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Phil Arkin, ESSIC University of Maryland and Pingping Xie, CPC/NCEP NOAA

The Diurnal Cycle of Precipitation Associated with the North American Monsoon System and its Seasonal and Interannual Variability . Phil Arkin, ESSIC University of Maryland and Pingping Xie, CPC/NCEP NOAA. Outline. Diurnal variability in precipitation and cloudiness over the U.S.

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Phil Arkin, ESSIC University of Maryland and Pingping Xie, CPC/NCEP NOAA

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  1. The Diurnal Cycle of Precipitation Associated with the North American Monsoon System and its Seasonal and Interannual Variability Phil Arkin, ESSIC University of Maryland and Pingping Xie, CPC/NCEP NOAA

  2. Outline • Diurnal variability in precipitation and cloudiness over the U.S. • Seasonal variability in precipitation in the NAME region • Diurnal variability in cloudiness in the NAME region – mean (1987-1997) and an individual year (1990) • Details of the diurnal cycle during the 2003 monsoon season

  3. Diurnal variation in precipitation and cloudiness over the U.S. – radar and geostationary IR • Radar data is an index based on the WSI composites – 4 km/10 minutes, averaged to 20 km/hourly • Satellite data is hourly fractional coverage of 20km areas by pixels colder than 235K (GPI) • Collaboration with Rit Carbone et al, NCAR • Analysis software courtesy John Tuttle, NCAR

  4. June 2003 Geostationary IR (<235K) Radar 6:00PM local 6:00PM local 6:00AM local 6:00AM local

  5. June 2002 Geostationary IR (<235K) Radar

  6. July 2002 Geostationary IR (<235K) Radar

  7. August 2002 Geostationary IR (<235K) Radar

  8. Conclusions: Part 1 • Radar coverage and coverage by IR pixels colder than 235K (GPI) exhibit similar diurnal behavior across the U.S. • Convection begins just before noon just west of 105ºW • After 4-8 hours, eastward propagation is seen in all four months • So it’s at least possible that diurnal variability in cloudiness can be used to diagnose diurnal variability in precipitation in the NAME domain (since we can’t use the radar there for the most part)

  9. Seasonal variability in precipitation in the NAME region from CMAP • CMAP is composite product using several satellite-derived estimates and gauge observations (Xie and Arkin, 1997) • Monthly, 2.5ºx 2.5º used here • Averaged over 1987 – 1997 to correspond to histogram data

  10. CMAP Precipitation 1987 - 1997 April – June July - August Ratio – percentage increase from AMJ to JA

  11. CMAP precipitation (1987-1997) for AMJ (top) and JA (bottom) Tip of Baja California goes from <0.2 to 2-3 mm/day; coastal point to the south goes from 1 to >6 mm/day

  12. Ratio (JA/AMJ) Annual cycle of precipitation averaged over the two boxes

  13. Climatology (1987-1997) of diurnal variability in cloudiness in the NAME region from geostationary IR • Based on geostationary IR histograms collected for GPCP (GOES-E used here) • Pentad averages over 2.5ºx 2.5º areas every three hours • Fractional coverage colder than 255K, 235K (GPI) and 215K (deep convection) shown

  14. 255K 235K (GPI) Climatology of diurnal cycle of cloudiness (pentads for full year) 215K Mean diurnal cycle in fractional coverage (87-97) at 255K (upper left), 235K (upper right) and 215K (lower left). Averaged over NAME region. 0000 UTC at bottom (4-6pm local), time increasing upward.

  15. LAND OCEAN

  16. 255K 235K (GPI) Climatology of diurnal cycle of cloud cover for land portion of the region 215K Mean diurnal cycle in fractional coverage (87-97) at 255K (upper left), 235K (upper right) and 215K (lower left). Averaged over land areas in the NAME domain. 0000 UTC at bottom (4-6pm local), time increasing upward.

  17. 235K (GPI) 255K Climatology of diurnal cycle of cloud cover for ocean portion of the region 215K Mean diurnal cycle in fractional coverage (87-97) at 255K (upper left), 235K (upper right) and 215K (lower left). Averaged over ocean areas in the NAME domain. 0000 UTC at bottom (4-6pm local), time increasing upward.

  18. Conclusions: Part 2 • NAME region characterized by abrupt increase in precipitation (5-10 times) from spring to summer • Diurnal cycle prominent during May – September when averaged over whole region • Huge differences between land and water • Begins in late June in both (northern/western subset of full domain) • Over land, clouds begin to increase in mid-afternoon and peak around 7-8pm; seasonal peak mid-July – mid-August • Over water,diurnal cycle much weaker but still clear – peaks around 4-6am; seasonal peak later – August – September • Much more intraseasonal variability in 11-year average over water

  19. 255K 235K (GPI) Diurnal cycle during 1990 for land portion of the region – pentads from May 1 through mid-October 215K Diurnal cycle in fractional coverage for 1990 at 255K (upper left), 235K (upper right) and 215K (lower left). Averaged over land areas in the NAME domain. 0000 UTC at bottom (4-6pm local), time increasing upward.

  20. 255K 235K (GPI) Diurnal cycle during 1990 for ocean portion of the region 215K Diurnal cycle in fractional coverage for 1990 at 255K (upper left), 235K (upper right) and 215K (lower left). Averaged over ocean areas in the NAME domain. 0000 UTC at bottom (4-6pm local), time increasing upward.

  21. Conclusions: Part 3 • 1990 seems to have weak onset in mid-June over land, with strong maximum in mid-July followed by irregular weaker pulses • Over water, episodes of cloudiness begin in June and seem to get steadily stronger into September • Strong intraseasonal variability over water; also clear over land • The diurnal variations look similar to the 11-year mean (although plots of the anomalies show that some of the maxima are atypical)

  22. Details of the diurnal cycle during the 2003 monsoon season using CMORPH • CMORPH is composite product using all available passive microwave-derived estimates with interpolation by advection inferred from geostationary IR (Joyce et al., 2003, submitted) • Basic dataset is 30 minute/8 km – 3 hour totals for 0.25ºx 0.25º areas used here • Pingping Xie made the figures I will show here; thanks also to Robert Joyce, John Janowiak, Mingyue Chen and Yelena Yarosh

  23. Conclusions: Part 4 • CMORPH allows us to visualize details of the influence of the terrain of the diurnal cycle of precipitation that probably have not been seen before • Precipitation dies away quickly to the west of the Sierra Madre Occidental; only a little rain makes it offshore • ITCZ south of Mexico has weak diurnal cycle with peak just after midnight; sharp demarcation right at coastline • Over the U.S., CMORPH exhibits clear eastward propagation from the Rockies, quite similar to Carbone et al. findings from several years of radar data

  24. Conclusions and Future Work • These results seem to hang together well: • Radar and IR give similar results over U.S. • CMORPH supports them • Results over NAME region consistent with earlier work • But there are still some loose ends: • Need a better way to characterize year to year changes in the diurnal cycle. • How can CMORPH be combined with other information (gauges, radars, …) to ensure the best quantitative results? • Can we extend these data back in time?

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