REPORT OF THE WORKING GROUP ON TROPICAL METEOROLOGY RESEARCH (WGTMR) ACTIVITIES SINCE JSC-V AND FUTURE PLANS

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1 WORLD METEOROLOGICAL ORGANIZATION COMMISSION FOR ATMOSPHERIC SCIENCES (CAS) 6 th Joint Science Committee of the World Weather Research Programme WMO Geneva, Switzerland (18-19 July 2013) CAS/WWRP/JSC6/Doc 3.1 (xx June 2013) Item: 3.1 REPORT OF THE WORKING GROUP ON TROPICAL METEOROLOGY RESEARCH (WGTMR) ACTIVITIES SINCE JSC-V AND FUTURE PLANS (Submitted to JSC-VI by WGTMR, June 18, 2013) 1. ACTIVITIES SINCE JSC-V 1.1 Research and Development Project (RDP) The South China Monsoon Rainfall Experiment (SCMREX) proposed by China was approved by JSC as a WWRP RDP. The scientific plan was reviewed and revised throughout summer and fall of 2012 with the WGTMR Monsoon Panel Expert Team working with the Scientific Team of SCMREX. The modified version of the SCMREX Implementation Plan was discussed during the Second Monsoon Heavy Rainfall Workshop held in Malaysia in December The Implementation Plan was subsequently finalized in early 2013 and presented to some members of the SCMREX Scientific Advisory Committee in a meeting held during the ICMC-9 conference organized by the Chinese Academy of Meteorological Sciences in Beijing on 28 March The SCMREX pilot phase for the purpose of testing the equipment, communications, operation procedure and all aspects related to the filed observations was conducted from 15 April to 31 May The period of the pilot field experiment includes an early sub-period for instrument inter-comparison (15-30 April) and two Intensive Observation Periods (IOPs): 8-17 May, and May. Data have been collected from the portable instruments that took part in this experiment, including two C-band and 1 X-band polarimetric radars, one micro rain radar (MRR), four radindrop disdrometers, two millimeter-wave cloud radars, and two wind profilers, in addition to a variety of measurements from the operational observing systems. Moreover, during the second IOP, HKO conducted an extra radiosonde observation at 06 UTC 27 May, and also a data flight collection over western Guangdong coastal waters was successfully carried out around UTC. The SCMREX website ( has been established to facilitate data sharing and international participation. Numerical experiments were conducted to investigate the impacts of additional radiosonde sounding observations in Guangdong province of China, and preliminary results suggested notable positive impacts of the additional observations on QPF. Based on the initial results, a new proposal was submitted to

2 CMA to apply for partial support for the 2014 field campaign and relevant NWP work. 1.2 Forecast Demonstration Project (FDP) The WMO Typhoon Landfall Forecast Demonstration Project (TLFDP) organized by Shanghai Typhoon Institute of China Meteorological Administration has made significant progress. Real-time TC forecast products were collected from a total of 13 agencies, including the TIGGE TC EPS products from 7 agencies. The real time verification platform of TC forecasts and the evaluation system for TC forecasts had been set up. A variety of TC forecast and verification products were disseminated through the WMO-TLFDP website ( A new landfall typhoon forecast flow based on the products of WMO-TLFDP had been experimentally set up in the ECRMC/CMA, in which the TC EPS products are playing an important role. In addition to the TC forecasts and verification, a series of surveys were carried out to assess the benefit of the WMO-TLFDP. The objects of the surveys included public users, industrial and special users and meteorological forecasters. Based on the benefit assessment of TC forecast services in Shanghai from 2010 to 2011, the overall satisfaction factor was fairly high up to 82.3%. Inspite of the significant success achieved by the project since it started in 2010, the TLFDP was extended to 2015 for the following reasons: a) to apply in the operational mode the forecast evaluation methods designed for the project, b) to conduct further tests on the stability of the operational performance of each typhoon forecast product and technique, c) to enhance integration of the forecast products with services for users, and d) to include studies on the tropical cyclone pre-formation period [a recommendation of the International Workshop on Tropical Cyclone Unusual Behaviour (Xiamen, China, October 2011)]. The project will continue to be jointly supported by WMO s World Weather Research Programme (WWRP), Tropical Cyclone Programme (TCP) and Public Weather Service Programme (PWS). 1.3 Workshops and meetings The Workshop and Training Course on Operational Tropical Cyclone Forecast associated with the WMO-TLFDP was held in Shanghai, China from June Eleven lecturers from 9 institutions (Chinese Academy of Meteorological Sciences, WWRP/WMO, Shanghai Meteorological Service of CMA, National Center for Atmospheric Research of USA, National Meteorological Center of CMA, Shanghai Typhoon Institute of CMA, Regional Specialized Meteorological Center Tokyo, Hong Kong Observatory, and National Typhoon Center of KMA) and more than 40 forecasters from 21 forecasting and warning centers in China (including Macao and Hong Kong, China) and Republic of Korea attended this Workshop. The Second Monsoon Heavy Rainfall Experiment Workshop was held in Kuala Lumpur, Malaysia from December The first part of this workshop (10 December to noon of 11 December) was devoted to scientific presentations about heavy rainfall research and forecast in the monsoon region. Participants included monsoon researchers from U.S., China, India, Japan, Korea, Hong Kong, and

3 monsoon forecasters from India, China, Malaysia, Philippines, Vietnam, Indonesia and Australia. The second part of the workshop (noon 11 December to 12 December) was devoted to the discussion of SCMREX science plan, international participation, and the framework of an implementation plan. The International Top Level Forum on Rapid Change Phenomena in Tropical Cyclones was held in Haikou, Hainan Province, China from 5-9 November The workshop was attended by more than 80 tropical cyclone researchers and forecasters from 6 NMHSs and 6 Universities (China, Australia, India, Philippines, Thailand and USA). The major topics included sudden changes in tracks, rapid changes in structure and intensity, rapid changes in rainfall, advances in forecasting and forecaster requirements. A large fraction of the presentations at the Forum were results of the National (Key) Basic Research and Development (973) Program of China. The workshop provided a welcome opportunity for researchers and forecasters to exchange information on recent research advances and forecaster needs and requirements. Advances in understanding of processes related to rapid intensification, secondary eyewall formation, mechanisms controlling inner-core structure and size changes, and structure and intensity changes at landfall have been achieved, but progress in prediction of rapid changes in structure and intensity has been slow. Forecasters at the Forum were generally encouraged by the progress being made in modeling TCs and understanding their complex interactions. They also expressed a strong desire to see a robust process for quickly transferring that new-found knowledge to numerical models and to operations. As part of the process, forecasters would benefit from more exposure to modeling and other research studies, which would enable them to improve their conceptual models of TCs. The more skilled TC forecasters are arguably those who have acquired good conceptual models. In return, researchers may also benefit from more communications with forecasters. Worthy of consideration here would be an exercise to understand and exploit forecaster rules of thumb. A side meeting of the workshop was also held where a new RDP about High-Resolution Numerical Prediction of Landfalling Typhoon Rainfall was proposed and discussed. It was recommended that the WGTMR/WWRP take the lead in finalizing the project proposal and coordinate future related activities. The Fourth International Workshop on Extratropical Transition was held in Montreal, Canada from May The workshop was sponsored by the WMO/WWRP through Tropical Meteorology Research and THORPEX. The aims of the workshop are to review recent progress in predicting and understanding extratropical transition and to set future priorities for research and development in response to operational needs. The workshop provided a forum for the research and forecasting communities to interact. In addition to presentations sufficient time was dedicated to discussions and practical sessions. 2 COLLABORATIVE ACTIVITIES A number of the WGTMR activities collaborate well with operational and modeling centers, and WMO programs.

4 In the WMO-TLFDP, WWRP, TCP, and PWSP are the WMO programs supporting this FDP. Shanghai Typhoon Institute of CMA, East China Regional Meteorological Center of CMA, and RSMC Tokyo Typhoon Center are the lead agencies of this Project. The Project of High-Resolution Numerical Prediction of Landfalling Typhoon Rainfall proposed in the International Top Level Forum on Rapid Change Phenomena in Tropical Cyclones will be jointly supported by WGTMR, WGMWFR, and JWGFVR of WWRP and guided by an International Scientific Steering Committee. The proposed RDP will collaborate with the on-going WMO-TLFDP and the Forecast Demonstration Project in Southeast Asia (SWFDP). The potential participants are from Australia, China, Hongkong, China, Japan, the Philippines, ROK, Thailand, USA, Viet Nam, etc. In the SCMREX, cooperation is made among three operational centers: (i) National Meteorological Center of China Meteorological Administration, (ii) Guangdong Meteorological Bureau, and (iii) Hong Kong Observatory. They will serve as operational centers and modeling forecast centers. Five NMHSs in the region are also participating in this RDP (Australia, India, Japan, Malaysia, Philippines and Republic of Korea). Aside from WGTMR, three other WWRP working groups are represented (JWGFVR, WGMWFR, and WGNR). Additionally, currently being discussed is a possible collaboration and coordinated operational program for aircraft observations between SCMREX and TCP s Typhoon Committee The 5th International Workshop on Monsoons (IWM-V) organized by WGTMR s Monsoon Panel in cooperation with WCRP CLIVAR Monsoon Panel, WCRP-WWRP MJO Task Force, and WCRP CLIVAR-GEWEX Asian Monsoon Year will be held from 28 October to 1 November. The first four days of the workshop will be held in Macao, China. The last day s (1 November 2013) sessions will take place in Hong Kong in which invited lectures that are of high relevancy to forecast applications will form the content of a WMO Monsoon Training Workshop. This quadrennial workshop had been providing monsoon researchers and forecasters with a forum to discuss recent advances and current issues covering all time scales (meso-, synoptic, extended range, intraseasonal, climate) that are relevant to the forecasts of high-impact weather in the monsoon regions around the world. It is also an effective means to transfer new science and technology to National Meteorological and Hydrological Services in these monsoon affected regions. The workshop will include both invited reviews and contributed oral and poster papers. The invited reviews will form the basis of manuscripts to be published as chapters in a WMO-sponsored book: The Global Monsoon System: Research and Forecast, III. 3 SCIENTIFIC AND RESEARCH CHALLENGES The main components of the WGTMR continue to work on various scientific and research challenges related to the understanding of monsoon and tropical cyclones. Both of these natural systems are associated with complicated multi-scale interactions, and likewise related to different components of the earth system. The essential

5 physical mechanisms for tropical cyclone genesis, rapid change, interactions with other weather systems, onset of monsoon, and monsoon rainfall are still not well understood. There still exist gaps in integrating and communicating forecast uncertainties so that decision makers can make better use of meteorological information to reduce disaster risks. A list of scientific and research challenges of interest to the WGTMR is given in Annex A. 4 FUTURE PLANS 4.3 Projects The WMO-TLFDP will continue till the end of Two academic teams will be formed to work on demonstrating the application of new TC forecast products, verification techniques and benefit assessment. A proposal for a WMO/WWRP Research and Development Project (RDP) was made during the International Top Level Forum on Rapid Change Phenomena in Tropical Cyclones held in Haikou, China (November 2012). The proposed project aims to (i) understand the current capability of high-resolution regional NWP models and Ensemble Prediction Systems in predicting rainfall related to landfalling TCs, and (ii) improve the model settings for better NWP of rainfall associated with landfalling tropical cyclones. The RDP will focus on high-impact and challenging landfalling tropical cyclones with good observations available for the forecast evaluation, establish a data center to archive the NWP and EPS forecasts and facilitate evaluations by the participants. The RDP also plan to take advantage of the databases that have been established for past field experiments, including the combined THORPEX Pacific Asia Regional Campaign (TPARC)/Tropical Cyclone Structure (TCS-08) and the Impact of Typhoons on the Ocean in the Pacific (ITOP)/TCS-10. A new proposal is submitted to CMA to apply for partial support for the SCMREX-related work including participation of Wuhan Heavy Rainfall Institute and Guangzhou Tropical Marine Meteorological Institute in the SCMREX 2014 field experiment, numerical weather prediction studies on data assimilation and ensemble prediction experiments, and mechanism studies of convective systems over south China. 4.4 Workshops and meetings The Annual Meeting of the WGTMR will be held in Macao, China from October The Fifth International Workshop on Monsoons (IWM-V) will be held in Macao, China from 28 October-1 November The IWM-V Training Workshop on Operational Monsoon Research & Forecast Issues will be held in Hong Kong, China on 1 November The Eighth International Workshop on Tropical Cyclones (IWTC-VIII) will be held in Jeju, Republic of Korea from 2-10 December 2014.

6 The Third International Workshop on Tropical Cyclone Landfall Processes (IWTCLP-III) will be also held in Jeju, Republic of Korea from 8-10 December 2014.

7 Annex A Scientific and Research Challenges for WGTMR WGTMR/WWRP, June 2013 Tropical cyclones By Johnny Chan 1. Motion While NWP models have led to significant improvements in the track forecasts of tropical cyclones (TCs) on the average, sudden changes in TC tracks, which can result in large forecast errors, remain a big challenge for both the scientific and operational communities. The physical mechanisms that lead to such sudden changes have yet to be identified. Such changes apparently occur more often when a TC is about to make landfall, which means that evacuation and disaster preparedness efforts could be implemented at the wrong locations. On the scientific side, such occurrences suggests that land-sea contrast and topography likely play a significant role. In addition, some recent studies have shown that sea-surface temperature anomalies could lead to track changes. In other words, the effect of air-sea interaction on TC motion needs to be investigated further. Recent studies on data sensitivity have identified regions around the TCs that are apparently critical to track changes. More efforts should therefore be made in exploring how these results could be used in operational forecasting. The use of ensembles in operational forecasting of TC tracks has become more widespread in forecast centres around the world. The most common way is to take the ensemble average, although ensemble spread is also helpful in determining forecast uncertainty. Verification of such use with large samples is therefore necessary to establish its usefulness. Research into other ways to use the ensemble approach should also continue. 2. Genesis Although TC genesis is of more interest to the scientific than the operational

8 community in general especially over the open ocean, genesis occurring in more confined oceans such as the South China Sea, the Gulf of Carpentaria, the Bay of Bengal, the Arabian Sea and the Gulf of Mexico can pose threat to land in one to two days. Forecasting of genesis in these oceans continues to be a big challenge for the operational community. The prediction of genesis in operational NWP models has improved significantly during the last few years with the increased resolution, improved physics and enhanced data availability and assimilation techniques. However, the skill in such predictions still needs to be evaluated systematically especially in regard to false-alarm rates and failures. Investigations of the reasons behind these situations should also be made, the results of which would be useful for forecasters and provide a basis for the understanding of genesis. In the past, most of the research on TC formation has focused on the large-scale conditions associated with genesis. Recently, with much better computing power, many simulations of genesis have been made using high-resolution models to study the meso- and micro-scale physical processes that lead to genesis. As a result, a number of theories have been proposed but a widely accepted theory has yet to emerge. This is partly related to the lack of observational analyses for verifying the validity of the theories, despite the availability of large amounts of data collected from aircraft reconnaissance and high-resolution satellite data. Thus, in addition to the continuation of model simulations, more research efforts in the use of these data are necessary. 3. Intensity Despite the advances in NWP models, the prediction of TC intensity changes has not improved significantly during the past few decades. Some of the reasons proposed for this lack of improvement include the inadequate representation of the physical processes in the model, the air-sea interaction not properly simulated (in coupled models), and the relatively small amount of data available for initialization, etc. Empirical and statistical forecast techniques, with their inherent limitations, have also likely reached their maximum potential in intensity forecasting. Thus, forecasting of intensity changes continues to have large errors especially in sudden intensification cases. Similar to the study of genesis, many theories on intensity change based on high-resolution model simulations have been proposed. In recent years, one focus of such studies is on the formation and dissipation of concentric eyewalls, which have been shown to be strongly related to rapid changes in intensity. However, there is still considerable debate as to which of these theories is more likely to be able to explain TC intensity changes. Again, one of the difficulties is to verify the theories using observations through data analyses.

9 Thus, future research efforts must include both simulations and observational analyses to identify the likely physical processes associated with intensity change, which would help improve model performance and lead to better intensity forecasts. 4. Size In the forecasts of TC winds, the focus has always been on TC intensity. However, the area of damaging winds is much larger, and thus forecasts of size (generally defined as the radius of gale-force winds) changes are also important. Despite such importance, very few studies have been carried out in documenting and understanding size changes until very recently, mainly because of the lack of data. Current operational forecasts of size changes are generally empirical with very little physical basis, and no systematic verifications have been carried out because of the lack of ground truth. With the availability of satellite-derived winds, more investigations on TC size have been carried out. Nevertheless, much more research efforts need to be made on this topic before good forecasts of size changes can be made. 5. Rainfall Apart from winds, the most damaging aspect of a TC is rainfall because of its effects on flooding and landslides. Recent events such as Typhoon Morokot have demonstrated that even if the TC track was well forecasted, the predicted rainfall amount could have significantly large errors. Currently, most of the operational forecasts of rainfall associated with TC landfall are statistical in nature. It is therefore a big challenge to the research community to (a) understand the physical processes that lead to different rainfall distributions associated with different TC landfall characteristics, and (b) develop better models and/or forecast schemes to make better predictions of rainfall. Much more research efforts are therefore essential to improve TC rainfall forecasts. 6. Storm surge Although storm surge models are available for predicting storm surge, systematic verifications of such predictions are rare, partly because of the availability of data. We need more research on understanding the accuracy of such predictions, part of which depends on the accuracy of the track and intensity forecasts, and how to improve the prediction further. 7. Temporal variations on longer-than-synoptic time scales After the Knutson et al. (2010, Nature Geoscience) paper that summarized the state of knowledge of the possible effects of climate change on TC activity, research in this topic has continued and should continue to be encouraged. This is important especially with the release of the Assessment Report Five to be out later this year.

10 More research is necessary to further refine the findings of the Knutson et al. paper. In addition, research on variations of TC activity on different time scales, from intraseasonal to interdecadal, has increased substantially in the last decade or so, which has enhanced our knowledge on such variations. These results should now be included in seasonal predictions of TC activity. Research on such variations, especially on decadal or longer time scales should also be encouraged. This might involve the search for proxy data prior to instrumental records of meteorological variables, which will be extremely useful for determining various oscillations in TC activity. In the past, most seasonal prediction schemes are generally statistical in nature. Recently, both statistical-dynamical prediction schemes and pure dynamical model predictions of TC activity have been developed, with rather reasonable accuracy. More efforts should continue to be devoted to these predictions especially with regard to the number of landfalling TCs in a particular region. Monsoon By Chih-Pei Chang, Ngar-Cheunc Lau, Richard Johnson, Ben Jou, Hiroshi Uyeda, and Matt Wheeler 1. Mesoscale Weather and Heavy Rainfall While the monsoon circulations of the world provide the large-scale environmental setting (moisture, instability, wind shear) conducive to monsoon precipitation, processes acting on the mesoscale determine the nature of the monsoon convective systems themselves their size, duration, and intensity and whether or not they will produce extreme rainfall and flash flooding. The magnitude and complexity of processes operating on the mesoscale present great challenges for both understanding and prediction of monsoon precipitation. Virtually every monsoon region of the globe faces similar challenges. Monsoon precipitation can occur in conditionally stable or unstable environments. In the former case, the most significant events occur during the winter monsoon, such as over the coastal mountain ranges of Viet Nam and Malaysia during the northeast monsoon flow over the South China Sea. In these instances, orographic lifting of the low-level, moist, stable flow along the slopes of the terrain is of critical importance to the generation of rainfall, as is the warm-process in the precipitation formation. In conditionally unstable environments, a multitude of processes are operative in determining the nature of the precipitation systems and whether or not they are

11 capable of producing extreme rainfall. a) Mesoscale factors influencing monsoon precipitation Convective organization Recent work has shown that the organization of mesoscale convective systems (MCSs) greatly influences the ability for such systems to produce extreme rainfall. Characteristics of heavy-rain-producing MCSs include training convective cells or back-building convection. These processes are influenced by local environmental shear and stability, but both understanding and forecast skill for such systems is limited. Orographic effects Mountain/valley circulations, flow blocking, and terrain lifting can all influence the location and intensity of monsoon precipitation systems. Some of the heaviest monsoon rainfall regions of the world are located where the summer monsoon flow intercepts steep terrain, e.g., the Western Ghats and the coastal mountains of Myanmar; however, both the precise locations of the heaviest rainfall (offshore, on the coast, or over the mountain slopes) and the mechanisms that determine them are not well understood. Diurnal cycle Daytime heating of the boundary layer, land/sea breezes, and nocturnal low-level jets play a key role in the formation of many monsoon precipitation systems. However, boundary layer parameterizations are often one of the weakest components of numerical models. They are even unable to properly represent the timing of the diurnal cycle of convection over both land and water. Mesoscale convective vortices (MCVs) Midlevel mesoscale circulations generated by topography (e.g., vortices downstream of the Tibetan Plateau) or by the stratiform latent heating profile within MCSs have been found to influence the location and intensity of subsequent deep convection and hence the production of extreme rainfall. New precipitation occurs on the downshear side of MCVs, and can be particularly intense when collocated with a strong low-level jet. Microphysics The structure, intensity, and horizontal extent of precipitation systems are strongly influenced by microphysics. Recent studies with increasingly complex microphysics parameterizations have shown that the structural properties of MCSs sensitively depend on the details of those parameterizations. Surface-atmosphere interactions Surface inhomogeneities, apart from terrain effects, have been found to influence the location and intensity of precipitation systems. Over land, inhomogeneities might be related to horizontal variations in land-use characteristics and over the ocean to sea-surface temperature gradients. Given the weaknesses in boundary layer parameterizations in models, the prediction of such effects remains a serious challenge.

12 The above mesoscale factors and others not mentioned (e.g., cloud-induced radiative effects, cumulus momentum transports, gravity currents, gravity waves) create great difficulties for accurate prediction of monsoon precipitation. To make progress, improvements in model physical parameterizations are needed, but these must be supported by direct observations. Hence, there is an important role for field campaigns targeted at one or more of these key processes in monsoon precipitation systems, with particular attention to those that produce extreme rainfall and flash flooding. b) The following areas of investigation and related subtopics are relevant to the use of radar observations for improving forecasts of monsoon rainfall. Kinematic and precipitation structure of MCSs (heavy rain producer) three-dimensional wind field and retrieved cloud scale thermodynamic and precipitation variables, the inflow and outflow structure, the convective and mesoscale vertical motions. Microphysical properties of MCSs the warm rain process, the role of ice cloud process, the hydrometeor identification and evolution, the drop-size distribution, polarimetric radar variables distribution and characteristics in MCSs. Quantitative precipitation estimation (QPE) radar network design and data display algorithm, real-time data quality control, radar calibration, QPE with multiple sensors. Quantitative precipitation forecasting (QPF) radar data assimilation techniques, nowcasting technique (storm identification, storm movement determination and prediction, storm rainfall prediction). Intensive field observational programs - Detailed and high-resolution data are collected for diagnosis purpose. The scientific issues include (1) to understand the kinematic and thermodynamic characters of the pre-frontal prevailing southwesterly flow; (2) to explore the boundary layer process related to the convection initiation; (3) to investigate the microphysical characteristics of the mesoscale convective systems (MCSs) related to heavy rain events; (4) to examine the effects of terrain and land/sea distribution on triggering and redeveloping of MCSs; and (5) to develop the radar data assimilation techniques and to improve the capability of quantitative precipitation estimation and forecasting (QPE/QPF) during the Asian monsoon period. 2. Intraseasonal Forecasting in the Tropics The importance of intraseasonal variations of tropical weather, especially of rainfall, for agricultural production and decision-making is becoming increasingly recognised. For instance, intraseasonal rainfall variations, and especially monsoon break conditions, have been demonstrated to have a pronounced impact on Indian groundnut

13 (peanut) production, which is one of the most economically important crops sown during the Indian monsoon. More generally, the types of agricultural decisions that can be made with intraseasonal forecasts include the scheduling of planting and harvest operations, maintenance works, and the application of fertilizers. Mining and other industries can also benefit from intraseasonal weather information, and a number of National Meteorological and Hydrological Services (NMHSs) now have products focusing on this timescale (i.e. in the range of 1 to 4 weeks). The Madden Julian Oscillation (MJO), with its day period, is the strongest and most predictable mode of intraseasonal variability in the tropics. However, an important contribution to the prediction skill on this timescale also comes from the El Nino-Southern Oscillation (ENSO), given its persistence on the shorter intraseasonal timescale. Therefore, highest levels of intraseasonal predictability are expected in regions that are most strongly affected by these phenomena. Both statistical and dynamical models are used for intraseasonal forecasting, but the focus on dynamical models is ever increasing given their ability to combine the influence from multiple interacting sources of known and potentially unknown sources of predictability. Statistical models, on the other hand, tend to concentrate on just one or a few distinct modes of variability, such as the MJO and ENSO. Many dynamical models have traditionally had difficulty with the simulation and prediction of the MJO, and it has been a great research challenge to improve this, usually through changes to the physical parameterizations of the models, especially those associated with clouds and convection. Until relatively recently, progress has also been hampered by the lack of tools for analysing the MJO in dynamical prediction models in real-time, but this has now changed with the development of the Real-time Multivariate MJO (RMM) index and its adoption by the modelling community. Much use is being made of the index for both real-time forecasting (e.g. as an input to the intraseasonal prediction products of the NMHSs) and to provide skill measures to guide model improvement. A number of dynamical forecast models are now showing improved MJO skill compared to a few years ago, however, the challenge remains to be able to do this without compromising other aspects of the models, such as their skill for midlatitude weather. In the case of climate model simulations, the challenge is to improve the MJO simulation without detrimental effects to the mean climate. The importance of air-sea interaction for MJO forecasting and simulation is still debated and both coupled atmosphere-ocean and atmosphere-only models are used. Model resolution does not appear to play a dominant role, except if the physical processes associated with clouds and convection are altered by a change in resolution. Building on the recent success with the RMM index for MJO prediction from dynamical models, efforts are underway to provide a similar product for the Boreal Summer Intraseasonal Oscillation (BSISO) of the Asian monsoon region. This

14 activity has faced additional challenges due to the more complicated nature of the BSISO than the MJO, involving both northward and eastward propagation as well as a greater range of timescales. Downscaling the predictions of the MJO and BSISO indices to useful information for applications also poses a challenge. For some applications (e.g. basin-scale intraseasonal tropical cyclone activity forecasting), the planetary-scale variations in convection and winds that are associated with these modes are of direct relevance. However, for applications requiring weather (e.g. rainfall) information at a point, there is not necessarily a strong match between the planetary-scale fields and the point-scale weather. For example, around the steep topography of the islands of the Maritime Continent, the peak rainfall with the MJO may occur in a very different phase to the peak large-scale convection over the nearby ocean. In these regions the interaction of the MJO with the local diurnal cycle of convection and rainfall is crucial to get the correct impact. The correct interaction of the island-scale diurnal cycle with the planetary-scale MJO will be a significant challenge to meet for the new generation of high-resolution dynamical prediction models. Among the various uses that have been made of tropical intraseasonal forecasts, one has been for organising staff rosters in NMHSs themselves. For example, when it is known that a peak in MJO convection and associated severe weather (e.g. tropical cyclone) activity is coming in the next couple of weeks, staff levels can be increased to meet the extra demand that is likely. Conversely, the number of on-duty staff may be reduced when it is known that the monsoon is entering break conditions. Besides the MJO and BSISO, there also exist the convectively coupled equatorial waves, such as the n=1 Rossby and Kelvin waves, that should provide additional predictability on the intraseasonal timescale. Like the MJO, they organise convection on large scales and can propagate along the equator as a coherent entity for many thousands of kilometres. However, the time scale of predictability they provide is less, due mostly to their shorter periods. Their coupling with convection also poses a challenge for dynamical models, and their many interactions poses a challenge for statistical models. With greater attention to the character of these waves in dynamical forecast models, gains in tropical skill should be expected in the 1-2 week range. 3. Indian Monsoon Onset Predicting the timing of the arrival of significant rainfall amounts to the Indian Subcontinent in early summer, often referred to as Indian monsoon onset, is a major problem. The onset has been linked to phenomena and processes with a broad range of time scales by various studies. These features include synoptic convective events such as tropical cyclones, passage of intraseasonal fluctuations such as Madden-Julian Oscillations (MJO), seasonal changes in the large-scale forcing associated with land-sea contrasts and orography, and interannual variations in the boundary

15 conditions in different remote and local sites, such as those accompanying El Nino-Southern Oscillation (ENSO). There is no agreement as to the relative importance of these features. There are also difficulties in defining the specific onset date. It is dependent on the particular location used for monitoring the precipitation signal. For instance, dates indicating onsets over Bay of Bengal are considered separately from those corresponding to onsets over Kerala. In addition to monsoon onset dates, other measures have been used to describe monsoon activity. An example is the All-India Rainfall Index (AIRI), which is based on the seasonally averaged precipitation over the entire Indian region. There is considerable disagreement on the utility of this index. It has been used for more than one hundred years to define onset, but many scientists point out that the heaviest rainfall is in the neighboring oceans and the large spatial variation of the monsoon rainfall within India makes the AIRI not very meaningful. Arguments are presented on the relative utility of all of the above measures as indicators of monsoon strength and evolution. There are also different views on the current skill and usefulness of the forecast products of various operational centers, which provide information on the regional rainfall distribution several weeks in advance. Some studies suggest that these NWP forecast products might be more useful to agricultural practices than the conventional prediction of onset dates and spatio-temporal averages such as AIRI, but some operational forecasters do not agree. There are also different views on the fundamental time scales and processes of the monsoon system. One school of thinking challenges the traditional view of treating the monsoon onset date as a climatological event. This suggests that if the monsoon is to be viewed primarily as a seasonal and interannual phenomenon, then perhaps more attention should be devoted on those longer time scales and their relationships with forcing mechanisms such as land-sea contrasts, orography and ENSO. In this case, shorter-term features such as synoptic disturbances and MJO might be treated as high-frequency perturbations superposed on the slowly varying basic state. 4. Relationships between Climate Change and Extreme Weather Climate change does not only affect the averaged conditions of the atmospheric environment, as represented by seasonal or annual means. Perturbations in the climate system also influence the characteristics of extreme weather events. The latter effects are of considerable interest due to the substantial cost of the extreme events to society, as measured in terms of human casualties and economic losses. Prominent examples of extreme weather in the monsoon regions include episodes of strong anomalies in temperature, precipitation or wind speed. These anomalous weather conditions are associated with weather phenomena such as heat and cold waves, floods and droughts, and tropical cyclones. Efforts are being made in the research community to document the long-term trends

16 of different types of extreme events by analyzing historical weather records. Particular emphasis is placed on the secular changes in various metrics of the extreme events, such as degree of severity, typical duration of individual episodes, frequency of occurrence, and averaged number of days affected by extreme weather in a given season. The mechanisms linking various drivers of climate change (such as greenhouse gases, aerosols, natural variability) to alterations of the behavior of extreme events are being investigated by diagnosing observational datasets and output from climate model experiments. Projections of the future evolution of different aspects of extreme events are also being made by climate model integrations conducted at various research and operational centers. Since the characteristics of extreme events exhibit strong geographical dependence, and the identification of individual extreme episodes entails diagnoses of high-frequency weather fluctuations, observational data archives with high spatial and temporal resolution are a prerequisite for thorough studies of extreme events. The lack of such data resources in some monsoon regions presents considerable challenges in establishing clear signals in the trends of various metrics for extreme events in such locations. In some monsoon regions, the attempt to detect trends in extreme episodes is also complicated by the high level of spatial and temporal noise in the observational estimates. Process studies and projections of future trends are dependent on the skill and robustness of the available model tools. There is still considerable room for improvement of the current generation of computer models in simulating various aspects of monsoon climate, including extreme events. Of particular concern is the ability of the climate models in replicating the precipitation field, which is a crucial parameter for monsoon studies. There still exists a wide scatter among projections from different models of trends in monsoon climate, thus yielding a low or moderate level of confidence in such projections. A critical issue in the detection and attribution of extreme events is whether the statistics of these events can mainly be accounted for by a shift in the mean conditions, or a change in the level of variability ( spread ) about the mean, or a combination of both factors. This question needs to be pursued in the context of extreme events in various monsoon regions, with particular attention being devoted to dependence on geographical location and the variable (e.g., temperature versus precipitation) being considered.