Effects of a convective GWD parameterization in the global forecast system of the Met Office Unified Model in Korea

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1 Effects of a convective GWD parameterization in the global forecast system of the Met Office Unified Model in Korea Young-Ha Kim 1, Hye-Yeong Chun 1, and Dong-Joon Kim 2 1 Yonsei University, Seoul, Korea 2 Korea Meteorological Administration, Seoul, Korea AGU Chapman Conference / 4 March 2011

2 Introduction Gravity-wave drag (GWD) has been considered as a crucial dynamical forcing to reproduce the observed features in global models (e.g. Lindzen, 1981; Holton, 1983). Orographic GWD was found to alleviate excessive polar-night jets and cold-pole biases in the northern winter stratosphere (e.g. Boer et al., 1984; Palmer et al., 1986; McFarlane, 1987). Recently, non-stationary GWD parameterizations have been implemented in global models (e.g. Scaife et al., 2000; Sassi et al., 2002; McLandress and Scinocca, 2005). - Scaife et al. (2002) : With a parameterization by Warner and McIntyre (1999), the maximum strength and tilt of stratospheric jets in both hemispheres are improved as well as the representation of the equatorial QBO.

3 Introduction Determination of the source-level gravity-wave momentum flux (GWMF) in the non-stationary GWD parameterization - An empirical constant in all model grids or a fixed latitudinal function is used in most parameterizations - Unfavorable for representing the spatiotemporal variability of GW sources Convective GWD (CGWD) parameterization (e.g. Kershaw, 1995; Chun and Baik, 1998; Beres et al., 2004; Song and Chun, 2005; Chun et al., 2008) - Song et al. (2007) : With the implementation of Song and Chun (2005) into a GCM, climate results of the stratospheric wind and temperature are improved significantly. - For weather forecasting models, only a few studies examine the effects of the CGWD parameterization (e.g. Kim et al., 2006). In this study, the impacts of CGWD in the global forecasting system of the Met Office Unified Model in Korea are examined by implementing the parameterization of Song and Chun (2005).

4 Experimental description Model Descriptions Unified Model (UM) vn6.6 Resolution N320L50 ( h ~ 40 km, z top ~ 63 km ) Initial field Forecast From 6-hr 4DVAR assimilation cycle 5-days forecasts from every 00/12 UTC Period January 2010 / y 2009 Experiments CTL CGWD CGWD parameterization None SC05 (Song and Chun, 2005) Song and Chun (2005) : spectral parameterization of CGWD that calculates the analytic solution of cloud-top GWMF using the information from the cumulus convection parameterization of the model. In the CGWD experiment, the magnitude of source-level GWMF in the nonstationary GWD parameterization by Warner and McIntyre (1999) is reduced in low latitudes in order to prevent double count of GWMF. Verification data ECMWF Interim reanalysis (ERA-Interim) data (Berrisford et al., 2009) ( resolution, hpa)

5 Results CTL experiment - Biases in zonal-mean U and T CGWD experiment - Cloud-top GWMF and convective GW drag - Effects of the CGWD parameterization on 5-day FCST the zonal-mean U and T the meridional circulation and precipitation the forecast skill scores (RMSE)

6 Jan U Zonal-mean bias (CTL_FCST) ERA CTL_5 CTL_5 ERA red : positive blue : negative m s -1 # Equatorial-wind biases # Less tilt of the polar-night jet in contour : monthly mean shading : standard dev.

7 Zonal-mean bias (CTL_FCST) T red : positive blue : negative Jan ERA CTL_5 CTL_5 ERA K # Similar structures except near the winter pole in contour : monthly mean shading : standard dev.

8 Cloud-top GW momentum flux (CGWD) Jan Zonal avg. Zonal avg.

9 Zonal convective GWD (CGWD) Jan # Equatorial forcing with a wave-like vertical structure (0.2 ~ 0.4 m s -1 day -1 at hpa) # Large negative forcing in the winter midlatitudes (~2 m s -1 day -1 at 1 hpa in y 2009) # Positive forcing in the subtropics of summer hemisphere

10 Zonal forcing in the TEM eqn. (CGWD) Jan solid : positive dotted : negative # Positive (negative) CGWD forcing above (below) 20 hpa where the phase of QBO is easterly maximum. U

11 Zonal forcing in the TEM eqn. (CGWD) solid : positive dotted : negative # Positive (negative) CGWD forcing above (below) 20 hpa where the phase of QBO is easterly maximum. U

12 CGWD effects on the zonal-mean U (5-day FCST) U red : positive blue : negative Jan CTL CTL ERA CGWD CTL 2 ~ m s -1 contour : monthly mean shading : standard dev. contour : monthly mean shading : 95, 99% sig. lev.

13 CGWD effects on the zonal-mean T (5-day FCST) T red : positive blue : negative Jan CTL CTL ERA CGWD CTL K contour : monthly mean shading : standard dev. contour : monthly mean shading : 95, 99% sig. lev.

14 CGWD effects on the meridional circulation Jan CTL CGWD CTL mm s -1 Residual-mean meridional circulation (vector) and vertical velocity (shading) # Enhanced Brewer-Dobson circulation in # Significant changes at the edges of Hadley cell in Jan

15 CGWD effects on 3-hr acc. precipitation Jan CTL 5-day forecasting CTL ANAL Zonal mean dotted : CTL ANAL solid : CGWD CTL Difference in vertical velocity (CGWD - CTL)

16 CGWD effects on forecast skill scores (RMSE) RMSE scores of 5-day FCST in CTL and CGWD are calculated for 9 variables in each month 850-hPa temperatures (T850) 500-hPa geopotential height (Z500) 200-hPa zonal wind (U200) for the TR, NH, SH TR : 20 N 20 S NH : 20 N 90 N SH : 20 S 90 S Jan T850TR T850TR Z500TR 5 variables are selected based on the confidence level of differences between CTL and CGWD larger than 90% (student t-test) : U200TR Jan T850TR T850TR, Z500TR, U200TR, Z500NH Z500NH RMSE differences (CTL CGWD) normalized by the averaged RMSE of the CTL experiment (%)

17 Summary and Conclusions The impacts of CGWD in the global forecasting system of the UM are examined by implementing the parameterization of Song and Chun (2005). The cloud-top GWMF reflects the spatial and seasonal variability of GW sources. The CGWD significantly forces the equatorial winds with a wave-like vertical structure and midlatitudinal winds in both hemispheres. The equatorial wind biases in the stratosphere are reduced in part by 10 20%. The tropical circulation is modified significantly at the edges of Hadley cell. Tropical precipitation in January 2010 is enhanced, and its difference from the analysis is reduced. RMSE skill scores of the 850-hPa T in both months and 500-hPa Z in y 2009 are reduced.

18 Thank you for your attention!