![Rossby wave source – What is it?
- first consider nonlinear vorticity equation on one level
- where v is horizontal velocity, ζ is vorticity, D is divergence and F is friction
- as we are looking at the Rossby wave source, we want to get [1] into form of
- where vψ
is rotational wind and S is the Rossby wave source
- this can be done by splitting wind into rotational and divergent components
- then [1] can be written like [2] and the Rossby wave source S is now
(Sardesmukh and Hoskins 1988)](https://image.slidesharecdn.com/76b865d8-d4d8-4d5c-9e80-650f8008695d-170115124852/85/Summary_of_the_summer_project_v2-13-320.jpg)
Summary_of_the_summer_project_v2
- 1. Summary of the summer project - task was to get some insight to the failure of day 6 forecast over Europe - especially, how does convection over N America behave? - first 1.5 months we were looking at the wrong MCS - the right day to consider is the 10th of April - in this project we used OpenIFS resolution T255L91
- 2. Verification of the bust - we ordered initial conditions for days beginning from the 1st and ending to the 13th of April - we needed to check that the bust occurs also in OpenIFS - we calculated anomaly correlation coefficient (ACC) for day six 500hPa geopotential height over Europe (the blue square)
- 3. - the upper one is from Rodwell et al. (2013) and the lower one is the reproduced figure - bust occurs also in OpenIFS - the overall ACC is lower - perhaps lower resolution might affect: T1279 vs T255
- 4. Synoptic scale evolution - next three slides including this one have been plotted from ERA-Interim reanalysis - a lifting trough locates over the Rockies and provides forcing for development of a surface low over the Central US - substantial wind shear in southern Nebraska where the western MCS initiates around 21.00 UTC
- 5. - strong southerly hot and humid airflow near surface ahead of the surface low - the northernmost part of the humid surface airmass is going under cooler mid and high tropospheric airmass → build up of CAPE
- 6. - the CAPE reaches very high values at the left flank of the MCS west of the Great Lakes - the maximum value is ~4000 J/kg - more than enough for strong convection
- 7. Errors in forecast convection - next we take a closer look at the control forecast initialised on the 10th of April at 00.00 UTC - this forecast is the worst in terms of ACC of day 6 forecast over Europe - we concentrate on type of errors and interactions between errors in different fields - CAPE and precipitation fields
- 8. - forecast CAPE and error of the forecast - there is severe displacement error and moderate magnitude error in hour 6 forecast already - error in CAPE leads to error in precipitation 3 hours later
- 9. - 3 hours after the large error in CAPE field has lead to growing displacement error in precipitation - area of heavy precipitation is moving too fast to east in the control forecast
- 10. - again 3 hours later the MCS is moving too south in the control forecast
- 11. - model sounding from Omaha, Iowa-Nebraska border 10 Apr 00 UTC - cloud layer in the upper troposphere, probably anvil cloud of the MCS
- 12. - actual sounding from Omaha - no sign of upper level clouds - considerably more CAPE than in the model sounding
- 13. Rossby wave source – What is it? - first consider nonlinear vorticity equation on one level - where v is horizontal velocity, ζ is vorticity, D is divergence and F is friction - as we are looking at the Rossby wave source, we want to get [1] into form of - where vψ is rotational wind and S is the Rossby wave source - this can be done by splitting wind into rotational and divergent components - then [1] can be written like [2] and the Rossby wave source S is now (Sardesmukh and Hoskins 1988)
- 14. - a couple of figures of the Rossby wave source - the upper one is from the control forecast and the lower one is from ERA-Interim reanalysis - OpenIFS overshoots the Rossby wave source - there is some overestimation of the Rossby wave source in most of the figures
- 15. - later the overshooting decreases but does not end entirely - note especially the trough over the east coast of the US
- 16. - OpenIFS is still overshooting the Rossby wave source - wave breaking created an omega block over N Europe in the forecast but there is no sign of blocking in reanalysis
- 17. Rossby wave breaking - as Rossby wave breaking (RWB) is considered to be potential onset for blocking, we decided to compare the breaking action between the control forecast and the reanalysis - RWB can be defined for example so that the gradient of PV has locally southward pointing component i.e. PV contours are locally overturned or reversed - we plotted PV on 315 Kelvin potential temperature surface which actually penetrates the dynamic tropopause at the midlatitudes - on the slide 18 there are PV on 315K θ surface (shaded) and 500 hPa geopotential height from the control forecast and on the slide 19 there are the counterparts from the reanalysis - the times are: (a) 10 Apr 00 UTC, (b) 11 Apr 00 UTC, (c) 12 Apr 00 UTC, (d) 13 Apr 00 UTC, (e) 14 Apr 00 UTC, (f) 15 Apr 00 UTC and (g) 16 Apr 00 UTC respectively in both figures - the first RWB´s over North America and the Atlantic occur quite similarly (figs a-c) but in figs d-f there is a large difference in the RWB over the Atlantic between the control forecast and the reanalysis; the wave breaks cyclonically in the forecast but anticyclonically in the reanalysis - the RWB is associated with the same trough which located over the Rockies at the time of the initialisation of the control forecast
- 18. a b c d e f g
- 19. a b c d e f g
- 20. 500 hPa geopotential height 16 Apr 2011 00 UTC The day 6 forecast vs. the reanalysis - Z500 hPa once again but now in colours - the upper one is from the control forecast and the lower one is from ERA-Interim reanalysis - the control forecast predicted blocking over Northern Europe - no clear sign of blocking in the reanalysis - note also the cut-off low west of the Canary Islands in the reanalysis
- 21. Ensemble - We ran first five members of the ensemble for the worst initialisation date of six day forecast - none of the members performed well - the fifth member was the best - the second member was the worst - finally we got some spread - one problem: there are perturbations all over the world in ensemble members so some contribution may come from elsewhere than from North America
- 22. - spaghetti plots of 5400 m contour of Z500 - green = the control forecast, black = the reanalysis, other colours = the ensemble members - every member of the ensemble is forecasting blocking at least to some degree - there is a ridge over N Europe also in reanalysis but every member over estimated it - the ensemble forecast and the reanalysis are remarkably consistent over the Atlantic and inconsistent over Europe
- 23. - often ensemble mean has better skill than a single deterministic control forecast - this is not the case with these five members - ACC of the ensemble mean is worse than of the control forecast (slide 21) - green = the control forecast, red = the ensemble mean, black = ERA- Interim reanalysis - the blocking is only displaced slightly to east
- 24. Ensemble initial conditions and short forecasts ● we have also looked at the initial conditions. A couple of quick notes: - characteristics of improving perturbation: amplified Rockies´ trough and weak surface low - characteristics of deteriorating perturbation: weak ridge east of the trough and strong surface low ● CAPE over North America 6 hours after the initialisation - at least I was a little surprised that difference in CAPE between the ensemble members and the control forecast is 3-5 times smaller than between the control forecast and the reanalysis - none of the CAPE fields is significantly better than in the control forecast ● 3h precipitation 12h after the initialisation - in most of the members the area of heavy precipitation over the Great Lakes is moving even faster east than in the control forecast ● OpenIFS was not especially sensitive to the perturbations in these five ensemble members
- 25. Sources Rodwell, M.J. and Coauthors, (2013). Characteristics of occasional poor medium- range weather forecasts for europe. Bull. Am. Meteorol. Soc., 94(9) 1393–1405. Sardesmukh, P. D. and Hoskins, B.J., (1988). The Generation of Global Rotational Flow by Steady Idealized Tropical Divergence. J. Atmos. Sci., 45(7), 1228–1251.