Simulations of Convergence Lines

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1 Simulations of Convergence Lines with Convection using COSMO-DE Markus Übel and Andreas Bott (University of Bonn) --- COSMO User Seminar

2 Outline Motivation Definition of convergence lines Simulation of case studies using COSMO-DE - 3 rd of July th of June th of July 2010 Conclusions 2

Motivation convergence lines may induce severe thunderstorms (heavy rain, large hail, gust

interest in a precise prediction www.much-heute.

the dynamic and thermodynamic mechanisms forcing convergence lines and its convection is

3 Motivation convergence lines may induce severe thunderstorms (heavy rain, large hail, gust fronts, tornadoes) and squall lines high potential of danger and destruction strong public interest in a precise prediction especially the forecast of convection is very difficult and uncertain detailed knowledge of the dynamic and thermodynamic mechanisms forcing convergence lines and its convection is important estimation of the model uncertainity or knowledge of typical model errors are helpful for the forecaster MOTIVATION 3

4 Definition A convergence line is an aligned area of a convergent wind field near the surface (e.g. in a mesoscale trough [german: Tiefdruckrinne]) rising air masses CONVERGENCE DEFINITION 4

5 Definition A convergence line is an aligned area of a convergent wind field near the surface (e.g. in a mesoscale trough [german: Tiefdruckrinne]) rising air masses convective cloud formation thunderstorms (convection) typical situations: - in a mesoscale trough in front of a cold front in summer - in a thermal low in an area with slight pressure gradients under a ridge in the upper troposphere CONVERGENCE DEFINITION 5

6 Case Studies simulations of convergence lines with different forms of convection model: COSMO-DE with the cumulus convection scheme HYMACS deep convection is parameterized! comparing of the simulations with measurements using the NinJo system presentation of limitations and failures of COSMO-DE CASE STUDIES 6

7 3 rd of July 2010 synoptic situation 500hPa geopotential, temperature DWD analysis 3 rd of July 2010, 12 UTC CASE STUDIES 7

8 3rd of July 2010 convergent wind field 10m wind field [m/s], surface pressure [hpa] divergence in 950 hpa [1/s] -1.50e-04 (Δ = 2.00e-05) 1.50e-04 3rd of July 2010, 15 UTC (COSMO-DE) CASE STUDIES 8

3 rd of July 2010 COSMO-DE Jul 03 2010 Sa 17:00 UTC precipitation Radar COSMO-DE

convective activity is 2 hours to late in COSMO-DE - structure of convection is

9 3 rd of July 2010 COSMO-DE Jul Sa 17:00 UTC precipitation Radar COSMO-DE (mm/h) - quite good agreement between simulation and observation - main convective activity is 2 hours to late in COSMO-DE - structure of convection is well represented: separate but very severe thunderstorms (dbz) Radar (dbz) CASE STUDIES 9

geopotential height [gpdm] 3 rd of July 2010, 15 UTC

10 3 rd of July 2010 vertical windshear 10m wind field [m/s], surface pressure [hpa] wind field 700hPa [m/s], geopotential height [gpdm] 3 rd of July 2010, 15 UTC (COSMO-DE) weak thermic wind separate cells CASE STUDIES 10

11 3 rd of July 2010 vorticity advection - averaging about 18 grid points ( 50km) necessary to see synoptic phenomena (convergence, vorticity advection) filtering of the model noise CASE STUDIES 11

12 3 rd of July 2010 vorticity advection - vorticity advection: v. Ȝ h 2 nd derivative of a basic model quantity (v) is needed - systematic model errors of COSMO-DE exceed the vorticity advection 3 rd of July 2010, 17 UTC (COSMO-DE) CASE STUDIES 12

13 3 rd of July 2010 vorticity advection - vorticity advection: v. Ȝ h? 2 nd derivative of a basic model quantity (v) is needed - systematic model errors of COSMO-DE exceed the vorticity advection 3 rd of July 2010, 17 UTC (COSMO-DE) interpretation very difficult model with a coarser grid resolution (e.g. GME, GFS) gives better results of synoptic scale forcings CASE STUDIES 13

14 6 th of June 2010 precipitation (1) - first convective cells in the luv of the Morvan in front of the cold front trigger: orography - squall-line-like and severe convection along the convergence line since noon - no/very weak precipitation along the cold front in the afternoon radar reflectivity [dbz] CASE STUDIES 14

heavy, squall-line-like thunderstorms along the cold front - too fast

15 precipitation [mm/h] 6 th of June 2010 precipitation (2) COSMO-DE COSMO-DE - only weak signals of convection in front of the cold front - heavy, squall-line-like thunderstorms along the cold front - too fast displacement of the cold front unsatisfied simulations of the convection CASE STUDIES 15

to strong change of direction (WNW S/SSW) COSMO-DE measurements, radar simulated convergence

16 6 th of June 2010 (one) reason of the failure in COSMO-DE: divergence only due to decrease of the wind speed and small change of direction observation: 10m wind field [m/s] divergence due to strong change of direction (WNW S/SSW) COSMO-DE measurements, radar simulated convergence too weak for triggering convection no reduction of CAPE, no decrease of temperature, CASE STUDIES 16

Radar 1 19 28 37 46 55 12 th of July 2010 precipitation COSMO-DE

line to SW (= time lag of 1 2 h) - orientation of the squall line is

southern part - convective form (squall line) is correct - air mass

17 Radar th of July 2010 precipitation COSMO-DE COSMO-DE (mm/h) Radar (dbz) in COSMO-DE: - dislocation of the squall line to SW (= time lag of 1 2 h) - orientation of the squall line is more zonal - intensity too strong in the northern and to weak in the southern part - convective form (squall line) is correct - air mass convection is correct simulated (little displacement) CASE STUDIES 17

geopotential height [gpdm] 12 th of July 2010, 12 UTC

18 12 th of July 2010 vertical windshear 10m wind field [m/s], surface pressure [hpa] wind field 700hPa [m/s], geopotential height [gpdm] 12 th of July 2010, 12 UTC (COSMO-DE) strong thermic wind destructive squall line CASE STUDIES 18

19 12 th of July 2010 (one) reason of the differences surface pressure [hpa] COSMO-DE measurements (SYNOP stations) CASE STUDIES 19

20 Conclusions COSMO-DE well represents the dependence of the convective structure with the vertical wind shear in the area of the convergence line: - isolated cells (weak vertical wind shear) - squall line (strong vertical wind shear) detailed analysis of the fine structure of convection possible (grid size: 2.8km!) precise simulations of synoptic scale fields (e.g. lows, troughs/ridge) only small deviations of mesoscale fields (e.g. mesoscale troughs) convection is very sensitive to these small deviations failure of convection in too weak mesoscale lows wrong intensity or orientation of squall lines averaging necessary for the analysis of synoptic phenomena interpretation of 2 nd derivatives of basic quantities (e.g. vorticity advection) very difficult convergence line is well simulated, convection is more difficult CONCLUSIONS 20

21 Thank you for your attention! ljhfclhgfkhfdkhgfgljhglhglglhgljhglj 21

Weather report 28 November 2017 Campinas/SP

Weather report 28 November 2017 Campinas/SP Summary: 1) Synoptic analysis and pre-convective environment 2) Verification 1) Synoptic analysis and pre-convective environment: At 1200 UTC 28 November 2017