TWP/SKYRAD/C1 - Comparison between shaded and unshaded PIRs

We notice a clear-sky diurnal pattern which shows PIR_shaded > PIR_total in the morning 
with a maximum difference of about 2.5 W/m^2. They are equal at about 10AM local time, 
PIR_total > PIR_shaded from 10AM until about 2PM reaching a max difference of about 5W/m^2 at 
noon. PIR_shaded > PIR_total after 2PM, reaching a max difference of about 2.5 W/m^2 
around 4PM. During the night, PIR_shaded is typically about 0.5 W/m^2 greater than 
PIR_total. We have thought about the difference at noon, and think that this difference may be due 
to a heating of the dome of the unshaded PIR, or perhaps a contribution from the IR 
portion of the solar spectrum.  Initial simple calculations show that this solar contribution 
may be of the magnitude as the difference in the two PIRs (consultation with Tom Stoffel 
at NREL indicates that the source of the discrepancy is likely the solar IR).  We cannot 
explain the structure over the remainder of the day.

There are also some general trends that should be noted: (1)cloudy conditions result in 
PIR_shaded > PIR_total (2)Larger positive spikes in PIR_total - PIR_shaded occur in rain, 
particularly when the rain rate >10mm/hr (even for a very short period).

In brief, the two radiometers usually agree within 1 W/m^2  at night and under cloudy 
conditions.  The two instruments tend to deviate during sunny conditions. The descrepancy 
between the shaded and unshaded instruments generally peaks around 5 W/m^2 near local noon 
with the unshaded instrument reporting higher values. 

Note: the unshaded PIR is incorrectly named "shaded2" during this time period.  This is 
covered in a separate DQR.

• Downwelling Longwave Hemispheric Irradiance, Pyrgeometer2, Minima(down_long_hemisp2_min)
• Downwelling Longwave Hemispheric Irradiance, Pyrgeometer1, Maxima(down_long_hemisp1_max)
• Downwelling Longwave Hemispheric Irradiance, Pyrgeometer1, Standard Deviation(down_long_hemisp1_std)
• Downwelling Longwave Hemispheric Irradiance, Pyrgeometer2, Standard Deviation(down_long_hemisp2_std)
• Downwelling Longwave Hemispheric Irradiance, Pyrgeometer2(down_long_hemisp2)
• Downwelling Longwave Hemispheric Irradiance, Pyrgeometer1(down_long_hemisp1)
• Downwelling Longwave Hemispheric Irradiance, Pyrgeometer2, Maxima(down_long_hemisp2_max)
• Downwelling Longwave Hemispheric Irradiance, Pyrgeometer1, Minima(down_long_hemisp1_min)

• Instantaneous Downwelling Pyrgeometer Case Thermistor Resistance, Shaded
  Pyrgeometer2(inst_down_long_shaded2_case_resist)
• Instantaneous Downwelling Pyrgeometer Dome Thermistor Resistance, Shaded
  Pyrgeometer1(inst_down_long_shaded1_dome_resist)
• Instantaneous Downwelling Pyrgeometer Case Thermistor Resistance, Shaded
  Pyrgeometer1(inst_down_long_shaded1_case_resist)
• Instantaneous Downwelling Pyrgeometer Dome Thermistor Resistance, Shaded
  Pyrgeometer2(inst_down_long_shaded2_dome_resist)
• Instantaneous Downwelling Pyrgeometer Thermopile, Shaded Pyrgeometer2(inst_down_long_hemisp_shaded2_tp)
• Instantaneous Downwelling Pyrgeometer Thermopile, Shaded Pyrgeometer1(inst_down_long_hemisp_shaded1_tp)