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Shrinivas Moorthi and R. Wayne Higgins

Abstract

An efficient, direct, second-order solver for the discrete solution of a class of two-dimensional separable elliptic equations on the sphere (which generally arise in implicit and semi-implicit atmospheric models) is presented. The method involves a Fourier transformation in longitude and a direct solution of the resulting coupled second-order finite-difference equations in latitude. The solver is made efficient by vectorizing over longitudinal wave-number and by using a vectorized fast Fourier transform routine. It is evaluated using a prescribed solution method and compared with a multigrid solver and the standard direct solver from FISHPAK.

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Kingtse C. Mo and R. Wayne Higgins

Abstract

Atmospheric circulation features and convection patterns associated with two leading low-frequency modes in the Southern Hemisphere (SH) are examined in multiyear global reanalyses produced by NCEP–NCAR and NASA–DAO. The two leading modes, referred to as the Pacific–South American (PSA) modes, are represented by the first two EOF patterns. The two patterns are in quadrature with each other and are dominated by wavenumber 3 in midlatitudes with large amplitudes in the Pacific–South American sector. In the Pacific, anomalies in the subtropics and in the midlatitudes are opposite in phase. Taken together, the two PSA modes represent the intraseasonal oscillation in the SH with periods of roughly 40 days. The evolution of the PSA modes shows a coherent eastward propagation.

A composite analysis was conducted to study the evolution of tropical convection and the corresponding circulation changes associated with the PSA modes. Outgoing longwave radiation (OLR) anomaly composites during the mature phase of the PSA modes resemble the first two leading EOFs of OLR anomalies (OLRA) in the Tropics. Composites of OLRA show an east–west dipole structure roughly 5–10 days prior to the onset of persistent PSA events. The PSA 1 mode is associated with enhanced convection in the Pacific between 140°E and 170°W and suppressed convection over the Indian Ocean. The PSA 2 mode is linked to tropical heating anomalies in the central Pacific extending from 160°E to 150°W just south of the equator and suppressed convection in the western Pacific with a maximum at 20°N. Contributions are from both interannual and intraseasonal bands.

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Charles Jones, Jon Gottschalck, Leila M. V. Carvalho, and Wayne Higgins

Abstract

Extreme precipitation events are among the most devastating weather phenomena since they are frequently accompanied by loss of life and property. This study uses reforecasts of the NCEP Climate Forecast System (CFS.v1) to evaluate the skill of nonprobabilistic and probabilistic forecasts of extreme precipitation in the contiguous United States (CONUS) during boreal winter for lead times up to two weeks.

The CFS model realistically simulates the spatial patterns of extreme precipitation events over the CONUS, although the magnitudes of the extremes in the model are much larger than in the observations. Heidke skill scores (HSS) for forecasts of extreme precipitation at the 75th and 90th percentiles showed that the CFS model has good skill at week 1 and modest skill at week 2. Forecast skill is usually higher when the Madden–Julian oscillation (MJO) is active and has enhanced convection occurring over the Western Hemisphere, Africa, and/or the western Indian Ocean than in quiescent periods. HSS greater than 0.1 extends to lead times of up to two weeks in these situations. Approximately 10%–30% of the CONUS has HSS greater than 0.1 at lead times of 1–14 days when the MJO is active.

Probabilistic forecasts for extreme precipitation events at the 75th percentile show improvements over climatology of 0%–40% at 1-day lead and 0%–5% at 7-day leads. The CFS has better skill in forecasting severe extremes (i.e., events exceeding the 90th percentile) at longer leads than moderate extremes (75th percentile). Improvements over climatology between 10% and 30% at leads of 3 days are observed over several areas across the CONUS—especially in California and in the Midwest.

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