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Monsoon Onset Dates on the Tohono O'odham Nation

Monsoon Onset Dates on the Tohono O'odham Nation. Josef Marlow Tohono O'odham Community College. Develop local (TON) monsoon onset dates for climatology. North American monsoon system regional atmospheric circulation over sw NAm in JJAS

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Monsoon Onset Dates on the Tohono O'odham Nation

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  1. Monsoon Onset Dates on the Tohono O'odham Nation Josef Marlow Tohono O'odham Community College

  2. Develop local (TON) monsoon onset dates for climatology

  3. North American monsoon system • regional atmospheric circulation over sw NAm in JJAS • 500 mb subtropical ridge moves northward into TX & NM • Winds shift from westerly to southerly & easterly, causing an influx of moisture • Thermal low and “Gulf surges” also important sources of moisture • Moisture influx leads to convection and precipitation • Areas along the W coast of mainland Mx between SMO and coast receive 60-70+% of annual precip during monsoon, TON receives 50% • SW AZ on northern fringe of NAMS → variability

  4. Onset Criteria for NAMS • Ellis et al (2004) – 3-day, area avg. dew point temp (50°F) and measureable precip (20% of reporting stations), 1st day • Xeng & Lu (2004) – 3-day, normalized precipitable H2O index > 0.618 in 7 of 9 centered 1°x1° grid cells, 1st day • Higgins et al (1997) - 3-day, area avg. precip.>0.5mm, 3rd day • NWS TFO- 3-day, 54°F avg. dew point temp., one station, 1st day • NWS PFO- 3-day, 55°F avg. dew point temp., one station, 1st day

  5. Data • NCEP North American Regional Reanalysis (NARR) – daily dataset, 32km grid cells, 1979-2002, data from DVD • Surface precipitation, temp. at 2m, rel. hum. at 2m • NOAA CPC (Climate Prediction Center) - 0.25°x0.25° Daily U.S. Unified Precipitation Data, 1979-2002 • Only U.S. coverage • NOAA NCDC (National Climatic Data Center) - U.S. Summary of Day, Dew Point Temperature, 1994-2002 • 1°x1° bins of surface meteorological station data

  6. Method for determining monsoon onset date • calculate precipitation index = daily area average precipitation • Precipitation index threshold = 0.5 mm/d • Monsoon onset date is the first date the threshold is met or exceeded for 3 consecutive days after June 1

  7. TON Monsoon Onset Dates Calculated from NARR

  8. Comparison of NARR and CPC Onset Dates for TON

  9. Red = CPC Green = NWSTFO Blue = NARR

  10. Summary • TON monsoon onset dates determined using precipitation index and 3-day threshold • First day of 3-day period would be improvement • Helpful to use dew point temperature to eliminate “early precipitation events” not related to monsoon, especially in larger area • Would be useful to have dew point temp. in daily NARR dataset • TON onset dates were used to prepare onset +/- 60 day datasets for climatology study

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