Real-time monitoring and forecast of the West African monsoon intraseasonal variability: from 2011 to 2013

Transcription

1 Real-time monitoring and forecast of the West African monsoon intraseasonal variability: from 2011 to 2013 P.Peyrillé 1, R. Roehrig 1, F. Couvreux 1, E. Poan 1, J.-P. Lafore 1, N. Chapelon 4, O. Ndiaye 2, A. Diongue-Niang 2, F. Favot 1, J.-L. Boichard 3, L. Fleury 3 1 CNRM-GAME, Météo-France/CNRS, Toulouse, France. 2 ANACIM, Dakar, Sénégal. 3 OMP, Toulouse, France 4 CISMF, Météo-France

2 Introduction and motivations Forecasts of convective events over West Africa: low skill at time scales > 48h Major progresses during the last decade in documenting and understanding the African IntraSeasonal Variability (ISV) Daily rainfall in 1968 (Sultan and Janicot 2003) 15 days 40 days 5-6 days Rain (mm/day) Monsoon IntraSeasonal Variability : 5-6 days: African Easterly Waves (Kiladis et al. 2006), Kelvin Waves (Mounier et al. 2007, Ventrice and Thorncroft 2012), Variability of precipitable water (Couvreux et al. 2010, Poan et al. 2013) 15 days : Guinean mode, Sahelian mode (Mounier and Janicot 2004, Janicot et al. 2010), Heat Low mode (Roehrig et al. 2011) 40 days : MJO (Matthews 2004, Janicot et al. 2009) 2

3 Introduction and motivations Forecasts of convective events over West Africa: low skill at time scales > 48h Major progresses during the last decade in documenting and understanding the African IntraSeasonal Variability (ISV) Daily rainfall in 1968 (Sultan and Janicot 2003) Objectives: Rain (mm/day) Monsoon IntraSeasonal Variability : 15 days 40 days 5-6 days Confront our climatological and statistical view of intraseasonal variability to the real world; Investigate the forecast skill of the monsoon ISV, based on our current knowledge Approach: Address these 2 points through a real-time exercice 5-6 days: African Easterly Waves (Kiladis et al. 2006), Kelvin Waves (Mounier et al. 2007, Ventrice and Thorncroft 2012), Variability of precipitable water (Couvreux et al. 2010, Poan et al. 2013) 15 days : Guinean mode, Sahelian mode (Mounier and Janicot 2004, Janicot et al. 2010), Heat Low mode (Roehrig et al. 2011) 40 days : MJO (Matthews 2004, Janicot et al. 2009) 3

4 Outline 1. Introduction 2. The project MISVA Data and methodology Available products 3. Overview of MISVA products day scale in Focus on 30 August 2013 Event 4. Conclusions

5 2. Project MISVA MISVA= real time Monitoring and forecast of IntraSeasonal Variability over Africa Collaboration between France & Senegal: involving forecasters from ANACIM initiated in 2011 A real-time website, simple but easily and rapidly evolving, according to the encountered needs and ideas Use of websites providing complementary information (broader context) e.g. MJO: Wheeler s website + NCEP, African Desk Regular briefing & reports (~1/week) between Toulouse and Dakar. 2 versions available: Light: for forecast purpose Expert: Research area to test ideas

6 2.a Data and methodology Seasonal anomalies Data: ECMWF 6-hourly analyses and forecasts (+10 days) Daily satellite OLR, with a 2-day delay (NOAA) 3-hourly TRMM precipitation, with a ~6h delay Observations: Senegal rain gauges, soundings, SST θ 850 Methodology: Computation of seasonal anomalies, using the mean seasonal cycle derived from ERA-Interim/NOAA OLR/TRMM-3B42 Filtering using forecast data (ECMWF) and zeropadding (as in Wheeler and Weickmann 2001) Indexes of ISV modes are computed through a spatial projection of (filtered or not) anomalies on their mean climatological (canonical) structure. Canonical structure Heat Low index Analyses Forecasts -20d 0 +10d

7 2.b Available Products At first (in 2011), we selected just a few products (maps, indices, hovmüller ) Sahelian mode QBZD mode Mid-latitude Rossby waves Equatorial Waves Heat Low mode Onset Precipitable Water AEWs Precipitation (TRMM & Obs) TRMM accumulated rainfall 24-27/ 08/12 (mm/day)

8 2.b Available Products At first (in 2011), we selected just a few products (maps, indices, hovmüller ) Sahelian mode QBZD mode Mid-latitude Rossby waves Equatorial Waves Heat Low mode Onset Precipitable Water AEWs Precipitation (TRMM & Obs) TRMM accumulated rainfall 24-27/ 08/12 (mm/day) Improved in 2012 and 2013: Interactions with forecasters from Météo-France and ANACIM Experience from AMMA Forecater s Handbook initiative (THORPEX) Meridional transport hpa Low level flow ( hPa) Monsoon depth Surface pressure

9 Outline 1. Introduction 2. The project MISVA Data and methodology Available products 3. Overview of MISVA products day scale in Focus on 30 August 2013 Event 4. Conclusions

10 3.a Analysis of the day scale Example: Heat Low Variability (East or West location) and its link with rainfall (Roehrig et al. 2011) Composite structure of the Heat Low Mode Weakened convection over Sahel ~4 days later West Phase East Phase Favored convection over Sahel ~4 days later

11 3.a Analysis of the day scale Example: Heat Low Variability (East or West location) and its link with rainfall (Roehrig et al. 2011) Composite structure of the Heat Low Mode Weakened convection over Sahel ~4 days later West Phase East Phase Favored convection over Sahel ~4 days later TRMM Rainfall index (7.5-15N,10W-10E)-(0-7.5N,10W-10E) 2011 Heat Low Index

12 3.a Analysis of the day scale Example: Heat Low Variability (East or West location) and its link with rainfall (Roehrig et al. 2011) Composite structure of the Heat Low Mode Weakened convection over Sahel ~4 days later West Phase East Phase Favored convection over Sahel ~4 days later TRMM Rainfall index (7.5-15N,10W-10E)-(0-7.5N,10W-10E) Heat Low Index

13 3.a Analysis of the day scale Composite structuredifferent of Heat Low Mode (Roehrig etdid al ~ close to the climatology, indexes work well West Phase East Phase Weakened 2012 ~ year more complex than the climatology, significant Favored convection of mid-latitude, with higher wavenumber wave train influence convection over Sahel over Sahel 2013 ~? In progress The Heat Low variability provides a large-scale context, in particular for the onset, and a conceptual model for impact of the mid-latitude on the monsoon

14 3.b Zoom on a heavy-precipitating event in 2013 Analysis of ISV can give some clue for enhanced convection: Example on 30 August Rainfall event with record value of rainfall accumulation over Senegal (~200mm/day on ) - Large consequences (10 victims, flood, loss of crops) Seasonal rain anomaly ( ) Accumulated until Seasonal rain anomaly ( ) Accumulated until TRMM precipitation (mm/d) Weekly SSt anomaly Large-scale environment days ISV: Neutral of the Heat low mode (not shown) Negative phase of the Sahelian mode (not shown) Positive SST anomalies along the coast Favorable conditions for strong convection to the West of Senegal

15 3.b Zoom on a heavy-precipitating event in day Variability Arrival of an African Easterly Wave: moist (dry) phase to the West (East) of Sahel A useful product: Precipitable Water (PW) good tracking of AEWs better forecast skill 850-hPa Vorticity 700-hPa Zonal Wind ECMWF, based on 8/27 Precipitable Water/Rainfall, ECMWF Precipitable water (mm) at Dakar, Forecast on 8/25 Time Analysis + soundings Forecast Day relative to 25 August

16 3.b Zoom on a heavy-precipitating event in day Variability Total precipitable Water, Precipitable water anomalies 925-hPa wind anomalies Total 925-hPa wind Analysis hpa 925 hpa Forecast for hpa 925 hpa Forecast models did work well 48h before the convective systems develops The arrival of the moisture anomaly and the vortex can be tracked from the 8/25 forecast using PW anomaly

17 3.b Zoom on a heavy-precipitating event in 2013 MISVA: a testbed for new diagnostics or forecast products 1-10-day Variability - New product developed in the framework of the international Forecaster s Handbook initiative (Parker et al.) have been tested in Example: hPa mean meridional flow (colour) + mean hpa horizontal wind (vectors), based on 8/25 Forecast : 08/30 08/29 08/27 08/25 Good skill to depict AEW trough and regions that favor deep convection

18 Conclusions MISVA : A now 3-year experience of West African Monsoon monitoring A cooperative effort between Senegalese and French research and forecast teams targeting short-to-medium range rainfall forecasts Much has been learned by sharing our different experiences of the monsoon Analysis and description of different time scales of monsoon rainfall: Synoptic scales - African Easterly Waves: Precipitable water is a good tracer for African Easterly Waves New diagnostics coming from research have been applied quite successfully to forecast days: Still some work to be done Monitoring of heat low variability provides some large-scale information on the monsoon system Analysis of ISV using observations and NWP models is a good guide for forecasting convection (large-scale conditions)

19 Perspectives Go East : Good potential of predictability over the Eastern Sahel (Tchad and Ethiopia) = region of generation of AEWs need to better understand the variablity in the region MISVA appears to be a usefull tool For applications purposes: ACASIS, French project funded to understand/forecast heat waves during Spring over the Sahel link with the heat low variability For collaborations with National Hydrological and Weather Services and African universities Forecast skills of MISVA products Quantify and improve the forecast skills of the MISVA products Go towards probabilistic forecast products (TIGGE) Gain of predictability?

20 Daily Accumulated rainfall (ground stations)