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Considerations on convection issues

Considerations on convection issues. Federico Grazzini, ARPA-SIMC Emilia-Romagna. some conclusions so far…. Benefit of aggregating precipitation (Eckert GM 2008, Tesini GM 2009) Overall better results for very high-res models for moderate-high precipitations intensity (Eckert GM 2008)

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Considerations on convection issues

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  1. Considerations on convection issues Federico Grazzini, ARPA-SIMC Emilia-Romagna

  2. some conclusions so far….. • Benefit of aggregating precipitation (Eckert GM 2008, Tesini GM 2009) • Overall better results for very high-res models for moderate-high precipitations intensity (Eckert GM 2008) • Benefit of high-res models QPF is rather to see in situations with strong synoptic forcing, flow interaction with orography, embedded convection. Strongest impact at scales of 30 to 50 km. Not always convection is a limiting factor.

  3. Heavy rain over the Appenines mountains: orographic forcing and embedded convection in a strong southerly flow.Operational precipitation forecasts from different model suites FC+36/48 Averaged precip over warning area E (3000Km2)

  4. COSMOI7 – Acc total precip Fc+12 – Fc+36 29/10 00-24UTC

  5. COSMOI7 – Acc Conv precip Fc+12 – Fc+36 29/10 00-24UTC

  6. COSMOI2 – Acc total precip Fc+12 – Fc+36 29/10 00-24UTC

  7. Heavy rain over the Appenines mountainsflow: despite relevant presence of convection and instability the COSMOI2 forecast was very accurate, why? Greater loss of predictability occurs over moist convectively unstable regions that are able to propagate energy against the mean flow (absolute instability). From linear analysis of absolute instability theory we see that for U > Ucrit=Nm2/(m2+k2)*sqrt /(m2+k2) upstream propagation of gravity waves is inibithed, growing perturbation are swept away by lateral boundary condition, increase of predictability. Hohenegger et al. 2006 Mon. Wea. Rev. For the choice of m and k (vertical and horizon wave number) we assume that gravity waves induced thermal perturbation are up to 10Km in vertical 50 Km in horizontal.

  8. Heavy rain over the Appenines mountainsOn this day very high precip were recorded, in the order of 150-200 mm/24h with peaks during convective events of 30mm/h Wind velocity U 500 hPa 29/10/2008 12UTC – White areas U>Ucrit U > Ucrit COSMO I2 integration domain Absolute instability* as defined in Hohenegger et al. 2006 Mon. Wea. Rev.

  9. some conclusions so far….. • Precipitation scores increase with box size (Eckert GM 2008) • Overall better results for very high-res models for moderate-high precipitations intensity (Eckert GM 2008) • This suggest that the benefits of very high-res models is rather to see in situations with strong synoptic forcing, flow interaction with orography, embedded convection. Strongest impact at scales of 30 to 50 km. Not always convection is a limiting factor. • This is confirmed by experience and case studies. On those cases COSMO2 is often better than COSMO7 due excessive convective precip upwind to the orography of the latter. Some time there is also an excessive feedback of parametrized convection in to the dynamics. • What about weakly forced convection ? We present a case study over N-Italy illustrating this situation.

  10. Weak synoptic forcing, cold air advection at upper levels north of the Alps

  11. A case of non equilibrium convection: building up of CAPE over longer time-scales, high spatial variability of CIN values, convection enhanced by cold pools from previous systems (Done et al. 2006 QJR Meteorol. Soc.) Floodings outside Parma 27/05/2009 Observed precipitation in 15 minutes • 27/05/2009 – 14:00 Colorno (PR) 21.2 • 27/05/2009 – 14:45 Parma T. P. (PR) 29.4 • 27/05/2009 – 14:45 Parma (PR) 31.4 • 27/05/2009 – 15:15 Casatico – Langhirano (PR) 36.8 • 27/05/2009 – 15:45 Collagna (RE) 20.2 • 27/05/2009 – 16:00 Ligonchio (RE) 30 • 27/05/2009 – 16:00 Collagna (RE) 25

  12. COSMOI2 FC+12 Precip, CAPE COSMOI2 FC+12 Precip, CIN Precipitation is anticipated and weaker

  13. Conv. Equilibrium: “Air Mass convection”, widespread convective precipitating single cells (or multi cells) developing simultaneously. Cell organization it is not that complex. In this case parametrized convection could be appropriate Non Equilibrium: CIN is strong, CAPE accumulates, convection develops only in few limited spots but can develop with a strong mesoscale organization (role of cell internal dynamics become more important). Explicit convection should be superior than parametrized. Flow dependant use of models ?

  14. COSMO2 had a very limited diurnal cycle, the afternoon maximum was very weak. DWD introduced turlen=150m together with a modification of the subgrid cloudiness q_krit=1.6, clc_diag=0.5. For Germany this change has a quite positive impact, with more small scale convection, which is in many cases more realistic. Also the diurnal cycle has been improved. However there is still a tendency underestimate the frequency of convective systems in weakly forced cases. Convection is a bit too strong in strongly forced situations. Also the organization, propagation and lifetime of convective systems is often not good enough in non-equillibrium situations, but sometimes it is surprisingly good. In 2009 also the initiation of convection was much better. We still miss some isolated cells or even systems in weakly forced non-equilibrium situations (Seifert, personal communication). At MeteoSwiss they find a negative impact of the reduced mixing length getting to much convection over the Alps.They have now operationally introduced turlen=250m combined with q_krit=1.6, clc_diag=0.5. At ARPA-SIMC we recently upgraded to version 4.9, running I2 with turlen=250m. This had a quite remarkable impact on the diurnal cycle of scattered convection with a substantial increase in number of cells. Distribution and frequency looks realistic now, the intensity seems a bit too strong. NEW Few weeks ago ETH (thanks to Peter Brockhaus and Daniel Luethi) released the IFS convection scheme library to be tested and implemented in COSMO (7). How to improve ?....better physical description of PBL A brief overview of COSMO convection permitting implementations

  15. Moist Processes in Future High Resolution NWPModels June 15-17, 2009 Norrköping, Sweden Topological issues: domains often are tailored on the “shape of the country”. Choice of the domain should be more “process” oriented. There will always be processes that act on scales smaller than the horizontal resolution, the need of parametrization will not ease with increased resolution. Importance of Areosols although they may add further complexity to the system Worrying that an artificial parameter such as horizontal diffusion has such a strong influence, is that showing the need for different parametrizations? 1D param are still useful at high resol? Is it a problem that we treat convection as fully mature, without intermediate stage? What is the “non-mature” stage of convection? Data assimilation - Which technique to use? - How to best use radar - Avoid touching large and synoptic waves, modify only small scales Verification

  16. Despite the clear benefit of running NWP models at convective-scale resolutions for the representations of local effects, weakly forced convection is still problematic with a general underestimation by COSMO2 A not correct representation of nocturnal PBL evolution could have a negative effect on diurnal convection preventing or anticipating the triggering. (sensitivity on PBL parameters, sensitivity to turlen and horizontal diffusion) More sophisticated microphysics schemes could help in the correct representation of life cycle and organization of deep convection, especially for the formation of downdrafts and of cold pools. (implementations of the new Seifert and Beheng hierarchical two-moment scheme) Running different implementations of the model may represents a great potential for diagnostic investigation. However it is difficult to exploit this potential due to the limited exchange of data between the different operational centres…………… Concluding remarks

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