INTRODUCTION

The second Microwave Temperature Profiler (MTP) instrument was built for the NASA Kuiper Airborne Observatory (KAO), a C-141 that was modified for airborne astronomy flights (with a telescope and slit opening). The KAO was based at NASA"s Ames Research Center, Moffett Field, CA.  This instrument was referred to as the Airborne Microwave Radiometer (AMR). The AMR began routine operation in May, 1981.

The rationale for using the KAO as the next platform for CAT studies is based on the belief that a temperature profiler can issue "where" warnigns (where in altitude CAT could be found) but it cannot issue the "when" advisory, and the the KAO routinely flew Pete Kuhn's "when" sensor.  These two instruments would work synergistically in a two-step manner:  1) Kuhn's IR radiometer would be used to give an alert of upcoming CAT, and 2) the AMR T(z) information would be used to guide the pilot away from likely CAT generating altitudes (the tropopause and inversion layers).

Another rationale for advancing to another aircraft with a new instrument was that the CV-990 experience was an obvious success that promised a possible solution to the CAT warning problem, but in order to statistically demonstrate its usefulness a better instrument flying more hours would be needed.  The CV-990 instrument was not built specifically for this task, and it suffered from many unnecessary limitations (an inevitable result given the 5-month interval between funding and flight).  The KAO instrument would be 5 times more sensitive, exhibit a greater altitude coverage, have smaller window corrections (for window absorptions and reflections), and would provide real-time displays on a monitor of T(z) profiles.

A final rationale for using the KAO was that it flew at higher altitudes than the CV-990 during the CAT flight test series. There were hints that when the CV-990 microwave temperature profiler showed that flight was near the tropopause there was a greater probability of CAT, but most of the CV-990 flight hours were at lower altitudes, where the Doppler lidar had stronger signals, so the CV-990 CAT test flight series was unsuited to confirming the microwave temperature profiler's ability to associate its tropopause profiles with CAT.

As it turned out, the 3-year KAO experience showed that the IR radiometer did a poor job of generating "when" alerts when Pete Kuhn was not aboard the KAO, and he was rarely aboard.  This was a mystery, and it suggests that Pete Kuhn had an intuition for when CAT was imminent and it that intuition was not encoded into his real-time analysis program.  Therefore, the synergism was never demonstrated.  Nevertheless, the AMR was able to acquire sufficient flight hours to enable the construction of a CAT incidence per unit time for a set of 11 common T(z) shapes.  This result gave additional confirmation of the association of CAT with the tropopause and inversion layers.

HARDWARE

The AMR consisted of a "sensor unit" (below) installed in the right wheel well of the C-141 and a rack-mounted computer and electronic control panel in the passenger compartment.   The sensor unit consisted of an electronics box with a feed horn that rotated about a horizontal axis.  The local oscillator frequency was 56.0 GHz, and was double sideband. A scalar feed horn with a 7.5 degree half-power beam was scanned through the elevation range -40 to +40 degrees in 10 steps.  Two calibration targets were placed beyond the sky scan positions; one target was allowed to drift just above ambient and the other, called a "hot target," was heated to a specific temperature much higher than ambient.  The radome window was much thinner than that used for the CV-990 instrument because the sensor unit was located in the wheel well, which is unpressurized. The window material was high density polyethylene, grooved on both sides with opposite polarity, and because of its thinness and having grooves on both surfaces it had a lower absorption and reflection than the CV-990 window.  Aircraft altitude, roll and pitch were acquired using a serial output from another computer on the KAO.  A vertical accelerometer was installed in the main equipment rack. A HP9825 desltop computer controlled the sensor unit scanner box, recieved data from the aircraft computer, created CRT displays of T(z) and recorded data on magnetic tapes.  The entire AMR system could be started by the flip of a switch, and this allowed us to obtain data by merely instructing someone on the flight crew to turn us on after take-off and turn us off before landing.

Additional material describing the C-141 can be found at ???

CAT INVESTIGATION

The principal result of the 157 flight hours of AMR measurements of T(z) and the presence of CAT is shown in the next figure.

This is a scoring matrix for all CAT encounters with the AMR.  To illustrate its use, the upper-left corner shows a score of 0 +/- 2.6; this means that when the T(z) shape was isothermal (which occurred on 27 occasions), 0 % of those situations produced CAT, and the statistical uncertainty of the 0% number is 2.6%.  The lower-left corner of this box shows that zero out of 27 occasions produced CAT.  The top row of 4 boxes are "stratospheric."  The middle row shows the two principal "tropospheric" T(z) shapes.  Shape "T3" is associated with CAT 23.0 +/- 3.8 % of the time.  The right-most column of 3 boxes shows the breakdown for regions of the atmosphere.  Clearly, the tropopause has the strongest association with CAT.

A fuller description of this study can be found at AIAA84, which is a copy of a publication describing these results.

A description of the 1982 Dec 8 encounter with severe CAT while flying within a tropospher inversion layer can be found at Most Severe C141 CAT
 

THE FIRST IAC

Prior to the MTP-type remote sensing of T(z) the only way to construct an isnetrope altitude cross-section, IAC, was to fly the same ground track at many altitudes and hope the temperature field didn't change during hte series of passes so that the in situ air temperature readings could be combined to provide an IAC with  crude altitude resolution.  Ed Danielsen was a master at creating such plots.  The MTP allows a better quality IAC to be constructed with just one pass through a region.  Here is the first IAC ever produced from an airborne instrument flying along a flight track just once.

It is evident that this IAC was constructed "by hand."  It was inspired by Ed Danielsen's IAC plots created from radiosondes.  In this case, it is created from the MTP/C141 AMR instrument for a flight segment with moderate/light CAT.  It is apparent that the isentropes separate prior to and following the CAT encounter for both CAT patches.  In other words, each CAT patch is embedded within a region of air that has been worked over to produce a sub-adiabatic lapse rate which in this case is ~1 km thick.  Presumably the region before and after the CAT patches that have been made sub-adiabatic were turbulent before the aircraft flew through it.

This 3-dimensional pattern of CAT surrounded horizontally by worked-over air that can be used to predict an upcoming CAT encounter resembles my "fried egg" hypothesis, suggested in 1979 after the CV-990 flight experience.  It is also compatible with several other conjectures at other web pages, such as Vertical Compression Hypothesis for CAT Generation.
 
 
 
 
 
 

Additional information on the origins of the MTP work at JPL can be found at MTP Beginnings

This site opened:  October 28, 2003.  Last Update:  October 28, 2003