|Frequently Asked Questions about Radiosonde Data Quality||
(1) Why must radiosonde data be checked for accuracy?
Stations in the NWS network take a total of about 75,000 observations each year. Each observation can contain thousands of measurements of air pressure, temperature, relative humidity, wind direction, and wind speed. Radiosonde defects, ground equipment problems, and/or atmospheric conditions aloft result in some soundings containing erroneous data. Some errors are easy to detect, such as excessive cooling of the temperature with height (i.e. super-adiabatic lapse rate). Others, such as identifying temperature errors caused by solar radiation, can be difficult.
(2) How does the NOAA/National Centers for Environmental Prediction (NCEP) quality control upper-air data?
Before ingesting rawinsonde data into numerical weather prediction models, NCEP corrects temperature data (for some radiosonde types such as Sippican) for solar and infra-red radiation and checks the quality of the data using objective and subjective techniques.
Solar and infra-red radiation significantly affects the accuracy of radiosonde temperature data at heights over 15 km. The temperature errors caused by radiation can exceed 1OC. These errors are difficult for upper-air observers to detect and they are not expected to correct the data. Full correction of the errors is difficult, but NCEP applies an adjustment to the temperature data to reduce the error.
For checking data quality, NCEP utilizes computerized comparisons of the actual upper-air data received from an upper-air station with that generated from a 6-hour numerical weather prediction model. These automated data quality control programs are then combined with knowledge of how the real atmosphere operates. NCEP meteorologists examine the validity of the upper-air data and check the continuity of model comparisons from cycle to cycle. This provides the human element necessary to balance the computer's prognosis. If the temperature, geopotential height, and/or wind data compare poorly, the upper-air data are either deleted or corrected. Typically, most data deletions are for temperature and height data above 10 km. Often NCEP meteorologists find bad data not flagged by the computer or determine that data flagged as bad by the computer are actually good and should be kept. It should be noted that at the present time, RH data are not as closely checked for accuracy as are the other radiosonde data because data analysis tools have yet to be developed.
Data quality control methods at NCEP are not perfect and errors do pass through undetected. Furthermore, NCEP does not quality control soundings before plotting them on upper-air charts. If bad data is disseminated from a station it will appear on the charts. Thus, it is very important that upper-air observers make every effort to check the validity of the sounding data before they are disseminated.
(3) Does the NOAA/National Climatic Data Center (NCDC) quality control the upper-air data archive?
Yes. Similar to NCEP, NCDC relies on a mix of objective and subjective procedures to quality control rawinsonde data before it is archived. Briefly, each sounding undergoes a series of checks to determine the plausibility of the data. If data is determined to be suspect or clearly erroneous, it is flagged as such, but is not deleted from the archive. Unlike NCEP, NCDC routinely checks RH data quality (primarily for rapid, unrealistic fluctuations in RH). As with NCEP data quality checks, NCDC data quality control is not perfect and erroneous data will not always be flagged.
(4) What can the upper-air station do to improve data quality?
Observers must avoid disseminating poor quality observations. Upper-air station staff should not assume that NCEP or NCDC will detect and correct all the errors in the soundings. Data quality control at NCEP and NCDC will not capture all data errors. Furthermore, NCEP does not quality control soundings before plotting them on upper-air charts. If bad data is disseminated from a station it will appear on the charts. Thus, it is very important that upper-air observers make every effort to check the validity of the sounding data before they are disseminated.
Station staff must follow all standard NWS and FMH #3 (Chapter 4) procedures for detecting and editing as much erroneous data as possible. However, not even the best observers can detect all data errors. Effective data quality control involves the upper-air observer, NCEP, NCDC, and NWS administrative office support on the regional and national level. Station staff should also have an adequate equipment maintenance program to keep the radiosonde ground tracking system in good working order. Assistance in troubleshooting problems with the radiosondes, ground tracking system or data communications should be directed to the appropriate NWS Regional Office.
(5) NCEP is frequently rejecting upper-air data from a particular station. Why is this happening and what can station staff do to correct the problem?
Here are some possible causes and corrective actions
(a) Defective radiosondes or ground equipment.
The NWS upper-air network uses about 75,000
radiosondes each year. Some of them will be defective. Likewise, the ground
tracking equipment is old and prone to failure or misalignment. Station staff
should check to see if there is a correlation between the date when the data
rejections started and when a new shipment of radiosondes was first used. If
there is clearly a match, the NWS Regional Upper-air Program Manager should be
contacted as soon as possible. If frequent winds errors are occurring, the
automated radiotheodolite (ART) should be checked for correct
(b) Solar/IR radiation on the radiosonde temperature sensor.
Solar/IR radiation can affect accuracy of the
temperature data (and height data) at altitudes typically above 200 hPa.
Corrections are applied to the data, but sometimes they don't work adequately.
If temperature/height rejections are mostly above 200 hPa and are cyclical in nature (high in the
Summer, low in the Winter or vice versa) then solar/IR radiation is probably
causing the errors. There is little station staff can do to correct this
problem. NWS HQ is working with the radiosonde manufacturers to improve
stratospheric temperature measurements.
(c) Incorrect station data.
All station data in the upper-air computer (release
point latitude/longitude, release point height, release point pressure
correction, etc, etc) must be correct and kept up to date. Station staff should
pay special attention to the pressure correction between the station barometer
height and the height of the balloon release point and make sure it is correctly
entered into the upper-air computer station data file. If this value is wrong,
upper-air height errors will result. NWS HQ works with NCEP to make sure the station data
used for the numerical weather prediction models is correct.
(d) Observer Error.
Improper handling of the radiosonde, balloon, or
flight train prior to release can introduce data errors. Also, incomplete or
incorrect data quality control by the observer can cause problems as well. This
- Incorrect measurement of the station pressure. This value must be as accurate as possible to ensure that the radiosonde is properly baselined.
- Incomplete deletion of bad radiosonde data. All observers are required to check the validity of their soundings prior to dissemination. The upper-air training guide provides instructions on how to identify and edit bad data.
- Incorrect deletion of radiosonde data, especially position data. While deleting elevation and azimuth angles (used for calculating winds), some observers will try to identify and keep one or two "good" points in a layer of clearly erroneous data so that the data profile is preserved. Trying to identify which position points are good or bad is very difficult and not necessary. All position data in the erroneous layer(s) must be deleted. The same procedure holds true for the temperature and RH data. If these data in a layer are all bad, delete the whole layer.
- Making the radiosonde flight train length too short. The sun can warm the balloon skin to temperatures well above that of the ambient atmosphere. The radiosonde flight train length must be of sufficient length to ensure that heat radiating from the balloon will not contaminate the temperature measurements, especially in the stratosphere.
- Incorrect inflation of the balloon. Under-inflation will cause slow ascension rates, which can lead to inadequate sensor ventilation and loss of wind data (from limiting angles). In most cases, balloons should be inflated so that they have an average ascension rate no less than 275 meters/minute.
- Manipulation of the radiosonde sensor boom or other radiosonde components in a manner not in accordance with the radiosonde preparation instructions. With good intent, some observers manipulate the sensor boom in the belief that it will provide better data or do no harm. In most cases these changes don't work and cause erroneous data. Following all the radiosonde preparation instructions will help ensure a problem free sounding.
Lastly, there have been cases where upper-air data errors are associated with one or two observers at the station who are making some honest mistakes. This can be verified by comparing the observer log with the data error reports issued by NCEP over a several week period. If there is a match, corrective action (i.e., retraining) needs to be taken by the station manager.
(6) What is the NWS upper-air station performance score?
The NWS Observing Systems Branch developed the station performance score program to help monitor and improve NWS upper-air data availability, quantity, and quality. One of the goals of the program was to keep the workload required to score the stations to a minimum and automate the procedure as much as possible. Realizing that many stations across the network use different ground equipment, balloon inflation gas, and radiosondes, the variables and weighting factors that make up the equation were adjusted accordingly to make the scoring as fair as possible to all upper-air stations.
(7) If a NWS upper-air station receives a low performance score are upper-air station staff to blame?
Not always. A low score may be caused by a number of factors beyond the control of station staff. Sometimes undetectable defects in radiosonde components and balloons, weather conditions, or problems with the data communications lines can cause a low score. Therefore, the station score reflects the performance of the entire upper-air program, which includes upper-air observers and program managers, as well as staff involved in equipment maintenance, logistics, and data communications. Upper-air station managers should use the scores as only one of many tools for assessing how well their station is performing.
(8) Why should a station's performance score be reduced when poor or missing observations were not caused by station staff?
First, NWS management realizes that missing, late, or
poor quality observations are not always the observers fault. ALL
upper-air stations, at one time or another, will get an "unfair" hit on their
monthly score. It is random and this is helping keep the network scores as fair
as possible. The final score will be an average over a full year and
this will help smooth out low scores that may have occurred during the year.
One of the goals of the station performance score program is keep workloads required to generate the scores to a minimum. Going through each sounding to determine who or what caused a missing/poor quality sounding and then adjusting the scores accordingly would be an overwhelming task (the NWS network disseminates over 5,500 soundings/month). Therefore, generation of the scores is automated as much as possible. Further improvements to the equation will be made as additional performance measures are automated.
(9) What is NWS HQ doing to help improve performance scores?
NWS HQ works with the NWS Regional Offices to identify stations with faulty equipment (ground tracking system, balloons, etc) or other problems in the network and provide any needed assistance. As an example, the upper-air performance score program revealed that data communications between the upper-air station and NCEP needed improvement. Stations were disseminating upper-air data on time, but NCEP received the data late or not at all. To help solve this problem, NWS HQ asked NCEP to implement a new web page that notifies, in real-time, those upper-air stations that NCEP has or has not received data from. This way, notified stations can retransmit missing data.
(10) What is the "Isaac M. Cline Award"?
The Isaac M. Cline award will be given yearly to the NWS upper-air station that best meets the following criteria:
(1) Provides reliable and high quality upper-air
observations on a routine basis. This includes:
- Complete and correct data quality control of all upper-air data
- Makes few second balloon releases
- Few failed soundings
(2) Adheres to ALL policies and procedures for
taking upper-air observations and operating and maintaining the upper-air
equipment. This includes:
- Correct preparation of the radiosonde and flight train
- Correct inflation of the balloon so that an average ascension rate of 275 meters/min from surface to flight termination is achieved. Under or over inflation of the balloon on a routine basis is not acceptable.
(3) Takes the initiative to work with NWS Regional Headquarters to improve the upper-air program at their office and identify/correct problems affecting the program.
(4) Readily participates in system tests and special projects as directed by Regional and National headquarters.
A station's average performance score for the year plays a role in determining the award, but it's only given a 40% weighting factor. Performance scores are not used as the only indicator for determining this award.
(11) Who do I contact if I have further questions?
Please send an email to firstname.lastname@example.org. We will try to answer your questions as soon as possible.
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