Uncertainty in weather and climate prediction

Following Lorenz's seminal work on chaos theory in the 1960s, probabilistic approaches to prediction have come to dominate the science of weather and climate forecasting. This paper gives a perspective on Lorenz's work and how it has influenced the ways in which we seek to represent uncertainty in forecasts on all lead times from hours to decades. It looks at how model uncertainty has been represented in probabilistic prediction systems and considers the challenges posed by a changing climate. Finally, the paper considers how the uncertainty in projections of climate change can be addressed to deliver more reliable and confident assessments that support decision-making on adaptation and mitigation

Modelling the Madden Julian Oscillation

The MJO has long been an aspect of the global climate that has provided a tough test for the climate modelling community. Since the 1980s there have been numerous studies of the simulation of the MJO in atmospheric general circulation models (GCMs), ranging from Hayashi and Golder (1986, 1988) and Lau and Lau (1986), through to more recent studies such as Wang and Schlesinger (1999) and Wu et al. (2002). Of course, attempts to reproduce the MJO in climate models have proceeded in parallel with developments in our understanding of what the MJO is and what drives it. In fact, many advances in understanding the MJO have come through modeling studies. In particular, failure of climate models to simulate various aspects of the MJO has prompted investigations into the mechanisms that are important to its initiation and maintenance, leading to improvements both in our understanding of, and ability to simulate, the MJO. The initial focus of this chapter will be on modeling the MJO during northern winter, when it is characterized as a predominantly eastward propagating mode and is most readily seen in observations. Aspects of the simulation of the MJO will be discussed in the context of its sensitivity to the formulation of the atmospheric model, and the increasing evidence that it may be a coupled ocean-atmosphere phenomenon. Later, we will discuss the challenges regarding the simulation of boreal summer intraseasonal variability, which is more complex since it is a combination of the eastward propagating MJO and the northward propagation of the tropical convergence zone. Finally some concluding remarks on future directions in modeling the MJO and its relationship with other timescales of variability in the tropics will be made

On the maintenance and initiation of the intraseasonal oscillation in the NCEP/NCAR and ECMWF reanalyses and in the GLA and UKMO AMIP simulations

Julian Intraseasonal (Madden-Julian) oscillations are a dominant model of tropical variability (Madden and 1971, 1972). Satellite derived outgoing longwave radiation (OLR) and reanalyses from NCEP/NCAR and ECMWF are used as verification data in a study of intraseasonal variability in the Goddard Laboratory for Atmospheres (GLA) and the United Kingdom Meteorological Office (UKMO) atmospheric general circulation models. Sling0 et al. (1996) indicated that no model was able to capture the dominance of the intraseasonal oscillation (IO) found in the ECMWF/JDP analyses. However, in the case of the GLA and UKMO AMIP integrations, when a clear eastward propagating signal is evident, the period of the oscillation is realistic.Therefore, in order to show the models in their best light, we examine the November-May period during which these models exhibited their strongest&most coherent IO`s. 1987/88 from observations and the reanalyses will be compared with 1986/87 from GLA and 1980/81 from UKMO. Case studies are important since specific processes/mechanisms may be evident which might otherwise be obscured by cornpositing over many years (e.g., Matthews et al. 1996). During the active phase of the IO, convection migrates from the Indian Ocean to the western/central Pacific Ocean, and into the SPCZ. To demonstrate this, we have calculated an IO index to be used for lagged correlation analysis. This pentad averaged time series is constructed from 20-100 day bandpass filtered 200hPa velocity potential over the region 1OO{degrees}- 140{degrees}E, lO{degrees}N- 10{degrees} S from the NCEP/NCAR reanalysis (not shown; the IO index from the ECMWF reanalysis is virtually identical with the NCEP/NCAR IO index [correlation coefficient=0.987]). This region was chosen since this is where the diabatic heating associated with the IO is greatest. This IO index is then correlated with pentad averaged OLR at various time lags. Convection first arises over the western Indian Ocean on day -15. Through day 0 the convective envelope matures quickly, dominating the eastern Indian Ocean, the Maritime continent and much of Australia. Subsequently, the extent of the convection decreases, with the strongest enhancement located in the SPCZ. The simulated convection, particularly in the GLA model, is most realistic over the western/central Pacific Ocean and the SPCZ. However, both models fail to simulate IO related convection over the Indian Ocean and the propagation eastward into the west Pacific. The maintenance and initiation of the intraseasonal oscillation has also been investigated. Evaporative wind feedback hypothesizes that evaporation to the east of the convection is fundamental for maintaining the eastward migration of the convection. To examine the viability of this hypothesis we have correlated the IO index with 20-100 day bandpass filtered latent heat flux from NCEP/NCAR reanalysis and ECWMF reanalysis. Both reanalyses indicate that evaporation is enhanced to the west of the convection, particularly from day -5 onward, with both reanalyses exhibiting virtually identical lag correlation patterns. This result indicates that evaporative wind feedback is not the dominant process by which the eastward propagation of the intraseasonal oscillation is maintained. Correlations of the simulated IO indexes with filtered latent heat flux from the GLA and UKMO integrations are also shown. In the GLA simulation, enhanced evaporation tends to develop in-place over the west Pacific warm pool, while in the UISMO simulation westward propagation of enhanced evaporation is evident. Thus, the models do not simulate the processes suggested by the reanalyses that occur during the eastward propagation of the IO. While our results suggest a wave-CISK type mechanism , the contribution due to frictional convergence is not apparent. It is suggested that lack of an interactive ocean may be associated with the models systematic failure to simulate the eastward transition of convection and the latent heat flux from the Indian Ocean into the western Pacific Ocean. Examination of observed SST and its relationship to the active phase of the intraseasonal oscillation suggests that air-sea interaction may be important during the course of the evolution of the IO. To explore this in more detail, we show the correlation of the IO index with filtered observed SST (and skin temperature over land from the ECMWF reanalysis) for the 1987/88 case study

Global meteorological influences on the record UK rainfall of winter 2013-14

The UK experienced record average rainfall in winter 2013–14, leading to widespread and prolonged flooding. The immediate cause of this exceptional rainfall was a very strong and persistent cyclonic atmospheric circulation over the North East Atlantic Ocean. This was related to a very strong North Atlantic jet stream which resulted in numerous damaging wind storms. These exceptional meteorological conditions have led to renewed questions about whether anthropogenic climate change is noticeably influencing extreme weather. The regional weather pattern responsible for the extreme UK winter coincided with highly anomalous conditions across the globe. We assess the contributions from various possible remote forcing regions using sets of ocean–atmosphere model relaxation experiments, where winds and temperatures are constrained to be similar to those observed in winter 2013–14 within specified atmospheric domains. We find that influences from the tropics were likely to have played a significant role in the development of the unusual extra-tropical circulation, including a role for the tropical Atlantic sector. Additionally, a stronger and more stable stratospheric polar vortex, likely associated with a strong westerly phase of the stratospheric Quasi-Biennial Oscillation (QBO), appears to have contributed to the extreme conditions. While intrinsic climatic variability clearly has the largest effect on the generation of extremes, results from an analysis which segregates circulation-related and residual rainfall variability suggest that emerging climate change signals made a secondary contribution to extreme rainfall in winter 2013–14

The role of the basic state in the ENSO-monsoon relationship and implications for predictability

The impact of systematic model errors on a coupled simulation of the Asian Summer monsoon and its interannual variability is studied. Although the mean monsoon climate is reasonably well captured, systematic errors in the equatorial Pacific mean that the monsoon-ENSO teleconnection is rather poorly represented in the GCM. A system of ocean-surface heat flux adjustments is implemented in the tropical Pacific and Indian Oceans in order to reduce the systematic biases. In this version of the GCM, the monsoon-ENSO teleconnection is better simulated, particularly the lag-lead relationships in which weak monsoons precede the peak of El Nino. In part this is related to changes in the characteristics of El Nino, which has a more realistic evolution in its developing phase. A stronger ENSO amplitude in the new model version also feeds back to further strengthen the teleconnection. These results have important implications for the use of coupled models for seasonal prediction of systems such as the monsoon, and suggest that some form of flux correction may have significant benefits where model systematic error compromises important teleconnections and modes of interannual variability

The sensitivity of the tropical circulation and Maritime Continent precipitation to climate model resolution

The dependence of the annual mean tropical precipitation on horizontal resolution is investigated in the atmospheric version of the Hadley Centre General Environment Model (HadGEM1). Reducing the grid spacing from about 350 km to 110 km improves the precipitation distribution in most of the tropics. In particular, characteristic dry biases over South and Southeast Asia including the Maritime Continent as well as wet biases over the western tropical oceans are reduced. The annual-mean precipitation bias is reduced by about one third over the Maritime Continent and the neighbouring ocean basins associated with it via the Walker circulation. Sensitivity experiments show that much of the improvement with resolution in the Maritime Continent region is due to the specification of better resolved surface boundary conditions (land fraction, soil and vegetation parameters) at the higher resolution. It is shown that in particular the formulation of the coastal tiling scheme may cause resolution sensitivity of the mean simulated climate. The improvement in the tropical mean precipitation in this region is not primarily associated with the better representation of orography at the higher resolution, nor with changes in the eddy transport of moisture. Sizeable sensitivity to changes in the surface fields may be one of the reasons for the large variation of the mean tropical precipitation distribution seen across climate models

An Unbiased Measurement of Ho through Cosmic Background Imager Observations of the Sunyaev-Zel'dovich Effect in Nearby Galaxy Clusters

We present Ho results from Cosmic Background Imager (CBI) observations of the Sunyaev-Zel'dovich Effect (SZE) in 7 galaxy clusters, A85, A399, A401, A478, A754, A1651, and A2597. These observations are part of a program to study a complete, volume-limited sample of low-redshift (z<0.1), X-ray selected clusters. Our focus on nearby objects allows us to study a well-defined, orientation unbiased sample, minimizing systematic errors due to cluster asphericity. We use density models derived from ROSAT imaging data and temperature measurements from ASCA and BeppoSAX spectral observations. We quantify in detail sources of error in our derivation of Ho, including calibration of the CBI data, density and temperature models from the X-ray data, Cosmic Microwave Background (CMB) primary anisotropy fluctuations, and residuals from radio point source subtraction. From these 7 clusters we obtain a result of Ho = 67^{+30}_{-18}, ^{+15}_{-6} km/s/Mpc for an unweighted sample average. The respective quoted errors are random and systematic uncertainties at 68% confidence. The dominant source of error is confusion from intrinsic anisotropy fluctuations.Comment: 49 pages, 8 figures. Accepted for publication in Ap

Fifty years of research on the Madden-Julian Oscillation: recent progress, challenges and perspectives

Since its discovery in the early 1970s, the crucial role of the Madden-Julian Oscillation (MJO) in the global hydrological cycle and its tremendous influence of high-impact climate and weather extremes have been well recognised. The MJO also serves as a primary source of predictability for global Earth system variability on subseasonal time scales. The MJO remains poorly represeted in our state-of-the-art climate and weather forecasting models, however. Moreover, despite the advances made in recent decades, theories for the MJO still disagree at a fundamental level. The problems of understanding and modeling the MJO have attracted significant interest from the research community. As part of the AGU's Centennial collection, this article provides a review of recent progress, particularly over the last decade, in observational, modeling and theoretical study of the MJO. A brief outlook for near-future MJO research directions is also provided

Evaluation of the model representation of the evolution of convective systems using satellite observations of outgoing longwave radiation

We introduce a technique for assessing the diurnal development of convective storm systems based on outgoing longwave radiation fields. Using the size distribution of the storms measured from a series of images, we generate an array in the lengthscale-time domain based on the standard score statistic. It demonstrates succinctly the size evolution of storms as well as the dissipation kinematics. It also provides evidence related to the temperature evolution of the cloud tops. We apply this approach to a test case comparing observations made by the Geostationary Earth Radiation Budget instrument to output from the Met Office Unified Model run at two resolutions. The 12km resolution model produces peak convective activity on all lengthscales significantly earlier in the day than shown by the observations and no evidence for storms growing in size. The 4km resolution model shows realistic timing and growth evolution although the dissipation mechanism still differs from the observed data