The formation of wider and deeper clouds through cold-pool dynamics

Transcription

1 The formation of wider and deeper clouds through cold-pool dynamics Linda Schlemmer, Cathy Hohenegger e for Meteorology, Hamburg Bergen COST Meeting Linda Schlemmer 1 / 27

2 1 Motivation 2 Simulations 3 Pathway to a parameterization 4 Conclusions COST Meeting Linda Schlemmer 2 / 27

3 Motivation Diurnal cycle of convection over land ow to get deep clouds?? Promotion of deeper and wider clouds that are less affected by entrainment: cold pools (Khairoutdinov and Randall, 2006, Kuang and Bretherton, 2006, Böing et al., 2012) COST Meeting Linda Schlemmer 3 / 27

4 Motivation Cold pools, driven by evaporation of recipitation and melting snow/graupel/hail 1 Dynamical lifting of convective cells along the edges of cold pools (e.g. Lima et al., 2008) 2 Occurrence of a band with high equivalent potential temperature and water vapor along the edge of cold pools (Tompkins, 2001) How do cold pools promote wider clouds? Different in different environments? Shallow <-> deep convection Figure from Cotton (2011) COST Meeting Linda Schlemmer 4 / 27

5 Motivation Structure of the downdraft models: Kamburova and Ludlam (1966), Betts (1979), Srivastava (1985, 1987) Stronger downdrafts for drier environments and a lapse rate that is close to the dry adiabat. In more stable conditions (wet adiabat) melting of graupel, hail or snow must be involved COST Meeting Linda Schlemmer 5 / 27

6 Simulations Large Eddy Simulations (LES) UCLA-LES Anelastic core Smagorinsky-type subgrid-scale diffusion x= y=250 m, z stretched grid with a minimum of 70 m Domain size km, model top at m Two-moment ice microphysics scheme (Seifert and Beheng, 2006) Interactive radiation scheme (Pincus and Stevens, 2009) Surface heat fluxes prescribed COST Meeting Linda Schlemmer 6 / 27

7 Simulations Different environments: cases Mid-latitude continental case (cf. Schlemmer et al., 2011) WET WET_noshear STABLE DRY Maritime case (cf. Waite and Khoudier, 2010) OCEAN COST Meeting Linda Schlemmer 7 / 27

8 Simulations The diurnal cycle COST Meeting Linda Schlemmer 8 / 27

9 Simulations Cold pools θe at 35 m height at 12 LST COST Meeting Linda Schlemmer 9 / 27

10 Simulations Cold pools under different conditions θ e size of cold pool COST Meeting Linda Schlemmer 10 / 27

11 Simulations Signature in moisture field Dry areas in the cold pools and moist damns or patches around them Clouds are located on these wet patches. COST Meeting Linda Schlemmer 11 / 27

12 Simulations Signature in moisture field COST Meeting Linda Schlemmer 12 / 27

13 Simulations Signature in moisture field Modifications of the moisture field have been mentioned by Tompkins (2001). Increase of buoyancy in moist areas. Seifert and Heus (2013) stressed the importance of cold-pool induced moisture modifications for the organization of shallow convection. COST Meeting Linda Schlemmer 13 / 27

14 Simulations Wet patches and clouds COST Meeting Linda Schlemmer 14 / 27

15 Simulations Wet patches and cloud sizes q size of wet patch COST Meeting Linda Schlemmer 15 / 27

16 Simulations Wet patches and cloud sizes q size of wet patch COST Meeting Linda Schlemmer 15 / 27

17 Simulations Turbulence vs precipitation Where do cold pools and wet patches come from? Perturbation and size increases with precipitation. Strong hysteresis effect for the sizes. COST Meeting Linda Schlemmer 16 / 27

18 Simulations Turbulence vs precipitation Where do cold pools and wet patches come from? Development not in line with turbulence. No difference between different simulations. COST Meeting Linda Schlemmer 17 / 27

19 Simulations Dynamical lifting COST Meeting Linda Schlemmer 18 / 27

20 Simulations Convergence & propagation speed cold pool intensity c i (Rotunno, 1988): c 2 i = 2 H cp 0 ( B) dz COST Meeting Linda Schlemmer 19 / 27

21 Simulations Cold pools - cloud width Both a dynamical triggering and an accumulation of moisture by cold pools seems to be important for the generation of wider convective cells, mechanisms hard to disentangle. Large wet patches yield wider clouds Cold pools act in a similar manner in different environments but are stronger and larger in a more humid environment COST Meeting Linda Schlemmer 20 / 27

22 Pathway to a parameterization Pathway to a parameterization Successful attempts to include the effects of cold-pool wakes and associated dynamical triggering into parametrization schemes (e.g. Qian et al., 1998, Mapes, 2000, Rio et al., 2009, Grandpeix and Lafore, 2010). include the role of a moisture aggregation. The aggregation leads to larger clouds feed back on entrainment rate (cf. Stirling and Stratton, 2011) COST Meeting Linda Schlemmer 21 / 27

23 Pathway to a parameterization Pathway to a parameterization: 1 redict θ e in cold pool using the origin of the downdraft: θ e = β (θ e (z orig ) θ e (z surf )) (1) z orig = min[z ct, z min ] (2) z ct z cb β = (3) m z cb z cb : cloud base height, z ct : cloud top height, z surf : surface height, z min height of the minimum value of θ e β Δθ e COST Meeting Linda Schlemmer 22 / 27

24 Pathway to a parameterization Prediction of θ e LES Prediction from domain mean values COST Meeting Linda Schlemmer 23 / 27

25 Pathway to a parameterization Pathway to a parameterization: 2 Relate size of cold pool to temperature depression: A cp = c fit θ 2 e (time for spreading included implicitly) COST Meeting Linda Schlemmer 24 / 27

26 Pathway to a parameterization Assume: size of wet patch = size of dry area = size of cold pool A cp = A wp Size of biggest cloud related to size of wet patch Size of cloud modified entrainment rate COST Meeting Linda Schlemmer 25 / 27

27 Pathway to a parameterization Cloud sizes LES Prediction from domain mean values COST Meeting Linda Schlemmer 26 / 27

28 Conclusions Conclusions Precipitation-driven cold pools set a new length scale in the sub-cloud layer in both the temperature and moisture field. Both a dynamical triggering and an accumulation of moisture by cold pools seems to be important for the generation of wider convective cells, mechanisms hard to disentangle. Cold pools act in a similar manner in different environments but are stronger and larger in a more humid environment Promising attempts towards a parameterization scheme. Intended for GCMs. Challenges: Positive feedback COST Meeting Linda Schlemmer 27 / 27