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Title:
Comparison Z-R Relationships in EPIC-2001
Authors:
Cifelli, R.; Baumgardner, D.; Petersen, W. A.; Rutledge, S. A.; Williams, C.; Johnston, P.; Gage, K.
Affiliation:
AA(Department of Atmospheric Science, Colorado State University, Fort Collins, CO 80523 United States ; ), AB(Centro de Ciencias de la Atmosfera, Universidad Nacional Aut¢noma de México, Ciudad Universitaria, 04510 Mexi Mexico ; ), AC(ESSC/NSSTC, University of Alabama-Huntsville, Huntsville, AL 35899 United States ; ), AD(Department of Atmospheric Science, Colorado State University, Fort Collins, CO 80523 United States ; ), AE(CIRES, University of Colorado, Boulder, CO 80309 United States ; ), AF(CIRES, University of Colorado, Boulder, CO 80309 United States ; ), AG(NOAA Aeronomy Laboratory, 325 Broadway, Boulder, CO 80305 United States ; )
Publication:
American Geophysical Union, Fall Meeting 2002, abstract #A22A-0053
Publication Date:
12/2002
Origin:
AGU
AGU Keywords:
1854 Precipitation (3354), 3339 Ocean/atmosphere interactions (0312, 4504), 3360 Remote sensing, 3374 Tropical meteorology
Abstract Copyright:
(c) 2002: American Geophysical Union
Bibliographic Code:
2002AGUFM.A22A0053C

Abstract

An important objective of the EPIC-2001 field campaign is to improve understanding of the air-sea coupling processes that occur in the east Pacific warm pool region. To accomplish part of this objective, the NOAA R.V. Ronald H. Brown (RHB) operated a C-band radar during EPIC-2001 to provide high temporal resolution near-surface rainfall estimates that can be used as input to ocean mixed layer studies. The radar collected data for rainfall estimates out to a range of 150 km at continuous 10-minute intervals while the RHB was on-station at 10N, 95W (12 September 1 October 2001). In this study, we examine the sensitivity of the radar rain mapping procedure to different reflectivity-rainfall (Z-R) relationships that were collected during EPIC-2001. Specifically, drop size distribution (DSD) data collected from a Joss Waldvogel (JW) disdrometer and 2D-P probe are compared in order to obtain the best Z-R relationship for the radar rain maps. Disdrometer data were collected on-board the R.V. New Horizon, located within about 100 km of the RHB. The 2D-P probe data were collected by the NCAR C130 research aircraft which flew a series of flight legs across the ITCZ region during the field experiment. For this study, all 2D-P data from flights within 1000 km of the RHB and heights below 1.5 km altitude were utilized. The drop diameter resolution of the 2D-P measurements is 0.2 6.2 mm. For the JW disdrometer, the upper size bin includes all drops exceeding 5.1 mm in diameter. The lower size threshold is sensitive to the amount of surrounding environmental noise. Preliminary estimates suggest that the low-end sensitivity of the JW deployed on the New Horizon is around 1 mm. Analysis of the DSD data and resulting Z-R (Z=A*R**B) relationships from the two instruments showed that the JW disdrometer had a coefficient (A) approximately a factor of two larger than the 2D-P (416 vs 218) and an exponent (B) approximately 25% lower than the 2D-P (1.2 vs. 1.6). For rain rates exceeding about 5 mm/hr, these differences translate into a higher reflectivity for a given rain rate using the 2D-P data. The trends in the differences in Z-R s are consistent with physical processes (i.e., evaporation) occurring between the in-cloud (2D-P) and near surface measurements of the disdrometer. Moreover, the differences could also reflect biases in precipitation environments sampled by each instrument (e.g., larger drop spectra associated in regions of melting aggregates sampled by the 2D-P probe compared to smaller drop spectra sampled by the JW disdrometer associated with melting of rimed particles) as well as issues related to sampling geometry. However, sensitivity analyses conducted with the 2D-P data strongly suggest that the resulting Z-R relationships largely reflect differences in the low-end drop diameter sensitivity of two instruments. As the low end sensitivity threshold of the 2D-P is increased from 0.2 to 1.0 mm, the coefficient and exponent in the resulting Z-R change in such a fashion as to nearly match the corresponding JW coefficient and exponent values. These results have important implications for the radar rain maps since the application of the JW Z-R produces higher rain rates and accumulations in the more intense radar echo regions (> 34 dBZ) while the opposite occurs at lower reflectivities. Further, for a given echo pattern the 2D-P Z-R tends to produce larger rain areas compared to the JW Z-R.
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