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Investigation of Heimann Probe Data
MJ Mahoney, July 24, 2002
During TexAQS 2000 two Heimann infrared (IR) temperature probes were flown
on the NCAR Electra. Data sets from two flights was examined to see if (in
cloud free conditions) these temperatures could be used as an additional
observable in the MTP temperature profile retrievals. The idea was that this
might in some way be able to ameliorate errors in the downward looking brightness
temperature measurements made by the MTP since the air temperature is generally
less than the skin temperature in the daytime. The skin temperature, when
flying at low altitudes, will clearly contribute to the measured brightness
temperature -- reducing it over water and increasing it over land.
August 25, 2000 Flight Data
The chart above plots the temperatures, Ts1 and Ts2, measured by each of
the two NCAR Electra Heimann probes, their temperature difference, Ts2 -
Ts1, and (on the right axis), the second-to-second temperature changes in
one of the probes, Ts1. It is interesting to note that the second-to-second
temperature changes in Ts1 (34.2 K peak-to-peak) are much larger than the
sample-to-sample probe differences (8.3 K peak-to-peak). The temperature
offset between the two probes was 0.25 K.
This flight was predominantly over land, with occassional passes over rivers
or Galveston Bay. As a result, there are few periods when the second-to-second
temperature changes on probe 1 are not large. An exception occurs near 78
ks when the Electra passed over the north end of Galveston Bay. The surface
air temperature on this day was about 30 C, which is basically the lower
limit to the IR temperature except for the early part of the flight over
Galveston Bay. In general the IR temperature over land can range from the
surface air temperature to about 20 K warmer, depending on the emissivity.
This will heat the air in immediate contact with the ground, but the layer
is so thin that the microwave emission is negligible. This skin emission,
however, is not.
August 27, 2000 Flight Data
The chart above plots the temperatures, Ts1 and Ts2, measured by each
of the two NCAR Electra Heimann probes, their temperature difference, Ts2
- Ts1, and (on the right axis), the second-to-second temperature changes
in Ts1. It is interesting to note that the second-to-second temperature changes
(33.7 K peak-to-peak) are much larger than the sample-to-sample probe differences
(5.8 K peak-to-peak). The temperature offset between the two probes was 0.32
K.
The Electra took off to the south on this flight, and remained over ocean
until nearly 67 ks. Since the ocean emissivity is much better behaved than
the emissivity over land, the second-to-second temperature variation is very
small and close to the sea surface temperature of 24 C. It rises as land
is approached and reaches ~30 C as in the flight of August 25. Again, the
peak temperatures over land are more than 20 C above the surface air temperature.
Summary
Over land, the IR skin temperature, because of changes in IR emissivity,
varies from near the surface air temperature to about 20 C warmer. Since
most of the TexAQS 2000 flight time was over land, it does not appear that
there is any obvious manner in which the IR temperature can be used to constrain
the MTP temperature profile retrievals. This problem is compounded because
there is no obvious relationship between the IR and microwave emissivities.