NCAR Technical Notes NATIONAL CENTER FOR ATMOSPHERI C RESEARCH

Transcription

1 NCAR-TN- 13 The NCAR Dropsonde Program Robert H. Bushnell Laboratory of Atmospheric Sciences National Center for Atmospheric Research Boulder, Colorado January, 1966 / L!BA R Y ' JAN NCAR Technical Notes NATIONAL CENTER FOR ATMOSPHERI C RESEARCH Boulder, Colorado? ' 1 0 I- ) NCAR Library / > 1 I ll 11lllll l II;II l Illl I IIIII 1 I I 1 II_0 III x ,

2 The National Center for Atmospheric Research (NCAR) is dedicated to the advancement of the atmospheric sciences for the benefit of mankind. It is operated by the University Corporation for Atmospheric Research (UCAR), a private, university-controlled, non-profit organization, and is sponsored and principally funded by the National Science Foundation. NCAR shares with other atmospheric research groups four interrelated, long-range objectives that provide justification for major expenditures of public and private funds: * To ascertain the feasibility of controlling weather and climate, to develop the techniques for control, and to bring about the beneficial application of this knowledge; * To bring about improved description and prediction of astrophysical influences on the atmosphere and the space environment of our planet; * To bring about improved description and prediction of atmospheric processes and the forecasting of weather and climate; * To improve our understanding of the sources of air contamination and to bring about the application of better practices of air conservation. The research and facilities operations of NCAR are conducted in four organizational entities: The Laboratory of Atmospheric Sciences The High Altitude Observatory The Facilities Laboratory The Advanced Study Program All visiting scientist programs and joint-use facilities of NCAR are available to scientists from UCAR-member and non-member institutions (including private and government laboratories in the United States and abroad) on an equal basis. The member universities of UCAR are: University of Alaska Florida State University University of Oklahoma University of Arizona University of Hawaii Pennsylvania State University University of California The Johns Hopkins University Saint Louis University University of Chicago Massachusetts Institute of Technology Texas A & M University Colorado State University University of Michigan University of Texas University of Colorado University of Minnesota University of Utah Cornell University New York University University of Washington University of Denver University of Wisconsin

3 NCAR-TN-13 : i. The NCAR Dropsonde Program as Robert H. Bushnell 2- / Laboratory of Atmospheric Sciences January, NCAR Technical Notes National Center for Atmospheric Research P/'7 /, Boulder, Colorado

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5 1 INTRODUCTION The NCAR dropsonde program, now actively under development, is intended to secure measurements within thunderstorms, by dropping several sondes nearly simultaneously into the top of a storm cell. Each sonde will be tracked and will telemeter sounding data to the ground. Data from several sondes, falling simultaneously, will be needed to make it possible to distinguish between spatial structures and development in time. The falling sondes must be considerably less than 500 m apart horizontally, a distance which we expect is the order of diameter of a strong active updraft. Eventually we hope to make a number of drops in succession into a single storm, to observe its time development. OBJECTIVES We hope to determine such things as: 1. Whether updrafts slant or are vertical. 2. Whether new updrafts grow on a favored side of a cell. 3. Spacing of updrafts. 4. Extent of non-turbulent draft regions in a cell. 5. Energy budget of a cell (with greater than present accuracy). 6. Motion field of a cell. 7. Water distribution in a cell. 8. Location of hail in a cell. PRELIMINARY STEPS We have designed a sonde to measure the vertical speed of the air in a storm and are proceeding with the design of the tracker. These considerations have the greatest effect on the sonde design and so are done first. When the initial design functions well, we will add other measurements. These may include measurement of droplet spectra and water content. We will consider a microphone-type hail detector and camera for observing large hydrometeors, and a photoelectric cell in-

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7 2 or-out-of-cloud indicator. An air thermometer will be included in the sonde but we do not expect the signal from it to be easily or even usefully interpreted because of the heat flow caused by supercooled water and wet bulb action. We will consider a lightning detector, and perhaps a potential gradient, or charge, measurement. TWO-STATION DROPSONDE SYSTEM This system is intended to measure the vertical speed of air in a thunderstorm as given by 10 dropsondes simultaneously. The vertical speed of the air is to be measured every 100 m of fall of the sonde with a standard error less than 0.5 m/sec when the sonde is as far as 50 km from the ground station. The horizontal position of each sonde will be given every 100 m of vertical drop with a standard error less than 10 m. The several sondes will be dropped within a circle of 10 km diameter from a height of about 12 km. They will drop about 10 km in 5 to 10 min at about 30 m/sec. TRACKING AND TELEMETRY Barometric pressure and rate of change of pressure combined will be used to calculate the vertical speed of the sonde while dynamic pressure from a heated pitot tube will be used to calculate the vertical air speed of the sonde. These two, subtracted, will give the vertical speed of the air in the storm. The signals for these calculations will be telemetered to the ground along with temperature signals. In addition, a transponder-type radio system will give the horizontal position of each sonde. Later models will contain the additional sensors mentioned above. Position will be measured by sending a train of 10 cycles of a 74,933.3 Hz signal, amplitude-modulated on a 1699 MHz carrier from the ground to all the sondes. This signal will be received by a crystalvideo receiver in each sonde. Each sonde will detect the 74,933 Hz modulation and use it to modulate a 400 MHz sonde transmitter. Each sonde transmitter will be crystal-controlled on a separate channel spaced 500 khz from the others. The two radio frequencies in each sonde will be two octaves apart to avoid interference from the sonde transmitter in the sonde receiver. The pressure and temperature signals will be generated by a block-

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9 3 ing oscillator whose frequency is controlled by either resistance or voltage determined by diaphragm-type pressure cells and thermistors. Each signal will be on for 0.5 sec in turn. The whole sequence, including internal temperature and monitoring signals, will take 4 sec. The 0.5 sec is needed to allow a servo-type recorder to follow the signals. The telemetered signals will be: 1. calibrate 2. zero 3. static pressure 4. air speed 5. rate of change of static pressure 6. air temperature internal temperatures 9.. The zero is the output of the blocking oscillator when the input is shorted. The signal spectrum will be cut off at 25 khz by a low pass filter to prevent disturbing the tracking signal. The signals are frequency-modulated on the 400 MHz carriers independently of the tracking pulse. Later models may contain subcarriers to handle signals which are not easily sampled. GROUND STATIONS Two stations 30 km apart on the ground will receive the signal and track it to measure the time delay. The ground transmitter will be located at one of the stations. The delay at that station will represent the distance to the sonde and back, while the delay at the other station will represent the distance from the first station to the sonde and from there to the second station. The main station will have a converter feeding 10 receivers and a 14-channel magnetic tape recorder. The signals will be recorded for later tracking analysis. The second station will have a converter feeding a microwave link to the main station where again 10 receivers will extract the signals and put them on the 10 channels of the tape recorder. This link is used to save the cost of a second recorder and

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11 4 to assure time synchronism of the records. The main station will have a tracker and telemeter frequency meter for monitoring during the soundings. The frequency meter in the ground station will produce a dc signal proportional to the sonde blocking oscillator frequency. This dc signal, when recorded graphically, will show all of the telemetered signals in sequence. The transmitted pulse will consist of sine waves to permit accurate tracking (by matching phase) without requiring large bandwidth. Each cycle will represent 2 km distance. The bandwidth of the receivers need only be wide enough so that we can determine which cycle is first. For modulation at khz the video bandwidth required is about 54 khz to 104 khz at 3 db down so that the receiver bandwidth need be only about 250 khz. SONDE DESIGN CONSIDERATIONS The sonde will weigh 2 kg. The drop time is 5 min which allows the use of a small battery. The sonde will 'have a drag device to produce about 30 m/sec falling speed although, due to the change in air density, the speed may decrease by two to one during the drop. The sonde will contain a pressure-actuated release to open a parachute and slow the sonde to less than 10 m/sec before it hits the ground. This release will be set from 600 mb to 1000 mb. Heat will be supplied to keep ice out of the pressure holes. There need be only one tracker at the ground station. All of the signals will be tracked' by reading the magnetic tape after the drop is completed. These readings of the tape will give two-channel paper records of position and pressures. These will then be read by hand to give the numbers from which calculations will be made. The sondes will be designed to require no adjustment in the airplane. They will operate on airplane power before release and switch automatically to batteries when dropped.

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13 5 OPERATION We will operate over the hail network of Colorado State University for our first storm measurements. The launching airplane will make vertical soundings on descent. We will have an M33 with S-band plan position and range-height display and camera and will have X-band tracking of the airplane. At least two different drop patterns can be used. One will be a straight line in which all the sondes will be dropped at intervals of about 4 sec. Another will be a turning path in which a sharp turn will be made to put half of the sondes alongside the others at less than 1000 m distance. The start of these patterns will be called from the ground but after the start they will be controlled by the aircrew. The airplane will be directed to the start by using the M33 X-band radar to track the airplane and the S-band radar to select the storm cell to be measured. Directions will be sent by VHF radio. During the drop the ground stations will simply record the signals. The receiving antennas need not be moved during the drop. One channel will be tracked to determine the location of the sondes on the ground. We will try to recover the sondes using portable direction finders on the sonde signals. While the tracking results will give the location within 10 m, this information will not be available for perhaps a week after the drop. ANALYSIS After the drop is complete all the sonde data will be on magnetic tape. This tape will be analyzed by reading each track individually. The distance signal will be tracked and the output recorded on chart paper. If necessary the record can be tracked more than once to reduce the noise. In addition, the tracking signal can be "aided" from a smoothed form of the first track to allow even better removal of noise. This is an important advantage of having the signals on tape. The telemetered signals will be read at the same time and put on another chart paper. The charts will be read by hand and the numbers will be put on

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15 6 punched cards. All position and temperature calculations will then be done by computer. The computer program will contain thermistor data, ground station positions and refractive index data. The computer output will give air speed, temperature, etc., as functions of position and time for each sonde. From these output lists, we will draw the cell structure as completely as possible. By comparing this structure with the radar photographs, ground data, and soundings, we will attempt to determine the points listed in the Objectives.

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17 7 74,933.3 Hz oscil lator 1699 MHz ^ rcps *+^^9~.o5 cps Transmitter ^~"gain a 10 gate p sec Converter MHz crystal-controlled, \ gain 10 M MHz from ' _-«sondes # receiver #2 receiver # 10 receiver 27.5 MHe 28.0 MHz \ 32.5 MHz 250 kc wide video 125 khz T O Switch to any 0 channel J ao pulse 75 khz Magnetic tape Scn has sai notransmitrecorder Fig._^~ -tt 1- w n dosne ytm gru channelstto#13 scope, 25 khz low pass Frequency meter khz ^ f filter p_ Tracker Graphic 60 cm/sec,recorders 1/4 sec full scale synch 3 km across paper. Manual switch in tracker, each 2 km. Second station has no transmitter. Fig. l--two-station dropsonde system, ground station #1

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19 MHz limiter Cavity filter Crystal-video signal strength reference pressure air speed 3 temperatures AGC Switch 0.5 sec khz filter U o Blocking oscilla t or Low pass filter MHz Channels Transmitter MHz crystal controlled Fig. 2--Dropsonde diagram

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