Extended Abstract

3 rd Conference on QPE /QPF and Hydrology. World Meteorological Organization, Nanjing, China, Oct.18-­‐22,2010. €Fig. 2. Apparent VPRs (blue dots) and associated linear VPRmodel (red lines) for (a) KABR at 1000UTC 06 Oct.2009, (b) KEAX at 1700UTC 19 Feb. 2010.Once the idealized BB VPR is obtained, a reflectivitycorrection factor, ΔZ(r), is computed from the VPRaccording to the following:⎧⎧ΔZ(r) = α • [h(r) − h ] + β • [h − h ];− − −h(r) > h p p b p⎨⎨⎩⎩ β • [h(r) − h b];− − − − − − − − − − h(r) ≤ h pZ c(r) r∈BBA= Z o(r) r∈BBA− ΔZ(r)(3)Here, r is the range from the radar to any given gateand h(r) is the height of the beam axis at the gate;Z o(r) andZ c(r) represents the observed and VPRcorrected reflectivities at any given gate within a BBA,observed reflectivity, the beam spreading effects areimplicitly accounted for. This approach was found to besuperior to the single-VPR approach, where one meanVPR is computed from observations of multipleelevations angles at closer ranges and then applied tothe far ranges. Experiments with the single-VPRapproach resulted in some circular discontinuities (notshown) because the VPRs could not accurately accountfor the beam spreading at different ranges. Using theVPRs for each tilt, the correction is adaptive to the rangevariation of the beam width and the BB distribution, anddiscontinuities in the corrected fields are minimized.Figure 3 shows example reflectivity fields before andafter the VPR correction. The inflated reflectivity inbright band area was corrected. The corrected fieldsshowed physically consistent distributions and were freeof circular discontinuities that were usually caused bynon-representative BB top/bottom heights in single-VPRapproaches.€€respectively. For a bright band situation,€€ΔZ(r) isusually positive, thus the correction would only reducethe observed reflectivities after the correction.€ΔZ(r)can become negative when h(r) – h p is so high that[ ] > β • [ h p− h b ]α • h(r) − h p, and equation (3) may bepotentially used to correct for underestimations in radarderivedQPE when the radar beam is sampling upperpart of precipitation clouds (i.e, the snow/ice region).However, this correction may not be very accurate if onlyone slope is considered above the BB peak.3. Case study resultsIn the current study, multiple VPRs are derived andthen applied for correction, one for each tilt. Thesemean observed VPRs account for the beam spreadingeffects because the reflectivity were averaged at thesame range on a single tilt, instead of over multiple tiltsat a certain height. When applying the correction back toFig. 3. Base level reflectivities (upper row: a 0 , and b 0 ) beforeAVPR correction, (low row: a 1 , and b 1 ) after AVPRcorrection. The two columns are images from (column 1)KABR at 10Z Oct 06, 2009, and (column 2) KEAX at17Z Feb 19, 2010.The hybrid scan reflectivity field is converted intorain rate using one Z-R relationship (Z=200R 1.6 ). Therain rates are aggregated into hourly rainfalls and-453-