An anthropogenic signal in Phoenix, Arizona winter precipitation

1
An anthropogenic signal in Phoenix, Arizona
winter precipitation

• Bohumil M. Svoma, Robert C. Balling, Jr.
• School of Geographical Sciences, Arizona State
  University, P.O. Box 874601, Tempe, AZ
  85287-4601, USA
• (Bohumil.Svoma_at_asu.edu robert.balling_at_asu.edu)

2
Precipitation and Aerosols

• PM2.5 linked to cold season precipitation
  suppression (Borys et al. 2003 Givati and
  Rosenfeld 2004 2005 Rosenfeld et al. 2007)
  (orographic precipitation downwind of Phoenix
  (Rosenfeld and Givati 2006 ))
• Sulfate/Sulfur aerosol concentrations
  significantly correlate with precipitation in PMA
  (Shutters and Balling 2006)
• Larger aerosols can act to enhance precipitation
  (Teller and Levin 2006)

3
Phoenix Metropolitan Area (PMA)

• Lacks a mass transit system with growth
  characterized as urban sprawl
• Vehicle emissions are a major source of PM2.5
  (Lewis et al. 2003 Kim and Hopke 2005 Hopke et
  al. 2006)
• PM2.5 concentrations and traffic have midweek
  maxima (Bell et al. 2008)

4
Phoenix Metropolitan Area

• Surrounded by mountains with spatially complex
  precipitation patterns
• Westerly winter wind pattern
• Infrequent but widespread winter precipitation
  events

5
Hypothesis

• The pronounced weekly cycle in traffic and
  therefore PM2.5 in Phoenix produces an inverse
  weekly cycle in winter precipitation
• Weekly cycles have been found in many
  meteorological variables worldwide (Bäumer and
  Vogel 2007 Gong et al. 2007 Bell et al. 2008)
• The absence of weekly cycles has also been noted
  (Schultz et al. 2007)

6
Precipitation Data

• Daily data for 116 FCDMC stations from Nov 1 to
  March 31 extending back to 1980
• All stations had at least 210 days with rain (gt
  1mm)

7
Analysis and Results

• Strong weekly cycle visible in precipitation
  frequency for rain gauge network as a whole.
• This pattern was not apparent for total
  precipitation

8
Harmonic Analysis

• Determined how well precipitation followed a
  sinusoidal cycle.
• Through harmonic analysis PM2.5 in PMA follows a
  significant weekly cycle (Bell et al. 2008)

9
Results

• The first harmonic explained 86.0 percent of the
  variance in the frequency data (p?lt 0.01)
• Among individual stations the explained variance
  ranged from 0.02 to 0.94 (mean 0.54)
• Virtually all stations showed maximum occurrence
  of precipitation on Sunday or Monday

10
Spatial Variability

• Morans I 0.07 explained variance (p??lt 0.01 )
• Morans I 0.06 standardized amplitude (p??lt
  0.01)
• High values of explained variance and
  standardized amplitude were significantly more
  clustered (p??lt 0.01) than the low values
  (Getis-Ord General G )

11
Spatial Variability

• Through linear regression explained variance and
  standardized amplitude decreased with distance
  from the city center (p?? 0.11) where PM2.5
  concentrations are highest (ADEQ, 2006)
• More easterly locations tended to have higher
  explained variances (p?? 0.09)

12
Discussion

• The significant weekly cycle found in winter
  precipitation frequencies is more prominent in
  the center of the PMA and in more easterly
  locations.
• This cycle inversely corresponds to the weekly
  PM2.5 and traffic cycles which have maxima on
  Thursday and Wednesday

13
Discussion

• PM2.5 suppresses precipitation in shallow and
  short-lived clouds (Rosenfeld and Givati 2006)
  and largely has been found to only enhance
  precipitation during convection (Andrea et al.
  2004 Khain et al. 2005 Lynn et al. 2005 Bell
  et al., 2008 )
• While PM10 follows a similar cycle to PM2.5, the
  suppressing capabilities of PM2.5 dominate the
  enhance influences of PM10 in subtropical cold
  season clouds (Teller and Levin 2006)

14
Conclusion

• Considering the suppressing influences of PM2.5,
  the weekly precipitation cycle is likely in
  response to anthropogenic activities in the PMA

15
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