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Mesoscale Weather Prediction with the RUC Hybrid Isentropic–Terrain

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Mesoscale Weather Prediction with the RUC Hybrid Isentropic–Terrain FEBRUARY 2004 BENJAMIN ET AL. 473 Mesoscale Weather Prediction with the RUC Hybrid Isentropic±Terrain-Following Coordinate Model STANLEY G. BENJAMIN,GEORG A. GRELL,* JOHN M. BROWN, AND TATIANA G. SMIRNOVA* NOAA/Forecast Systems Laboratory, Boulder, Colorado RAINER BLECK Los Alamos National Laboratory, Los Alamos, New Mexico (Manuscript received 17 April 2003, in ®nal form 5 August 2003) ABSTRACT A mesoscale atmospheric forecast model con®gured in a hybrid isentropic±sigma vertical coordinate and used in the NOAA Rapid Update Cycle (RUC) for operational numerical guidance is presented. The RUC model is the only quasi-isentropic forecast model running operationally in the world and is distinguished from other hybrid isentropic models by its application at fairly high horizontal resolution (10±20 km) and a generalized vertical coordinate formulation that allows model levels to remain continuous and yet be purely isentropic well into the middle and even lower troposphere. The RUC model is fully described in its 2003 operational version, including numerics and physical param- eterizations. The use of these parameterizations, including mixed-phase cloud microphysics and an ensemble- closure-based cumulus parameterization, is fully consistent with the RUC vertical coordinate without any loss of generality. A series of experiments con®rm that the RUC hybrid u±s coordinate reduces cross-coordinate transport over a quasi-horizontal s coordinate. This reduction in cross-coordinate vertical transport results in less numerical vertical diffusion and thereby improves numerical accuracy for moist reversible processes. Finally, a forecast is presented of a strong cyclogenesis case over the eastern United States in which the RUC model produced an accurate 36-h prediction, especially in a 10-km nested version. Horizontal and vertical plots from these forecasts give evidence of detailed yet coherent structures of potential vorticity, moisture, and vertical motion. 1. Introduction part, by a vision to translate the simplicity of the is- entropic perspective of three-dimensional (3D) baro- Numerical atmospheric prediction using an isentropic clinic structures as shown by Rossby et al. (1937), Na- vertical coordinate was ®rst introduced in 1968 (Elias- mias (1939), and others (see review by Gall and Shapiro sen and Raustein), and for the last 35 years, isentropic 2000) into atmospheric modeling. Although most ap- modeling has received a modest yet steady stream of plications of isentropic models up to this time have been research attention. Much of this work has centered on in research, an exception is the Rapid Update Cycle recasting atmospheric representation to better handle the (RUC), a regional mesoscale high-frequency data as- lower boundary condition, resulting in the development similation (Benjamin et al. 2004) and short-range nu- of a variety of hybrid isentropic±terrain-following co- merical prediction system running operationally at the ordinate models (e.g., Bleck 1978; Deaven 1976; Gall National Centers for Environmental Prediction (NCEP). 1972; Uccellini et al. 1979; Johnson et al. 1993; Bleck The RUC model is also distinctive in its application at and Benjamin 1993, hereafter BB93; Konor and Ar- akawa 1997). These efforts were all driven, at least in a much ®ner horizontal resolution (currently 10±20 km) than previous isentropic models. Isentropic-coordinate models are potentially advan- * Additional af®liation: Cooperative Institute for Research in tageous in that they reduce vertical transport through the Environmental Sciences, University of Colorado, Boulder, coordinate surfaces. In these models, lateral mixing is Colorado. carried out on isentropic surfaces rather than across them, meaning that no unwanted cross-isentropic mix- Corresponding author address: Stan Benjamin, NOAA/FSL, 325 ing occurs. Spurious growth of entropy in non-u models, Broadway, R/E/FS1, Boulder, CO 80305. resulting in a cold bias in climate applications (Johnson E-mail: [email protected] 1997) or lack of ensemble spread (Egger 1999), is avoid- Unauthenticated | Downloaded 09/29/21 03:20 PM UTC 474 MONTHLY WEATHER REVIEW VOLUME 132 ed. Atmospheric variables frequently display strong with terrain-following coordinates (Phillips 1957) grad- contrast in statically stable layers, including wind (e.g., ually turning into isobaric coordinates above some in- Shapiro 1978), water vapor (e.g., Johnson et al. 1993), termediate constant pressure level, we limit our discus- hydrometeors, and trace constituents (e.g., Zapotocny sion here speci®cally to isentropic±sigma (u±s) hybrid et al. 1997b; Reames and Zapotocny 1999). Weaver et coordinate de®nitions. al. (2000) showed that laminar structures of ozone in Bleck (1978) describes four different types of hybrid polar regions are better represented with a 3D of¯ine u±s models. All of these designs de®ne a speci®c co- chemistry and transport model con®gured in isentropic ordinate surface where s representation near the ground coordinates than with a similar model using a sigma- makes a transition to u representation aloft. One of these pressure coordinate with more vertical levels. Diabatic options (B), with the transition level at a ®xed pressure effects from radiative ¯ux divergence, always present distance above the surface, is found in the University in the atmosphere, and intermittent latent heating/cool- of Wisconsin (UW) hybrid u±s model (Johnson et al. ing cause departures of material ¯ow from isentropic 1993). This model has been the basis for a number of surfaces. However, to a ®rst-order approximation, is- studies that have demonstrated various numerical ad- entropic surfaces are material surfaces in the free at- vantages of the hybrid coordinate representation over mosphere. Moreover, the moisture and potential vortic- the pure s representation (e.g., Johnson et al. 1993, ity environment for areas of precipitation may be fore- 2000; Zapotocny et al. 1994). Only one of the hybrid cast with better coherence using an isentropic model design options (option D) described by Bleck (1978) (Johnson et al. 1993; BB93). Isentropic coordinates also avoids coordinate intersections at the interface between have an advantage in that they provide adaptive vertical u and s domains. In option D, an above-ground isen- resolution, greater in layers of higher static stability trope is speci®ed as the lower boundary of the u domain, where strong vertical gradients of other variables are with the atmosphere below that interface resolved by s likely to occur. The concentration of isentropes in frontal coordinates. An example of an option D hybrid model zones of high thermal contrast means that these zones is the global circulation model described by Zhu et al. appear as larger-scale features in both alongfront and (1992) that used a lowest pure isentropic level of 352 cross-front dimensions when viewed from an isentropic K, a value necessitated by the fact that observed surface perspective (e.g., Dutton 1976; Benjamin 1989; Gall and potential temperatures over the Himalayas in summer Shapiro 2000). All of these advantages of isentropic reach as high as 335 K. Johnson and Yuan (1998) have modeling in the atmosphere have counterparts in iso- developed an option D variant of the UW hybrid global pycnal modeling in the ocean (e.g., Bleck and Boudra model (called a u±h model) with a lowest pure isentro- 1981; Bleck 2002; Drijfhout 1992). This has led over pic level of 336 K. the years to a two-way technology transfer that has ben- Two other hybrid u±s designs have emerged since e®tted both ®elds. the Bleck (1978) hybrid design classi®cation, neither of The RUC numerical forecast model is an advanced which uses ®xed transition levels. The ®rst (which we version of the hydrostatic primitive equation model de- label as option E) is the two-condition (isentropic; su- scribed by BB93. Its vertical coordinate is also used in perseded by minimum layer thickness constraint) de®- the RUC assimilation system described in Benjamin et nition described by BB93, which provides isentropic al. (2004). The use of a generalized vertical coordinate representation far closer to the ground than previous in a numerical weather prediction model introduced in hybrid coordinate de®nitions. As described in BB93, BB93 is demonstrated again in this paper, but now in this de®nition marked a signi®cant departure from pre- an application with much higher horizontal resolution vious de®nitions, avoided any interface level, and, in (10±20 km compared to 100±160 km in BB93) and fact, used an equation set written for a fully generalized increased emphasis on modeling of diabatic processes. vertical coordinate. The BB93 design, used in the 20- Here we emphasize issues in the design of hybrid and 10-km versions of the RUC model, is derived from isentropic models and describe experiments with the a hybrid isopycnic approach in the ocean model of Bleck RUC model to investigate the particular issue of cross- and Boudra (1981), of which the Hybrid Community coordinate vertical transport. Ocean Model (HYCOM) ocean circulation model (Bleck 2002) is a modern descendant. The details of this design are described in greater detail in section 4. 2. Design of hybrid isentropic±sigma models Perhaps the best way to characterize the present hybrid Various hybrid isentropic model schemes have been scheme is to call it arbitrary Lagrangian±Eulerian developed with the goal of retaining the advantages of (ALE), a term originally coined by Hirt et al. (1974). isentropic representation while eliminating the main Note however, that our scheme goes far beyond ALE shortcoming of a pure isentropic scheme, which lies in in that it adds the element of ``coordinate maintenance,'' the treatment of the planetary boundary layer or other that is, the migration of coordinate surfaces toward a layers where entropy is constant or even may decrease set of prede®ned target surfaces, in this case isentropes, with height. While other types of hybrid coordinate wherever minimum thickness conditions allow. models exist, especially hybrid sigma±pressure models A second, more recent hybrid u±s design (which we Unauthenticated | Downloaded 09/29/21 03:20 PM UTC FEBRUARY 2004 BENJAMIN ET AL.
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