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Factors Affecting the Evolution of Hurricane Erin (2001) and the Distributions of Hydrometeors: Role of Microphysical Processes Ϩ GREG M

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Factors Affecting the Evolution of Hurricane Erin (2001) and the Distributions of Hydrometeors: Role of Microphysical Processes Ϩ GREG M JANUARY 2006 M CFARQUHAR ET AL. 127 Factors Affecting the Evolution of Hurricane Erin (2001) and the Distributions of Hydrometeors: Role of Microphysical Processes ϩ GREG M. MCFARQUHAR,* HENIAN ZHANG,* GERALD HEYMSFIELD, ROBBIE HOOD,# JIMY DUDHIA,@ ϩ JEFFREY B. HALVERSON, AND FRANK MARKS JR.& *Department of Atmospheric Sciences, University of Illinois at Urbana–Champaign, Urbana, Illinois ϩNASA Goddard Space Flight Center, Greenbelt, Maryland #NASA Marshall Space Flight Center, Huntsville, Alabama @National Center for Atmospheric Research, Boulder, Colorado &NOAA/Hurricane Research Division, Miami, Florida (Manuscript received 11 November 2003, in final form 8 March 2005) ABSTRACT Fine-resolution simulations of Hurricane Erin are conducted using the fifth-generation Pennsylvania State University–NCAR Mesoscale Model (MM5) to investigate roles of thermodynamic, boundary layer, and microphysical processes on Erin’s structure and evolution. Choice of boundary layer scheme has the biggest impact on simulations, with the minimum surface pressure (Pmin) averaged over the last 18 h (when Erin is relatively mature) varying by over 20 hPa. Over the same period, coefficients used to describe graupel fall speeds (Vg) affect Pmin by up to 7 hPa, almost equivalent to the maximum 9-hPa difference between microphysical parameterization schemes; faster Vg and schemes with more hydrometeor categories generally give lower Pmin. Compared to radar reflectivity factor (Z) observed by the NOAA P-3 lower fuselage radar and the NASA ER-2 Doppler radar (EDOP) in Erin, all simulations overpredict the nor- malized frequency of occurrence of Z larger than 40 dBZ and underpredict that between 20 and 40 dBZ near the surface; simulations overpredict Z larger than 25 to 30 dBZ and underpredict that between 15 and 25 or 30 dBZ near the melting layer, the upper limit depending on altitude. Brightness temperatures (Tb) computed from modeled fields at 37.1- and 85.5-GHz channels that respond to scattering by graupel-size ice show enhanced scattering, mainly due to graupel, compared to observations. Simulated graupel mixing ratios are about 10 times larger than values observed in other hurricanes. For the control run at 6.5 km averaged over the last 18 simulated hours, Doppler velocities computed from modeled fields (Vdop) greater Ϫ than 5 m s 1 make up 12% of Erin’s simulated area for the base simulation but less than 2% of the observed Ϫ area. In the eyewall, 5% of model updrafts above 9 km are stronger than 10 m s 1, whereas statistics from Ϫ other hurricanes show that 5% of updrafts are stronger than only 5 m s 1. Variations in distributions of Z, vertical motion, and graupel mixing ratios between schemes are not sufficient to explain systematic offsets between observations and models. A new iterative condensation scheme, used with the Reisner mixed- phase microphysics scheme, limits unphysical increases of equivalent potential temperature associated with many condensation schemes and reduces the frequency of Z larger than 50 dBZ, but has minimal effect on Z below50dBZ, which represent 95% of the modeled hurricane rain area. However, the new scheme Ϫ1 changes the Erin simulations in that 95% of the updrafts are weaker than 5 m s and Pmin is up to 12 hPa higher over the last 18 simulated hours. 1. Introduction cesses in models. To improve QPFs for hurricanes, im- proved knowledge of cloud microphysical, thermody- Quantitative precipitation forecasts (QPFs) require namic, and turbulent processes; land surface– knowledge of synoptic, mesoscale, and microscale pro- atmosphere interactions; improved measurements of cesses, and an adequate representation of these pro- atmospheric water vapor; a better understanding of me- soscale dynamics; and further development of meso- scale numerical models and cumulus parameterization Corresponding author address: Prof. Greg M. McFarquhar, Dept. of Atmospheric Sciences, University of Illinois at Urbana– schemes are required. Although several studies have Champaign, 105 S. Gregory Street, Urbana, IL 61801. investigated the influence of many such processes on E-mail: [email protected] the evolution of hurricanes, fewer recent studies have © 2006 American Meteorological Society Unauthenticated | Downloaded 10/09/21 04:06 AM UTC 128 JOURNAL OF THE ATMOSPHERIC SCIENCES VOLUME 63 examined the impacts of cloud microphysical processes 3.5), are used to examine impacts of boundary layer and on the structure and evolution of these systems and on microphysical parameterization schemes on the growth their QPFs. and maintenance of Erin. Impacts of varying coeffi- Previous studies showed that representations of mi- cients that describe the fall velocities of graupel par- crophysical processes affect simulations of hurricanes. ticles on hurricane dynamics are described. A new it- Willoughby et al. (1984) showed that hurricane simula- erative condensation scheme, developed here to limit tions with parameterized ice microphysics had a differ- the artificial increase of equivalent potential tempera- ⌰ ent structure and evolution compared to those with liq- ture e that occurs during the adjustment step of many uid water microphysics. Lord et al. (1984) and Lord and condensation schemes (Bryan and Fritsch 2000) is also Lord (1988) used an axisymmetric, nonhydrostatic tested. model to show that cooling associated with melting ice Observations made on 10 September 2001 during particles initiates and maintains model downdrafts, the flights of the National Aeronautics and Space Admin- extent and intensity of which are determined by hori- istration (NASA) ER-2 and National Oceanic and At- zontal advection of hydrometeors from convection to- mospheric Administration (NOAA) P-3 aircraft during gether with fall speeds of snow and graupel. The down- the Fourth Convection and Moisture Experiment drafts contribute to the formation of multiple convec- (CAMEX-4) provide a framework for interpretation of tive rings that in turn modify storm development model results. Vertical profiles of radar reflectivity fac- (Willoughby et al. 1984). McCumber et al. (1991) evalu- tor (Z) and Doppler velocity (Vdop) obtained from the ated the performance of several ice parameterizations ER-2 Doppler radar (EDOP; Heymsfield et al. 1996) in both tropical squall-type and nonsquall-type systems, are compared against Z and Vdop derived from modeled concluding that their simulations were more strongly fields. Distributions of Z observed by the P-3 lower influenced by differences in descriptive microphysical fuselage radar are also compared against modeled parameters (e.g., size distribution intercept parameter fields. Brightness temperatures (Tb) measured at four and particle density) than by differences in the way frequencies by the Advanced Microwave Precipitation microphysical processes were treated in the ice Radiometer (AMPR; Spencer et al. 1994) on the ER-2 schemes. They suggested that the application of bulk are compared with Tb calculated using modeled hy- ice microphysics in cloud models might be case specific, drometeor fields as input to a microwave radiative indicating that microphysical sensitivity studies for transfer model (C. Kummerow 2004, personal commu- other cloud systems may not apply to hurricanes. Un- nication). Temperature T and moisture q␷ profiles from certainties associated with microphysics must also be dropsondes released in the eye are compared against placed in the context of uncertainties associated with modeled profiles to determine their sensitivity to the other processes. condensation scheme. Modeled fields are also com- Other studies have focused on how initial conditions, pared against updraft and downdraft statistics observed rainfall assimilation, and cumulus parameterization in other hurricanes (Black et al. 1996) and against grau- schemes (Karyampudi et al. 1998), boundary layer pel mixing ratios estimated from in situ observations in schemes (Braun and Tao 2000), and the role of a gra- other storms (McFarquhar and Black 2004). dient of angular momentum above regions of maximum The remainder of this paper is organized as follows. convective heating (Krishnamurti et al. 1998) affect Section 2 provides information on the structure and hurricane simulations. Simulations with high resolution evolution of Erin based on observations acquired dur- (Liu et al. 1997, 1999) showed that the track, intensity, ing CAMEX-4, concentrating on those observations and inner-core structures of Hurricane Andrew could used to assess the simulations and sensitivity studies be reproduced using realistic model physics and proper outlined in section 3. Section 4 describes simulation initial vortices. Liu et al. (1997) also suggested that the results and impacts of microphysical, thermodynamic, axisymmetric models used in earlier studies did not ad- and boundary layer processes on the structure and evo- equately represent storm–environment interactions, lution of Erin. The significance of the results is summa- suggesting previous microphysical sensitivity studies us- rized in section 5. ing axisymmetric models might not be applicable. Rog- ers et al. (2004) also report on the sensitivity of hurri- 2. Observations of Hurricane Erin cane processes to the representations of microphysics. In this paper, simulations of Hurricane Erin 2001, Hurricane Erin (2001) was the first tropical cyclone conducted using the fifth-generation Pennsylvania in the Atlantic Ocean Basin to reach hurricane status in State University–National Center for Atmospheric Re- 2001 and achieved
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