Comments on the ER-2 Microwave Temperature Profiler (MTP) Data Analysis for CAMEX-4

by   MJ Mahoney, Principal Investigator

MTP data is archived in "MP-files," which use the common Gaines-Hipskind data format. The details regarding what is present in each file is included in the file header. 

WARNING: Previous users of MP-files should be aware that the data is now given with scale factors of 1.0, except for the number density. Altitudes (pressure and geometric) are in meters (not 0.01 km units) and temperature is in Kelvin (not 0.1 K units). 

There were serious problems with the ER-2 Nav Data Recorder data, and this is described in detail at:

http://mtp.jpl.nasa.gov/missions/camex4/Science/NDR_Corrections.html

Without accurate pressure data, the MTP retrievals can be seriously compromised. A summary of this problem follows.
In the process of calibrating MTP measurements against radiosondes, it was discovered that there were substantial errors in the ER-2 Nav Data Recorder (NDR) temperature and pressure measurements reported during CAMEX-4. Examination of SOLVE data, shows that these errors have existed for at least two years. (This was not a problem for SOLVE however since MMS data was available.) By studying the behavior of the difference between geometric altitude (Zg) and pressure altitude (Zp ), (that is, Zg-Zp), for the NDR compared to radiosondes and dropsondes, the following expression was derived for the corrected pressure altitude (Zp') in terms of the NDR pressure altitude ( Zp ):

Zp' = fPtoZ( fZtoP(Zp) + Pcorr ) + UTcorr * ( UT(s) - UTtakeoff(s) )/3600 + Zoffset
where the pressure correction (Pcorr), the temporal correction (UTcorr), and the pressure altitude offset ( Zoffset), vary from flight to flight. UTtakeoff is the Universal Time that the ER-2 launches, UT is the Universal Time of the current NDR sample, and fPtoZ(p) and fZtoP(Z) are functions that convert between pressure and pressure altitude, and pressure altitude and pressure, respectively, using the US Standard Atmosphere 1976. (All altitudes are in km and all pressures are in hPa.) An approximate expression in which only the Zoffset term varies from flight to flight is:

Zp' = fPtoZ( fZtoP(Zp) - 2.28 hPa) - 0.01864 * ( UT(s) - UTtakeoff(s) )/3600 + Zoffset
It cannot be emphasized enough how important it is that a reliable and accurately calibrated meteorological system be flown on all atmospheric research missions, since so many instruments, if not all, rely on these fundamental parameters for their data analysis and interpretation. Accurate temperatures and pressures are also essential for assimilation of aircraft measurements into various meteorological models.

A word on temperature calibration is also in order. More details can be found at:

http://mtp.jpl.nasa.gov/missions/camex4/Science/CAMEX4_Temperature_Comparisons.html

The MTP temperature calibration is referenced to radiosondes near the ER-2 and DC-8 flight tracks during the course of a campaign. As a result, there is excellent agreement when the MTP temperature profiles are compared to radiosondes. However, when the MTP data is compared to dropsondes launched during the campaign, it is found that the dropsondes temperatures are 1-2 K warmer than the MTP (and radiosonde) profiles. This was true for both aircraft.

It was also found that the DC-8 ICATS data was 0.83 K too warm compared to radiosondes, and that the ER-2 Nav Data Recorder (NDR) was 1.51 K too warm. We believe that we understand the nature of the NDR error. It was shown elsewhere that there was a leak in the ER-2 NDR static pressure line. This would introduce an error in the calculated Mach Number, and hence in the recovered static air temperature, which depends on the measured total air temperature and the Mach Number. The static air temperature would be overestimated because of this, but this was somewhat ameliorated because a recovery factor of unity (instead of 0.98 appropriate for flight at Mach 0.715) was used. The net result would be a net static air temperature overestimate of 1.5 K, in agreement with our radiosonde comparisons. It is also evident from the NDR ASC-files, that the reported total pressure is in error (it does not equal the sum of the static and dynamic pressure), and that the Mach Number is low by about 0.015 (the ER-2 is always flown at Mach 0.715 +/- 0.005 at cruise altitude and these values are never seen in the CAMEX-4 NDR data files).

2001.08.15	Transit flight from DFRC to JAX. No dropsondes. MTP pointing was not correct; MP-file was not submitted

2001.08.18	Andros Island intercomparison flight. GPS altitude not available to make pressure altitude correction; MP-file was not submitted

2001.08.20	Hurricane Chantel. Could not do normal instrument gain calibration because there were no RAOBs to use for calibration. 

2001.08.26	No dropsondes. OAT spikes at ~63.1 and 63.4 ks, suggesting freezing. MTP retrievals look good.

2001.09.03	Low level radio frequency interference (RFI). No dropsondes
		Good agreement with EYW 9.01.0000UT and 9.05.0000UT radiosondes

2001.09.07	OAT bad at ~63 ks. 
Good agreement with TBW 9.07.1200UT and 9.08.0000UT radiosondes

2001.09.09	Good agreement with TBW 9.09.1200UT and 9.10.0000UT, and EYW 9.09.1200UT and 9.10.0000UT radiosondes

2001.09.10	Hurricane Erin

2001.09.16	Hurricane Erin.

2001.09.19	Good agreement with EYW 9.19.1200UT and 9.20.1200UT radiosondes

2001.09.22	OAT spikes at ~62.6, 64.4, and 68.0 ks, probably due to probe icing.

2001.09.23	OAT spikes at 70.8, 72.6, 74.5, 75.3 and 76.8 ks. Dropsondes at 1854UT and 2157UT warmer than MTP below 3 km.	

2001.09.24	Low level RFI. No OAT spikes. 

2001.09.26	Good agreement with 09.26.1200UT and 09.27.0000UT radiosondes from TLH, LIX, LCH, SHV, DFW and ABQ