Nowcasting Applications African SWIFT Summer School Morné Gijben [email protected] Weather Research South African Weather Service Doc Ref no: RES-PPT-SWIFT-20190729-GIJ002-001.1 2020/01/10 1 What is the difference between nowcasting and forecasting? 2020/01/10 2 Definition of time ranges • Nowcasting: A description of current weather parameters and 0 to 2 hours’ description of forecast weather parameters • Very short-range weather forecasting: Up to 12 hours’ description of weather parameters • Short-range weather forecasting: Beyond 12 hours’ and up to 72 hours’ description of weather parameters • Medium-range weather forecasting: Beyond 72 hours’ and up to 240 hours’ description of weather parameters • Extended-range weather forecasting: Beyond 10 days’ and up to 30 days’ description of weather parameters. Usually averaged and expressed as a departure from climate values for that period 2020/01/10 3 Definition of time ranges • Long-range forecasting: From 30 days up to two years • Month forecast: Description of averaged weather parameters expressed as a departure (deviation, variation, anomaly) from climate values for that month at any lead-time • Seasonal forecast: Description of averaged weather parameters expressed as a departure from climate values for that season at any lead-time • Climate forecasting: Beyond two years • Climate variability prediction: Description of the expected climate parameters associated with the variation of interannual, decadal and multi-decadal climate anomalies • Climate prediction: Description of expected future climate including the effects of both natural and human influences 2020/01/10 4 Why do we need nowcasting? • Mandate of all weather services globally is to save lives and prevent losses • Monitoring weather events in real time and where they will move to in the next 2 hours (nowcasting) should lead to warnings to the public on the action which is needed to save lives and prevent damage to property • Nowcasting can warn the public on the impact of severe weather events (such as local flooding or wind/hail etc) • International trends are focusing more and nowcasting due to the impact this has on people 2020/01/10 5 • On the nowcasting to very short range forecasting time scale (first 12 hours): – we rely heavily on remote sensing since this gives us real time information – This is used for warnings • On the short to medium (and longer time scales): – NWP/EPS is more important – This is used for watches and advisories (more than a day ahead) • Between real time and 12 hours we can make use of remote sensing blended/combined with NWP to extend the remote sensing tools 2020/01/10 6 Improved observation of real time events: What can satellite provide to us for nowcasting purposes? 2020/01/10 7 The Meteosat Satellite channels Channel Band (μm) VIS0.6 0.56 – 0.71 VIS0.8 0.74 – 0.88 NIR1.6 1.50 – 1.78 IR3.9 3.40 – 4.20 IR8.7 8.30 – 9.10 IR10.8 9.80 – 11.80 IR12.0 11.00 – 13.00 WV6.2 5.35 – 7.15 WV7.3 6.85 – 7.85 IR9.7 9.38 – 9.94 IR13.4 12.40 – 14.40 HRV 0.4 – 1.1 ~3 km data sampling intervals, except HRV (~1 km) Images every 15 minutes 2020/01/10 8 Single Channels IR10.8 channel • Gives some idea of the type of clouds • Bright white color cold temperatures • Grey colors warmer clouds 2020/01/10 9 Color enhancement of IR imagery • A color palette can be added to IR channel displays to enable forecasters to see cloud top temperatures in color • This color palette makes it easier to see cloud top features • Since tall, cold clouds can be associated with severe weather, this is of interest to us all. 2020/01/10 10 Cold-U/V & Cold-ring shaped storms Definitions: Embedded warm area (spot): • smaller region of higher BT, • enclosed by a (more or less) continuous region of lower temperatures, • forms downwind of overshooting tops or in vicinity of elevated domes. The cold-U/V and cold-ring: • features are cold parts of a regular storm anvil only, • surrounding longer-lived (~ 30-40 minutes at least) and • larger-sized embedded warm areas. • It is the character and form of the embedded warm area which determines if the storm is labeled as a cold-ring-shaped or cold-U/V-shaped one. 2020/01/10 11 Cold-U/V & Cold-ring shaped storms • Short-lived embedded warm spots/areas (~ 5-20 minutes) • More frequent • Do not indicate possible severe weather • Long-lived embedded warm spots/areas (~ 1-2 hours) • Do indicate possible severe weather Documented cases showed a very close correlation with severe weather or supercells. However, this feature alone does not automatically classify a storm as a supercell !!! If observed, it indicates a possible severity of the storm, it is not a prove of the severity! 2020/01/10 12 Cold-U/V & Cold-ring shaped storms • Mechanism of cold-U/V and cold-ring formation still not quite well (unambiguously) explained. • Both types are most likely generated by similar mechanisms. It seems that their occurrence is supported by some specific airmass types: A strong thermal inversion above the tropopause. Upper-level wind shear (cold rings typical for lower shear, cold-U/V for higher values of wind shear. 2020/01/10 13 Cold-U/V & Cold-ring shaped storms • Embedded warm area (spot) - part of the storm above the tropopause, with warmer temperatures due to the temperature inversion above the tropopause. • The highest tops are located at the upwind side of the cold ring, and the central warm spot develops with time downwind of these, above the stratiform part of the anvil. 2020/01/10 14 Cold-U/V & Cold-ring shaped storms HRV Meteosat-9 (MSG2) 15:00 UTC IR 10.8 BT ENH DISTANT WARM AREA (DWA) CLOSE-IN WARM AREA (CWA) COLD-U 2020/01/10 26 May 2007, Germany 15 Cold-U/V & Cold-ring shaped storms HRV Meteosat-8 (MSG1) 13:45 UTC IR 10.8 BT ENH CENTRAL WARM SPOT (CWS) COLD RING 25 June 2006, Czech Republic and Austria 2020/01/10 16 MSG Channel Differences Useful to Monitor Convection Channel Diff. Application IR8.7 - IR10.8 Day/Night: optical thickness, phase IR10.8 - IR12.0 Day/Night: optical thickness NIR1.6 - VIS0.6 Day: phase (ice index), particle size IR3.9 - IR10.8 Day: particle size Night: particle size (only for warm clouds) WV6.2 - IR10.8 Day/Night: overshooting tops 2020/01/10 17 IR10.8 – IR12.0 • Uses two MSG channels (IR 10.8 and 12.0) • Identify moisture ridges and drylines • BTD IR10.8-12.0 gives indication of total moisture content • Focuses on surface features recommended by EUMETSAT Dry Moist 0600Z 0 to +1 K +2 to +4 K 1200Z 0 to +2 K +4 to +6 K 2020/01/10 18 IR10.8 – IR12.0 MOIST DRY 2020/01/10 19 IR10.8 – IR12.0 • 09:00 UTC • 13:00 UTC MOIST DRY 2020/01/10 20 IR10.8 – IR12.0 limitations • Clear skies • Influenced by diurnal variations • Low moisture hot surface = high moisture cold surface • Does not work at night • Does not work in high mountain areas • Contaminated by sandy surfaces 2020/01/10 21 MSG Red-Green-Blue(RGB) combinations 2020/01/10 22 Standard RGBs RGB Composite Applications Time RGB 10-09,09-07,09: Dust, Clouds (thickness, phase), Contrails Day & Night Fog, Ash, SO2, Low-level Humidity RGB 05-06,08-09,05 Severe Cyclones, Jets, PV Analysis Day & Night RGB 10-09,09-04,09: Clouds, Fog, Contrails, Fires Night RGB 02,04r,09: Clouds, Convection, Snow, Fog, Fires Day RGB 05-06,04-09,03-01: Severe Convection Day RGB 02,03,04r: Snow, Fog Day RGB 03,02,01: Vegetation, Snow, Smoke, Dust, Fog Day 2020/01/10 23 RGB 05-06, 04-09, 03-01 (Convective Storms) R = Difference WV6.2 - WV7.3 G = Difference IR3.9 - IR10.8 B = Difference NIR1.6 - VIS0.6 Applications: Severe Convective Storms Area: Full MSG Viewing Area Time: Day-Time 2020/01/10 24 RGB 05-06,04-09,03-01: Interpretation of Color Deep precipitating cloud Deep precipitating cloud Thin Cirrus cloud Thin Cirrus cloud (precip. not necessarily (Cb cloud with strong reaching the ground) updrafts and severe (large ice particles) (small ice particles) weather)* - high-level cloud - high-level cloud - large ice particles - small ice particles *or thick, high-level lee cloudiness with small ice particles Ocean Land 2020/01/10 25 RGB 05-06, 04-09, 03-01 (Convective Storms) Thin Ice Cloud (small ice) Maputo Thin Ice Cloud (large ice) Thick Ice Cloud Thick Ice Cloud (large ice) (small ice) MSG-1, 6 November 2004, 12:00 UTC, RGB 05-06, 04-09, 03-01 2020/01/10 26 Overshooting Top RGB • Overshooting tops are the most intense part of thunderstorms • This is where the strongest updrafts are and thus also possible severe weather • To identify this part of the thunderstorm can help with severe weather warnings. 2020/01/10 27 Overshooting Top RGB Recipe 2020/01/10 28 Example Overshooting Top RGB = Overshooting Tops Airmass RGB Overshooting Top RGB 14 September 2010, 19:45 UTC (Hurricane Julia) Slide by Jochen Kerkmann 2020/01/10 29 Satellite based instability indices 2020/01/10 30 Instability Indices • Why do we need to measure instability in the atmosphere? • How do we do it? • Is this good enough? • Typical indices? 2020/01/10 31 Instability Indices • The Global Humanitarian Forum states: • “Developing countries, which are most likely to suffer the brunt of climate change impacts, have the least number of ground-level weather data observation systems, the critical basis for efficient delivery of weather information.