains the difference of scattering signatures between rains over the two different surface types. It is also demonstrated that the sub-pixel variability of rains is largely responsible for the mismatch between plane-parallel model simulations and actual satellite observations.

Possible impacts of Indian Ocean Dipole mode events on global climate

Impacts of Indian Ocean Dipole mode (IOD) events on global climate are estimated by correlation/regression analysis. The analysis examined land rain and temperature and 3-dimensional atmospheric variables for a 42 yr period from January 1958 to December 1999. The correlation between IOD and the El Nino Southern Oscillation (ENSO) is accounted for using the multiple regression technique. We used partial correlation coefficients to describe the unique contribution of IOD to climate variability, independent of ENSO. In the Indian Ocean rim countries, IOD is associated with significant temperature and rain variability manifesting 2 large-scale patterns. In one, land tem- perature and rain are anomalously high over countries west of the Indian Ocean and anomalously low to its east. In the second pattern, enhanced rainfall is found over the Asian monsoon trough, extending from Pakistan up to southern China. Also noted are IOD impacts on several regions remote from the Indian Ocean. Strong correlation is found over Europe, northeast Asia, North and South America and South Africa concurrent with IOD events. Over these regions, positive IOD events are associated with warm land surface anomalies and reduced rainfall. The troposphere above the Indian Ocean exhibits strong variability during IOD events characterized by the following structures: (1) a Walker cell anomaly over the equator; (2) a deep modulation of monsoon westerlies; and (3) a Hadley cell anomaly over the Bay of Bengal. In the extratropics, IOD is associated with equivalent barotropic geopotential anomalies. These assume annular structure in the northern hemisphere, but Rossby wave train structure in the southern hemisphere.

Empirical predictability study of October–December East African rainfall

AbstractRelationships between September atmospheric dynamics over the global tropics and the East Africa short rains over the domain 4°S–4°N,30–39°E were investigated to define a seasonal rainfall forecasting scheme. From total and partial correlation analyses over the period 1968–97, three major and useful signals appear in September. Precursor components of the October–December east–west circulation cell appear over the equatorial Indian Ocean (EIO). They were discovered through various indices of sea‐surface temperature (SST), zonal wind, vertical velocity and moist static energy computed over the western, central and eastern parts of the EIO. Principal component analyses were applied to these indices to form two synthetic indices which discriminate ocean–atmosphere variability in the eastern and western parts of the basin. The Indian monsoon dynamics control the SSTs off the Somalia coast which are known to play an important role in the variability of the short rains. Ridge–trough systems develop off South Africa which, when persistent enough, drive SST anomalies in the eastern tropical and equatorial Atlantic; these in turn are associated with anomalies in the middle levels over an equatorial band ranging from the Guinea Gulf to the western Indian Ocean. The three September signals used as predictors in a multiple linear regression model explain 64% of the rainfall variance; a comparable efficiency is also achieved with a linear discriminant analysis model. Copyright © 2002 Royal Meteorological Society

Varabilidade temporal da precipitação pluvial: nível de nitrogênio no solo e produtividade de cultivares de girassol

Objetivando avaliar os efeitos provocados pela variabilidade temporal da precipitação pluvial sobre o desenvolvimento e produtividade da cultura do girassol (Helianthus annuus L.) em função da adubação nitrogenada e cultivares, realizaram-se experimentos nos anos de 1995, 1996, 1998, em Alagoinha-PB, e, no ano de 1998, em Esperança-PB, com a cultura do girassol, analisando-se quatro níveis de nitrogênio (0, 30, 60 e 90kg.ha-1), dois cultivares (uma precoce e uma tardia), em blocos casualizados, com 4 repetições. Durante a condução dos experimentos, foram analisados os estádios fenológicos, fitomassa, rendimento e precipitação pluvial durante o ciclo da cultura. Com relação aos resultados, verificou-se para o município de Alagoinha-PB uma precipitação pluvial média anual do período de 1.018,6mm. As respostas dos tratamentos adotados para a cultura sobre as variáveis analisadas estiveram na dependência da distribuição da precipitação pluvial durante as fases de desenvolvimento dos cultivares. Foi observada também uma grande variação na resposta do girassol em relação aos níveis de nitrogênio e às cultivares em função do tempo.

Multivariate ENSO index and Indian monsoon rainfall: relationships on monthly and subdivisional scales

El Nino/Southern Oscillation (ENSO) is known to cause world-wide weather anomalies. It influences the Indian Monsoon Rainfall (IMR) also. But due to large spatial and temporal variability of monsoon rains, it becomes difficult to state any single uniform relationship between the ENSO and IMR that holds good over different subdivisions of India, though the general type of relationship between all India monsoon rainfall and ENSO is known since long. The selection of the most suitable ENSO index to correlate with the IMR is another problem. The purpose of the present study is twofold, namely, to examine the relationship between the ENSO and IMR for entire monsoon season by using an ENSO index which represents the ENSO phenomenon in a comprehensive way, namely, the Multivariate ENSO Index (MEI) and to establish the relationships between MEI and IMR for every meteorological subdivision of India for each monsoon month; i.e. June, July, August and September. A comparison of MEI/IMR correlations has been made with Southern Oscillation Index (SOI)/IMR correlations. The result may find applications in the long range forecasting of IMR on monthly and subdivisional scales, especially over the high monsoon rainfall variability regions of Northwestern and the Peninsular India.

¿Catástrofes ecológicas en la estepa? Arqueología del Paisaje en el complejo minerometalúrgico de Kargaly (Región de Orenburg, Rusia)

Kargaly is one of the most important centers of mining and metallurgy in the great Eurasian steppe. Dr. E.N. Chernykh and his team (Institute of Archaeology, Russian Academy of Sciences, Moscow) and various researchers at the CSIC and other Spanish institutions have developed a joint project to undertake a comprehensive study of the site's two main phases of occupation, the Bronze Age (2nd millenium BC) and the first Russian industrialization (1745-1900 AD). The Russian members of the joint team are in charge of the archaeological investigations, while the Spanish members are studying metallurgical and mining technology and production, on the one hand, and the environmental context and impact of these activities, on the other. This article presents the research design and first results of the Palaeoenvironmental research at Kargaly. This work has two aspects. The first consisted of obtaining one of the most complete palaeoenvironmental data sets from the steppes through both the systematic sampling of archaeological sites to recover charcoal, seeds, fruits and pollen and the taking of palynological cores from natural deposits, on the other Both sampling programs were supported by radiocarbon dates. The second aspect, to which the greater part of this article is devoted, was dedicated to contextualizing the palaeobotanical evidence by studying the present-day landscape, with particular attention to understanding the processes which shape the variability of the pollen rain. Our purpose was to obtain explicit and measurable calibrative criteria which would enable us to answer the palaeoenvironmental questions raised by our archaeological and archaeometallurgical research. These questions include, most importantly, the following: what was the extent of forest (the energy base for the mining/metallurgical complex) during the Bronze Age? and how do we evaluate subsistence practices? (an issue related to the origins of agriculture on the steppe). Answering both questions require us to go beyond conventional palaeoenvironmental practice, since they require very specific information about the past spatial distribution of vegetation on both local and regional scales. We propose a methodological perspective that places the practice of palynology in archaeology within the goals, theoretical premisses, and methods of Landscape Archaeology. Using this framework we evaluate and identify the limitation of conventional palaeoenvironmental practice (particularly the use of pollen analysis in archaeology) and we develop an intensive application of model-based approach to palynology, one that combines study of the pollen rain with mathematical modelling of the landscape. To put it into practice we used modern methods of terrestrial observation, such as satellite imagery, grounded in the use of Geo graphical Information Systems (GIS) and global positioning (GPS) technology.

Time variability in rainfal events observed by Pludix

Pludix is a raingage-disdrometer based on the analysis of an X-band Continuous Wave radar signal backscattered by hydrometeors. The Doppler shift of the individual falling particle, which is supposed to randomly cross the radar volume, is detected. The 2048 readings of the signal are inverted to generate a hydrometeor size distribution subdivided into 21 bands. Pludix operates integrating 60-sec. backscattered signal and so it provides a more detailed information with respect to the classical tipping bucket rain gage. Moreover it's possible to detect the actual rainfall rate as an indirect product.Since September I999 the instrument has been in operation aside with a tipping bucket and a weighting scale rain gauge downtown Bologna. More than 200 rainfall events have been detected and subgroup of 21 events has been investigated. The spectral intensity of the signal is analysed with different algorithms for both time variability and precipitation type.A network of Pludix seems to be the right tool for investigating the space variability of rain in addition to time variability.

Moisture Effects on the Morphology of Ears, Cobs and Kernels of a South-western U.S. Maize (Zea maysL.) Cultivar, and Implications for the Interpretation of Archaeological Maize

Studies of the relative effects of growing conditions on specific maize (Zea maysL.) characters enhance interpretation of archaeological maize specimens. To understand better the contribution of moisture as a source of maize variation, this study documents morphological variability of ear, cob and kernel characters by examining a single south-western U.S. cultivar (Tohono O'odham flour maize) grown in 2 consecutive years under five controlled irrigation treatments. Variability in timing and amounts of irrigation water, as well as differences in the timing of rain between the 2 years, resulted in several statistically significant differences in morphological characters. Particularly relevant to the south-western archaeological record of predominantly dryland farming, data from the treatment irrigated only at planting demonstrated differences in maize morphology and grain yield attributable primarily to variability in rain. One character generally recorded on archaeological specimens, row number, was among those least affected by irrigation treatment and rain timing. Cupule width, another often recorded archaeological character, was moderately affected by moisture. For the combined 2-year experiment, characters listed in order from least to most affected were: kernel weight, pith diameter, row number, kernel volume, ratio of kernel width to kernel length, ratio of ear diameter to ear length, kernel thickness, cupule depth, ratio of kernel thickness to kernel length, ear diameter, ear weight, cupule width, cob diameter, ear length, rachis segment length, kernel length, and kernel width. This single cultivar study complements earlier studies of environmental effects on the morphology of maize characters. To determine if moisture effects can mask race-identifying characters, additional controlled-environment studies of other south-western maize cultivars should be conducted. The contribution of moisture, either as irrigation treatment or rain, to reproductive part morphology and grain yield must be considered in the interpretation of archaeological maize.

Application of the NCAR Regional Climate Model to eastern Africa: 2. Simulation of interannual variability of short rains

We have applied the NCAR RegCM2 to the simulation of the interannual variability of precipitation over eastern Africa for the short‐rains season by performing a set of experiments for the years 1982 to 1993. The model reproduced the observed interannual variability of precipitation in most of the years. The results show that remote factors play a dominant role in determining the precipitation anomalies. Interannual variability of precipitation over Tanzania is closely related to El Niño events in their mature phase and sea surface temperature (SST) anomalies over the Indian and Atlantic Oceans. The southward shift of the Arabian High results in a southward shift of the zonal component of the Intertropical Convergence Zone (ITCZ), which is responsible for early onset of the rainy season (e.g., 1982 and 1986). The enhanced St. Helena High and weaker Mascarene High lead to the eastward shift of the meridional branch of the ITCZ for the wet years. Model simulations confirmed a strong positive correlation between precipitation anomalies over Lake Victoria and the warm El Niño‐Southern Oscillation events, by which enhanced moist westerly flow from the Atlantic Ocean and the mainly easterly flow from the Indian Ocean converge over Lake Victoria during wet years. The interannual variability of precipitation over Lake Victoria and the western Kenya Highlands (WKH) are strongly coupled. Positive precipitation anomalies over the WKH region are usually associated with weaker Arabian High and Mascarene High, which weaken the large‐scale divergence over the WKH region and favor the development of convection. The interannual variability of precipitation over eastern Kenya Highlands (EKH) is not directly related to the El Niño events, but the association with a warm SST anomaly pattern over the western Indian Ocean is evident during wet years. An El Niño signal is, however, evident for wet years over the Turkana Channel, warm SST anomalies over the northern Indian Ocean contribute enhanced water vapor transport over the region.

Inter-annual variability of short rains over northern Tanzania

The structure and variability of inter-annual circulation anomalies and precursors over northern Tanzania are investigated. Northern Tanzania experiences bimodal rains centered on: March to May ('long' rains) and October to December ('short' rains), our focus here. Areal rainfall departures in the period 1960 to 1990 are used to identify wet and dry years. Composite fields of sea surface tem- perature (SST), outgoing longwave radiation (OLR) and winds in the lower and upper troposphere are analysed at lags 0, -2, -4 and -6 mo. The historical mean is subtracted to produce anomalies. The pat- terns indicate an east-west gradient in SST anomalies across the Indian Ocean which corresponds with a zonal circulation and enhanced convection over the western basin, and increased rainfall over north- ern Tanzania. A multiple regression model is developed using 3 tropical predictors in the July to Sep- tember period. The model accounts for about half of short-rain rainfall variance in the 1970s and 1980s.

Experimental manipulation of forest ecosystems: lessons from large roof experiments

Environmental impacts on forest ecosystems can be studied by manipulating energy, water, and element input or by changing the internal element cycling. In practice, the intended manipulations in a complex ecosystem such as a forest are followed by unintended manipulations of other factors that may cause artifacts in the experiment. The character and extent of such unintended changes were assessed in five major roof manipulation studies in coniferous forests in Europe. In all five cases the roofs were placed beneath the canopy 2–5 m above the ground and designed to study the response to reduced N and S deposition and effects of drought. Photosynthetic light was reduced 15–50% below the roofs and might have contributed to an observed decrease in forest floor moss cover. Soil temperature differences were up to ± 0.5°C, colder than outside during summer and warmer during winter. Climatic differences were least at the smallest roofs. The sprinkling system was the most critical component in the experimental design. The sprinklers could not reproduce the temporal and spatial variability of natural rain; event size and rain intensity increased, and the number of rain events decreased. It proved particularly difficult to reproduce small rain events. The stemflow proportion of the water input was increased by sprinkling. Observed decreases of litter decomposition and mineralisation under some of the roofs were probably caused by a reduced moisture content of the surface litter due to the differences from natural rain. Exclusion of throughfall by the roof disturbed the internal cycle of nutrients leached from the canopy (Ca, K, Mg) or present in suspended material (N, P, Mg). The circulation of these elements had to be restored by addition or recycling of suspended matter. The unintended changes probably delayed the soil response to reduced acidity input and accelerated the decline of nitrate leaching in response to reduced N input.

Weather Variables Associated with Infection of Tomato Fruit by Colletotrichum coccodes.

Field experiments were conducted to determine the relationship of tomato anthracnose to weather variables. Sixteen potted tomato plants were exposed to field conditions within rows of tomato plants for 4 consecutive days at various time periods during the 1993 and 1994 summer growing seasons. Incidence of fruit infection by Colletotrichum coccodes was correlated with rain variables (amount and duration of rain) alone and in combination with other meteorological factors. The best fitting regression equation, accounting for 72% of the variation in anthracnose incidence (arcsine-square root transformed), was Y = 111.77 - 1.16 HNRo, in which HNRo is the numbers of hours during which no rainfall occurs within 4-day intervals that tomato fruit were exposed to field conditions in central Pennsylvania.

Three-site space-diversity experiment at 20 GHz using ACTS in the Eastern United States

A three-site rain-fade space-diversity measurements experiment at 20 GHz has been in near continuous operation since September 1, 1994. Two receiver sites (at the Applied Phys. Laboratory ~APL\ of The Johns Hopkins University and COMSAT Laboratories) are located in Central Maryland, and the other is located in Virginia. The APL and COMSAT locations are separated by 33 km. The Virginia site is separated from APL by 44.5 km and from COMSAT by 30.5 km. Receivers at each of the sites measure the 20-GHz CW signal level from the radiating beacon onboard the geostationary Advanced Communications Technology Satellite (ACTS). Because of the lateral variability of rain, the likelihood is diminished that intense rain cells will simultaneously intersect all three Earth-satellite paths. There is also a reduced likelihood that intense rain will simultaneously intersect two Earth-satellite paths associated with different two-site scenarios. Hence, a substantially smaller rain-fade margin is required when the sites operate in a diversity mode (e.g., connected together such that the largest signal is used) vis-a-vis uncoupled single terminal operation. In this effort, we examine the efficacy of employing three-site and two-site space-diversity systems to reduce the required fade margin. Single- and joint-terminal rain-fade distributions are calculated for both the three-site and different combinations of two-site scenarios. Diversity gains, which give a measure of the reduced fade margin, are determined from these distributions. The distributions and diversity gains are also individually determined for two six-month periods of the year for which precipitation is predominantly convective or stratiform, respectively. Comparisons between measured diversity gains and those estimated using the model of the radiocommunications sector of the International Telecommunications Union are made.

Climate Variability in a Coupled GCM. Part II: The Indian Ocean and Monsoon

Abstract We have investigated the seasonal cycle and the interannual variability of the tropical Indian Ocean circulation and the Indian summer monsoon simulated by a coupled ocean-atmosphere general circulation model in a 26- year integration. Although the model exhibits significant climate drift, overall, the coupled GCM simulates realistically the seasonal changes in the tropical Indian Ocean and the onset and evolution of the Indian summer monsoon. The amplitudes of the seasonal changes, however, are underestimated. The coupled GCM also simulates considerable interannual variability in the tropical Indian Ocean circulation, which is partly related to the El Nino/Southern Oscillation phenomenon and the associated changes in the Walker circulation. Changes in the surface wind stress appear to be crucial in forcing interannual variations in the Indian Ocean SST. As in the Pacific Ocean, the net surface beat flux acts as a negative feedback on the SST anomalies. The interannual variability in monsoon rain...