Browsing by Subject "meteorology & atmospheric sciences"

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    Influence of Modes of Climate Variability on Global Precipitation Extremes
    (2010) Kenyon, Jesse; Hegerl, Gabriele C
    The probability of climate extremes is strongly affected by atmospheric circulation This study quantifies the worldwide influence of three major modes of circulation on station based indices of intense precipitation the El Nino-Southern Oscillation the Pacific interdecadal variability as characterized by the North Pacific index (NPI) and the North Atlantic Oscillation Northern Annular Mode The study examines which stations show a statistically significant (5%) difference between the positive and negative phases of a circulation regime Results show distinct regional patterns of response to all these modes of climate variability however precipitation extremes are most substantially affected by the El Nino-Southern Oscillation The effects of the El Nino-Southern Oscillation are seen throughout the world, including in India Africa South America, the Pacific Rim North America and weakly Europe The North Atlantic Oscillation has a strong continent wide effect on Eurasia and affects a small but not negligible percentage of stations across the Northern Hemispheric midlatitudes This percentage Increases slightly if the Northern Annular Mode Index is used rather than the NAO index In that case a region of increase in intense precipitation can also be found in Southeast Asia The NPI influence on precipitation extremes is similar to the response to El Nino and strongest in landmasses adjacent to the Pacific Consistently indices of more rare precipitation events show a weaker response to circulation than indices of moderate extremes, the results are quite similar but of opposite sign for negative anomalies of the circulation indices
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    ItemOpen Access
    The Ocean-Land-Atmosphere-Model: Optimization and Evaluation of Simulated Radiative Fluxes and Precipitation
    (2010) Medvigy, David; Walko, Robert L; Otte, Martin J; Avissar, Roni
    This work continues the presentation and evaluation of the Ocean Land Atmosphere Model (OLAM), focusing on the model's ability to represent radiation and precipitation. OLAM is a new, state-of-the-art earth system model, capable of user-specified grid resolution and local mesh refinement. An objective optimization of the microphysics parameterization is carried out. Data products from the Clouds and the Earth's Radiant Energy System (CERES) and the Global Precipitation Climatology Project (GPCP) are used to construct a maximum likelihood function, and thousands of simulations using different values for key parameters are carried out. Shortwave fluxes are found to be highly sensitive to both the density of cloud droplets and the assumed shape of the cloud droplet diameter distribution function. Because there is considerable uncertainty in which values for these parameters to use in climate models, they are targeted as the tunable parameters of the objective optimization procedure, which identified high-likelihood volumes of parameter space as well as parameter uncertainties and covariances. Once optimized, the model closely matches observed large-scale radiative fluxes and precipitation. The impact of model resolution is also tested. At finer characteristic length scales (CLS), smaller-scale features such as the ITCZ are better resolved. It is also found that the Amazon was much better simulated at 100- than 200-km CLS. Furthermore, a simulation using OLAM's variable resolution functionality to cover South America with 100-km CLS and the rest of the world with 200-km CLS generates a precipitation pattern in the Amazon similar to the global 100-km CLS run.