MAY 2002 STEINER AND SMITH 673 Use of Three-Dimensional Re¯ectivity Structure for Automated Detection and Removal of Nonprecipitating Echoes in Radar Data MATTHIAS STEINER AND JAMES A. SMITH Department of Civil and Environmental Engineering, Princeton University, Princeton, New Jersey (Manuscript received 16 May 2001, in ®nal form 12 September 2001) ABSTRACT This study aims at assessing the potential of anomalous propagation conditions to occur, reviews past attempts to mitigate ground clutter contamination of radar data resulting from anomalous signal propagation, and presents a new algorithm for radar data quality control. Based on a 16-yr record of operational sounding data, the likelihood of atmospheric conditions to occur across the United States that potentially lead to anomalous propagation of radar signals is estimated. Anomalous signal propagation may lead to a signi®cant contamination of radar data from ground echoes normally not seen by the radar, which could result in serious rainfall overestimates, if not recognized and treated appropriately. Many different approaches have been proposed to eliminate the problem of regular ground clutter close to the radar and temporary clutter resulting from anomalous signal propagation. None of the reported approaches, however, satisfactorily succeeds in the case of anomalous propagation ground returns embedded in precipitation echoes, a problem that remains a challenge today for radar data quality control. Taking strengths and weaknesses of past approaches into consideration, a new automated procedure has been developed that makes use of the three-dimensional re¯ectivity structure. In particular, the vertical extent of radar echoes, their spatial variability, and vertical gradient of intensity are evaluated by means of a decision tree. The new algorithm appears to work equally well in situations where anomalous propagation ground returns are either separated from or embedded within precipitation echoes. Moreover, sea clutter echoes are identi®ed as not raining and successfully removed. 1. Introduction control is combined with terrain-based visibility and vertical precipitation structure, and gauge adjustments Quality control is essential to meaningful radar-based to achieve the most reliable rainfall estimates. Fulton et rainfall estimation. Radar echoes may be contaminated al. (1998) report on the data quality control and rainfall by nonmeteorological echoes that need to be identi®ed estimation procedures of the operational radar network and removed before rainfall estimation. This is partic- in the United States. Despite elaborate and sophisticated ularly true for operational applications such as precip- efforts in data quality assurance, however, evaluations itation nowcasting and (¯ash) ¯ood warning. A well- by Smith et al. (1996) show that anomalously propa- trained person may successfully recognize nonmeteo- gated ground returns remain a serious problem, espe- rological contamination in radar echoes, such as ground cially for situations where AP is embedded in precipi- clutter or anomalously propagated ground returns tation echoes. (called ``AP'' or ``Anaprop'' echoes). For of¯ine case The aim of this paper is threefold: to investigate the studies, manual editing of the data may be feasible and potential of anomalous propagation conditions to occur appropriate; however, for operational applications au- throughout the United States from an atmospheric per- tomated procedures need to be used. spective (section 2), review past efforts in dealing with Harrison et al. (2000) present recent efforts under way ground clutter and AP contamination in radar data (sec- in the United Kingdom that show how extensive quality tion 3), and present and discuss a new approach for control may effectively reduce the root-mean-square automated radar data quality control (sections 4 and 5). (rms) difference between gauge-measured and radar-es- timated rainfall amounts. Joss and Lee (1995) discuss elaborate procedures in place for operational radar data 2. Anomalous propagation of radar signals processing in Switzerland, where extensive data quality a. Refractive index and signal propagation At microwave frequencies, the propagation of elec- Corresponding author address: Dr. Matthias Steiner, Department of Civil and Environmental Engineering, Princeton University, tromagnetic signals is in¯uenced by atmospheric con- Princeton, NJ 08540. ditions. A commonly used quantity to describe the prop- E-mail: [email protected] agation behavior of electromagnetic signals is the index q 2002 American Meteorological Society Unauthenticated | Downloaded 09/24/21 05:11 PM UTC 674 JOURNAL OF ATMOSPHERIC AND OCEANIC TECHNOLOGY VOLUME 19 of refraction n, or the refractivity N, which can be ap- warm dry air from land over cooler bodies of water proximated by (e.g., ocean), causing a temperature inversion in the boundary layer. At the same time, moisture is added by 77.6p 3.73 3 105e (n 2 1) 3 106 5 N 51 , (1) evaporation from the water surface, producing a mois- TT2 ture gradient (Skolnik 1980; Puzzo et al. 1989). Evap- where p is the barometric pressure in millibars, e the oration ducts are common just above the surface of the partial pressure of water vapor in millibars, and T the sea, where air may become saturated by evaporation absolute temperature in kelvins (Gossard 1977; Skolnik from the sea surface. Over land, ducting is often caused 1980; Babin 1996; Fabry et al. 1997). by radiational cooling during clear nights, particularly It is the vertical gradient of the refractivity within the in the summer when the ground is moist. Thus, over lowest several hundred meters above the ground that is land, ducting is most noticeable at night and tends to especially important for characterizing radar signal disappear during the warmest part of the day (e.g., propagation (e.g., Pratte et al. 1995). A decrease in at- Moszkowicz et al. 1994). Superrefraction or ground mospheric refractivity with altitude, dN/dh, tends to ducts may also be produced by the diverging downdraft bend the radar rays so as to extend coverage beyond under a thunderstorm and resulting gust fronts. The rel- that expected with a uniform atmosphere. This abnormal atively cool air, which spreads out from the base of a propagation of electromagnetic waves is called anom- thunderstorm, may produce a temperature inversion alous propagation. Four basic modes of propagation are within the lowest, possibly several hundred meters. The distinguished: moisture gradient along the out¯ow boundary is also appropriate for the formation of a duct. The conditions 21 x subrefraction dN/dh . 0m , favorable for the formation of a thunderstorm duct are 21 x normal refraction 0 . dN/dh .20.0787 m , relatively short-lived and have timescales on the order x superrefraction 20.0787 . dN/dh .20.157 of 30 min to 1 hr, although in extreme cases such con- 21 m , and ditions may last for hours (Weber et al. 1993). x trapping or ducting dN/dh ,20.157 m21. Trapping or ducting is the most severe case of anom- b. Climatological assessment of vertical refractivity alous signal propagation, and results in ground returns gradients (AP echoes) from locations where the radar beam in- tersects the ground or objects at the earth's surface. Using a 16-yr record of operational sounding data In order to propagate energy within the duct, the angle (1973±88), the potential of anomalous propagation con- the radar ray makes with the duct should be small, usually ditions to occur throughout the continental United States less than a degree (e.g., only the lowest elevation scans is assessed climatologically. Similar studies, for ex- of surface-based radar are affected). Only those radar rays ample, have been conducted by Bech et al. (2000) using launched nearly parallel to the duct will be trapped. At- soundings for Mediterranean coastal sites, Babin (1996) mospheric ducts are generally of the order of tens to using helicopter-based refractivity measurements off the hundreds of meters (Gossard 1977; Cook 1991; Babin coast of Wallops Island (Virginia), and Gossard (1977) 1996; Brooks et al. 1999). A simpli®ed approximate mod- using airmass analyses. For each sounding of the da- el of propagation in atmospheric ducts (Skolnik 1980) taset, the average refractivity gradient within the lowest predicts a maximum wavelength lmax that can be prop- 500 m above ground level (AGL) is determined. (The agated in a surface duct of depth d as given by maximum gradient might be more relevant for the radar signal propagation problem; however, the limited and l 5 2.5(2dn/dh)1/2d 3/2, (2) max variable vertical resolution of the operational sounding where lmax, dh, and d are in the same units (e.g., meters). data may result in questionable maximum gradient val- For an operational Weather Surveillance Radar-1988 ues.) These values are then compiled into a climatology Doppler (WSR-88D) of the Next Generation Weather of average refractivity gradients for each operational Radar (NEXRAD) network (Heiss et al. 1990; Baer 1991; sounding station and used to study the likelihood of Crum et al. 1998) with wavelength of 10 cm (S band), atmospheric conditions across the United States that are the duct must be at least 22 m thick in order for trapping susceptible to anomalous propagation of radar signals. to occur. Often only parts of the radar beam may be The sounding-based climatology will highlight large- trapped. scale temperature inversions and moisture gradients, yet A duct is produced when the index of refraction rap- only by chance capture conditions favorable to anom- idly decreases with height. In order to achieve this, the alous propagation produced by thunderstorm out¯ow temperature must increase and/or the humidity (water boundaries. Moreover, relatively thin layers of strong vapor content) must decrease with height. Temperature vertical gradients may cause anomalous propagation, inversions must be very pronounced in order to produce but the operational sounding data (variable resolution superrefraction, while water vapor gradients are more of one to several hundred meters) do not resolve tens effective than temperature gradients alone (Fabry et al.