Fundamental influences of large-scale wave dynamics on tropical weather systems

Lead Research Organisation: University of Reading
Department Name: Meteorology


Scarcity of water has been identified as the most serious environmental threat facing the health and security of people living in the tropics. Pressures on water supply undermine stability through stresses on food availability and spread of disease e.g., malaria and meningitis. Yet predictions of precipitation show very high uncertainty in the Tropics and especially the arid climatic zones such as sub-Saharan Africa where the vulnerability of the population is amongst the highest in the world. The outlook for Sahel precipitation in coupled simulations of the twenty-first century remains very uncertain with no consensus as to whether there will be more or less rain in the future, or how the frequency and intensity of high impact weather will change. Skillful forecasts of rainfall would be of enormous benefit across all timescales ranging from hours to decades. In particular, short-range forecasts (up to 2 days) for the public and aviation industry, and medium-range forecasts (10-30 days) for agriculture, hydrology and health information. This is challenging since rainfall is organised through a complex interplay of large-scale wave patterns, weather systems and isolated deep convective updrafts. The locations of individual convective updrafts are not predictable and even the occurrence and evolution of mesoscale weather systems are poorly represented in Numerical Weather Prediction (NWP) models. However, there is some hope for greater predictability since the pattern of active and inactive regions of convection is often determined by large-scale wave structure. Examples of such phenomena include African Easterly Waves (AEWs) and equatorial waves. Owing to the waves, there is potential predictability for high impact weather events occurring simultaneously in several locations at once. For example, if several storms are spawned within a large-scale wave, each one of them could present a significant natural hazard, with safety and financial implications (such correlated events are not accounted for in the risk models used by the insurance industry). Unfortunately, there are severe deficiencies in the simulation of tropical large-scale waves which typically decay far too quickly in forecasts and propagate too slowly. However, it is difficult to amend models to improve the simulation of large-scale tropical waves because underpinning theory for tropical waves is currently too weak to unpick the problem. It is necessary to formulate better how different processes influence wave evolution so that modifications can be aimed at improving wave representation. The aim of the proposed project is to develop the theory behind large-scale waves in the tropics to the level where it can be applied in the quantitative diagnosis of observed weather systems. In doing so we aim to identify the processes that are most important in wave initiation, maintenance and propagation, and ways in which they are misrepresented in models, with a view to improving weather forecasts. The research will study the interplay between large-scale waves and convective rainfall through three stages of complexity: A) the dynamics of waves assuming small amplitude, B) large-amplitude aspects including vacillations between jet strength and wave amplitude, and C) explaining deficiencies in state-of-the-art forecasts of tropical waves using the new theory developed. The anticipated benefit of the research is improvement in weather forecasts of rainfall throughout the tropics at lead times of a day to a season. Stage C will be advanced through collaboration with project partners from two world-leading operational forecast centres: the Met Office and European Centre for Medium-Range Weather Forecasts.


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Description This project has examined the fundamental influences of large-scale wave dynamics on tropical weather. Two tropical wave phenomena have been the centre of attention: equatorial waves (propagating around the tropics, especially across the oceans) and African Easterly Waves which develop primarily across West Africa and across the North Atlantic Ocean. Variability in rainfall across the tropical oceans and West Africa is dominated by the occurrence and properties of these waves. However, in the weather forecasts produced by numerical models, these waves decay rapidly and also do not propagate at the correct speeds. The net result is that weather forecasts for the tropics (up to weekly timescales) have very little skill. The chief exception is the longer range skill associated with the slowly propagating Madden Julian Oscillation, but this has not been the focus of this project.

The basic theories of equatorial and African Easterly Waves are derived for a dry atmosphere, not accounting for the influence of water (vapour, liquid and ice) and latent heating associated with changes in phase. Furthermore, equatorial wave theory assumes a resting atmosphere, while it is known that horizontal and vertical shear in the background zonal flow is a major influence on wave structure and development. This project has conducted new research to shed light on the role of moisture and shear in tropical wave dynamics and the precipitation embedded within them.

A new design of experiment using the Met Office Unified Model has demonstrated the effects that the entrainment of moisture into convection embedded within waves has on their intensity and propagation speed. It has been shown that the variation of humidity in phase with the wave is essential to coherent wave existence. If the moisture entrained is artificially made uniform (as seen by convection in the model) the relationship between convection and the waves is lost and convection exhibits self-aggregating behaviour typically of an environment without contrasts. A new local energetics framework has shown how the influence of disturbances in the subtropics excites tropical waves in the presence of a shear flow, while the heating within the waves acts to damp wave activity. So although the humidity variation is crucial to propagation and the coherent organisation of rainfall and weather systems, it acts to reduce the amplitude of wave energy. Research is ongoing to connect these findings to the observed deficiencies in operational weather forecasts in the tropics.

Work on African Easterly Waves has revealed the typical relationships between wave "troughs" and the location of deep convection (with its attendant thunderstorms, heavy precipitation and squall lines). It has been found that most climate models do not represent the composite structures captured in meteorological analyses. The tracks of African Easterly Wave centres are found to differ markedly between models and even between analyses because the observational constraint over Africa is weak due to paucity of data. We have also investigated the relationships between AEWs and equatorial waves and shown that the horizontal structure of AEWs projects strongly onto Rossby wave modes with eastward tilt in height expected for the baroclinic growth mechanism. This therefore links theory with the observed wave evolution. Current investigation is focusing on the interactions between different equatorial wave types and AEWs which appear to be enabled by the shear of the African Easterly Jet.
Exploitation Route Global numerical weather prediction centres (our project partners the Met Office and ECMWF) are likely to be the most immediate beneficiaries since the research points to the way in which convection and dynamics interact within large-scale waves and the sensitivity to the representation of those processes. The work on African Easterly Wave climatologies in climate models and analyses is very relevant to the robustness of climate change projections for the region and planning resilient societies and adaptation to climate change. Through a recently funded Newton WCSSP project, the dynamical insight and equatorial wave projection technique from this NERC project will be translated into forecaster tools for meteorological agencies in Southeast Asia, including Malaysia and the Philippines, as well as wider utilisation by the Met Office in their global projects.
Sectors Agriculture, Food and Drink,Communities and Social Services/Policy,Education,Environment,Financial Services, and Management Consultancy,Government, Democracy and Justice

Description The novel technique used to project global atmospheric data onto equatorial wave structures (derived from theory) is a central facet of a new Newton Fund project that has just been funded to translate the research into applications for Southeast Asia. The technique has been developed over the last decade by Guiying Yang and advanced most recently within this project to obtain composite wave structures associated with high impact weather phenomena. For example, in this project the cyclonic circulation centres within African Easterly Waves were tracked in global meteorological analysis data and then the different equatorial wave mode structures were composited in a frame moving with the circulation centres. In this way it was shown that a particular type of equatorial wave, the westward mixed Rossby-gravity wave, is of key importance to seasons with greater African Easterly Wave activity and therefore rainfall across the region. Now in the Newton Fund WCSSP Southeast Asia project the technique will be used to identify relationships between high impact weather events over Malaysia and the Philippines with equatorial wave activity and remote teleconnections. The aim is to extend predictive capabilities for the severe weather in the region by using the greater predictability associated with large-scale equatorial wave propagation.
First Year Of Impact 2017
Sector Environment
Impact Types Policy & public services

Description Newton Fund, Weather and Climate Science for Service Partnership Programme
Amount £300,000 (GBP)
Organisation Newton Fund 
Sector Public
Country United Kingdom
Start 11/2017 
End 11/2019
Description WCSSP SE Asia: High impact weather in the context of weather regimes over SE Asia
Amount £164,886 (GBP)
Organisation Newton Fund 
Sector Public
Country United Kingdom
Start 04/2020 
End 03/2021
Description WCSSP Southeast Asia: Systematic analysis of real time forecasts. FORcasting for SouthEast Asia (FORSEA)
Amount £900,000 (GBP)
Organisation Newton Fund 
Sector Public
Country United Kingdom
Start 04/2019 
End 03/2021
Title New diagnostic of time spent in each phase of the Madden-Julian oscillation, for comparing models with observations 
Description Developed new diagnostics which analyse the nature of Madden-Julian Oscillation propagation, for comparison with observations. The MJO is the leading mode of atmospheric variability in the tropics. The first diagnostic is a measure of how many days the MJO tends to stay in one Wheeler-Hendon phase before either decaying or moving into another phase (the "MJO inertia"). The second measures the longevity of an MJO event in terms of how many Wheeler-Hendon phases it evolves through in succession. 
Type Of Material Data analysis technique 
Year Produced 2015 
Provided To Others? Yes  
Impact These diagnostics have been added by the Met Office to their "auto-assess system" to analyse how realistic MJO propagation is in the Met Office operational forecasts. 
Description African Risk Capacity 
Organisation African Risk Capacity
Country South Africa 
Sector Charity/Non Profit 
PI Contribution Multi-lateral collaboration. Commissioned by Evidence on Demand to evaluate the use of the ARC sovereign insurance in Senegal and Mali during 2014. The report was submitted to DfID as part of the "Independent Annual Review of ARC " carried out at the end of ARC's first full year of operations (Cornforth, Lele, Tarhule, 2015).
Collaborator Contribution Multi-lateral collaboration.
Impact The report was submitted to DfID as part of the "Independent Annual Review of ARC " carried out at the end of ARC's first full year of operations (Cornforth, Lele, Tarhule, 2015).
Start Year 2014
Description Met Office 
Organisation Meteorological Office UK
Country United Kingdom 
Sector Academic/University 
PI Contribution Sean Milton, the head of the global model evaluation group of the Met Office is a project partner. We have two meetings per year regarding the project.
Start Year 2011
Description Participation in the Process Evaluation Group - Africa 
Organisation Meteorological Office UK
Country United Kingdom 
Sector Academic/University 
PI Contribution A working group collaboration between the Met Office and UK universities
Start Year 2012
Description Project partner - ECMWF 
Organisation European Centre for Medium Range Weather Forecasting ECMWF
Country United Kingdom 
Sector Public 
PI Contribution Mark Rodwell from the model diagnostics group at the ECMWF is a project partner. We have had meetings twice a year regarding the project.
Start Year 2011
Description UK Commercial Partnership 
Organisation MicroEnsure
Country United Kingdom 
Sector Private 
PI Contribution Multi-lateral partnership established with Micro-Ensure, a small micro-insurance company based in Cheltenham, providing affordable insurance for farm cooperatives in Africa. There is now a new project linked to the TAMSAT (Tropical Applications of Meteorology using SATellite data) group at Reading focused on providing satellite rainfall estimates to smallholder farmers in Africa
Start Year 2013
Description Impact activities take time as well as money 
Form Of Engagement Activity A magazine, newsletter or online publication
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Other academic audiences (collaborators, peers etc.)
Results and Impact Case study on the Pathways to Impact, public engagement section of the RCUK website.

The case study was accessed via the RCUK website.
Year(s) Of Engagement Activity 2012
Description Workshop on Intraseasonal Processes and Prediction in the Maritime Continent 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact Simon Peatman (PDRA) presented talk "Scale interaction between the MJO and the diurnal cycle of precipitation over the Maritime Continent" as part of a World Meteorological Organisation Working Group meeting on Intraseasonal Processes and Prediction in the Maritime Continent. The workshop was in Singapore, April 2016.
Year(s) Of Engagement Activity 2016