EVCEE7~1.DOC
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EVCEE7~1.DOC
Evaluation of Forecasted Southeast Pacific Stratocumulusin the NCAR, GFDL and ECMWF Models.Cécile Hannay, David L Williamson, James J Hack, Jeffrey T EVCEE7~1.DOC Kiehl, Jerry G Olson, National Center for Atmospheric Research, Boulder, Colorado-Stephen A Klein,Lau rence Livermore National Laborator)', Livermore, California.Christopher s Bretherton,Department of Atmospheric Sciences. University of Washington. Seattle, Washington.and Martin KohlerEuropean Cent EVCEE7~1.DOCer for Medium-range Weather Forecasts. Reading. England.* The National Center for Atmospheric Research is sponsored by the National Science FoundationEVCEE7~1.DOC
.Corresponding author address: Cecile Hannay, National Center for Atmospheric Research, 1850Table Mesa Drive, Boulder, co 80305E-mail: hannay@ucar.eduEvaluation of Forecasted Southeast Pacific Stratocumulusin the NCAR, GFDL and ECMWF Models.Cécile Hannay, David L Williamson, James J Hack, Jeffrey T EVCEE7~1.DOCte (EPIC) cruise of October 2001 with the ECMWF model, the Atmospheric Model (AM) from GFDL. the Community Atmosphere Model (CAM) from NCAR. and the CAM with a revised atmospheric boundary layer formulation from the University of Washington (CAM-UW). The forecasts are initialized from ECMWF analyses EVCEE7~1.DOC and each model is nin for 3 to 5 days to determine the differences with the EPIC field observations.Observations during the EPIC cruise show a well-mEVCEE7~1.DOC
ixed boundary' layer under a sharp inversion. The inversion height and the cloud layer have a strong and regular diurnal cycle. A key problem common tEvaluation of Forecasted Southeast Pacific Stratocumulusin the NCAR, GFDL and ECMWF Models.Cécile Hannay, David L Williamson, James J Hack, Jeffrey T EVCEE7~1.DOChts are achieved with more physically realistic PBL schemes: at one end. CAM uses a dry and surface driven PBL scheme and produces a very shallow PBL while the ECWMF model uses eddy-diffusivity.'mass-flux approach and produces a deeper and better-mixed PBL. All the models produce a strong diurnal cy EVCEE7~1.DOCcle in the liquid water path (LWP) but there are large differences in the amplitude and the phase compared to the EPIC observations. This, in turn, afEVCEE7~1.DOC
fects the radiative fluxes at the surface and the surface energy budget. This is particularly relevant for coupled simulations as this can lead to a lEvaluation of Forecasted Southeast Pacific Stratocumulusin the NCAR, GFDL and ECMWF Models.Cécile Hannay, David L Williamson, James J Hack, Jeffrey T EVCEE7~1.DOC eans with cold sea surface temperature (SST). They form al (he lop of the planetary boundary' layer (PI3Ỉ.) and are capped by a sharp inversion of temperature and moisture (e. g., Klein and Hartmann, 1993). Due to lheir high reflectivity, stratocumulus c louds strongly decrease the solar radiation EVCEE7~1.DOCthat reac hes the surlac e. Also, due to their large optic al thickness, they emit like a black body in the infrared. The net radiative elfec t is a sEVCEE7~1.DOC
trong cooling of the surface and the PBI. relative to clear skies. These radiative properties make stratocumulus a cnicial factor ill the surface and Evaluation of Forecasted Southeast Pacific Stratocumulusin the NCAR, GFDL and ECMWF Models.Cécile Hannay, David L Williamson, James J Hack, Jeffrey T EVCEE7~1.DOCud amount and liquid water path (LWP) with an early morning maximum and an early afternoon minimum in both quantities (Wood et al.. 2002). At night, the strong longwave cooling near the top of the cloud creates turbulence. This produces a well-mixed PBL. which transports moisture from the surface in EVCEE7~1.DOCto the PBL and maintains the cloud. During daytime, in-cloud absorption of solar radiation largely compensates the longwave cooling. As a result, theEVCEE7~1.DOC
turbulence decreases after sunrise leading to a decoupling between the cloud and the surface accompanied by a thinning of the cloud layer, rhe diurnalEvaluation of Forecasted Southeast Pacific Stratocumulusin the NCAR, GFDL and ECMWF Models.Cécile Hannay, David L Williamson, James J Hack, Jeffrey T EVCEE7~1.DOCneral Circulation Models (GCMs) simulate accurately the diurnal cycle of these clouds. In the Southeast Pacific, the diurnal cycle of stratocumulus is very pronounced and stronger than in other stiatocumulus regions (Rozendaal et al., 1995: Zuidcma and Hartmann. 1995; Wood et al., 2002). Other mecha EVCEE7~1.DOCnisms may amplify the stratocumulus diurnal cycle in the Southeast Pacific. In particular, Garreaud et al. (2004) show3that (he diurnal cycle in subsiEVCEE7~1.DOC
dence plays an important role in this region and increases (he amplitude of the diurnal cycle of the stratocumulus amount with respect to the cycle foEvaluation of Forecasted Southeast Pacific Stratocumulusin the NCAR, GFDL and ECMWF Models.Cécile Hannay, David L Williamson, James J Hack, Jeffrey T EVCEE7~1.DOC04), the stratocumulus are among the worst-simulated tropical clouds in GCMs (Bony and Dufresne, 2005). The cloud amount Is usually underestimated, even when the SSTs are observationally prescribed. Moreover, serious model biases exist in the representation of vertical structure. Several studies ass EVCEE7~1.DOCessing stratocumulus in climate and weather forecast models showed that the PBL was typically too shallow and the LWP too low compared with observatioEVCEE7~1.DOC
ns. Duynkerke and Teixeira (2001) showed that the European Center for Medium-range Weather Forecasts (ECWMF) reanalysis ERA 15 (Gibson et al., 1997) sEvaluation of Forecasted Southeast Pacific Stratocumulusin the NCAR, GFDL and ECMWF Models.Cécile Hannay, David L Williamson, James J Hack, Jeffrey T EVCEE7~1.DOCufficiently deep enough into the cloud layer, possibly partly due to the poor model vertical resolution. However, Stevens et al. (2007) showed that the liquid water path and the PBL depth were also underestimated in the ERA40 reanalysis (Uppala, 2005) despite an enhanced vertical resolution compared EVCEE7~1.DOC (0 the ERA 15, suggesting that the overly shallow PBL was not simply of a problem of vertical resolution. They argued that the inability of the ERA40EVCEE7~1.DOC
to produce sufficiently deep PBL came from its K-profile scheme that does not recognize moist processes, and improvement can be expected by better acEvaluation of Forecasted Southeast Pacific Stratocumulusin the NCAR, GFDL and ECMWF Models.Cécile Hannay, David L Williamson, James J Hack, Jeffrey T EVCEE7~1.DOC004) showed that the PBL depth and cloud LWP were underestimated in world-class leading GCMs and operational analyses. Siebesma et al. (2004) found a similar result in the Northeast Pacific and they concluded that the underprediction of clouds was likely due to too intense drizzle and/or too much en EVCEE7~1.DOCtrainment. Since the stratocumulus regions have a significant cooling effect on (he underlying ocean, an4underestimation of the cloud amount causes anEVCEE7~1.DOC
overestimation of the net heat surface flux into the ocean. This may contribute to positive SST biases of several degrees in coupled models (Mechoso Evaluation of Forecasted Southeast Pacific Stratocumulusin the NCAR, GFDL and ECMWF Models.Cécile Hannay, David L Williamson, James J Hack, Jeffrey T EVCEE7~1.DOC feedbacks can then exacerbate the coastal warm SST bias and further reduce the cloudiness, wind speed, evaporation, and upwelling near the model coasts. This Is a particular concern for ENSO predictions, since such errors can strongly affect the circulation.A series of large-eddy simulations (LES) EVCEE7~1.DOCand single-column model (SCM) intercomparison studies of stratocumulus and cumulus-cloud top boundary layers based on wellobserved test cases Iras expEVCEE7~1.DOC
lored some of the reasons behind the low values of LWP. The first intercomparison from the GCSS Boundary Layer Cloud Working Group of LES and SCM simuEvaluation of Forecasted Southeast Pacific Stratocumulusin the NCAR, GFDL and ECMWF Models.Cécile Hannay, David L Williamson, James J Hack, Jeffrey T EVCEE7~1.DOCses to unrealistic small values after as little as one hour of simulation, suggesting excessive entrainment of dry air. More recently, LES and SCM simulations of the Dynamics and Chemistry of Marine Stratocumulus (DYCOMS-II) Research Flight RF01 (Stevens et al., 2003) shows that despite an improveme EVCEE7~1.DOCnt of entrainment rates, the LWP still differed by an order of magnitude between models (Stevens et al., 2005; Zhu el al., 2005). Duynkerke et al. (20EVCEE7~1.DOC
04) found a similar result in the European Project on Cloud Systems (EUROCS) intercomparison of stratocumulus off the coast of California. Meanwhile, Evaluation of Forecasted Southeast Pacific Stratocumulusin the NCAR, GFDL and ECMWF Models.Cécile Hannay, David L Williamson, James J Hack, Jeffrey T EVCEE7~1.DOC 02 (vanZanten and Stevens, 2005) shows that drizzle substantially decreases the LWP for many models (Wyant et al., 2007).shttps://khothuvien.cori!Despite the undeniable value of SCM studies, they are not always able to assess the performance of a physical parameterization within a GCM because there EVCEE7~1.DOC are situations where the systematic errors of the GCM and the SCM differ due to differences in the feedbacks of the dynamics on the physics (Pet ch eEVCEE7~1.DOC
t al., 2007). Understanding the causes of the stratocumulus bias in climate simulations is difficult because of the complexity and non-linear interactGọi ngay
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