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Ratna S.B.,University of Pune | Sikka D.R.,Indian Institute of Tropical Meteorology | Dalvi M.,University of Pune | Venkata Ratnam J.,Research Institute for Global Change | Venkata Ratnam J.,Application Laboratory
International Journal of Climatology | Year: 2011

This 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. © 2010 Royal Meteorological Society. Source

Richards K.J.,Pacific University in Oregon | Natarov A.,Hawaii Pacific University | Firing E.,University of Hawaii at Manoa | Kashino Y.,Research Institute for Global Change | And 5 more authors.
Journal of Geophysical Research C: Oceans | Year: 2015

We investigate the characteristics of shear-generated turbulence in the natural environment by considering data from a number of cruises in the western equatorial Pacific. In this region, the vertical shear of the flow is dominated by flow structures that have a relatively small vertical scale of O(10 m). Combining data from all cruises, we find a strong relationship between the turbulent dissipation rate, ε{lunate}, vertical shear, S, and buoyancy frequency, N. Examination of ε{lunate} at a fixed value of Richardson number, Ri=N2/S2, shows that ε{lunate}∝ut2N for a wide range of values of N, where ut is an appropriate velocity scale which we assume to be the horizontal velocity scale of the turbulence. The implied vertical length scale, ℓv=ut/N, is consistent with theoretical and numerical studies of stratified turbulence. Such behavior is found for Ri<0.4. The vertical diffusion coefficient then scales as κv∝ut2/N at a fixed value of Richardson number. The amplitude of ε{lunate} is found to increase with decreasing Ri, but only modestly, and certainly less dramatically than suggested by some parameterization schemes. Provided the shear generating the turbulence is resolved, our results point to a way to parameterize the unresolved turbulence. © 2015. American Geophysical Union. All Rights Reserved. Source

Thompson A.M.,Pennsylvania State University | Miller S.K.,Pennsylvania State University | Tilmes S.,U.S. National Center for Atmospheric Research | Kollonige D.W.,Pennsylvania State University | And 30 more authors.
Journal of Geophysical Research: Atmospheres | Year: 2012

We present a regional and seasonal climatology of SHADOZ ozone profiles in the troposphere and tropical tropopause layer (TTL) based on measurements taken during the first five years of Aura, 2005-2009, when new stations joined the network at Hanoi, Vietnam; Hilo, Hawaii; Alajuela/Heredia, Costa Rica; Cotonou, Benin. In all, 15 stations operated during that period. A west-to-east progression of decreasing convective influence and increasing pollution leads to distinct tropospheric ozone profiles in three regions: (1) western Pacific/eastern Indian Ocean; (2) equatorial Americas (San Cristóbal, Alajuela, Paramaribo); (3) Atlantic and Africa. Comparisons in total ozone column from soundings, the Ozone Monitoring Instrument (OMI, on Aura, 2004-) satellite and ground-based instrumentation are presented. Most stations show better agreement with OMI than they did for EP/TOMS comparisons (1998-2004; Earth-Probe/Total Ozone Mapping Spectrometer), partly due to a revised above-burst ozone climatology. Possible station biases in the stratospheric segment of the ozone measurement noted in the first 7 years of SHADOZ ozone profiles are re-examined. High stratospheric bias observed during the TOMS period appears to persist at one station. Comparisons of SHADOZ tropospheric ozone and the daily Trajectory-enhanced Tropospheric Ozone Residual (TTOR) product (based on OMI/MLS) show that the satellite-derived column amount averages 25% low. Correlations between TTOR and the SHADOZ sondes are quite good (typical r2 = 0.5-0.8), however, which may account for why some published residual-based OMI products capture tropospheric interannual variability fairly realistically. On the other hand, no clear explanations emerge for why TTOR-sonde discrepancies vary over a wide range at most SHADOZ sites. © 2012. American Geophysical Union. All Rights Reserved. Source

Ratnam J.V.,Research Institute for Global Change | Ratnam J.V.,Application Laboratory | Behera S.K.,Research Institute for Global Change | Behera S.K.,Application Laboratory | And 5 more authors.
Climate Dynamics | Year: 2012

The main aim of this paper is to evaluate the Advanced Research Weather Research and Forecasting (WRF) regional model in simulating the precipitation over southern Africa during austral summer. The model's ability to reproduce the southern African mean climate and its variability around this mean state was evaluated by using the two-tier approach of specifying sea surface temperature (SST) to WRF and by using the one-tier approach of coupling the WRF with a simple mixed-layer ocean model. The boundary conditions provided by the reanalysis-II data were used for the simulations. Model experiments were conducted for twelve austral summers from DJF1998-99 to DJF2009-10. The experiments using both the two-tier and one-tier approaches simulated the spatial and temporal distributions of the precipitation realistically. However, both experiments simulated negative biases over Mozambique. Furthermore, analysis of the wet and dry spells revealed that the one-tier approach is superior to the two-tier approach. Based on the analysis of the surface temperature and the zonal wind shear it is noted that the simple mixed-layer ocean model coupled to WRF can be effectively used in place of two-tier WRF to simulate the climate of southern Africa. This is an important result because specification of SST at higher temporal resolutions in the subtropics is the most difficult task in the two-tier approach for most regional prediction models. The one-tier approach with the simple mixed-layer model can effectively reduce the complicacy of finding good SST predictions. © 2011 Springer-Verlag. Source

Hargreaves J.C.,Research Institute for Global Change | Annan J.D.,Research Institute for Global Change | Yoshimori M.,Atmosphere and Ocean Research Institute | Abe-Ouchi A.,Atmosphere and Ocean Research Institute
Geophysical Research Letters | Year: 2012

We investigate the relationship between the Last Glacial Maximum (LGM) and climate sensitivity across the PMIP2 multi-model ensemble of GCMs, and find a correlation between tropical temperature and climate sensitivity which is statistically significant and physically plausible. We use this relationship, together with the LGM temperature reconstruction of Annan and Hargreaves (2012), to generate estimates for the equilibrium climate sensitivity. We estimate the equilibrium climate sensitivity to be about 2.5C with a high probability of being under 4C, though these results are subject to several important caveats. The forthcoming PMIP3/CMIP5 models were not considered in this analysis, as very few LGM simulations are currently available from these models. We propose that these models will provide a useful validation of the correlation presented here. © 2012. American Geophysical Union. All Rights Reserved. Source

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