Diurnal cycle of rain rate and rain coverage at TRMM ground validation sites

The Tropical Rainfall Measuring Mission (TRMM) Satellite Validation Office (TSVO) has four primary ground validation (GV) radar sites: Melbourne, Florida; Houston, Texas; Darwin, Australia; and Kwajalein, Republic of the Marshal Islands. The TSVO performs quality control procedures on radar data from each site and creates rainfall products in an effort to provide validation for developers of space-based precipitation retrievals. Examination of the diurnal cycle of rainfall using most passive microwave platforms, such as AMSR-E, SSM/I and AMSU-B are inadequate due to inherent sampling issues resulting from their sun-synchronous orbits. While the TRMM satellite orbit was designed specifically to aid in sampling the diurnal cycle, it does so only with a period of about 40 days, while robust sampling, such as is done at GV sites, would require significantly more time. Provided is a quantitative analysis of diurnal precipitation cycles, intensity, and areal coverage. In this study we analyze the diurnal cycle of rain rate and the percentage of areal coverage for five different precipitation intensity thresholds. The TSVO 2A-53 (rain rate) product from 2000-2007 were used to calculate the conditional mean rain rate and the percentage of area with rain rates greater than 0, 1, 2, 5, and 10 mm hr^-1. Additional calculations were performed to find percentage of the rain area with rain rates greater than 1, 2, 5, and 10 mm hr^-1 during diurnal minimums and maximums.

Melbourne's rainfall is characterized by sea breeze induced convection, large organized tropical storms, easterly waves, and frontal boundaries that occur during winter months. A large amplitude diurnal cycle is present at Melbourne, a maximum occurs near 1500 LT with a mean rain rate of 5.5 mm hr^-1 and a minimum near 0100 LT with a mean rain rate of 3 mm hr^-1. Sea breeze induced convection is the major contributor to the afternoon maximum where 4.2% of the area is receiving rain and 17.4% of that rain being 10 mm hr^-1 or greater . Only 1.8% of the area is receiving rain during Melbourne's minimum with 11.7% of that rain being 10 mm hr^-1 or greater .

Rainfall in Houston is dominated by air mass and sea breeze convection, easterly waves, tropical storms, synoptic systems, and mesoscale convective systems (MCS). Houston's diurnal cycle is bimodal with one mode near 0600 LT with the dominant mode near 1400 LT and a minimum near 2100 LT. The early morning mode is due to MCSs that develop overnight in central/eastern Texas during the spring and fall and dissipate over the Houston area during the early morning. During Houston's maximum only 2.4% of the area is receiving rainfall, though within that area 31.3% has a rain rate of 10 mm hr^-1 or greater . Within Houston's diurnal minimum, heavy rain is a large influence with 1.2% of the area receiving rain and 29.2% of that rain being 10 mm hr^-1 or greater . Of the four sites analyzed in this study, Houston has the largest relative percentage areal coverage of rain intensity 10 mm hr^-1 or greater. Throughout the day on average 30% of Houston's rain area is receiving rain of 10 mm hr^-1 or greater.

Darwin's rainfall is characterized by large organized squall lines, sea breeze induced convection; some of the deepest in the world, air mass convection, and Madden-Julian Oscillation (MJO) induced convection. Darwin's diurnal cycle is bimodal. One mode occurs near 1000 LT with a mean rain rate of 7.0 mm hr^-1, and a dominant mode near 2000 LT with a mean rain rate of 11.5 mm hr^-1. A peak in maritime convection during the late morning contributes to the morning mode. Darwin has the largest rain coverage area of the four sites, with 5.8% of its area receiving rain during its diurnal maximum and 22.1% of that rain being 10 mm hr^-1 or greater.

Annual rainfall at Kwajalein is dominated by convective systems that form in the intertropical convergence zone (ITCZ), MCSs, MJO, and easterly waves. Kwajalein's diurnal cycle has a nocturnal maximum near 0200 LT and a daytime minimum near 1000 LT. The Kwajalein diurnal cycle of conditional mean rain rate is rather weak, with the difference between maximum and minimum hourly rain rates of only 0.35 mm hr^-1; however, the diurnal cycle of rain area shows a somewhat stronger cycle with a peak of about 5% areal coverage near 0500 LT, a second peak of 4.3% near 1400 LT, and a distinct minimum of 3.8% near 2100 LT. The second peak is due to an initial increase in convective cells due to day time heating. Solar radiation incident on cloud tops provides a warming and stabilizing influence thereby leading to the late day minimum.

The eight year analysis of high temporal resolution data provides robust representation of the diurnal cycle of both rain rates and areal coverage of precipitation from these four validation sites.