High-resolution Atmospheric Model Research Articles

Waves in the Red Sea: Response to monsoonal and mountain gap winds

An unstructured grid, phase-averaged wave model forced with winds from a high resolution atmospheric model is used to evaluate wind wave conditions in the Red Sea over an approximately 2-year period. The Red Sea lies in a narrow rift valley, and the steep topography surrounding the basin steers the dominant wind patterns and consequently the wave climate. At large scales, the model results indicated that the primary seasonal variability in waves was due to the monsoonal wind reversal. During the winter, monsoon winds from the southeast generated waves with mean significant wave heights in excess of 2m and mean periods of 8s in the southern Red Sea, while in the northern part of the basin waves were smaller, shorter period, and from northwest. The zone of convergence of winds and waves typically occurred around 19–20°N, but the location varied between 15 and 21.5°N. During the summer, waves were generally smaller and from the northwest over most of the basin. While the seasonal winds oriented along the axis of the Red Sea drove much of the variability in the waves, the maximum wave heights in the simulations were not due to the monsoonal winds but instead were generated by localized mountain wind jets oriented across the basin (roughly east–west). During the summer, a mountain wind jet from the Tokar Gap enhanced the waves in the region of 18 and 20°N, with monthly mean wave heights exceeding 2m and maximum wave heights of 14m during a period when the rest of the Red Sea was relatively calm. Smaller mountain gap wind jets along the northeast coast created large waves during the fall and winter, with a series of jets providing a dominant source of wave energy during these periods. Evaluation of the wave model results against observations from a buoy and satellites found that the spatial resolution of the wind model significantly affected the quality of the wave model results. Wind forcing from a 10-km grid produced higher skills for waves than winds from a 30-km grid, largely due to under-prediction of the mean wind speed and wave height with the coarser grid. The 30-km grid did not resolve the mountain gap wind jets, and thus predicted lower wave heights in the central Red Sea during the summer and along the northeast coast in the winter.

Development of local-scale high-resolution atmospheric dispersion model using large-eddy simulation. Part 3: turbulent flow and plume dispersion in building arrays

We have developed a LOcal-scale High-resolution atmospheric DIspersion Model using Large-Eddy Simulation (LOHDIM-LES) to assess the safety at nuclear facilities and to respond to emergencies against accidental or intentional release of radioactive materials (e.g., a terrorist attack in an urban area). In Part 1, the unsteady behavior of a plume over a flat terrain was successfully simulated. In Part 2, a new scheme to generate a spatially developing turbulent boundary layer flow was proposed. Then, the large-eddy simulation (LES) model for turbulent flow and plume dispersion around an isolated building was validated. In this study, we extend the LES model to turbulent flows and plume dispersion in various building arrays that represent typical urban surface geometries. Concerning the characteristics of flow and dispersion in building arrays, the flow patterns associated with obstacle densities and the distribution patterns of mean and root-mean-square (r.m.s.) concentrations agree well with those of the wind tunnel experiments. It is shown that the LES model successfully simulates the unsteady behaviors of turbulent flows and plume dispersion in urban-type surface geometries.

Multi-model climate projections of ocean surface variables under different climate scenarios—Future change of waves, sea level and wind

This study analyzes future changes in average upper ocean physics such as sea level rise, sea surface winds and ocean wave heights by using a climate data set combining IPCC (2007) results and the latest high-resolution atmospheric general circulation model results from the Meteorological Research Institute, Japan. Ocean wave height Hs is statistically projected by using multi-model ensemble method based on an empirical formula as a function of sea surface winds. The ensemble means and the standard deviations of upper ocean physics, which can be used in coastal and ocean engineering, are projected for the period between 2000 and 2100. The magnitude of the ratio of future changes in Hs to the value in the present climate is at most ±15%, which exceeds the projected changes in sea surface pressure and surface wind speed, U10. A large uncertainty in the projected Hs can be observed around the Equator and in the Antarctic Ocean. The synoptic scale of atmospheric pressure distribution is found to be important for estimating and understanding the future changes in sea level rise, U10 and Hs.

Simulated response to inter-annual SST variations in the Gulf Stream region

Recent studies show that mid-latitude SST variations over the Kuroshio-Oyashio Extension influence the atmospheric circulation. However, the impact of variations in SST in the Gulf Stream region on the atmosphere has been less studied. Understanding the atmospheric response to such variability can improve the climate predictability in the North Atlantic Sector. Here we use a relatively high resolution (∼1°) Atmospheric General Circulation Model to investigate the mechanisms linking observed 5-year low-pass filtered SST variability in the Gulf Stream region and atmospheric variability, with focus on precipitation. Our results indicate that up to 70 % of local convective precipitation variability on these timescales can be explained by Gulf Stream SST variations. In this region, SST and convective precipitation are strongly correlated in both summer (r = 0.73) and winter (r = 0.55). A sensitivity experiment with a prescribed local warm SST anomaly in the Gulf Stream region confirms that local SST drives most of the precipitation variability over the Gulf Stream. Increased evaporation connected to the anomalous warm SST plays a crucial role in both seasons. In summer there is an enhanced local SLP minimum, a concentrated band of low level convergence, deep upward motion and enhanced precipitation. In winter we also get enhanced precipitation, but a direct connection to deep vertical upward motion is not found. Nearly all of the anomalous precipitation in winter is connected to passing atmospheric fronts. In summer the connection between precipitation and atmospheric fronts is weaker, but still important.

Seasonal Predictions of Tropical Cyclones Using a 25-km-Resolution General Circulation Model

Abstract Retrospective seasonal predictions of tropical cyclones (TCs) in the three major ocean basins of the Northern Hemisphere are performed from 1990 to 2010 using the Geophysical Fluid Dynamics Laboratory High-Resolution Atmospheric Model (HiRAM) at 25-km resolution. Atmospheric states are initialized for each forecast, with the sea surface temperature anomaly (SSTA) “persisted” from that at the starting time during the 5-month forecast period (July–November). Using a five-member ensemble, it is shown that the storm counts of both tropical storm (TS) and hurricane categories are highly predictable in the North Atlantic basin during the 21-yr period. The correlations between the 21-yr observed and model predicted storm counts are 0.88 and 0.89 for hurricanes and TSs, respectively. The prediction in the eastern North Pacific is skillful, but it is not as outstanding as that in the North Atlantic. The persistent SSTA assumption appears to be less robust for the western North Pacific, contributing to less skillful predictions in that region. The relative skill in the prediction of storm counts is shown to be consistent with the quality of the predicted large-scale environment in the three major basins. It is shown that intensity distribution of TCs can be captured well by the model if the central sea level pressure is used as the threshold variable instead of the commonly used 10-m wind speed. This demonstrates the feasibility of using the 25-km-resolution HiRAM, a general circulation model designed initially for long-term climate simulations, to study the impacts of climate change on the intensity distribution of TCs.

Can we evaluate a fine-grained emission model using high-resolution atmospheric transport modelling and regional fossil fuel CO2 observations?

Quantifying carbon dioxide emissions from fossil fuel burning (FFCO2) is a crucial task to assess continental carbon fluxes and to track anthropogenic emissions changes in the future. In the present study, we investigate potentials and challenges when combining observational data with simulations using high-resolution atmospheric transport and emission modelling. These challenges concern, for example, erroneous vertical mixing or uncertainties in the disaggregation of national total emissions to higher spatial and temporal resolution. In our study, the hourly regional fossil fuel CO2 offset (ΔFFCO2) is simulated by transporting emissions from a 5 min×5 min emission model (IER2005) that provides FFCO2 emissions from different emission categories. Our Lagrangian particle dispersion model (STILT) is driven by 25 km×25 km meteorological data from the European Center for Medium-Range Weather Forecast (ECMWF). We evaluate this modelling framework (STILT/ECMWF+IER2005) for the year 2005 using hourly ΔFFCO2 estimates derived from 14C, CO and 222Radon (222Rn) observations at an urban site in south-western Germany (Heidelberg). Analysing the mean diurnal cycles of ΔFFCO2 for different seasons, we find that the large seasonal and diurnal variation of emission factors used in the bottom-up emission model (spanning one order of magnitude) are adequate. Furthermore, we show that the use of 222Rn as an independent tracer helps to overcome problems in timing as well as strength of the vertical mixing in the transport model. By applying this variability correction, the model-observation agreement is significantly improved for simulated ΔFFCO2. We found a significant overestimation of ΔFFCO2 concentrations during situations where the air masses predominantly originate from densely populated areas. This is most likely caused by the spatial disaggregation methodology for the residential emissions, which to some extent relies on a constant per capita-based distribution. In the case of domestic heating emissions, this does not appear to be sufficient.

Long-distance dispersal of spruce budworm (Choristoneura fumiferana Clemens) in Minnesota (USA) and Ontario (Canada) via the atmospheric pathway

Dispersal can play an important role in the population dynamics of forest insects, but the role of long-distance immigration and emigration remains unclear due to the difficulty of quantifying dispersal distance and direction. We designed an agent-based spruce budworm flight behavior model that, when interfaced with temperature, wind speed, and precipitation output from a high-resolution atmospheric model, produces detailed flight trajectories and deposition patterns over large landscapes. Rules and relationships describing budworm adult (moth) lift-off, ascent, horizontal flight, and descent were parameterized using a detailed empirical study of budworm dispersal behavior and corresponding meteorological conditions during a 1970s outbreak in New Brunswick, Canada. Simulated moth landings were assumed to be dependent in part on the availability of suitable host tree species. We applied the model to a 6.4million ha landscape at the border between northern Minnesota (USA) and Ontario (Canada) during an eight-day flight window in late June 2007. Specimens collected during and after this flight window indicated moths emerging from an inland source of outbreak populations dispersed over 150km to trap sites near the north shore of Lake Superior, where localized cooling was predicted to have delayed emergence of locally-produced budworm moths. Simulations suggested immigration of moths to lakeshore sites from the outbreak source was plausible on three of eight dates within the flight window, but the relatively narrow deposition footprints implied immigration occurred on different dates across lakeshore sites. Apart from wind speed and direction, precipitation and low temperatures limited dispersal to substantially shorter distances for a few dates within the simulated flight window. Key uncertainties limiting our understanding of atmospheric transport of spruce budworm include behavioral responses to vertical heterogeneity in the air temperature profile, the precipitation threshold required for the forced descent of moths from the air column, and search mechanisms affecting host and/or mate location during long-distance flight.

Changes in climate classification and extreme climate indices from a high-resolution future projection in Korea

We investigate the future changes in the climate zone and six extreme temperature indices in Korea, using the 20-km high-resolution atmospheric general circulation model (MRI-AGCM3.1S). The Trewartha and Koppen climate classification schemes are applied, and four summer-based extreme temperature indices (i.e., summer days, tropical nights, growing degree days, and cooling degree days (CDD) and two winter-based indices (frost days and heating degree days (HDD) are analyzed. To represent significantly the change in threshold indices, the monthly mean bias is corrected in model. The model result reasonably captures the temporal and spatial distribution of the present-day extreme temperatures associated with topography. It was found that in the future climate, the area of the subtropical climate zone in Korea expands northward and increases by 21% under the Trewartha classification scheme and by 35% under the Koppen classification scheme. The spatial change in extreme climate indices is significantly modulated by geographical characteristics in relation to land-ocean thermal inertia and topographical effects. The change is manifested more in coastal regions than in inland regions, except for that in summer days and HDD. Regions with higher indices in the present climate tend to reveal a larger increase in the future climate. The summer-based indices display an increasing trend, while the winter-based indices show a decreasing trend. The most significant increase is in tropical nights (+452%), whereas the most significant decrease is in HDD (−25%). As an important indicator of energy-saving applications, the changes in HDD and CDD are compared in terms of the frequency and intensity. The future changes in CDD reveal a higher frequency but a lower temperature than those in HDD. The more frequent changes in CDD may be due to a higher and less dispersed occurrence probability of extreme temperatures during the warm season. The greater increase in extreme temperature events during the summer season remains an important implication of projecting future changes in extreme climate events.

Some Counterintuitive Dependencies of Tropical Cyclone Frequency on Parameters in a GCM

AbstractHigh-resolution global climate models (GCMs) have been increasingly utilized for simulations of the global number and distribution of tropical cyclones (TCs), and how they might change with changing climate. In contrast, there is a lack of published studies on the sensitivity of TC genesis to parameterized processes in these GCMs. The uncertainties in these formulations might be an important source of uncertainty in the future projections of TC statistics.This study investigates the sensitivity of the global number of TCs in present-day simulations using the Geophysical Fluid Dynamics Laboratory High Resolution Atmospheric Model (GFDL HIRAM) to alterations in physical parameterizations. Two parameters are identified to be important in TC genesis frequency in this model: the horizontal cumulus mixing rate, which controls the entrainment into convective cores within the convection parameterization, and the strength of the damping of the divergent component of the horizontal flow. The simulated global number of TCs exhibits nonintuitive response to incremental changes of both parameters. As the cumulus mixing rate increases, the model produces nonmonotonic response in global TC frequency with an initial sharp increase and then a decrease. However, storm mean intensity rises monotonically with the mixing rate. As the strength of the divergence damping increases, the model produces a continuous increase of global number of TCs and hurricanes with little change in storm mean intensity. Mechanisms for explaining these nonintuitive responses are discussed.

Development of daily gridded rainfall dataset over the Ganga, Brahmaputra and Meghna river basins

The India Meteorological Department (IMD) gridded rainfall dataset, the 47 Bangladesh gauge rainfall observations and the Tropical Rainfall Measuring Mission (TRMM) 3B42V6 satellite data are used in the present analysis. The nearest neighbour interpolation scheme is used, wherein the interpolated values are computed from a weighted sum of observations. The Bangladesh daily gauge measured rainfall is interpolated into regular grids of 0.5° × 0.5° resolution every day from January 1988 to December 2007 and appended with the daily gridded dataset of the IMD over the Indian region. A similar resolution dataset of 0.5° × 0.5° for the TRMM-3B42V6 data from January 1998 to December 2007 is created from the original data of 0.25° × 0.25° resolution. To produce a merged rainfall product, all the gridded datasets are merged. The merging of datasets is done in such a way as to include the highest rainfall at each grid point from the three products. Based on the three available sets of daily observations (IMD dataset (1° × 1°), TRMM-3B42 (0.25° × 0.25°) and 46 daily station observations over Bangladesh), a dataset of 0.5° × 0.5° resolution on a daily scale is generated. The focus of this study is to compare the TRMM-3B42V6 rainfall data over the Ganga, Brahmaputra and Meghna (GBM) domain with observed point gauge data, and assess the possibility of using them for application in real time flood forecasting as well as to serve as a comparison tool for the baseline simulation of high resolution atmospheric models aimed at flood forecasting and climate change projections. Copyright © 2012 Royal Meteorological Society

Modulation of Tropical Cyclones over the Eastern Pacific by the Intraseasonal Variability Simulated in an AGCM

Abstract This study illustrates that observed modulations of tropical cyclone (TC) genesis over the eastern Pacific (EPAC) by large-scale intraseasonal variability (ISV) are well represented in a recently developed high-resolution atmospheric model (HiRAM) at the NOAA/Geophysical Fluid Dynamics Laboratory (GFDL) with a horizontal resolution of about 50 km. Considering the intrinsic predictability of the ISV of 2–4 weeks, this analysis thus has significant implications for dynamically based TC predictions on intraseasonal time scales. Analysis indicates that the genesis potential index (GPI) anomalies associated with the ISV can generally well depict ISV modulations of EPAC TC genesis in both observations and HiRAM simulations. Further investigation is conducted to explore the key factors associated with ISV modulation of TC activity based on an analysis of budget terms of the observed GPI during the ISV life cycle. It is found that, while relative roles of GPI factors are dependent on ISV phase and location, lower-level cyclonic vorticity, enhanced midlevel relative humidity, and reduced vertical wind shear can all contribute to the observed active TC genesis over the EPAC during particular ISV phases. In general, the observed anomalous ISV patterns of these large-scale GPI factors are well represented in HiRAM. Model deficiencies are also noted particularly in the anomalous midlevel relative humidity patterns and amplitude of vertical wind shear associated with the EPAC ISV.

Global Ocean Surface Wave Simulation Using a Coupled Atmosphere–Wave Model

Abstract This study describes a 29-yr (1981–2009) global ocean surface gravity wave simulation generated by a coupled atmosphere–wave model using NOAA/GFDL’s High-Resolution Atmosphere Model (HiRAM) and the WAVEWATCH III surface wave model developed and used operationally at NOAA/NCEP. Extensive evaluation of monthly mean significant wave height (SWH) against in situ buoys, satellite altimeter measurements, and the 40-yr ECMWF Re-Analysis (ERA-40) show very good agreements in terms of magnitude, spatial distribution, and scatter. The comparisons with satellite altimeter measurements indicate that the SWH low bias in ERA-40 reanalysis has been improved in these model simulations. The model fields show a strong response to the North Atlantic Oscillation (NAO) in the North Atlantic and the Southern Oscillation index (SOI) in the Pacific Ocean that are well connected with the atmospheric responses. For the NAO in winter, the strongest subpolar wave responses are found near the northern Europe coast and the coast of Labrador rather than in the central-northern Atlantic where the wind response is strongest. Similarly, for the SOI in the Pacific Ocean, the wave responses are strongest in the northern Bering Sea and the Antarctic coast.

Changes in Earth’s Energy Flows and Clouds in 228-Year Simulation with a High-Resolution AGCM

We have examined long-term changes in Earth’s energy flows at top of the atmosphere (TOA) and at Earth’s surface (land and ocean) by using 228-year simulation of a high-resolution global atmosphere model, MRI-AGCM3.2. It is found that the net downward short wave (SW) radiation (absorbed solar radiation, ASR) at TOA significantly increases during twenty-first century in agreement with a previous study. However, in the present study, the reason for the change is an increase in clear sky SW absorption by increased water vapor in the atmosphere, while it is a decrease in cloud amount in the previous study. It is also found that the long wave (LW) cloud radiative forcing for atmosphere is positive and increasing during twenty-first century in agreement with a previous study. The reason for the change in the present study is an increase in absorption by water vapor of the downward LW radiation emitted from clouds, while it is reductions of cloud amount in the middle troposphere in the previous study.

Ascent dynamics of large phreatomagmatic eruption clouds: The role of microphysics

We examine the impact of abundant surface water interaction on the development of volcanic clouds from large‐scale (>108 kg s−1 magma) phreatomagmatic eruptions, presenting the first 2‐D large‐eddy simulations of “wet” volcanic plumes that incorporate the effects of microphysics. The Active Tracer High‐Resolution Atmospheric Model was forced with field‐derived inputs from an exceptionally large phreatomagmatic eruption: the 27 ka Oruanui supereruption (Taupo volcano, New Zealand). Surface water contents were varied from 0 to 40 wt% for eruptions with equivalent magma eruption rates of ∼1.3 × 108 and 1.1 × 109 kg s−1. Our findings confirm that increased surface water has a pronounced impact on column stability, leading to transitional column behavior and hybrid clouds resulting from simultaneous ascent of material from stable columns and pyroclastic density currents (PDCs). Contrary to the suggestion of previous workers, however, abundant surface water does not systematically lower the spreading level or maximum height of volcanic clouds, owing to vigorous microphysics‐assisted lofting of PDCs. The simulated behavior and ash cloud dimensions provide a close match to field evidence from the Oruanui case study. Cloud heights from the collapsing eruption columns also show a notable sensitivity to changes in the altitude of the tropopause, while ambient humidity primarily impacts the abundance of airborne hydrometeors (particularly ice) associated with the volcanic clouds. General relationships between eruption style, meteorological conditions, and the resulting vertical profiles of volcanic emissions outlined here could also be adapted for use in operational volcanic ash forecasting and deposit reconstruction techniques already in existence.

Computational Performance of the High-Resolution Atmospheric Model FAMIL

AbstractThis paper describes the model speed and model In/Out (I/O) efficiency of the high-resolution atmospheric general circulation model FAMIL (Finite-volume Atmospheric Model of IAP/LASG) at the National Supercomputer Center in Tianjin, China, on its Tianhe-1A supercomputer platform. A series of three-model-day simulations were carried out with standard Aqua Planet Experiment (APE) designed within FAMIL to obtain the time stamp for the calculation of model speed, simulation cost, and model I/O efficiency. The results of the simulation demonstrate that FAMIL has remarkable scalability below 3456 and 6144 cores, and the lowest simulation costs are 1536 and 3456 cores for 12.5 km and 6.25 km resolutions, respectively. Furthermore, FAMIL has excellent I/O scalability and an efficiency of more than 80% on 6 I/Os and more than 99% on 1536 I/Os.

Experimental Tropical Cyclone Prediction Using the GFDL 25-km-Resolution Global Atmospheric Model

Abstract This paper describes a forecasting configuration of the Geophysical Fluid Dynamics Laboratory (GFDL) High-resolution Atmospheric Model (HiRAM). HiRAM represents an early attempt in unifying, within a global modeling framework, the capabilities of GFDL’s low-resolution climate models for Intergovernmental Panel on Climate Change (IPCC) type climate change assessments and high-resolution limited-area models for hurricane predictions. In this study, the potential of HiRAM as a forecasting tool is investigated by applying the model to the near-term and intraseasonal hindcasting of tropical cyclones (TCs) in the Atlantic basin from 2006 to 2009. Results demonstrate that HiRAM provides skillful near-term forecasts of TC track and intensity relative to their respective benchmarks from t = 48 h through t = 144 h. At the intraseasonal time scale, a simple HiRAM ensemble provides skillful forecasts of 21-day Atlantic basin TC activity at a 2-day lead time. It should be noted that the methodology used to produce these hindcasts is applicable in a real-time forecasting scenario. While the initial experimental results appear promising, the HiRAM forecasting system requires various improvements in order to be useful in an operational setting. These modifications are currently under development and include a data assimilation system for forecast initialization, increased horizontal resolution to better resolve the vortex structure, 3D ocean model coupling, and wave model coupling. An overview of these ongoing developments is provided, and the specifics of each will be described in subsequent papers.

Dynamical simulation of Indian summer monsoon circulation, rainfall and its interannual variability using a high resolution atmospheric general circulation model

AbstractThis paper discusses the simulations of Indian summer monsoon (ISM) using a high‐resolution National Center for Environmental Prediction (NCEP) T170/L42 model for a 20‐year period (1985–2004) with observed Sea Surface Temperature (SSTs) as boundary conditions and using five initial conditions in the first week of May. Good agreement is found between the observed and simulated climatologies. Interannual variability (IAV) of the ISM rainfall as simulated in individual ensemble members and as provided by ensemble average shows that the two series are found to agree well; however, the simulation of the actual observed year‐to‐year variability is poor. The model simulations do not show much skill in the simulation of drought and excess monsoon seasons. One aspect which has emerged from the study is that where dynamical seasonal prediction has specific base for the large areal and temporal averages, the technique is not to be stretched for application on short areal scale such as that of a cluster of a few grid point. Monsoon onset over Kerala (MOK) coast of India and advance from Kerala coast to northwest India is discussed based on ensemble average and individual ensemble member basis. It is suggested that the model is capable of realistically simulating these processes, particularly if ensemble average is used, as the intermember spread in the ensemble members is large. In short, the high‐resolution model appears to provide better climatology and its magnitude of IAV, which compares favourably with observations, although year‐to‐year matching of the observed and simulated seasonal/monthly rainfall totals for India as a whole is not good. Copyright © 2010 Royal Meteorological Society

Next generation atmospheric model improves hurricane forecasting

Accurately predicting hurricane development from one season to the next is a problem that crosses multiple temporal and spatial scales. The driving force behind the overall activity level for a year depends on broad climate dynamics, such as the El Niño–Southern Oscillation or the Madden‐Julian Oscillation (MJO), while the generation of an individual tropical cyclone is a product of small‐scale fluctuations in atmospheric moisture or sea surface temperature. Classically, these broad‐ or small‐scale events have fallen within the realm of climate or weather forecasting models, respectively. However, researchers have recently developed the High‐Resolution Atmospheric Model (HiRAM), which can flip between weather forecasting and climate modeling. HiRAM has 25‐kilometer horizontal resolution and can properly represent clouds, both important improvements over current models. (Geophysical Research Letters, doi:10.1029/2011GL047629, 2011)

On improving the ability of a high-resolution atmospheric general circulation model for dynamical seasonal prediction of the extreme seasons of the Indian summer monsoon

The paper is devoted to examine the ability of a high-resolution National Center for Environmental Prediction (NCEP) T170/L42 Atmospheric General Circulation Model (AGCM), for exploring its utility for long-range dynamical prediction of seasonal Indian summer monsoon rainfall (ISMR) based on 5-members ensemble for the hindcast mode 20-year (1985-2004) period with observed global sea surface temperatures (SSTs) as boundary condition and 6-year (2005-2010) period in the forecast-mode with NCEP Coupled Forecast System (CFS) SSTs as boundary condition. ISMR simulations are examined on five day (pentad) rainfall average basis. It is shown that the model simulated ISMR, based on 5-members ensemble average basis had limited skill in simulating extreme ISMR seasons (drought/excess ISMR). However, if the ensemble averaging is restricted to similar ensemble members either in the overall run of pentad-wise below (B) and above (A) normal rainfall events, as determined by the departure for thethreshold value given by coefficient of variability (CV) for the respective pentads based on IMD observed climatology, or during the season as a whole on the basis of percentage anomaly of ISMR from the seasonal climatology, the foreshadowing of drought/excess monsoon seasons improved considerably. Our strategy of improving dynamical seasonal prediction of ISMR was based on the premise that the intra-seasonal variability (ISV) and intra-annual variability (IAV) are intimately connected and characterized by large scale perturbations westward moving (10-20 day) and northward moving (30-60 day) modes of monsoon ISV during the summer monsoon season. As such the cumulative excess of B events in the simulated season would correspond to drought season and vice-versa. The paper also examines El Niño-Monsoon connections of the simulated ISMR series and they appear to have improved considerably in the proposed methodology. This strategy was particularly found to improve for foreshadowing of droughts. Based on results of the study a strategy is proposed for using the matched signal for simulated ISMR based on excess B over A events and vice-versa for drought or excess ISMR category. The probability distribution for the forecast seasonal ISMR on category basis is also proposed to be based on the relative ratio of similar ensemble members and total ensembles on percentage basis. The paper also discusses that extreme monsoon season are produced by the El Niño-Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD) modes in a combined manner and hence stresses to improve prediction of IOD mode in ocean-atmosphere coupled model just as it has happened for the prediction ENSO mode six to nine months in advance.

The remarkable predictability of inter-annual variability of Atlantic hurricanes during the past decade

[1] A newly developed global model, the Geophysical Fluid Dynamics Laboratory (GFDL) High-Resolution Atmospheric Model (HiRAM) which is designed for both weather predictions and climate-change simulations, is used to predict the tropical cyclone activity at 25-km resolution. Assuming the persistence of the sea surface temperature anomaly during the forecast period, we show that the inter-annual variability of seasonal prediction for hurricane counts in the North Atlantic basin is highly predictable during the past decade (2000–2010). A remarkable correlation of 0.96 between the observed and model predicted hurricane counts is achieved. The root-mean-square error of the predicted hurricane number is less than 1 per year after correcting the model's negative bias. The predictive skill of the model in the tropics is further supported by the successful prediction of a Madden-Julian Oscillation event initialized 7-day in advance of its onset.