Peter Webster

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Papers by Peter Webster

Evolution and modulation of tropical heating from the last glacial maximum through the twenty-first century

Climate Dynamics, Nov 6, 2011

Twentieth century observations show that during the last 50 years the sea-surface temperature (SS... more Twentieth century observations show that during the last 50 years the sea-surface temperature (SST) of the tropical oceans has increased by *0.5°C and the area of SST [26.5 and 28°C (arbitrarily referred to as the oceanic warm pool: OWP) by 15 and 50% respectively in association with an increase in green house gas concentrations, with non-understood natural variability or a combination of both. Based on CMIP3 projections the OWP is projected to double during twenty-first century in a moderate CO 2 forcing scenario (IPCC A1B scenario). However, during the observational period the area of positive atmospheric heating (referred to as the dynamic warm pool, DWP), has remained constant. The threshold SST (T H), which demarks the region of net heating and cooling, has increased from 26.6°C in the 1950s to 27.1°C in the last decade and it is projected to increase to *28.5°C by 2100. Based on climate model simulations, the area of the DWP is projected to remain constant during the twenty-first century. Analysis of the paleoclimate model intercomparison project (PMIP I and II) simulations for the Last Glacial maximum and the Mid-Holocene periods show a very similar behaviour, with a larger OWP in periods of elevated tropical SST, and an almost constant DWP associated with a varying T H. The constancy of the DWP area, despite shifts in the background SST, is shown to be the result of a near exact matching between increases in the integrated convective heating within the DWP and the integrated radiative cooling outside the DWP as SST changes. Although the area of the DWP remains constant, the total tropical atmospheric heating is a strong function of the SST. For example the net heating has increased by about 10% from 1950 to 2000 and it is projected to increase by a further 20% by 2100. Such changes must be compensated by a more vigorous atmospheric circulation, with growth in convective heating within the warm pool, and an increase of subsiding air and stability outside the convective warm pool and an increase of vertical shear at the DWP boundaries. This finding is contrary to some conclusions from other studies but in accord with others. We discuss the similarities and differences at length. Keywords Tropical warm pool Á Tropical convective heating Á Tropical climate, areas of convection in last glacial maximum, Mid-Holocene and present Á Projections of the warm pool for the twenty-first century

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Estructura Espaciotemporal De La Variabilidad Intraestacional en El Trópico

Proyecto académico sin fines de lucro, desarrollado bajo la iniciativa de acceso abierto

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Report of a workshop for the Winter Monsoon Experiment, 14-15 June 1982, Monterey, California

Interannual variability of cold surges 2.5 Future research 3. Large Scale Motion (Forcing) 3.1 Re... more Interannual variability of cold surges 2.5 Future research 3. Large Scale Motion (Forcing) 3.1 Review of scientific objectives 3.2 The large scale heating distribution and the atmospheric response 3.3 The influence of orography 3.4 Large-scale interaction and the winter monsoon 3.5 Future research 4. Regional Scale Motion r 4.1 Review of scientific objectives 4.2 Cold surges in the Winter-MONEX region 4.3 Near-equatorial convective disturbances 4.4 Regional numerical prediction 4.5 Future research 5. Clouds and Frecipitation and Relationship to Large and Regional Scale Motions 5.1 Cloud population and characteristics 5.2 Cloud cluster structure 5.3 Cloud cluster heating effects 5.4 Synoptic and mesoscale forcing of cloud formation 5.

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AGRONOMY AND SOILS Using Precipitation Forecasts to Irrigate Cotton

In this experiment, precipitation forecasts were used to schedule irrigation for cotton (Gossypiu... more In this experiment, precipitation forecasts were used to schedule irrigation for cotton (Gossypium hirsutum L.). Four irrigation treatments and two cotton varieties were evaluated at Stripling Irrigation Research Park located near Camilla, GA in 2014. Two treatments were irrigated based on forecasts from the European Centre for Medium-Range Weather Forecasts (ECMWF) Ensemble Prediction System (EPS) and The Weather Channel ® 's mobile app. Irrigation amounts for these two treatments were determined by the Cotton Irrigation Schedule Suggested for High Yields (check-book recommendations). The third treatment was irrigated by the checkbook method and the fourth was rainfed. Irrigation applied for each treatment was 34.8 cm, 26.7cm, and 31.9 cm for the ECMWF EPS, weather.com, and checkbook, respectively. Rainfed cotton received 16.56 cm in precipitation. All irrigation methods resulted in significantly higher yields than the rainfed cotton. Results suggest using precipitation forecasts to schedule irrigation could provide a convenient alternative to the checkbook method. R ecent droughts throughout the country and the continuing water disputes among the states of Georgia, Alabama, and Florida have made agricultural producers more aware of the importance of managing irrigation systems efficiently. Some southeastern states are beginning to consider laws that will require monitoring and regulation of water used for irrigation. In fact, Georgia recently suspended issuing irrigation permits in some areas to try and limit the amount of water used in irrigation (Hollis, 2013). Even in southwest Georgia, which receives on average 59.06 cm (23.25 in) of rain during the grow

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The Curious Nature of the Hemispheric Symmetry of the Earth’s Water and Energy Balances

Current climate change reports, Jul 26, 2016

This paper presents new estimates of the hemispheric energy balance based on an assembly of radia... more This paper presents new estimates of the hemispheric energy balance based on an assembly of radiative flux and ocean heat data. Further, it provides an overview of recent simulations with fully coupled climate models to investigate the role of its representation in causing tropical precipitation biases. The energy balance portrayed here features a small hemispheric imbalance with slightly more energy being absorbed by the Southern hemisphere. This yields a net transport of heat towards the NH composing of a northward crossequatorial heat transport by the oceans and a southward heat flow in the atmosphere. The turbulent fluxes and hemispheric precipitation balance to about 3 Wm −2 with slightly larger total accumulation occurring in the NH. CloudSat data indicate more frequent precipitation in the SH implying more intense precipitation in the NH. Fully coupled climate model simulations show that reducing hemispheric energy balance biases does little to reduce existing biases in tropical precipitation. Keywords Hemispheric energy balance. Surface heat budget. Ocean heat storage. Cross-equatorial heat transport. Fully coupled climate model. Tropical precipitation bias This article is part of the Topical Collection on Global Energy Budgets

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Forecasting Severe Floods for the Meghna River Basin

AGU Fall Meeting Abstracts, Dec 1, 2010

Accurate prediction of extreme floods in Bangladesh is vital for the agricultural practices and p... more Accurate prediction of extreme floods in Bangladesh is vital for the agricultural practices and planning in the region, and to provide warnings for evacuation in case of flooding. Hopson and Webster (2010) developed and implemented a short-term flood forecasting scheme in Bangladesh for the Ganges and Brahmaputra basins that performs significantly better than the climatological and persistence forecasts at all

Forecasting Flooding in the Brahmaputra and Ganges Delta of Bangladesh on Short (1-10 days), Medium (20-30 days) and Seasonal Time Scales (1-6 months)

AGUFM, Dec 1, 2007

Following the devastating flood years of 1998 during which 60% of Bangladesh was under water for ... more Following the devastating flood years of 1998 during which 60% of Bangladesh was under water for a period of 3 months, the Climate Forecast Applications in Bangladesh (CFAB) project was formed with funding by USAID and NSF which eventually resulted in a joint project with the European Centre for Medium Range Weather Forecasting (ECMWF), the Asian Disaster Preparedness Centre (ADPC)

Operational Short-Term Flood Forecasting for Bangladesh: Application of ECMWF Ensemble Precipitation Forecasts

AGUFM, Dec 1, 2004

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A 1–10-Day Ensemble Forecasting Scheme for the Major River Basins of Bangladesh: Forecasting Severe Floods of 2003–07*

Journal of Hydrometeorology, Jun 1, 2010

This paper describes a fully automated scheme that has provided calibrated 1-10-day ensemble rive... more This paper describes a fully automated scheme that has provided calibrated 1-10-day ensemble river discharge forecasts and predictions of severe flooding of the Brahmaputra and Ganges Rivers as they flow into Bangladesh; it has been operational since 2003. The Bangladesh forecasting problem poses unique challenges because of the frequent life-threatening flooding of the country and because of the absence of upstream flow data from India means that the Ganges and Brahmaputra basins must be treated as if they are ungauged. The meteorological-hydrological forecast model is a hydrologic multimodel initialized by NASA and NOAA precipitation products, whose states and fluxes are forecasted forward using calibrated European Centre for Medium-Range Weather Forecasts ensemble prediction system products, and conditionally postprocessed to produce calibrated probabilistic forecasts of river discharge at the entrance points of the Ganges and Brahmaputra into Bangladesh. Forecasts with 1-10-day horizons are presented for the summers of 2003-07. Objective verification shows that the forecast system significantly outperforms both a climatological and persistence forecast at all lead times. All severe flooding events were operationally forecast with significant probability at the 10-day horizon, including the extensive flooding of the Brahmaputra in 2004 and 2007, with the latter providing advanced lead-time warnings for the evacuation of vulnerable residents.

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Remote Sensing and River Discharge Forecasting for Major Rivers in South Asia (Invited)

AGUFM, Dec 1, 2013

Comparing Postprocessing Approaches to Calibrating Operational River Discharge Forecasts

AGU Fall Meeting Abstracts, Dec 1, 2010

With rare exceptions, current operational ensemble weather and hydrologic forecast systems requir... more With rare exceptions, current operational ensemble weather and hydrologic forecast systems require a final post-processing step to steer the forecast products towards satisfying the twin constraints of greater reliability while retaining (or enhancing) forecast sharpness. Such post-processing of model output can be viewed as an extension of the modeling effort itself, such as in the case of under-dispersive ensemble forecasts,

Three-tier operational probabilistic precipitation and hydrological forecasting of large-scale un-gauged river basins: Developing a basis for strategic and tactical decisions for water management, agricultural planning

AGU Spring Meeting Abstracts, May 1, 2005

The problem is that a user or user community must make deterministic (yes/no) decisions on multip... more The problem is that a user or user community must make deterministic (yes/no) decisions on multiple time scales about environmental systems containing considerable uncertainty. Such decisions range from strategic to tactical. A useful product takes into account forecasts of the probabilities of a state of the environment (e.g., rain rate, river discharge, flood potential and so on) and quantitative or

The effect of potential vorticity fluxes on the circulation of the tropical upper troposphere

Quarterly Journal of the Royal Meteorological Society, 2018

With simple analytical arguments, we suggest that the subtropical westerly jets, the equatorial e... more With simple analytical arguments, we suggest that the subtropical westerly jets, the equatorial easterly winds, the westerly ducts (i.e. regions of westerly winds over the Equator), and the tropical upper tropospheric troughs, are coupled via potential vorticity (PV) dynamics. We suggest that deep tropical convection leads to advective PV fluxes towards the Poles over the subtropical westerly jets, triggering Rossby waves on the jet, and leading to advective PV fluxes towards the Equator when the Rossby waves break over the westerly ducts. Moreover, we show that the advective PV fluxes towards the Poles closely balance those towards the Equator, and suggest that this close balance is to be expected from the impermeability theorem. We then argue that small imbalances of these advective PV fluxes play an important role in maintaining the upper tropospheric mean flow of the tropical atmosphere.We test our ideas using a shallow‐water model of the atmosphere, where diabatic heating terms...

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Multiscale analysis of three consecutive years of anomalous flooding in Pakistan

Quarterly Journal of the Royal Meteorological Society, 2014

A multiscale investigation into three years of anomalous floods in Pakistan provides insight into... more A multiscale investigation into three years of anomalous floods in Pakistan provides insight into their formation, unifying meteorological characteristics, mesoscale storm structures and predictability. Striking similarities between all three floods exist, from planetary and large‐scale synoptic conditions down to the mesoscale storm structures, and these patterns were generally well‐captured with the European Centre for Medium‐Range Weather Forecasts (ECMWF) Ensemble Prediction System (EPS). Atmospheric blocking events associated with high geopotential heights and surface temperatures over Eastern Europe were present during all three floods. Quasi‐stationary synoptic conditions over the Tibetan plateau allowed for the formation of anomalous easterly midlevel flow across central India into Pakistan that advected deep tropospheric moisture from the Bay of Bengal into Pakistan, enabling flooding in the region. The Tropical Rainfall Measuring Mission (TRMM) Precipitation Radar observat...

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MJO Propagation across the Maritime Continent in the ECMWF Ensemble Prediction System

Journal of Climate, 2016

The characteristics of the MJO propagation across the Maritime Continent are investigated using a... more The characteristics of the MJO propagation across the Maritime Continent are investigated using a 20-yr reforecast dataset from the ECMWF ensemble prediction system. Analysis of the MJO events initialized over the Indian Ocean (phase 2) shows that the initial MJO amplitude and prediction skill relationship is not linear, particularly when the predictions start in moderate (between strong and weak) MJO amplitude category. To examine the key factors that determine the prediction skill, reforecasts in the moderate category are grouped into high- and low-skill events, and the differences in their ocean–atmospheric conditions as well as the physical processes during reforecast period are examined. The initial distribution of OLR anomalies in high-skill events shows a clear dipole pattern of convection with an enhanced convective anomalies over the Indian Ocean and strongly suppressed convective anomalies in the western Pacific Ocean. This dipole mode may support the MJO propagation acros...

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A three-tier overlapping prediction scheme: tools for strategic and tactical decisions in the developing world

Predictability of Weather and Climate

Upstream satellite remote sensing for river discharge forecasting: Application to major rivers in South Asia

Remote Sensing of Environment, 2013

In this work we demonstrate the utility of satellite remote sensing for river discharge nowcastin... more In this work we demonstrate the utility of satellite remote sensing for river discharge nowcasting and forecasting for two major rivers, the Ganges and Brahmaputra, in southern Asia. Passive microwave sensing of the river and floodplain at more than twenty locations upstream of Hardinge Bridge (Ganges) and Bahadurabad (Brahmaputra) gauging stations are used to: 1) examine the capability of remotely sensed flow information to track the downstream propagation of river flow waves and 2) evaluate their use in producing river flow nowcasts, and forecasts at 1-15 days lead time. The pattern of correlation between upstream satellite data and in situ observations of downstream discharge is used to estimate wave propagation time. This pattern of correlation is combined with a cross-validation method to select the satellite sites that produce the most accurate river discharge estimates in a lagged regression model. The results show that the well-correlated satellite-derived flow (SDF) signals were able to detect the propagation of a river flow wave along both river channels. The daily river discharge (contemporaneous) nowcast produced from the upstream SDFs could be used to provide missing data estimates given its Nash-Sutcliffe coefficient of 0.8 for both rivers; and forecasts have considerably better skill than autoregressive moving-average (ARMA) model beyond 3-day lead time for Brahmaputra. Due to the expected better accuracy of the SDF for detecting large flows, the forecast error is found to be lower for high flows compared to low flows. Overall, we conclude that satellite-based flow estimates are a useful source of dynamical surface water information in data-scarce regions and that they could be used for model calibration and data assimilation purposes in near-time hydrologic forecast applications.

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Application of Thorpex-Tigge Multi-Center Weather Forecasts for River Discharge Forecasting: Application to Major Rivers in South Asia

Accounting for Uncertainties in Generating Reliable Probabilistic Flood Forecasts for Bangladesh

The country of Bangladesh experiences life-threatening floods in the basins of the Ganges and Bra... more The country of Bangladesh experiences life-threatening floods in the basins of the Ganges and Brahmaputra rivers flowing through the country with tragic regularity. These floods result in loss of life on a scale that often greatly eclipses the deaths due to natural disasters in developed countries. Flooding in these basins can occur on weekly time scales (as occurred during the

Extended-Range Probabilistic Forecasts of Ganges and Brahmaputra Floods in Bangladesh

by Peter Webster and Thomas Hopson

Bulletin of the American Meteorological Society, 2010

M any of the largest rivers on the planet emanate from the Tibetan Plateau and the Himalayas , fe... more M any of the largest rivers on the planet emanate from the Tibetan Plateau and the Himalayas , fed by glacial and snow melting and monsoon rainfall. Nearly 25% of the global population reside in the vast agrarian societies in the Yellow, Yangtze, Mekong, Irrawaddy, Ganges, Brahmaputra, and Indus river basins, each of which is subject to periods of widespread and long-lived flooding. Flooding remains the greatest cause of death and destruction in the developing world, leading to catastrophic loss of life and property. While almost every government in Asia has made substantial progress over the past two decades in saving the lives of victims of slow-onset flood disasters, such events remain relentlessly impoverishing. In India alone, an average 6 million hectares ha) of land (approximately equivalent to the size of Texas) is inundated each year, affecting 35-40 million people (Dhar and Nandargi 2000; CWC 2008). Because the flooding occurs in the fertile flood plains of major rivers, the loss in agricultural inputs (seed, fertilizer, and pesticides) alone costs in excess of 1 billion U.S. dollars (USD; henceforth all costs will be given in USD) in an average flood or drought event. Smallholders nearly always purchase these agricultural inputs on credit against repayment after the expected harvest. The loss of crops and the purchased agricultural inputs  Following the 2008 floods, extensive surveys of how the probabilistic flood forecasts were used at the village level. Here, a team from the Asian Disaster Preparedness Centre interviews the Imam from the Mosquein koijuri Union of Sirajgong District in Bangladesh who relates how the forecasts were disseminated and used.

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