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A Method to Merge WSR-88D Data with ARM SGP Millimeter Cloud Radar Data by Studying Deep Convective Systems

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A Method to Merge WSR-88D Data with ARM SGP Millimeter Cloud Radar Data by Studying Deep Convective Systems 958 JOURNAL OF ATMOSPHERIC AND OCEANIC TECHNOLOGY VOLUME 26 A Method to Merge WSR-88D Data with ARM SGP Millimeter Cloud Radar Data by Studying Deep Convective Systems ZHE FENG,XIQUAN DONG, AND BAIKE XI Department of Atmospheric Sciences, University of North Dakota, Grand Forks, North Dakota (Manuscript received 18 June 2008, in final form 8 December 2008) ABSTRACT A decade of collocated Atmospheric Radiation Measurement Program (ARM) 35-GHz Millimeter Cloud Radar (MMCR) and Weather Surveillance Radar-1988 Doppler (WSR-88D) data over the ARM Southern Great Plains (SGP) site have been collected during the period of 1997–2006. A total of 28 winter and 45 summer deep convective system (DCS) cases over the ARM SGP site have been selected for this study during the 10-yr period. For the winter cases, the MMCR reflectivity, on average, is only 0.2 dB lower than that of the WSR-88D, with a correlation coefficient of 0.85. This result indicates that the MMCR signals have not been attenuated for ice-phase convective clouds, and the MMCR reflectivity measurements agree well with the WSR-88D, regardless of their vastly different characteristics. For the summer nonprecipitating convective clouds, however, the MMCR reflectivity, on average, is 10.6 dB lower than the WSR-88D mea- surement, and the average differences between the two radar reflectivities are nearly constant with height above cloud base. Three lookup tables with Mie calculations have been generated for correcting the MMCR signal attenuation. After applying attenuation correction for the MMCR reflectivity measurements, the averaged difference between the two radars has been reduced to 9.1 dB. Within the common sensitivity range (210 to 20 dBZ), the mean differences for the uncorrected and corrected MMCR reflectivities have been reduced to 6.2 and 5.3 dB, respectively. The corrected MMCR reflectivities were then merged with the WSR- 88D data to fill in the gaps during the heavy precipitation periods. This merged dataset provides a more complete radar reflectivity profile for studying convective systems associated with heavier precipitation than the original MMCR dataset. It also provides the intensity, duration, and frequency of the convective systems as they propagate over the ARM SGP for climate modelers. Eventually, it will be possible to improve understanding of the cloud-precipitation processes, and evaluate GCM predictions using the long-term merged dataset, which could not have been done with either the MMCR or the WSR-88D dataset alone. 1. Introduction with global climate models (GCMs) (Cess et al. 1996; Randall et al. 2006). The deep convection parameteriza- Clouds are one of the most fundamental aspects of tion of GCMs has the greatest uncertainty because the our climate system and have significant impacts on cli- formative and dissipative processes of convective clouds mate change by reflecting solar radiation back to space are poorly understood (Xu and Krueger 1991). A deep and absorbing longwave radiation from the surface and convective system (DCS) normally consists of two parts: the atmosphere (Wielicki et al. 1996). They are also one the convective core (precipitation) and the stratiform of the most important elements in the hydrology and region (cirrus anvil with or without precipitation). The energy cycle of our planet, particularly in the process of former is important for the atmospheric hydrological cy- producing precipitation (Del Genio et al. 2005). Yet, cle, whereas the latter is dominant in atmospheric radia- understanding of the interaction between clouds and the tion because of its large spatial coverage (Lin et al. 2006). climate, namely, cloud feedback, remains the major To understand the fundamental physics and the in- source of uncertainty in estimating climate sensitivity teractions between clouds and their radiative feedbacks in the atmosphere, the Atmospheric Radiation Mea- surement Program (ARM; Ackerman and Stokes 2003) Corresponding author address: Mr. Zhe Feng, Dept. of Atmo- spheric Sciences, University of North Dakota, 4149 University Dr., supported by the U.S. Department of Energy (DOE) Stop 9006, Grand Forks, ND 58202-9006. established the research site at the Southern Great Plains E-mail: [email protected] (SGP) in 1992. The centerpiece of the ARM cloud DOI: 10.1175/2008JTECHA1190.1 Ó 2009 American Meteorological Society Unauthenticated | Downloaded 09/27/21 01:13 AM UTC MAY 2009 F E N G E T A L . 959 instrument array is the millimeter wavelength cloud the MMCR reflectivity (corrected for attenuation) to fill radar (MMCR; Moran et al. 1998), which has excellent in the gaps during the heavy precipitation periods. This sensitivity and large dynamic range to detect small hy- merged dataset provides a more complete radar re- drometeors, such as cloud droplets and ice crystals, but flectivity profile for convective systems associated with the signals can be significantly attenuated during the heavier precipitation than the original MMCR dataset. heavy precipitation periods (Lhermitte 1990). The It also provides the intensity, duration, and frequency of MMCR provides long-term continuous profiles of the convective systems as they propagate over the ARM equivalent radar reflectivity factor and vertical velocity SGP for climate modelers. Eventually, we can improve at the SGP site. When combined with other ARM in- our understanding of the cloud-precipitation processes, struments, such as the laser ceilometer, micropulse li- and evaluate GCM predictions using the long-term dar, or microwave radiometer (MWR), the hydrome- merged dataset, which could not have been done with teor profiles of the atmosphere can be retrieved (e.g., either the MMCR or the WSR-88D dataset alone. For Dong and Mace 2003; Matrosov et al. 2006). These example, the precipitating part of DCS might be missed valuable long-term datasets facilitate the progress of if using the MMCR only, whereas the cirrus anvil from research on understanding cloud microphysics and their the DCS might not be detected by the WSR-88D. effects on climate (e.g., Clothiaux et al. 2000; Mace and Benson-Troth 2002; Dong et al. 2005, 2006). Because the MMCR signals could be attenuated 2. Data during the heavy precipitating periods, the MMCR a. ARM data reflectivity measurements have some gaps and discon- tinuities, especially when deep convective clouds pass The ARM MMCR operates at a wavelength of 8 mm over the MMCR. Therefore, the MMCR measurements (Ka band) in a vertically pointing mode and measures during the precipitation periods, even for nonprecipitating continuous reflectivity profiles as clouds and hydrome- DCSs with large cloud liquid water path (LWP), are teors pass over the radar field of view (Clothiaux et al. questionable (Moran et al. 1998; Kollias et al. 2007; 2000). It records equivalent radar reflectivity factors Dong et al. 2008). Dong et al. (2008) found that the (Ze) with a 90-m vertical resolution; a total of 167 levels ARM MMCR-derived cloud-top height is much lower starting from 105 m above ground level. The MMCR (;2 km) than the Moderate Resolution Imaging Spec- uses a 3-m-diameter antenna and has a 0.28 beamwidth, troradiometer (MODIS)-retrieved effective cloud height which yields lateral resolution of 35 m at the height of at the ARM Tropical Western Pacific (TWP) site for 10 km for a vertically directed beam. The minimum nonprecipitating DCSs. They suspected that some radar detectable Ze of MMCR are 255 dBZ at 1 km and signals were attenuated near cloud tops because of the 235 dBZ at 10 km (Moran et al. 1998). For cumulus clouds, 22 high LWP (;5000 g m ) measured at the ARM TWP the MMCR can detect Ze up to about 120 dBZ, but Mie site. The results in the Dong et al. (2008) study moti- scattering due to increasing hydrometeor size and signal vated us to investigate precipitating and nonprecipitating attenuation make it difficult to penetrate deep convec- DCSs at the ARM SGP site. As demonstrated in Fig. 1, tive clouds within heavy precipitation. A constant cor- the MMCR signals have been severely attenuated by rection of 3 dB to all levels as well as near-field cor- heavy rainfall (rain rate . 40 mm h21), and completely rections have been added to the MMCR reflectivities attenuated above 2 km between 0250 and 0300 UTC. To (Sekelsky 2002). The MMCR has four operational provide a more complete reflectivity profile, that is, to modes optimized for various clouds types and runs fill in the gaps of the MMCR reflectivity measurements, consecutively in a 36-s cycle (Clothiaux et al. 1999). The other types of radars designed for detecting precipita- data used in this study are 5-min averages (the Mace PI tion are required. product; more information is available online at http:// The Weather Surveillance Radar-1988 Doppler iop.archive.arm.gov/arm-iop/0pi-data/). The Mace PI (WSR-88D) near the SGP site is a good complementary product (Mace et al. 2006) mimics the Active Remote instrument to the MMCR because it has the particular Sensing of Clouds (ARSCL) product (Clothiaux et al. advantage of providing the vertical distribution and 1999, 2000), but the key difference between the two horizontal coverage of precipitation (Miller et al. 1998). products is how the profiles from the four operational In this study, we used the collocated WSR-88D radar modes are merged. For the ARSCL product, they per- volumetric scans over the SGP site and reconstructed formed interpolation to a 9-s temporal grid (the tem- the WSR-88D reflectivity into a time–height series that poral spacing of the individual modes), whereas for the matched the MMCR data format during the period of Mace PI product, they estimated the most reasonable 1997–2006. The WSR-88D data were then merged with measurements for a given 90-m vertical bin from one of Unauthenticated | Downloaded 09/27/21 01:13 AM UTC 960 JOURNAL OF ATMOSPHERIC AND OCEANIC TECHNOLOGY VOLUME 26 FIG.
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