Earth and Atmospheric Sciences. Sky condition. Prof. J. Haase EAS535 EAS535

Transcription

1 Sky condition Prof. J. Haase

2 Federal Meteorological Handbook

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4 Estimating cloud height Report cloud levels to nearest 100 feet below 5000 feet (in practice, human observers usually report to nearest 500 ft). 1. PIREPS (pilot reports) - height is based on reports from pilots in nearby aircraft (near the airport is more reliable than 20 km out). 2. ceiling lights (which involve triangulation using a light during darkness) 3. Pilot Balloons (PIBALS) 4. convective cloud tables 5. usage of objects of known heights 6. estimation of cloud heights based upon observer experience

5 PIREP Coded PIREP: ALO UA /OV ALO /TM 1256 /FL400 /TP C560 /TA M45 /WV 25072KT /TB NEG UA/OV OKC /TM 1522/FL 080/TP CE172/SK 020 BKN 045/060 OVE 070/TA -04/WV /TB LGT/RM IN download pireps

6 PIREP Coding

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8 Coded PIREP: ALO UA /OV ALO /TM 1256 /FL400 /TP C560 /TA M45 /WV 25072KT /TB NEG Decoded PIREP: 94 nm 90 deg radial (East) of Waterloo, Iowa, at 12:56 UTC. flight level (pressure altitude based on altimeter setting at ALO). Type of aircraft is a C560. Outside air temperature -45C. Wind speed 72 kt from 250 degrees. Negligble turbulence. UA/OV OKC /TM 1522/FL 080/TP CE172/SK 020 BKN 045/060 OVE 070/TA -04/WV /TB LGT/RM IN CLR Decoded PIREP: 64 nm on the 63 o radial from Oklahoma City VOR. At 1522 UTC. Flight altitude 8,000 ft. type of aircraft is a Cessna 172 base of broken cloud layer at 2,000 ft with tops at 4,500 ft. base of overcast layer at 6,000 ft with tops at 7,000 ft outside air temperature is minus four degrees C wind is from 245 true at 40 kt light turbulence and clear skies.

9 Ceiling lights Ceiling lights are based upon simple methods of measuring the distance from the ground to a cloud base. With the ceiling light, the observer turns on a light at a known distance (baseline) from the observer, pointed vertically. The observer then uses a theodolite to measure the angle formed from the observer to where the light reflects off of a cloud base, and then solves an equation to determine the distance from the light to the cloud base. When using this method, the measurement has an error estimate of 5% of the cloud height. It can be used to measure ceilings up to ten times the length of the baseline. If distance is 500 feet, can be used to measure ceilings between 50 feet and 5,000 feet with an error estimate at 5,000 feet of 250 feet.

10 Ceilometer

11 Ceilometer backscatter profile Vaisala CT75K ceilometer Wavelength 905 nm Time resolution 30 s, height resolution 30m Up to three cloud base heights

12 Ceilometer operation The cloud height indicator (CHI) transmits skyward approximately 9,240 pulses in 12 seconds. ASOS then assigns the returned signals, cloud base hits (CHIs), to one of foot interval bins. After the 12 seconds, ASOS produces a profile of the back-scattered signal to help determine if the returned signals were from cloud bases. The system reports cloud layers up to an altitude of 12,000 feet. Each minute the algorithm processes 30 minutes of the 30- second data samples to create values for sky coverage and cloud height for the observation. 30 minutes of data is representative of an area 3-5 miles around the sensor site. To be more responsive to the latest changes in the weather, the last 10 minutes of the data are double-weighted in the algorithm calculations. ASOS identifies the recorded "hits" by height and may create up to three layers. The system assigns a coverage value of CLR (clear <5%), FEW (few), SCT (scattered), BKN (broken), or OVC (overcast). The computer algorithm also tests the sensor return for obscurations (can t see the cloud base because of fog, smoke, rain) and variable ceilings.

13 Packing effect The observer does not see the openings in the cloud decks near the horizon due to the viewing angle. Pilots tend to overestimate the coverage even more than ground observers because of visual compression. When flying at 300 to 400 kph, the breaks in the clouds appear smaller. Ceilometer is not biased, BUT clouds need to be moving => doesn t work so well in tropics.

14 Pilot Balloons

15 Convective cloud base chart Cloud base = 6000 T= 27 o C T dew = 7 o C

16 Example from ASOS KLAF 2007/08/21 19:00 UTC Station Elev(m) Lat(N) Lon(E) Grid I Grid J ObTime Provider TD QCD QCA QCR V-TD KLAF _1854 ASOS S 59 0 Station Elev(m) Lat(N) Lon(E) Grid I Grid J ObTime Provider RH QCD QCA QCR V-RH KLAF _1854 ASOS S 59 0 Station Elev(m) Lat(N) Lon(E) Grid I Grid J ObTime Provider ALTSE QCD QCA QCR V-ALTSE KLAF _1854 ASOS S 51 0 Station Elev(m) Lat(N) Lon(E) Grid I Grid J ObTime Provider SLP QCD QCA QCR V-SLP KLAF _1854 ASOS S 59 0 Station Elev(m) Lat(N) Lon(E) Grid I Grid J ObTime Provider P QCD QCA QCR V-P KLAF _1854 ASOS S 51 0 Station Elev(m) Lat(N) Lon(E) Grid I Grid J ObTime Provider T QCD QCA QCR V-T KLAF _1854 ASOS S 59 0 Station Elev(m) Lat(N) Lon(E) Grid I Grid J ObTime Provider DD QCD QCA QCR V-DD KLAF _1854 ASOS S 35 0 Station Elev(m) Lat(N) Lon(E) Grid I Grid J ObTime Provider FF QCD QCA QCR V-FF KLAF _1854 ASOS S 51 0 Station Elev(m) Lat(N) Lon(E) Grid I Grid J ObTime Provider VIS QCD QCA QCR V-VIS KLAF _1854 ASOS C 3 0 Station Elev(m) Lat(N) Lon(E) Grid I Grid J ObTime Provider ELEV QCD QCA QCR V-ELEV KLAF _1854 ASOS Z 0 0 Station Elev(m) Lat(N) Lon(E) Grid I Grid J ObTime Provider LAT QCD QCA QCR V-LAT KLAF _1854 ASOS Z 0 0 Station Elev(m) Lat(N) Lon(E) Grid I Grid J ObTime Provider LON QCD QCA QCR V-LON KLAF _1854 ASOS Z (archive for data assimilation tests) (archive for LAF only)

17 Convective cloud base chart T=28.0 Tdew=23.0 *what was the convective cloud base on 8/21/2007

18 Example from ASOS KLAF 2007/08/21 19:00 UTC Station Elev(m) Lat(N) Lon(E) Grid I Grid J ObTime Provider TD QCD QCA QCR V-TD KLAF _1854 ASOS S 59 0 Station Elev(m) Lat(N) Lon(E) Grid I Grid J ObTime Provider RH QCD QCA QCR V-RH KLAF _1854 ASOS S 59 0 Station Elev(m) Lat(N) Lon(E) Grid I Grid J ObTime Provider ALTSE QCD QCA QCR V-ALTSE KLAF _1854 ASOS S 51 0 Station Elev(m) Lat(N) Lon(E) Grid I Grid J ObTime Provider SLP QCD QCA QCR V-SLP KLAF _1854 ASOS S 59 0 Station Elev(m) Lat(N) Lon(E) Grid I Grid J ObTime Provider P QCD QCA QCR V-P KLAF _1854 ASOS S 51 0 Station Elev(m) Lat(N) Lon(E) Grid I Grid J ObTime Provider T QCD QCA QCR V-T KLAF _1854 ASOS S 59 0 Station Elev(m) Lat(N) Lon(E) Grid I Grid J ObTime Provider DD QCD QCA QCR V-DD KLAF _1854 ASOS S 35 0 Station Elev(m) Lat(N) Lon(E) Grid I Grid J ObTime Provider FF QCD QCA QCR V-FF KLAF _1854 ASOS S 51 0 Station Elev(m) Lat(N) Lon(E) Grid I Grid J ObTime Provider VIS QCD QCA QCR V-VIS KLAF _1854 ASOS C 3 0 Station Elev(m) Lat(N) Lon(E) Grid I Grid J ObTime Provider ELEV QCD QCA QCR V-ELEV KLAF _1854 ASOS Z 0 0 Station Elev(m) Lat(N) Lon(E) Grid I Grid J ObTime Provider LAT QCD QCA QCR V-LAT KLAF _1854 ASOS Z 0 0 Station Elev(m) Lat(N) Lon(E) Grid I Grid J ObTime Provider LON QCD QCA QCR V-LON KLAF _1854 ASOS Z 0 0 Sky Cover Layer Base: Station Elev(m) Lat(N) Lon(E) Grid I Grid J ObTime Provider SKYCVLB QCD QCA QCR V-SKYCVLB KLAF _1854 ASOS 488. Z Z Z Z Z Z (archive for data assimilation tests) (archive for LAF only)

19 Observer experience Estimate height based on cloud type and the height that it typically occurs