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<br />Central Oklahoma is characterized by a weaker diurnal variation than those <br />regions to the west. In spring there is little variation in rain cloud <br />frequencies throughout the day. A rather pronounced nocturnal minimum is <br />observed in late summer (around 2200 l.s.t.). This nocturnal minimum per- <br />sists throughout the month of September. In north-central Oklahoma, spring <br />mornings have frequent rain clouds, with a gradual transition to an afternoon <br />maximum in south-central Oklahoma. <br /> <br />A semidiurnal variation in rain cloud frequencies is apparent in the data for <br />northeastern Oklahoma, with the maximum around 0400 and 1600 l.s.t; the <br />minimum around 0900 and 2200 l.s.t. <br /> <br />As in the northwest, southeastern Oklahoma also has a pronounced diurnal <br />variation throughout the year. The highest rain cloud frequencies in the <br />State are found here around 1500 l.s.t. in August and early September. The <br />diurnal variation is lowest in July. <br /> <br />Four sets of calendars have been prepared: (1) frequencies of ~ll rainfall <br />intensities; (2) frequencies of echoes indicating rainfall of less than <br />2.5 mm/h (0.1 in/h); (3) frequencies of echoes indicating rainfall of between <br />2.5 and 25 mm/h (0.1 and 1.0 in/h); and (4) frequencies of echoes indicating <br />rainfall of more than 25 mm/h (1.0 in/h). <br /> <br />The study also compared the daily rainfall measurements from rain gages with <br />rainfall estimated from the MDR data. One would not expect a high degree of <br />agreement since the rain gages are point estimates and the MDR data give <br />area-averaged estimates. For a daily precipitation of less than 5 cm (2 in), <br />the regression relationships were good, explaining almost 50 percent of the <br />variance for the 1977 season. <br /> <br />Rawinsonde data compiled between April and September 1977, from Amarillo, <br />Dodge City, Oklahoma City, and Stephenville, were used to compute daily <br />divergences of mass and water vapor for various pressure surfaces over <br />western Oklahoma. These divergences were compared with the areal extent and <br />mean intensity of radar echoes over the region during the succeeding 24-hour <br /> <br />24 <br />