TEPPS Sonde Data NOTE: To see the full document including the figures please see: http://www.joss.ucar.edu/pacs/tepps_doc/sounding_data.html 1. General Description Two hundred and fourty nine rawinsondes were successfully launched during the 49 days of the PACS TEPPS 1997 cruise on the NOAA Ship Ronald H. Brown. All of the soundings were made with Vaisala RS80 sondes. Of these 45 used Omega tracking to derive the winds (model 15N.15), like those used for the Integrated Sounding Systems (ISS) sites in TOGA COARE. The majority of the sondes (204) used GPS tracking to derive the winds (model 15G). The sonde data were recorded using Vaisala software running on a DigiCora sonde system, which was set up to store the data at 2-s intervals. Vertical resolution was determined by the rate of data transmission and by the filtering applied to the data within the DigiCora before the data are stored. The Omega sonde wind data are transmitted on 10 s intervals and held in a 4 min buffer, from which they are then filtered and stored with a final vertical resolution of approximately 300 m. The GPS sonde wind data are transmitted at 2-s intervals and are also filtered, in this case to eliminate the pendulum motion of the sensor detectable by this more sensitive system. The GPS sonde data are filtered over 60-s intervals, yielding a vertical resolution of approximately 30 m. Both systems transmit thermodynamic data every 1-2 s, which are then median filtered over 11-s intervals. Both wind and thermodynamic variables are interpolated to 2-s intervals before being written to disk. Omega sonde launches were distributed randomly throughout the cruise, except for the 00 UTC and 12 UTC sonde launches which were always GPS sondes. The order of the particular sondes launched was randomized relative to their shipping cases in order to minimize the effect of biases associated with a particular sonde batch. Independent surface measurements of temperature, relative humidity, pressure, wind speed and wind direction were provided by the Woods Hole Oceanographic Institution integrated suite of improved meteorological (IMET) instruments and other ship's sensors. The 1-min average data at sonde launch time from the surface meteorology time series are used as the first sounding data point. The IMET temperature and relative humidity instruments were located at 13 m above sea level, and the wind instruments were at 14 m above sea level. The ship's barometer was at 15.5 m above sea level. The procedure for preparing and launching the sonde was designed to minimize heating or cooling of the sonde sensor prior to launch. Ground checks of the temperature, relative humidity and pressure from the sonde were compared to reference instruments. The sondes were released via a launch tube contained in a non-airconditioned van located approximately 10 m from the sea surface. The van door was usually left open to equilibrate the inside of the van as closely as possible to the ambient outside environment. A second set of independent surface points for air temperature and relative humidity was collected at the time of launch using a hand-held sling psychrometer. The soundings were taken at regular intervals throughout the cruise. The launch schedule was as follows: Sondes/ Total Total Location Dates day GPS Omega ------------ ------------- ------- ----- ----- Panama to 970728-970808 6 34 33 8N, 125W on station, 970808-970823 6 93 1 (8N, 125W) 8N, 125W 970824-970825 12 22 0 to San Diego San Diego to 970826-970829 2 9 0 subtropics subtropics 970830-970906 8 49 11 The sonde data were quality controlled at the Joint Office for Science Support (JOSS) at NCAR, using methods similar to those used to quality control the TOGA COARE sounding data (Loehrer et al., 1996; their sections 3a-e). The only significant departure was in the automated quality control check for large inversions as described in Section 3.2 below.). 2. Errors in surface measurements Post-cruise corrections to air temperature and relative humidity have been applied to the WHOI IMET data based on a recalibration of the instruments at Woods Hole immediately following the TEPPS cruise. The correction increased the humidity by a few percent, giving better overall agreement with the hand-held sling psychrometer measurements. Figure 1 shows the sling psychrometer measurements centered about the one-to-one line with the corrected IMET data. The uncorrected IMET data (solid blue circles) lie below the one-to-one line, indicating a dry bias. Few sonde measurements are above the one-to-one line in Figure 1, implying fewer instances where the relative humidity above the surface layer is greater than that measured at the surface (see sections 3.1 and 3.4 for details). Air temperature corrections were small, increasing air temperatures by an average of 0.2 deg C, or equivalent to less than a 1% decrease in relative humidity (Figure 2). The errors for the IMET surface meteorological instruments, used for the first sounding data point, are expected to be similar or greater on a ship than those on a WHOI IMET buoy. The estimated accuracies of IMET buoy measurements (Weller and Anderson, 1996) are given in the table below. Parameter Instantaneous accuracy Units ------------------- ---------------------- ----- wind speed 5 % wind direction 10 deg barometric pressure 0.5 hPa air temperature 0.2 deg C relative humidity 4 % 3. Error analysis 3.1 Omega sonde biases A sounding is considered to have a high or low moisture bias if the offset between the IMET surface measurement and the sonde mixed layer point is greater than that predicted from surface similarity theory for the surface layer moisture gradient by an amount exceeding the combined measurement errors of the IMET and sonde sensors. To investigate the occurance of biases, a mixed layer moisture value was estimated from the 960 mb level. The difference between this value and the surface IMET measurement is compared to surface similarity theory, which predicts that for the conditions encountered during TEPPS, a moisture gradient of about 1 g/kg is expected across the surface layer. The sonde relative humidity sensor has a measurement error on the order of 4% - 5%, and thus increases with increasing humidity. This error also applies to the calibration of the sonde which implies that even if the sonde measurements are well within the measurement accuracy of the instrument, the calibration error could offset these precise measurements by 4% - 5%. The IMET relative humidity sensor has an average accuracy of 4%, with higher errors when the instrument is heated in conditions of direct sun and low winds (Anderson and Baumgartner, 1998). A 4% to 5% error in relative humidity is equivalent to 0.7 g/kg to 1 g/kg in mixing ratio, given the typical range of observed surface relative humidity values in TEPPS of 70% to 80%. Therefore, in order to consider a sounding moist bias, the difference between the mixed layer and surface moisture values must exceed the predicted value of 1 g/kg by an amount equivalent to the sum of the errors of both the IMET and sonde instruments, or 2 g/kg. Thus soundings with a difference greater than 3 g/kg are considered to have a moist bias. Likewise, soundings with a difference between the mixed layer and surface moisture values of less than -1 g/kg are considered to have a dry bias. Most of the Omega sondes launched had been on the shelf for several years and were donated to TEPPS for use before the Omega system was turned off on 15 September 1997. These older Omega sondes account for all but 1 occurance of mixed layer relative humidity values higher than those of their corresponding 10-m IMET measurements (Figure 3), with 26 out of the 45 Omega sondes recording these high mixed layer values. Of these 26 Omega sondes, only 6 have differences less than -1 g/kg (moist bias), while only 1 out of 204 GPS sondes exhibit a moist bias. Drift in the original calibration due to the age of the sensor could be a cause of this problem for the Omega sondes. Also observed in Figure 3 are soundings for which the difference between the surface and mixed layer moisture values is greater than 3 g/kg, indicating a dry bias. There are 8 soundings which meet this criterion, of which all are Omega sondes. The Omega sonde batches, determined from their serial numbers, are seen to be well correlated with the occurance of a moist or dry bias in the sounding. Different batches are indicated in Figure 3 by different colored open squares about the Omega points (red diamonds). The magenta and yellow batches tend to be dry biased, while the cyan, blue and green batches tend to be moist biased. This result favors the interpretation that the biases result from a calibration error of the particular batch. As we have no method to correct these data for moist or dry biases, they stand uncorrected. 3.2 Inversions Part of the NCAR/JOSS quality control procedure is to flag the temperature profiles for large inversions. During COARE, a questionable data flag was used for inversions greater than 5 C/km and a bad data flag was used for inversions greater than 30 C/km. Given the different environmenal conditions in the eastern Pacific, the flagging procedure is slightly modified for the TEPPS tropical soundings (970726-970828). A questionable data flag is used for inversions greater than 30 C/km and no bad data flag is used. For the subtropical soundings (970829-970906), a questionable data flag was 50 C/km above 700 mb and 100 C/km below 700 mb. The tolerance was increased since strong inversions at the top of the boundary layer are typical for the subtropics, and to avoid flagging dry layers observed periodically above the boundary layer which are believed to be real. 3.3 Superadiabatic lapse rates A subset of the temperature data exhibited superadiabatic lapse rates in both the tropical and subtropical regions. NCAR/JOSS examined the data on 6-s intervals and flagged lapse rates greater than 15 deg C/km as questionable, and lapse rates greater than 30 deg C/km as bad data. In addition, following their quality control procedure, where the temperature data are flagged for superadiabatic lapse rates, the humidity data are also flagged with a lesser warning flag. Out of 751028 data points, the NCAR/JOSS analysis flagged 9661 as superadiabatic, or about 1.3% of the data. However, nearly all fo the data points flagged as superadiabatic were questionable (9456, or ~98%) and only 205 (~2%) were flagged bad. From our own analysis of these data, 80% of the superadiabatic lapse rates in the surface layer are accompanied by a dry surface layer which may be erroneous (Figure 4). 3.4 Low-level temperature and humidity errors The low-level temperature and humidity data exhibited questionable behavior for the first few data points for about half of the soundings collected in each region. Dry surface layers are observed with and without superadiabatic temperature gradients and occur with varying recovery depths and shapes in the moisture profile (Figure 5). These dry, and at times warm, surface layers are similar to those observed at some of the ISS sites in COARE. Following COARE, this problem was investigated (Cole, H., 1993) and corrections were developed. Further investigation into the dry surface layers has also been done by the University of Washington on the TEPPS data. In general it is found that while a dry layer appears in a fair number of TEPPS soundings, the magnitude is small, on average 1.2 g/kg, and relaxes fairly quickly, in about 8 s, to the ambient value. A more detailed description of the problem and a discussion of the results are given in the following sections. 3.4.1 Description of the moisture problem The anomalous moisture data are characterized by a decrease in humidity from the independently measured surface point at approximately 10 m to the first sonde point, which is typically measured anywhere between 20 and 40 m. This decrease is then followed by an increase in humidity with height, as the sonde relaxes to the ambient mixed layer value, estimated from the value of the mixing ratio at 960 mb (Figure 5b). Such dry layers are found in 62% of the soundings in the ITCZ and 81% of the soundings in the stratocumulus region, or in 72% of the soundings overall. For our best soundings, those which are not exhibiting a dry or moist bias (section 3.1), and do not contain a dry layer structure, the average difference between the first sonde measurement and the mixed layer value, in terms of the mixing ratio, is 0.3 g/kg. For these same soundings the average difference between the surface and mixed layer values is 1.0 g/kg, in agreement with surface similarity theory, giving confidence in our choice of this subset of soundings for determining an acceptable difference between the first sonde and mixed layer moisture values of 0.3 g/kg. For the subset of soundings that exhibit a dry layer, this difference is in the range of -3 g/kg to -0.01 g/kg, with an average difference of -0.6 g/kg. While these dry layers appear to be a real and erroneous pattern in the data, a value of -0.6 g/kg is within the 4% - 5% measurement error of the sonde humidity sensor. Including measurement error the range of measured values for this difference is then -1.7 g/kg to 2.3 g/kg, based on an expected value of 0.3 g/kg. Only 4 soundings have dry layers which exceed a -1.7 g/kg difference between the first sonde measurement and the mixed layer value. No soundings exceed a 2.3 g/kg difference, indicating no moist layers are observed. 3.4.2 Description of the temperature problem At times a dry layer is accompanied by a superadiabatic lapse rate at the surface, determined from the first three sonde measurements and excluding the independent surface point. The method of least squares was used to determine the lapse rates, for which those greater than 10 deg C/km are considered superadiabatic for the purposes of this discussion. These superadiabatic lapse rates have been determined as part of the analysis done at the University of Washington and are independent from the NCAR/JOSS calculations that determine which data are flagged in the final released data set (section 3.3). Overall, superadiabatic temperature gradients occured at the surface 46% of the time in the ITCZ, and 56% of the time in the stratocumulus region. Using the 10 deg C/km criterion, anomalous dry layers are seen 83% of the time in conjunction with superadiabatic lapse rates at the surface. Dry layers for this overlap statistic are defined as moisture structures for which the first sonde moisture value is less than the mixed layer value, this difference not necessarily being less than -1.7 g/kg. 3.4.3 TOGA COARE error analysis Many TOGA COARE soundings had a similar dry surface layer structure in their temperature and humidity profiles at low levels (see Cole, H., 1993, Figures 6 and 10). After much post-experiment analysis, it was concluded that the sonde sensors were heated by the sun, resulting in incorrectly low relative humidity measurements. This results because the relative humidity on the sonde is determined relative to the sensor arm. If this arm is heated, the higher arm temperature produces the erroneous humidity values. Relative humidity is very sensitive to small changes in temperature. Thus, this error may not necessarily be as evident in the temperature measurements, explaining why superadiabatic temperature gradients are not always measured for all instances of dry layers. Once the problem was determined, the sondes were run through experiments in a wind tunnel to determine the heat transfer function of the sensor arm. The time constant for the sonde to adjust to ambient temperatures given wind speeds typically seen on ascent is 20-25 seconds. Within these 20-25 seconds, the relative humidities exponentially relax to ambient. If the initial temperature of the humidity sensor are known, the correct humidity can be backed out of the erroneous data from this relationship. However, for TEPPS we do not have the original temperature data recorded by the sonde at launch time, so it is not possible to extract the correct values of the humidity in this way. 3.4.4 TEPPS error analysis We suspect that the sonde sensor arm was heated relative to ambient by a small temperature difference between the launching chamber and the outside air. It is fairly certain that the sensors were not heated by solar radiation, first, because the sondes were not exposed to the sun until after they were launched, and second, because of the lack of correlation between the near surface temperature and humidity gradients and incoming solar radiation (Figure 6). The correlation coefficient between the incoming solar radiation at the surface and the near surface temperature gradient is 0.46, while that between the humidity gradients and the incoming solar radiation is -0.32. Even lower correlation coefficients are found between the air temperature and the near surface temperature gradient. These correlations do not indicate a relationship between the behavior of the measurements at the surface and the incoming solar radiation. Our analysis indicates that the likely source of the heating of the sensor arm above ambient prior to launch was related to the launch chamber temperature, and that the chamber temperature is only loosely correlated to the incoming solar radiation. A study is currently underway to measure the temperature difference between the launch chamber and the van and outside temperatures, the results of which will hopefully clarify this issue. The recovery rate of the affected TEPPS soundings has also been estimated by finding the time and height at which the difference decreased by 1/e from its maximum value. The recovery is similar for both regions, with values of 53 m and 8 seconds for the ITCZ and 51 m and 7 seconds for the stratocumulus region. Thus, the TEPPS sonde low relative humidity values recovered more quickly than the COARE sondes, likely because of a smaller sensor arm temperature perturbation from ambient in comparison to COARE. 4. Summary of the sonde data quality analysis Overall we have good confidence in the TEPPS sonde data above 100 m, with the exception of the subset of Omega soundings (14) and one GPS sounding which exhibit a significant dry or moist bias (Figure 3). Below 100 m, anomalous temperature and moisture structure are observed, and although these anomalies are generally within measurement error, they appear to be systematic and erroneous. Therefore we recommend viewing each sounding before any analysis is undertaken involving the surface layer. Anomalous temperature structure, defined as surface layers with superadiabatic temperature gradients, were observed for roughly 50% of the soundings (Figure 4a). Superadiabatic lapse rates greater than 30 deg C/km, the bad data criterion, occurred for 4% of the soundings (Figure 6a). Anomalous moisture structure in the form of dry layers, defined as soundings with a higher mixed layer moisture value than the corresponding first sonde measurement, were observed for roughly 70% of the soundings (Figure 5b). Observed moisture differences varied between just less than zero to -3 g/kg, with an average value of -0.6 g/kg. Of these cases only 4 have moisture differences outside of the error range of the sonde moisture sensor (Figure 6b). The moisture deficit recovers to the mixed layer value within an average of 60 m. The overlap between the occurance of a superadiabatic layer and a dry layer is high, with roughly 80% of the anomalous temperature structures also having an anomalous moisture structure at the surface. As these particular statistics were not calculated for the COARE sounding data, it is difficult to determine how the TEPPS anomalous surface layers compare. We can state that the higher vertical resolution of the GPS sondes provides a more accurate estimate of the the depth of the anomalous layer in TEPPS. Contacts and data access For information on how to obtain these data see our web page at http://www.atmos.washington.edu/gcg/MG/tepps/ For more information regarding quality control issues or general questions on the sonde data email Yolande L. Serra at yserra@u.washington.edu. Data are also available via the UCAR/JOSS CODIAC data management system at http://www.joss.ucar.edu/cgi-bin/codiac/dss?2.223 Here subsets of the data set may be ordered for any selected period of the cruise, and the data may be previewed via skew T/log P or x-y plots. Citation for data set Yuter, S. E., 1998: Convection and stratus in the tropical eastern Pacific: The 1997 Pan American Climate Studies Tropical Eastern Pacific Process Study. In preparation. References Anderson, S.P. and M.F. Baumgartner, 1998: "Radiative heating errors in naturally ventilated air temperature measurements made from buoys", JAM, in press. Cole, H., 1993: "The TOGA COARE ISS Radiosonde Temperature and humidity sensor errors". Technical Report, Surface and Sounding Systems Facility (SSSF), National Center for Atmospheric Research (NCAR), Boulder, 26 p. Loehrer, S. M., T. A. Edmands and J. A. Moore, 1996: "TOGA COARE upper-air sounding data archive: development and quality control procedures", BAMS, 77, 2651-2671. Lucas, C. and E.J. Zipser, 1996: "The Variability of Vertical Profiles of Wind, Temperature and Moisture During TOGA COARE". Seventh Conference on Mesoscale Processes, September 9-13, 1996, Reading, UK. American Meteorological Society, Boston, 125-127. Weller, R. A. and S. P. Anderson, 1996: "Surface meteorology and air-sea fluxes in the western equatorial Pacific warm pool during the TOGA Coupled Ocean Atmosphere Response Experiment", J. Climate, 9, 1959-1990. Figure captions Figure 1: This figure shows 3 sets of points; the sling psychrometer relative humidity values verses the IMET corrected relative humidity, the first sonde relative humidity measurement (the second point in the sounding; n=2) verses the IMET corrected relative humidity and the uncorrected IMET relative humidity verses the corrected IMET relative humidity. Sling measurements were collected on all ship decks as well as on the RHIB (rigid hulled inflatable boat) which went out upwind of the ship once a day for measurements of ocean temperature, air temperature and humidity. Figure 2: Same as Figure 1 only for air temperature. Figure 3: The difference in mixing ratio (r) between the corrected IMET sensor at the surface and the mixed layer value (estimated at 960 mb) is plotted verses time. Data from the Omega sondes constitute all but one of measurements where this difference is negative. A negative difference indicates a possible moist bias in the sonde data, with differences less than -1 g/kg being significant. A difference greater than 1 g/kg indicates a possible dry bias in the sonde data, with differences greater than 2 g/kg being significant for these biases. The occurance of either a dry or moist bias is seen to be associated with particular Omega sonde batches, indicated by different colored open squares around the Omega points. Figure 4: Examples of anomalous low level temperature and moisture structure. Temperature (a) and mixing ratio (b) profiles for 970817 2229 UTC. The lower portion of the temperature profile is superadiabatic, as seen when compared to the dry adiabat shown. This superadiabatic structure is coincident with a dry surface layer seen in the moisture profile. Such anomalous dry layers often accompany superadiabatic lapse rates in the TEPPS soundings. Figure 5: Two examples of temperature profiles (a) and their corresponding anomalous dry surface layers (b) found in the TEPPS soundings. The strongest dry layer observed, which occured on 970816 2238 UTC (green) has a -3 g/kg difference between the first sonde measurement and the mixed layer. On average, this difference was much lower, or approximately -0.6 g/kg. The example from 970820 1840 UTC (blue), represents a more typical dry layer, with a recovery to mixed layer humidity values by 60 m. Figure 6: The lapse rate (a) and difference in mixing ratio between the first sonde measurement and the mean mixed layer value (b) versus incoming shortwave radiation is shown. There is no observed correlation between surface gradients of temperature and moisture and the incoming solar radiation, indicating that heating of the sonde sensor during TEPPS was not related to the external environment, as was the case in COARE.