Inter-comparison of Raingauges on Rainfall Amount and Intensity Measurements in a Tropical Environment

The HKO （Hong Kong Observatory） has been carrying out an inter-comparison of automatic raingauges since 2011 for identifying raingauges that can meet the ~ 5% accuracy requirement of the WMO （World Meteorological Organization） in measuring rainfall amount. The inter-comparison was conducted at HKO＇s meteorological stations at King＇s Park and Hong Kong International Airport in Hong Kong. Two 0.1-mm resolution Pluvio-OTT weighing gauges were introduced in 2013. This type of raingauges has outperformed others in the WMO＇s field inter-comparison held between October 2007 and April 2009. The performances of 14 raingauges, comprising five different measurement methods, viz. drop-counting, weighing, tipping bucket with software correction, tipping bucket with extra pulse correction and tipping bucket without correction, were evaluated. The focus was to study their performances in rainfall intensity measurement, especially during heavy rain situations. Different high rainfall intensity episodes were selected for analysis. Among these episodes, the maximum 1-minute rainfall intensity as high as around 130 mm/hr was recorded by the Pluvio-OTT raingauges. This paper serves to conclude the 3-year （2011-2013） inter-comparison exercise for rainfall amount measurements and to provide preliminary 1-year （2013） comparison results on rainfall intensity measurements.

The Value of Using Unofficial Measurements of Rainfall: The Dublin Storm and Flood of June 1963

Rainfall measurements are vital for the design of hydraulic structures, climate change studies, irrigation and land drainage works. The most important source of design rainfall data comes from convective storms. Accurate assessment of the storm rainfall requires a fairly dense network of raingauges. In 1963, such a storm took place over Dublin in Ireland. However, the existing raingauge network was insufficient to identify both the depth and pattern of rainfall. An appeal was made by Met Eireann for additional unofficial rainfall data. The result was remarkable in that the estimated maximum rainfall depth was found to be more than double the official value and that the resulting depth area analysis suggested a rainfall volume over a large area much bigger than the original isohyet map indicated. This result has huge implications for the estimation of maximum rainfall and dam safety assessment, especially in countries where the raingauge network has a low density. This paper first provides a description of the synoptic conditions that led to the storm, second an analysis of the rainfall data and how the unofficial measurements produced a very different depth area relationship;third, the social consequences of the resulting flood are described. Fourth, the storm is then placed in the context of other storms in the British Isles Finally the implications for rainfall measurement, gauge density and an example of how revised estimates of probable maximum precipitation (PMP) have been used to improve the safety and design standard of a flood detention dam are discussed.

MJO Modulation of Station Rainfall in the Semiarid Serido,Northeast Brazil

The influence of the Madden Julian Oscillation (MJO) on station rainfall over the Seridó region of Rio Grande do Norte state, Northeast Brazil is examined based on 17 raingauge daily data over 30-year period (1 January-31 December, 1981-2010). The Seridó is one of the driest regions in Northeast Brazil and is recognized as particularly vulnerable to desertification by the United Nations Convention to Combat Desertification. Firstly, daily anomalies were calculated by removing the 30-year daily climatology. Then, to distinguish the MJO signal from other patterns of climate variability, the daily anomalies were band pass filtered in the frequency domain (20 - 90 days) by applying Fast Fourier Transform (FFT). Composites of rainfall anomalies were computed for each of the eight phases of the MJO during February-May (FMAM) rainy season, based on the Jones-Carvalho MJO index. Only days classified as MJO events were considered in the composites. For each phase composite, statistical significance tests were computed independently at each individual station by applying a two-tailed Student’s t-test at 5% significance level. Preliminary results showed that the rainfall anomalies have a spatial coherence throughout the MJO cycle. Extreme positive (negative) anomalies occurred in phase 2 (phase 5), where 13 (12) out of the 17 stations showed statistically significant anomalies in the range of 0.9 - 1.9 mm/day (0.8 - 1.7 mm/day). The typical difference between the wet MJO phase 2 and dry phase 5 represented at least 50% modulation of the daily mean rainfall.

A FUSING TECHNIQUE WITH SATELLITE PRECIPITATION ESTIMATE AND RAINGAUGE DATA

Satellite rainfall estimate can provide rainfall information over large areas,and raingauge can provide point-based ground observations with high accuracy.With the combination of satellite and raingauge data together,the estimated rainfall fields are greatly improved.This combination method,called 'fusing technique',is discussed in this paper,and the validation for this technique is accomplished with HUBEX IOP data.

Real-Time Synchronous Integration of Radar and Raingauge Measurements Based on the Quasi Same-Rain-Volume Sampling

A technique for real-time synchronous integration of radar and raingauge measurements based on the concept of the quasi same-rain-volume sampling（QSVS） is presented.Because of the temporal and spatial discrepancies and resolution differences,the integration of radar measurements with raingauge observations has long been a difficult task.Observations indicate that there exists a correlation that conforms to the power law between hourly accumulated raingauge measurement（Q_G） and detected radar echo（Z_（OH）） over the raingauge.On the basis of this,a concept of the QSVS and five direct correspondent formulas of radar and raingauge samples are built up,aiming to eliminate the temporal and spatial discrepancies.A convenient and practical sampling method—the time integral vertical synchronous sampling（TIVS） is proposed and the Z_（OH）—Q_G relationship is studied.It is significant that under the fixed exponent,the coefficient A_B or A_M varies flexibly in accordance with the temporal and spatial variability of natural precipitation,having the function of synchronously integrating the Z—R conversion and the gauge adjustment into a single equation,and thus the precipitation estimation errors caused by detecting resolution differences between radar and raingauge can be obviously mitigated.The real-time synchronous integration technique using the Z_（OH）—Q_G relationship to estimate the ground hourly rainfall accumulation is called the radar-gauge synchronous integration method（RASIM）.The experiments of two cases show that the accuracy of estimated surface hourly rainfall accumulation within 230 km is about 90%,and the average relative error for the point estimation over the whole process is about 20%. Through the detailed analysis of the applicability of TIVS in three environmental fields with various wind drifts,the physical essence of TIVS is explored：it is an approximate QSVS.By analyzing the data pairs of radar and raingauge,an effective quality-control procedure is established,which can greatly improve the stability and rationarity of the Z_（OH）—Q_G relationship.The forecasting product of hourly rainfall accumulation derived from the RASIM has been put into operation.It is demonstrated that the RASIM plays an important role in the quantitative monitoring and forecasting of short-term torrential rainfall.