Estimation of probable maximum precipitation for a 2-day duration: Part 2 — north Indian region

Summary In a previous study the authors have estimated the probable maximum precipitation (PMP) for a 2-day duration using Hershfield's formulaX _PMP =X _n +k _{m n} for stations in the southern Indian region. In this paper, the study is extended to estimate the PMP for stations in the north Indian region, north of 20°N. Maximum annual 2-day rainfall data for an 80-year period, from 1901, were obtained for 286 stations in the region. A mathematical relationship between the frequency factor (k _m ) and the mean annual extreme rainfall (X _n ) was developed to bek _m = 18.15 exp (–0.0448X _n ). This equation was used to obtaink _m for different values ofX _n and, subsequently, to estimate 2-day PMP values for the 286 stations. Using these PMP estimates, a generalised chart was prepared, showing the spatial distribution of 2-day PMP. It was found that 2-day PMP over the north Indian region varied from 60 cm to 130 cm, and the average ratio of the 2-day PMP to the highest observed 2-day rainfall was found to be 1.91. The results show that there have been instances when almost 2-day point PMP have occurred at some of the stations in the region. These results indicate that the statistically estimated PMP rainall are not therefore, mere theoretical estimates, but they can occur under optimum meteorological conditions.With 4 Figures     

Estimation of probable maximum precipitation for catchments in eastern India by a generalized method

Summary A generalized method to estimate the probable maximum precipitation (PMP) has been developed for catchments in eastern India (80° E, 18° N) by pooling together all the major rainstorms that have occurred in this area. The areal raindepths of these storms are normalized for factors such as storm dew point temperature, distance of the storm from the coast, topographic effects and any intervening mountain barriers between the storm area and the moisture source. The normalized values are then applied, with appropriate adjustment factors in estimating PMP raindepths, to the Subarnarekha river catchment (upto the Chandil dam site) with an area of 5663 km². The PMP rainfall for 1, 2 and 3 days were found to be roughly 53 cm, 78 cm and 98 cm, respectively. It is expected that the application of the generalized method proposed here will give more reliable estimates of PMP for different duration rainfall events.With 5 Figures     

Pre-monsoon surface pressure and summer monsoon rainfall over India

Summary The relationship between the surface air pressure field during the pre-monsoon months and the Indian summer monsoon rainfall is analysed using climate data from 105 stations situated in Eurasia between 0°–60° N and 20°–100° E. Moreover, grid-point data for the whole northern hemisphere are used. Pressure during April over an area around 50° N and 35° E is found to show a significant negative correlation with the subsequent monsoon rainfall. During May the pressure over a large part of the study area south of 40° N shows a significant correlation with its highest value in the heat low region over Pakistan. It is assumed that monitoring of pressure variations over this region may be useful in predicting monsoon rainfall, particularly the rainfall during the first half of the season. Certain limitations of the climate data in this region are also discussed.With 5 Figures     

Surface and upper air temperatures over India in relation to monsoon rainfall

Summary The relationship between the all-India summer monsoon rainfall and surface/upper air (850, 700, 500 and 200 mb levels) temperatures over the Indian region and its spatial and temporal characteristics have been examined to obtain a useful predictor for the monsoon rainfall. The data series of all-India and subdivisional summer monsoon rainfall and various seasonal air temperatures at 73 surface observatories and 9 radiosonde stations (1951–1980) have been used in the analysis. The Correlation Coefficients (CCs) between all-India monsoon rainfall and seasonal surface air temperatures with different lags relative to the monsoon season indicate a systematic relationship.The CCs between the monsoon rainfall and surface-air temperature of the preceding MAM (pre-monsoon spring) season are positive over many parts of India and highly significant over central and northwestern regions. The average surface air temperature of six stations i.e., Jodhpur, Ahmedabad, Bombay, Indore, Sagar and Akola in this region (Western Central India, WCI) showed a highly significant CC of 0.60 during the period 1951–1980. This relationship is also found to be consistently significant for the period from 1950 to present, though decreasing in magnitude after 1975. WCI MAM surface air temperature has shown significant CCs with the monsoon rainfall over eleven sub-divisions mainly in northwestern India, i.e., north of 15 °N and west of 80 °E.Upper air temperatures of the MAM season at almost all the stations and all levels considered show positive CCs with the subsequent monsoon rainfall. These correlations are significant at some central and north Indian stations for the lower and middle tropospheric temperatures.The simple regression equation developed for the period 1951–1980 isy = – 183.20 + 8.83x, wherey is the all-India monsoon rainfall in cm andx is the WCI average surface air temperature of MAM season in °C. This equation is significant at 0.1% level. The suitability of this parameter for inclusion in a predictive regression model along with five other global and regional parameters has been discussed. Multiple regression analysis for the long-range prediction of monsoon rainfall, using several combinations of these parameters indicates that the improvement of predictive skill considerably depends upon the selection of the predictors.With 9 Figures     

A hydrometeorological analysis of the severe rainstorm of 20–22 September 1900 over the Gangetic West Bengal

A hydrometeorological study is made of the September, 1900 severe rainstorm which led up to the record rain-falls over Gangetic West Bengal with subsequent disastrous flooding in the Damodar and the Hooghly rivers. The spatial extent of the rainstorm for different durations has been examined by constructing the isohyetal patterns based on rainfall records of stations affected by the storm. Areal rainfalls for 1,2 and 3-day periods are calculated and the values have been compared with similar values from other major rainstorms of the region. The comparison revealed that the September, 1900 rainstorm was the heaviest for 1,2 and 3-day durations for all the areas. The storm contrib-uted rainfalls of 33.0 cm, 52.0 cm and 62.0 cm over an area of 10,000 km2 in 1,2 and 3 days respectively. This rainstorm could, therefore, be considered as an important input in flood and design storm studies in the Gangetic West Bengal region. A relationship between point to areal rainfall has also been developed with a view to evaluate areal PMP estimates.     

Homogeneous zones of heavy rainfall of 1-day duration over India

Summary The highest recorded 24-hour rainfall totals from 1875 to 1982 for about 300 stations were plotted and isohyets drawn to delineate the homogeneous zones of heavy rainfall. The isohyetal pattern indicated an unsteady increase from less than 20 cm in the far west and far north to over 50 cm on or near the coasts. At a few inland stations outstanding amounts have been recorded but these are randomly distributed in space and time. Besides these, there existed a densely gauged area in the Central Peninsula between latitudes 8 °N to 21 °N within which more than 20 cm of rain in 24 hours have never been recorded. The areas of heavy rainfall of one day duration include the entire Indian region except the far western extremity, the northern area bordered by the Himalayas and the central peninsula. The correlation coefficient between the highest rainfall and elevation indicated no significant relationship.With 2 Figures     

Coupling between the El-Niño and planetary-scale waves and their linkage with the Indian monsoon rainfall

Summary The relationships between the El-Niño phenomenon and the planetary-scale waves, and the interannual variations in the Indian monsoon (June–September) rainfall have been analysed in order to investigate how the sea surface temperature (SST) in the equatorial eastern Pacific associated with the El-Niño can produce reduced monsoon rainfall over India by teleconnections.The longitude of ridge location over the Indian region of the integrated planetary waves (numbers 1–3) along 15° N latitude circle in the height field of 200 mb pressure level in May is significantly (r=0.93, significant at 98% CL) related to the May SST anomaly at Puerto Chicama. This implies that warmer (colder) SST anomalies are associated with eastward (westward) longitude of the ridge location. The variations of the ridge location in May appear to be significantly inversely (r=–0.95, significant at 99% CL) related to the Indian monsoon rainfall, with rainfall tending to be less (more) than normal during eastward (westward) longitude of the ridge location suggesting some predictive value for the Indian monsoon rainfall. The Indian monsoon rainfall and May SST anomaly at Puerto Chicama are inversely related (r=–0.90, significant at 96% CL).In terms of the observed relationships, a plausible mechanism for linking El-Niño with the reduced Indian monsoon rainfall is discussed. The relationships noted suggest that excessive warm SST anomalies associated with El-Niño induce an eastward shift in the planetary waves which in turn reduce the Indian monsoon rainfall.With 8 Figures     

Estimation of bulk transfer coefficient for latent heat flux (Ce)

The bulk transfer coefficient for latent heat flux (Ce) has been estimated over the Arabian Sea from the moisture budget during the pre-monsoon season of 1988.The computations have been made over two regions (A: 0–8 ° N; 60–68 ° E; B: 0–4 ° N; 56–60 ° E) with the upper computational boundary fixed at the 300 mb level. The precipitation amount (P) was negligible for region A while the observed values of P have been used for region B. The Ce estimates have been compared with those obtained with other available schemes (Kondo, 1975: Bunker, 1976). which are based on wind speed and atmospheric stability within the surface layer. Our value of Ce is higher in region A and lower in region B than the other estimates.     

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

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

Trends in the annual extreme rainfall events of 1 to 3 days duration over India

Summary In this paper, the annual extreme rainfall series in the time scale of 1 to 3 days duration at 316 stations, well distributed over the Indian region, covering 80-years of rainfall data from 1901 to 1980 were analysed for trend and persistence using standard statistical tests. It has been found that the annual extreme rainfall records of most stations are free from trend and persistence. However, the extreme rainfall series at stations over the west coast north of 12°N and at some stations to the east of the Western Ghats over the central parts of the Peninsula showed a significant increasing trend at 95% level of confidence. Stations over the southern Peninsula and over the lower Ganga valley have been found to exhibit a decreasing trend at the same level of significance. The data series of the stations which showed trends were subjected to a 10-year moving average and the resulting smoothed series have been discussed. It may be said that this increasing or decreasing trend in the annual extreme rainfall events at a few places will have tremendous implications in the hydrologic studies and dam design projects.With 9 Figures     

Light hydrocarbons in the surface water of the mid-Atlantic

During a cruise of RV Polarstern over the Atlantic in September/October 1988, C₂–C₄ hydrocarbons were measured in surface sea water. The ship passed through three different ocean regions divided by divergences at 8° N and 3° S. Hydrocarbon concentrations differed considerably in these regions. The highest values were obtained for ethene with mean concentrations of 246 pMol/l between 35° N and 8° N, 165 pMol/l between 8° N and 3° S, and 63 pMol/l between 3° S and 30° S. Low values were found for i- and n-butane and acetylene between 32 pMol/l and 1 pMol/l. The alkene concentrations were in general higher than the concentrations of their saturated homologs. Concentrations decreased with increasing carbon numbers. The various alkenes were well correlated with one another as were the various alkanes. Oceanic emission rates of the light hydrocarbons were calculated from their sea water concentrations using an ocean atmosphere exchange model. The averaged fluxes ranged from about 10⁸ molec cm^-2 s^-1 for the alkenes and ethane to less than 10⁷ molec cm^-2 s^-1 for the C₄ alkanes. Acetylene emissions were below 3×10⁶ molec cm^-2 s^-1. Based upon these rates budget estimates of NMHC in the ocean surface layer were made with a simple model considering production and destruction processes in the water. The emissions to the atmosphere appear to be the dominant loss process between 35° N and 8° N, whereas destruction in the water seems to be dominant in the latitude ranges 8° N-3° S and 3° S-30° S.     

Pattern characteristics of Indian monsoon rainfall using principal component analysis (PCA)

In the present study the Principal Component Analysis (PCA) is used to determine the dominant rainfall patterns from rainfall records over India. Pattern characteristics of seasonal monsoon rainfall (June–September) over India for the period 1940 to 1990 are studied for 68 stations. The stations have been chosen on the basis of their correlation with all India seasonal rainfall after taking the ‘t’ Student distribution test (5% level). The PCA is carried out on the rainfall data to find out the nature of rainfall distribution and percentage of variance is estimated. The first principal component explains 55.50% of the variance and exhibits factor of one positive value throughout the Indian subcontinent. It is characterized by an area of large positive variation between 10°N and 20°N extending through west coast of India. These types of patterns mostly occur due to the monsoon depression in the head Bay of Bengal and mid-tropospheric low over west coast of India. The analysis identifies the spatial and temporal characteristics of possible physical significance. The first eight principal component patterns explain for 96.70% of the total variance.     

Surface NO and NO2 mixing ratios measured between 30° N and 30° S in the Atlantic region

Surface NO and NO₂ mixing ratios were measured aboard the research vessel Polarstern during the mission ANT VII/1 from 24 September to 5 October 1988. The measurements were taken along the meridian at 30° W in the Atlantic region covering latitudes between 30° N and 30° S. The average mixing ratios were about 12 pptv NO/30 pptv NO₂ in the Northern Hemisphere and about 7 pptv NO/22 pptv NO₂ in the Southern. Elevated mixing ratios of 20 pptv NO/70 pptv NO₂ were found at 12° N (probably due to air masses originating from the surface of West Africa) and in the region of the ITCZ between 8° N and 5° N. Because of probable contamination by the ship, the measured mixing ratios mostly represent upper limits.     

Design storm studies for the Upper Krishna river catchment upstream of the Almatti dam site

Summary The Almatti dam is the major engineering feature in the development of water resources in the Upper Krishna river forming a storage reservoir of 6425 million m³ at spillway crest level. In this paper, the design storm rainfalls for different return periods and also the Probable Maximum Precipitation (PMP) for the catchment above Almatti dam have been estimated to review the adequacy of the flood spillway design for the dam. The design storm rainfalls of various return periods have been computed from a statistical analysis of point and areal time series of annual maximum rainfall. In evaluating the PMP, the maximum observed rainfall obtained by Depth Duration method were maximized as the orography of the Western Ghats plays profound influence over the catchment. It was found that (area 35925 km²) the highest areal rainfalls over the catchment were 14.0 cm, 21.5 cm and 24.6 cm in 1, 2 and 3-day durations, respectively. These are scaled up by a factor of 1.23 to obtain the PMP rainfalls. The areal PMP estimates for the upper Krishna River (UKR) catchment above Almatti dam have been found to be 18.0 cm, 27.0 cm and 31.0 cm, respectively.With 6 Figures     

Temporal and spatial variability of SSTs in the tropical Atlantic and Indian Oceans

Summary The objective of this study is to describe spatial and temporal patterns of sea-surface temperature (SST) variability in the Atlantic and Indian Oceans. The analysis domain extends from 40°S to 25°N and 50°W to 80°E, hence the tropical and most of the South Atlantic and central and western Indian Oceans. The investigation, covering the years 1948 to 1979, utilizes the COADS marine data set. Empirical orthogonal functions and spectral analysis are used to analyze SST fields.A major finding of this investigation is that SSTs vary coherently throughout most of the analysis domain. The greatest coherence is evident from 10°N to 30°S in the Atlantic and from 20°N to 35°S in the western Indian Ocean. Spectral analysis of regional time series shows that throughout this region the time scale of 5–6 years is the dominant one in the fluctuations; this is also the case for the Southern Oscillation and for equatorial rainfall. SST variations are roughly in-phase within each ocean and the two oceans are roughly in-phase with each other, i.e., the lags which exist are much smaller than the dominant time scale of the fluctuations. The SST anomalies appear to propagate eastward from NE Brazil; the eastern Atlantic lags the western by two to six months and the Indian Ocean lags the western Atlantic by four to eight months.With 15 Figures     

Sea Level Pressure Departures in the Mediterranean and their Relationship with Monthly Rainfall Conditions in Israel

Summary Monthly rainfall conditions in Israel were determined, using data from 12 stations, during 30 years (1961–90). The definition of a month to be dry, normal or wet, was done using standardized rainfall totals. Pressure departures for each of the three rainfall categories for each month of the rainy season, were calculated and mapped. Correlation between rainfall totals at each of the 12 stations and monthly mean sea level pressure at 72 grid points in the area delimited by the 20° W and 50° E meridians and the 20° N and 60° N parallels, was performed. For each month, 12 correlation maps were prepared (one of each station). Similar maps were averaged together to form coherent rainfall regions. At the beginning of the rainy season (October) the rainfall in Israel is sporadic and spotty without a distinctable coherent region. At the end of the rainy season (April) the rainfall is more widespread, forming a large coherent region covering most of the country. Dry rainfall conditions in Israel, were found to be characterized by positive pressure departures in the eastern Mediterranean and over Israel and/or by easterly or southerly circulation over the eastern Mediterranean. Wet rainfall conditions in Israel, were found to be characterized by negative pressure departures in the eastern Mediterranean and over Israel and/or by westerly or northerly circulation over the eastern Mediterranean. Moreover, in many cases dry conditions in Israel, were associated with below normal pressure conditions over central or western Europe, while wet conditions in Israel, with above normal conditions over the same region, thus, reflecting the so-called Mediterranean Oscillation. Finally, normal rainfall conditions are characterized by very slight to negligible pressure departures over the entire Mediterranean and Europe. Received November 18, 1997 Revised March 3, 1998     

Tropospheric ozone and oxides of nitrogen over the north-western Pacific in summer

In summer, atmospheric ozone was measured from an aircraft platform simultaneously with nitric oxide (NO), oxides of nitrogen (NO _y ), and water vapor over the Pacific Ocean in east Asia from 34° N to 19° N along the longitude of 138±3°E. NO _y was measured with the aid of a ferrous sulfate converter. The altitude covered was from 0.5 to 5 km. A good correlation in the smoothed meridional distributions between ozone and NO _y was seen. In particular, north of 25° N, ozone and NO _y mixing ratios were considerably higher than those observed in tropical marine air south of 25° N. NO _y and O₃ reached a minimum of 50 pptv and 4 ppbv respectively in the boundary layer at a latitude of 20° N. The NO concentration between 2 and 5 km at the same latitude was 30 pptv. The profiles of ozone and water vapor mixing ratios were highly anti-correlated between 25° N and 20° N. In contrast, it was much poorer at the latitude of 33° N, suggesting a net photochemical production of ozone there.     

Latitudinal variation of measured NO2 photolysis frequencies over the Atlantic Ocean between 50° N and 30° S

Using a filter radiometer, the meridional profile of the NO₂ photolysis frequency, J(NO₂), was measured between 50° N and 30° S during the cruise ANTVII/1 September/October 1988 of the research vessel Polarstern on the Atlantic Ocean. Simultaneously, global broadband irradiance and acrosol were monitored. Clean marine background air with low aerosol loads (b _sp=(1–2)×10^-5 m^-1) was encountered at the latitudes 25° N–30° N and 18° S–27° S, respectively. Under these conditions and an almost cloudless sky J(NO₂) reached 7.3×10^-3 s^-1 (2 sr) for a zenith angle of 30°. Between 30° N and 30° S, the latitudinal variation of the J(NO₂) noontime maxima was less than ± 10%, while the mean value at noon was 7.8×10^-3 s^-1. For the set of all data between 50° N and 30° S, a nearly linear correlation of J(NO₂) vs. global broadland irradiance was found. The slope of (8.24±0.03)×10^-5 s^-1/mW cm^-2 agrees within 10% with observations in Jülich (51° N, 6.2° E).     

Annual mean statistics of the surface fluxes of the tropical Indian Ocean

The computed long-term annual mean and intramonthly variances of air and sea surface temperature, wind stress, effective radiation at the surface, heat gain over the ocean and the total heat loss for the tropical Indian Ocean between 30 °N and 30 °S are presented. These estimates, which are based on about one million weather reports for the period 1948–1972, indicate a mean annual meridional heat transport in agreement with previous estimates in direction though different in magnitude. The annual mean E-P chart shows that the Bay of Bengal region is highly conducive to large-scale convergence.     

Rainfall and the 6–9 day wave-like disturbance in West-Africa during summer 1989

Summary Using ECMWF analyses and daily rain amounts of 569 stations in Western Africa for summer 1989, the study documents the composite structure of the 6–9 day oscillation and its influence on rain. Rain is modulated by vorticity as displayed in the wave composite. There are rainfall maxima coincident with cyclonic vorticity and rainfall minima coincident with anticyclonic vorticity at the 700hPa level, at 17.5°N and 7.5°N.With 6 Figures