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Shower advection
Non-advection inland of showers (equilibrium convection)
Fig9.6.2: This old example was with "equilibrium convection", that is no longer present in the IFS: 30h convective precipitation totals (mm) with mean sea level pressure verifying at 18UTC 29 Nov 2010, forecast data time 12UTC 28 Nov 2010. The convection scheme is diagnostic and works on a grid box column, so cannot produce large amounts of precipitation over the relatively dry and cold (stable) wintery land areas. Showers are shown as limited to the sea alone while in nature these showers penetrate inland on the brisk easterly wind.
Non-advection inland of showers (non-equilibrium convection)
Fig9.6.3: This newer example was with non-equilibrium convection, introduced into the IFS in 2013: precipitation fields when showers developed over the Great Lakes in very cold air on a westerly wind (mm rainfall equivalent). Significant showers are shown where strong convection is initiated over the relatively warm waters of the Great Lakes, but give very small amounts of showery precipitation down-wind where instability is weakly initiated or not initiated over the cold land. In reality the showers developing over the Great Lakes persisted long enough to be blown well inland as active convection. Note in particular the difference in precipitation east of Lake Michigan.
Inland penetration of wintertime maritime showers
Wintertime maritime convection can penetrate further inland than indicated by IFS forecasts.
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Fig9.6.4C: Schematic illustration of systematic precipitation biases in onshore maritime convection. Too much precipitation is forecast for windward coastal zones and too little precipitation is forecast for areas leeward of high ground. These areas expand and move downwind with stronger winds.
Convective Severity - CAPE and CAPE-shear
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Observations available for this case suggested quite a lot of localised variability, though peak 24h totals were no more than 20mm. It may be that although the CAPE-shear EFI was anomalously large, the absolute values of CAPE-shear may not have been very high.
Impact of differing land surfaces
Land surface characteristics (soil moisture, leaf area index) have an impact upon land and temperature forecasts. Changes in land characteristics is especially important where there is a sharp discontinuity in ground type or vegetation cover. This can produce significant difference in temperatures or moisture content of the lower atmosphere over a short distance, and hence to air temperature and/or the development of convection. Users should inspect model information on land surface, soil moisture and leaf area index to identify areas where significant changes in precipitation or other weather phenomena over short distances may occur.
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Fig 9.6.8: Illustration of the impact of differing land cover and type in the vicinity of Flagstaff, Arizona. Showers broke out over the vegetated west part of the area but not over the rocky region to the east. The central diagram shows the ensemble 98th percentile of "point rainfall", with tephigrams DT 00UTC 18 July 18 T+24 VT 00UTC July 19. The parcel curves have very different CAPE values - greater in the west and hence greater risk of very wet weather, but lesser in the east even though temperatures were higher over the bare surface. This illustrates high sensitivity to humidity mixing ratios and altitude. Humidity mixing ratios can reflect land surface processes related to evapotranspiration which control the moisture exchange with the lower troposphere. And in turn these relate to the soil moisture which controls moisture availability. Also of critical importance on the soundings are the light winds with shear. Here the land surface characteristics changed rapidly across a short distance (forest to rock), which is in fact reflected on the deep (1m) soil moisture plots from the IFS, and also in the leaf area index (LAI), which is a multiplying factor for evaporation.
Impact of low-level moisture
The release of convection is strongly dependent upon correct analysis and forecasting of boundary layer humidity and land surface characteristics. This can result in a mismatch, mainly in arid coastal regions, between the location and severity of forecasts of active convection and verifying observations. Showers may be forecast in the wrong location or not forecast at all. Users should consider the possible effects of more moist air feeding into the boundary layer, perhaps by considering the potential for moist marine air to spread inland in a more pronounced way than CONTROL-10/HRES forecasts suggest. Users should consider the possibility of an influx of low level air that is dissimilar to forecast values - i.e. moist air across coastal areas that might allow release of convection, or the converse if an influx from drier areas occurs. Daytime heating in upland locations and/or upslope flow over the mountains can also cause destabilisation that may not be captured by the forecast models.
Poorly forecast heavy showers in Oman
Fig9.6.9: Large and vigorous convection over eastern Oman 6 July 2018 bringing heavy showers.
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Fig9.6.10: Observed (black) and forecast (red) vertical profiles at approximately the same time as the satellite picture (Fig9.6.9) for a radiosonde location (Seeb, WMO:41256) just northwest of Muscat. The lowest layers were observed to be quite moist while the forecast vertical profile indicated much drier conditions. The low level winds are shown as drifting air from the nearby sea on both observed and forecast profiles. Higher moisture at low levels would allow deep and active convection to be released with sufficient energy input to overcome the convective inhibition (CIN), either by surface heating or by uplift over the mountains. Heavy showers did develop but were not well forecast, if at all.
Poorly forecast heavy showers in SE England
Fig9.6.1: An example of the effect of under-representation of low-level temperature and dew point by the IFS near an urban heat island. Forecast values of near-surface temperature and dew point are about 3C cooler than observed. Modifying the forecast vertical profile using observed values results in significantly greater CAPE than forecast which, together with the forecast shear, would indicate much more active convection. Flash flooding and a tornado were observed near the urban heat island.
Medium level instability in drier areas
It is important that moist medium level instability is modelled sufficiently as even relatively small CAPE can produce precipitation. Users should check forecast vertical profiles against local observations and profiles.
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In Central Australia, no precipitation is indicated; any precipitation in the model is being evaporated before reaching the ground. However, the lightning activity chart suggests that, though the deep moist convection isn't very well-organised, scattered thunderstorms appear likely. This was bourne out by observations. Note also that the model greatly over-predicted lightning activity over northwest Australia.
Use of available forecast data and derived products
CAPE and CAPE-shear don't tell the full story
Large values of CAPE lie in a zone across the Aegean Sea and parts of mid-Greece coincident with a belt of strong vertical wind shear resulting in very high values of CAPE-SHEAR (Figs 2.1.24 & 2.1.25). In particular, high forecast values of CAPE and CAPE-SHEAR are indicated at Pilio while much lower forecast values are shown at Kavala. This might suggest at first sight that any instability that is released in the region of Pilio would be very active with the possibility of severe storms and rainfall. At the same time much less showery activity might be expected at Kavala on the northern flank of the CAPE and CAPE-SHEAR zone. Such a snap assessment would be incorrect.
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Fig2.1.28: Forecast vertical profiles for Kavala and Pilio, Greece. T+24 VT00UTC 11 July 2019, DT00UTC 10 July 2019.
Use of a sequence of data as early warning
A sequence of forecast EFI charts gives early indication of forthcoming severe weather potential (Fig2.1.29), and some idea of the confidence that may be placed on the forecast event. In this case, northern Greece is identified as being at moderately high risk of an extreme event (EFI ~ 0.6) four days before, rising steadily to a very high risk of an extreme event (EFI ~ 0.9) two days before the occurrence of the severe weather. Note how there is consistent indication of a very high risk of an extreme event (EFI ~ 0.9) over the Balkan states through the sequence of forecast runs. The consistency in the areas shown at risk leads to a higher confidence in forecasts of severe weather. Users should inspect forecast fields using ecCharts and vertical profiles as outlined above to assess forecast details, and also add in the influence of additional factors using local knowledge (e.g. regarding topographic influences) wherever possible.
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Fig2.1.29: Sequence of EFI precipitation charts from four EFI runs at 24hr intervals (DT 12UTC on 6, 7, 8, 9 July 2019). Increasingly high EFI precipitation values identify the areas at greatest risk.
Model output associated with extreme convection
CONTROL-10/HRES tends to over-forecast extreme convection, especially in the maritime tropics. Spurious quasi-circular waves (sometimes rings) in convective precipitation fields can emanate from the forecast rapid uplift that is associated. These spread outwards. The gravity waves can even move well upwind, giving a false impression of an eastward-moving trough. The source of these false features should be recognised and their effects discarded from forecasts. Changes to the IFS moist physics in 2019 have mitigated but not entirely removed this effect.
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