FINAL, CORRECTED ACE sounding data set: - This includes soundings taken from the two NCAR/ATD/SSSF ISS sites in ACE-1 (Cape Grim and the NOAA Discoverer) and the CLASS soundings taken from the ship "Southern Surveyor". For each sounding there are three files, the "m" file, the "x" file, and the "i" file. The "m" file is the raw thermodynamic data file containing radiosonde thermodynamic data obtained approximately every 1.5 seconds. The "x" file is the ten second data file containing thermodynamic, wind, and position data. NOTE: With the new reprocessing scheme we have recently implemented, the "x" file pressure, temperature, and relative humidity (PTU) data are currently nearly as good as the heretofore final product, the "i" file, PTU data. However, the wind data in this product are NOT finalized. All calculated wind data are included accompanied by a quality flag. The "i" file, the 5mb interpolated data file, is the final processed data file generated from the ten second data file. Quality control constraints are applied in the creation of this file and it is considered the final product. The file naming convention for all these files is the same - "m", "x", or "i" followed by: "mddhhmm.sit" where: m = month (1-9,a,b,c for Jan -> Dec), dd = day of month, hh = hour of day GMT, mm = minute of hour, and ".sit" refers to the site. Only the "x" files are on line on CODIAC. Other datasets available by request only. ************************************************************* ************************************************************* For detailed information on the general sounding processing refer to our information on the "Web": There is a document online with a chapter on sounding data processing: "SSSF Observing Facilities: Description and Specification Version 1" The URL is: http://www.atd.ucar.edu/sssf/facilities/sssf_facility_descrip/sssf.html Chapter 6.0 deals with sounding data processing. This will give you a flavor of what is done in our processing and details of the different files produced. However, please note that a new automated PTU data outlier removal scheme has been implemented in reprocessing but that scheme is not discussed in the documentation online. That will be updated in the near future. For now, some brief IMPORTANT processing information: Ten-second data file ("x" file): - A newly implemented PTU data outlier removal scheme has greatly improved the ten-second data file PTU data quality. - The first point is surface data point (independent of sonde data, it does not affect or influence sonde data in normal processing). - Each ten-second PTU point is obtained from smoothing the raw sonde thermodynamic data over a ten-second interval centered on that ten-second point. (e.g. the point at 10 seconds is obtained from data obtained between 5 and 15 seconds). THE SURFACE POINT DOES NOT influence the first ten-second data point (PTU) in normal data processing. The surface thermodynamic data and sonde thermodynamic data are completely independent, neither is influenced or altered by the other in normal processing. (NOTE: The surface point DOES influence the subsequent 10 second PTU data when the low-level humidity correction is applied. That surface value is required for the correction. Also note that when the correction is applied, the corrected values converge to the old uncorrected values within the first minute of the sounding.) - The wind data in the ten-second data file are NOT considered the final wind data. All wind data appear in this file as does a wind data quality flag (see online documentation for details). Note that wind data through the first 120 seconds are obtained by interpolation between the surface wind and the first OMEGA wind (obtained from a 240 second smoothing interval). 5mb Interpolated Data File ("i" file): This file is generated from the ten-second data file, the "x" file. The first point is again the surface data point. Subsequent points in the sounding are obtained by linear interpolation from the ten-second data file data points. Thus only the first point above the surface in this file is affected by the surface data point. The resolution of this data file is 5mb - the points coming every 5mb at pressure levels divisible by 5 -- i.e. 1005, 1000, 995, 990, and etc. The first point is at the measured surface pressure. The wind data in the 5mb Interpolated data file are considered the final wind data. They are obtained from using only acceptable wind data (based on the quality numbers) from the ten-second data file. (See the online documentation for details of how the quality parameters are used in the production of the 5mb interpolated data file. Note that the wind data are smootherd over 240 seconds. Thus the true wind data resolution is 240 seconds, much coarser than the 5mb data rate provided. NOTE: Bad Wind Data (with good quality flags): This can happen. Bad wind data can get into the 5mb data file. This is due to the nature of the wind data quality flag. That flag is simply the standard deviation of the data in the smoothing interval. Thus, it is possible for the data to be bad for the entire interval resulting in a small, acceptable standard deviation. The flag will then not flag it as bad and it will be included in the final 5mb data file. This is something to be aware of. ************************************************************* ************************************************************* - ACCESS to the sounding data set: We have them here at NCAR. Contact Mr. Bob Rilling (NCAR RDP - rilling@ucar.edu) or myself (millere@ucar.edu). UCAR OFPS has the final data set as well - accessed through CODIAC. ************************************************************* SPECIFICS WITH REGARD TO ACE-1 ************************************************************* 1) Reprocessing and Low Level Temperature and RH Correction: Briefly: Many of the soundings have been corrected for low-level humidity sensor errors. These errors were due to sonde sensor arm heating that was a result of the pre-launch environment. This occured in both the Cape Grim and NOAA Discoverer soundings. The first line of the header in all re-processed files indicates if that sounding had this low-level humidity correction applied or not. Sonde Sensor Arm Heating: In sonde sensor arm heating, the sensor arm itself heats up resulting in too high a temperature reading and too low a rela- tive humidity reading. The humidity reading is low due to the way in which the sensor works. The humicap sensor gives a reading of humidity relative to the temperature of the sensor arm itself. If the sensor arm is warmer than the environment, that humidity reading will be lower than ambient as the ambient vapor pressure remains unchanged, but the saturation vapor pressure (based on the temperature of the sensor arm) is higher than ambient. At launch, the sonde is ventilated by the rising balloon. The temperature sensor is mounted in a very thin cylinder at the end of the radiosonde sensor arm. The thermal time constant of this cylinder is such that the temperature data recovers completely within the first ten seconds of ascent. The humidity sensor is mounted on a flat plate which has a slower thermal time constant. (The thermal time constant of the flat plate was determined to be nearly 14 seconds at 3.0 m/s ventilation in wind tunnel tests. NOTE: this is NOT the relative humidity sensor time constant.) Thus, it takes from up to 60 seconds for the humidity sensor to come to equilibrium with the environment. A correction has been developed for the sensor arm heating. That correction uses the SURFACE DATA and determined thermal characteristics of the sensor arm to correct the data over the first portion of the sounding. The details of the correction can be obtained in the following reference: Cole, H. L., and E. R. Miller, 1995: A Correction for Low-Level Radiosonde Temperature and Relative Humidity Measurements. Proc., Ninth Symp. on Meteorological Observations and Instrumentation, Charlotte, Amer. Meteor. Soc., 32-36. 2) NOTE: There was no reliable surface humidity data available for the soundings taken on the "Southern Surveyor". The processed soundings from the "Southern Surveyor" used sonde RH data prior to launch as the surface relative humidity data point for the sounding. 3) OMEGA WINDS: There is one major inherent problem with OMEGA winds that is clearly present in the ACE soundings. That problem is one of "modular" interference (beyond the scope of this presentation). The result of this is an oscillation in the wind data with height. This oscillation in apparent in several, but not all soundings and is proportedly dependent on station geometry. Steps WERE taken to reduce this effect - 1) an elevated signal to noise cuttoff threshold and 2) elimination of a station from the solution if it is too close to the sounding site. These steps obviouly did not completely mitigate the situation and the investigator should be aware of this when dealing with the data. *************************************************************