An attempt to calibrate the UHF strato-tropospheric radar at Arecibo using NexRad radar and disdrometer data P. Kafando, Monique Petitdidier To cite this version: P. Kafando, Monique Petitdidier. An attempt to calibrate the UHF strato-tropospheric radar at Arecibo using NexRad radar and disdrometer data. Annales Geophysicae, European Geosciences Union, 2004, 22 (12), pp.4025-4034. 10.5194/angeo-22-4025-2004. hal-00159569 HAL Id: hal-00159569 https://hal.archives-ouvertes.fr/hal-00159569 Submitted on 31 Dec 2015 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. Annales Geophysicae (2004) 22: 4025–4034 SRef-ID: 1432-0576/ag/2004-22-4025 Annales © European Geosciences Union 2004 Geophysicae An attempt to calibrate the UHF strato-tropospheric radar at Arecibo using NexRad radar and disdrometer data P. Kafando1,2 and M. Petitdidier1 1Centre d’Etudes des Environ. Terr. et Planetaire/Inst.´ Pierre Simon Laplace, 10–12 Av. de l’Europe, 78140 Velizy,´ France 2on leave to: Ouagadougou University, 04 B.P.8306 Ouagadougou 04, Burkina Faso Received: 2 December 2003 – Revised: 18 June 2004 – Accepted: 20 July 2004 – Published: 22 December 2004 Abstract. The goal of this paper is to present a methodology Key words. Meteorology and atmospheric dynamics (trop- to calibrate the reflectivity of the UHF Strato-Tropospheric ical meteorology; remote sensing; instruments and tech- (ST) radar located at NAIC in Puerto Rico. The UHF niques) lower relevant altitude is at 5.9 km, the melting layer being at around 4.8 km. The data used for the calibration came from the observations of clouds, carried out with Strato- Tropospheric dual-wavelength (UHF and VHF) radars and a 1 Introduction disdrometer; those instruments being located on the NAIC site in Arecibo, Puerto Rico. The National Weather Ser- Calibration of UHF/VHF wind profilers is essential to obtain vice operates other instruments like the radiosondes and the absolute radar reflectivities and their variation as a function NexRad Radar in other sites. of time and meteorological conditions. Reflectivity is used 2 The proposed method proceeds in two steps. The first to characterize the turbulence intensity through Cn, the hy- consists of the comparison between the NexRad reflectivity drometeors precipitating or non-precipitating. At VHF the and the reflectivity computed from the drop size distributions contribution of the sky noise and its diurnal variation could measured by the disdrometer for one day with a noticeable be used to calibrate ST radars while at UHF the sky noise rainfall rate. In spite of the distance of both instruments, the contribution is relatively small and is overwhelmed by the agreement between the reflectivities of both instruments is instrument noise (Campistron et al., 2001; Petitdidier et al., enough good to be used as a reference for the UHF ST radar. 1997). At UHF when the lowest reliable height is below the The errors relative at each data set is found to be 2.75 dB for melting layer, precipitation parameters are compared with the disdrometer and 4 dB for the NexRad radar, following the the ones measured with ground-based precipitation measure- approach of Hocking et al. (2001). The inadequacy between ments like a disdrometer (Gage et al., 2000; William et al., the two sampled volume is an important contribution in the 2000). In case of the UHF wind profiler at the National As- errors. tronomy and Ionospheric Center (NAIC) in Arecibo on the island of Puerto Rico, the lowest reliable height is above the The second step consists of the comparison between the melting layer, and then it is not possible to relate the hydrom- NexRad radar reflectivity and the UHF non-calibrated reflec- eteor characteristics to precipitation ones measured at ground tivity at the 4 altitudes of common observations during one level. event on 15 October 1998. Similar features are observed and a coefficient is deduced. An offset around 4.7 dB is observed The purpose of this work is to present an attempt to cali- and the correlation factor lies between 0.628 and 0.730. Ac- brate indirectly the backscattered signal of a ST UHF radar. cording to the errors of the data sets, the precision on the The data concerned by the calibration were taken at NAIC, calibration is of the order of 2 dB. This method works only in September/October 1998 during a campaign to observe when there are precipitation hydrometeors above the NAIC tropical thunderstorms (Petitdidier et al., 2000). The obser- site. However, the result of the calibration could be applied to vations of several clouds were carried out with the Strato- other data obtained during the campaign, the only constraint Tropospheric dual-wavelength (UHF and VHF) radars, an being the same value of the transmitter power. electric field mill, and a disdrometer, those instruments be- ing located on the NAIC site. The National Weather Service has operated other instruments like the rain gauges, radioson- Correspondence to: M. Petitdidier des and the Doppler weather radar, NexRad. Thus, the data ([email protected]) collected will be used to describe tropical thunderstorms. 4026 P. Kafando and M. Petitdidier: Calibration of the UHF strato-tropospheric radar The proposed calibration approach combines data from the Table 1. Geographical coordinates of the different instruments. disdrometer, the NexRad radar and the ST UHF radar. In the calibration of scanning weather radar, Gage et al. (2000) ex- tended the disdrometer results to larger scales and to higher Latitude Longitude Altitude (m) altitudes by means of a vertically directed UHF wind profiler. ◦ 0 ◦ 0 In our situation, the approach to calibrate the ST UHF radar NexRad 18 6.94 N 66 4.68 W 881.5 Disdrometer 18◦ 20.850N 66◦ 45.250W 364.24 must be different and the method consists of using the re- UHF Profiler 18◦ 20.610N 66◦ 45.180W 230 flectivity computed from the drop-size distribution measure- ments by the disdrometer, to calibrate the reflectivity of the weather Doppler radar, NexRad, which, in turn, is used to calibrate the ST UHF reflectivity. Thus, it is vital that the samples. Ulbrich et al. (1999) carried out a first analysis of initial relation, upon which this sequence of calibrations is the disdrometer data, obtained during the campaign. based, be as accurate as possible. The JWD records the number of raindrops, nd , of diame- This paper describes the calibration of the ST UHF radar i ter D , in each of its 20 diameter channels, for a 1-min sam- by using the NexRad radar located in Cayey (Puerto Rico). In i pling period. The DSD, N(D ), expressed in m−3 mm−1, is the first part of the paper, the simultaneous measurements are i computed from these counts nd by using: selected and the data obtained by the different instruments i presented. The second part addresses the NexRad calibra- ndi N(Di) = , (1) tion. In order to avoid any assumption for the Z−R relation- S ∗ dt ∗ v(Di) ∗ ddi ship that it is highly variable during one event, reflectivities 2 are compared. The third part concerns the UHF ST radar cal- where the measuring area, S, is equal to 0.0050 m ; dt is the ibration. The comparison between the reflectivity observed sampling period equal to 60 s; Di is the mean diameter (mm) by the UHF radar and the NexRad radar is carried out. As for the ith channel, ddi is the ith channel width (mm); v(Di) is the fall speed in still air of a drop of diameter Di, expressed the attenuation of the UHF backscattered signal varies as a −1 function of time, to avoid the receiver saturation the gain dur- in m s . ing the different periods has to be evaluated using the noise From the DSD, the rainfall rate and the radar reflectivity power spectral density. Then the backscattered signal is cor- are deduced: rected by this factor and the comparison is done in the com- Z ∞ R = 6π×10−4π v(D) D3 N(D) dD (2) mon range. 0 where R is expressed in mm/h; v(D) in m/s, D and dD in mm, − − 2 Data and N(D) in m 3mm 1. ∞ 2.1 Description (disdrometer, NexRad, UHF/VHF wind Z Z = D6N(D)dD , (3) profilers) 0 During the whole campaign, outside the period of the hurri- where Z, the equivalent reflectivity, is expressed in mm6m−3 cane Georges’ passage, there were 18 days out of 29 when with N(D) in m−3mm−1 and D, dD in mm. the disdrometer detected rain but only 7 days with a rain The drawback of the instrument occurs during strong amount larger than 10 mm. The Doppler weather radar, rains. The impact of large drops causes such a strong in- NexRad, data were available only for a few days (10 common ternal noise response that the signal due to any small drop periods with the ST radar observations out of 18) during the arriving during the ensuing few milliseconds – the dead time campaign. As a consequence, simultaneous measurements of – is not detectable (e.g. Sauvageot et Lacaux, 1995). Ap- disdrometer, ST UHF radar and NexRad Radar occurred only plying the dead time correction proposed by the manufac- on 3 days, 18 and 30 September and 15 October, out of 7 no- turer has the effect of increasing the numbers of small drops, ticeable rainfall events.