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Numerous investigations have been made in the last two decades from both a direct measurement of the radar reflectivity and the rainfall amount, as well as indirect measurements of the raindrop size spectra. Calculations of the reflectivity factor and rainfall rate from these spectra can be made and the relationships determined. Both methods are discussed and a summary of the relationships presented. These relationships show differences in excess of 500% in rainfall rate at the same reflectivity. These large differences are primarily associated with differences in geographic locality. In addition, there are smaller differences on the order of 150% that can be attributed to different types of rain or different synoptic conditions. Some data are available which are indicative of the differences in the relationship on a given day, depending upon the location within the storm which is sampled. This is briefly described and in only one case out of 18 is there a significant difference. Estimates of the...

To provide a foundation for other radar studies in the Miami areas, 50 comparisons were made between shower rainfall recorded by raingages and observed with radar to evaluate the reflectivity Z, rainfall rate R relation, Z = 300R1,4, referred to here as the Miami Z-R relation. Total shower rainfalls measured by recording raingages were compared with estimates derived from the Miami Z-R relation in conjunction with radar reflectivity measurements with iso-echo contouring and the analysis scheme described. Rainfall rate comparisons were not possible because of the poor time resolution of the raingage observations. The radar and raingage estimates of shower rainfall were highly correlated (+0.93, significant at the 1% level); they had an average difference of 8% and a mean absolute difference of 30%. Stratification by shower amount revealed that the radar estimate of gage-recorded rainfall was too high for small shower amounts (

Raindrop-size distributions obtained with the drop camera have been used to determine rainfall rate-radar reflectivity relationships for nine different locations throughout the world. Since the climates sampled were quite varied, an extrapolation of these Z-R relationships to other areas of the world with similar “drop-spectra climates” can be performed. Two climatic parameters, the mean annual per cent of rain days that are thunderstorm days, and the mean annual relative humidity at 0.5 km above ground, were found to be highly correlated with the coefficient A and exponent b in the Z-R equation, Z = ARb. Regression equations based on the two climatic parameters were determined, permitting an estimation of the Z-R relationship for any area once the parameters are obtained.

Raindrop-size spectra obtained with the raindrop camera have been analyzed from two locations, Island Beach, N. J., and Franklin, N. C. The spectra were analyzed with respect to total number of drops per average rain rate per cubic meter of sample, geometric mean diameter, mode diameter, and the diameter of drops at which half the liquid water content lies above that diameter and half below. The results indicate that the distributions from both locations are quite similar for corresponding rainfall rates. Rainfall rate-radar reflectivity relationships indicate that cold frontal rains in these areas generally have smaller drops than warm frontal rains. In addition, it was found that upslope rains are composed of smaller drops than rains of similar synoptic conditions without upslope effects. Finally, a small sampling of a tropical storm rain revealed that small drops may be characteristic of this type of rain.

In evaluating the environmental limitations on microwave signal transmission, it was necessary to determine the occurrence of rainfall rates for a number of regions in different parts of the world. Clock-hourly precipitation data were used where available. Where these data were not available, a known empirical technique was employed which correlates clock-hourly rainfall at individual rates to a climatological index. This index uses parameters which are almost universally obtainable—namely, mean annual rainfall and number of days with measurable rain. In the course of this work, the authors found a more general relation between clock-hourly rainfall rate frequencies and the climatological index. Using this relation, rainfall frequencies at any rate are obtained directly from a linear equation and two nomograms. It is also possible to synthesize the complete frequency distribution of rainfall rates and obtain, as a check, the mean annual rainfall. Examples, supplemented by illustrations, are given in the development of this technique and its utilization.

A research program involving the study of the attenuation and backscattering by rain of millimeter wavelength radiation has resulted in the requirement for very accurate measurements of rainfall characteristics along paths ranging from 1000 to 2000 ft. A special continuous-wire raingage has been developed which gives an essentially continuous rainfall response by measuring the water level in a cylindrical gage. An accuracy of 0.001 in of rain is obtainable. The responses from two of these gages are compared with those from a number of modified tipping bucket and other gages for several rains in the Austin area.

We discuss the design and operation of a dense rain gauge system for obtaining statistical data on both the temporal and spatial distribution of heavy rainfall. The data are collected every ten seconds from a network of approximately one hundred rain gauges spaced 1.34 kilometer in a square array which covers an area of 130 square kilometers. The rain gauge is a continuous, flow type with a response time of the order of one second. We describe the system used for recording the data on magnetic tape and give typical computer-generated rain maps for large area storms and for localized showers.

A new type of rainfall gage has been built to read instantaneous rates of rainfall and resistivity. Construction details are given. Maximum rate of rainfall recorded during the period May-November 1961 at East Pittsburgh was 0.07 inch per minute. The minimum resistivity was 1,500 ohms per inch cube and the maximum resistivity 40,000 ohms per inch cube. These values are to be compared with standard wet-test values of 0.2 inch per minute and 7,000 ohms per inch cube.

One hundred fifty-four tables are presented which show the relationships between rainfall rate, radar reflectivity, and attenuation. These parameters have been calculated from drop-size distributions obtained by the drop camera technique. Calculations of attenuation and reflectivity were made using the Mie theory for wavelengths of 10.0, 4.0, 3.2, 1.87, 0.86, and 0.43 cm. Logarithmic regressions have also been calculated for these parameters. These tables have been prepared for data obtained from Illinois, Florida, Oregon, Majuro, Alaska, Indonesia, New Jersey, and North Carolina. Other tables showing the relationships of the drop-size medium volume diameter and liquid water content to reflectivity are presented for New Jersey and North Carolina at 4.0 and 3.2 cm. A table of the relationship of 4.0-cm attenuation to 3.2-cm reflectivity is included for New Jersey and North Carolina.