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Poster Session 1
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Observed Spatial Variation in Perceptible Water Vapor by a Ground-Based, Dual-Channel Radiometer
Observed PWV from Radiosondes and WVR
A ground-based water vapor radiometer (WVR) operating at 23.8 and 31.4 GHz was utilized to observe spatial variations in PWV at the Taipei weather station on March 18-25, 1998. The WVR was loaned by Radiometries Corporation, USA. During the field campaign, the WVR was initially installed due east, while it was turned to due north periodically. A directions perpendicular to the WVR itself so that atmospheric emissions can be measured at eleven elevation angles from 15° to 165° at an interval at 15°.
Figure1 shows (a) brightness temperatures and (b) opacities at 23.8 and 31.4 GHz, and (c) the corresponding PWV derived from radiosonde data collected at Taipei weather station in Marches of 1988-1997. In general, brightness temperature and opacities are increased with increasing PWV. High value of brightness temperature and opacity are associated with the presence of cloud liquid water and precipitation. Otherwise, brightness temperatures at both frequencies are generally lower than 100 K, and the corresponding opacities are lower than 0.5.
Figure 2 compares (a) brightness temperatures at 23.8 and 31.4 GHz, and (b) PWV derived from both WVR and radiosonde soundings collected on March 18-25, 1998. WVR-observed brightness temperatures are generally lower than those derived from radiosonde soundings. Standard deviations in differences between them are 14.2 K for 23.8 GHz and 27.3 K for 31.4 GHz. The corresponding standard deviations for the PWV are 0.51 and 0.49 cm for the brightness temperature- and opacity-based retrieval algorithms, respectively . Major differences between WVR and radiosonde observations of brightness temperatures and PWV appear in the mornings of March 23 and 25 (day numbers of 82 and 84) probably because measurements from radiosondes and WVR lack of spatial and temporal consistency.
Since the opacity-based retrieval algorithm provides better estimates of PWV than the brightness temperature-based approach in the self-consistent test, the opacity-based scheme is used to derive PWV in the rest of the paper. Figure 3 shows WVR-observed PWV for elevation angles from 15° to 165° on March 18-25, 1998 when the WVR is due east. As expected, higher values of PWV appear at angles further departing from the zenith. Minimum PWV at a given time in general occurs at zenith as expected. Noise-like signals can be seen from day numbers 79 to 81 (March 20 to 22) so that the observed PWV from midnight to noon from day numbers 79 to 81 (March 20 to 22) so that the observed PWV from midnight to noon of March 20 are shown in Figure 4 for better visualization. Figure 4 indicates that there was a northward weather system in the line of sight of the WVR between 6 to 8 a.m.
Figure 5 shows the difference in PWV between WVR observations and those derived through a secant mapping of the angle between the observing angle and eh zenith (cosecant of the observing angle) with respect to the PWV at zenith on March 18-25, 1998.
There are 345 observations for each of the eleven elevation angles. The atmosphere deviates from spherically-symmetrical assumption by 8.9% at angle of 165°. The average PWV at angle of 165° is 14.2 cm.
To examine the differences between PWV observations when the WVR is due north and those when the WVR is due east, the observed PWV at the two situations are compared with. The differences between them for the eleven corresponding pairs of angles range from 0.28 cm at zenith to 0.55 cm at 15° as listed in Table 1. The discrepancies in PWV for the zenith pair are primarily due to temporal inconsistency. This indicates that PWV significantly varies with time since each pair of data were taken at an interval of 10 minutes of so.
Table 1: Differences between PWV observations when the WVR is due north and those when the WVR is due east, and the corresponding percentage.
| Angle degree |
15 |
30 |
45 |
60 |
75 |
90 |
105 |
120 |
135 |
150 |
165 |
| Difference, cm |
0.66 |
0.29 |
0.29 |
0.31 |
0.32 |
0.28 |
0.32 |
0.34 |
0.29 |
0.38 |
0.54 |
| Difference/mean,% |
4.8 |
4.1 |
6.1 |
8.0 |
9.2 |
8.2 |
9.40 |
8.8 |
6.6 |
5.5 |
3.8 |
Conclusions
Brightness temperature- and opacity-based retrieval algorithms were developed to infer PWV from ground-based WVR observations. They reproduced PWV with accuracy of 93.5%, and 94.8% respectively. Based upon the opacity retrieval approach, WVR observations collected at CWB's Taipei weather station on March 18-25, 1998 show that the atmosphere deviates from a spherically-symmetrical assumption by as much as 8.9% at angle of 165°. In addition, PWV changes rapidly with time on the order of 0.28 cm (8.2%) per 10 minutes.
Acknowledgments
The author appreciates much the National Space Program Office grant NSC87-NSPO(A)-PC-FA07-05. The author also thanks Dr. F. Solheim, Radiometrics Corporation for the WVR used in our experiment.
Reference
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Liou, Yuei-An, 1998b, "Ground-based radiometric sensing of atmospheric in homogeneity in precipitable water vapor, "Atmospheric Sciences, 1998. (revised)
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Schroeder, J. A., and E. R. Westwater, 1991: Users' guide to WPL microwave radioative transfer software. NOAA Technical Memorandum ERL WPL-213, 84 pp.
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Sierk, B., Burki, H. Beckner-Ross, S. Florek, R. Neubert, L. P. Kruse, and H. Kahle, 1998: Tropospheric water vapor derived from solar spectrometer, radiometer, and GPS measurements. J. Geophys. Res. (in press)
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Solheim, F., J. R. Godwin, E. R. Westwater, Y. Han, S. J. Keihm, K. Marsh, and R. Ware, 1998: Radiometric profiling of temperature, water vapor and cloud liquid water using various inversion methods. Radio Sci., 33, 393-404.
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Ulaby, F. T, R. K. Moore, and A. K. Fung, 1981: Microwave Remote Sensing: Active and Passive, Vol. I, Artech House Inc, Norwood.
Figure 1: (a) Brightness temperatures (Tb), and (b) opacities (Tau) at 23.8 and 31.4 GHz, and (c) precipitable water vapor (PWV) estimated from radiosonde soundings observed at CWB's Taipei weather station for March of 1988-1997.
Figure 2: WVR-observed (a) brightness temperatures at 23.8 and 31.4 GHz, and (b) PWV by the Tau-based retrieval scheme at zenith versus their corresponding radiosonde observations at the CWB's Taipei weather station on March 18-25, 1998.
Figure 3: WVR PWV at all angles derived by the Tau-based retrieval scheme on March 18-25, 1998.
Figure 4: WVR PWV at all angles derived by the Tau-based retrieval scheme on March 20, 1998.
Figure 5: Difference in PWV between WVR observations and those derived through a secant mapping of the angle between the observing angle and the zenith (cosecant of the observing angle) with respect to the PWV at zenith on March 18-25, 1998.
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