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Min Wen, Song Yang, Augustin Vintzileos, Wayne Higgins, and Renhe Zhang

Abstract

A series of 60-day hindcasts by the Climate Forecast System (CFS) of the National Centers for Environmental Prediction is analyzed to understand the impacts of atmospheric model resolutions and initial conditions on predictions of the Asian summer monsoon. The experiments, for the time period 2002–06 and with 14 ensemble members, are conducted at resolutions of T62, T126, and T254. They are initialized every 5 days from May to August, using the operational global atmospheric data assimilation system and operational global ocean data assimilation. It is found that, in predicting the magnitude and the timing of monsoon rainfall over lands, high model resolutions overall perform better than lower model resolutions. The increase in prediction skills with model resolution is more apparent over South Asia than over Southeast Asia. The largest improvement is seen over the Tibetan Plateau, at least for precipitation. However, the increase in model resolution does not enhance the skill of the predictions over oceans. Overall, model resolution has larger impacts than do the initial conditions on predicting the development of the Asian summer monsoon in the early season. However, higher model resolutions such as T382 may be needed for the CFS to simulate and predict many features of the monsoon more realistically.

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Kingtse C. Mo, J. Nogues Paegle, and R. Wayne Higgins

Abstract

Persistent wet and dry events over the central United States are examined during summer. Composites based on selected persistent wet and dry events reveal common atmospheric processes and circulation features. During summer, heavy precipitation in the central United States is accompanied by less precipitation to the south, in a band that extends from the tropical eastern Pacific through the Gulf of Mexico into the western Atlantic. Dry conditions also occur along the western coasts of Canada and Mexico during persistent wet episodes in the central United States. This rainfall pattern is supported by an inverse temperature–rainfall relationship over North America. During dry events, high pressure extends throughout a vertical column in a pattern that covers North America from 30° to 60°N. In contrast, during wet events, the high pressure is confined to the eastern half of North America, with low pressure prevailing in the western half. Increased northward meridional winds are found between this cyclonic–anticyclonic dipole, leading to increased moisture flux from the Gulf of Mexico at low levels.

A significant precursor to wet events is the enhancement of westerlies over the eastern Pacific and western North America from 30° to 40°N. Synoptic-scale eddies intensify prior to onset and accelerate this westerly flow as revealed by Eliassen–Palm flux diagnostics. One pentad before onset, rainfall begins in Texas, and the low level jet (LLJ) in the Great Plains strengthens. The intensified LLJ transports moisture into the central United States and the moisture convergence downwind from the LLJ maintains rainfall. For dry events, heating occurs in the tropical eastern Pacific associated with the northward shift of the ITCZ roughly one pentad prior to onset. The prevailing easterly flow over subtropical portions of North America is not conducive to moisture transport into the United States, and without the support of moisture influx from the Gulf of Mexico, dry conditions prevail.

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

Abstract

The Madden–Julian oscillation (MJO) is the most prominent form of tropical intraseasonal variability that impacts weather and climate. Forecast skill of extreme precipitation in the contiguous United States (CONUS) during winter is higher when the MJO is active and has enhanced convection over the Western Hemisphere, Africa, and/or the western Indian Ocean. This study applies a simple decision model to examine the relationships between the MJO and the relative value of deterministic forecasts of extreme precipitation. Value in the forecasts is significantly higher and extends to longer leads (2 weeks) during active MJO.

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Emily J. Becker, Ernesto Hugo Berbery, and R. Wayne Higgins

Abstract

This study examines the seasonal characteristics of daily precipitation over the United States using the North American Regional Reanalysis (NARR). To help understand the physical mechanisms that contribute to changes in the characteristics of daily precipitation, vertically integrated moisture flux convergence (MFC) and precipitable water were included in the study. First, an analysis of the NARR precipitation was carried out because while observed precipitation is indirectly assimilated in the system, differences exist. The NARR mean seasonal amount is very close to that of observations throughout the year, although NARR exhibits a slight systematic bias toward more-frequent, lighter precipitation. Particularly during summer, the precipitation intensity and the probability distribution function (PDF) indicate that NARR somewhat underestimates extremes and overestimates lighter events in the eastern half of the United States. The intensity and PDF of moisture flux convergence exhibit a strong similarity to those of precipitation, suggesting a link between strong MFC and precipitation extremes. On the other hand, the relationship between the precipitable water and precipitation PDFs is weaker, based on the lack of agreement of their gamma distribution parameters.

The dependence of the precipitation PDF on the lower-frequency modulation of ENSO was examined. During El Niño winters, the Southwest and central United States, Gulf of Mexico region, and southeastern coast have greater precipitation intensity and extremes than during La Niña, and the Ohio River and Red River basins have lower intensity and fewer extreme events. During summer, the northern Rocky Mountains receive higher intensity precipitation with more extreme events. Most areas where the change in the daily mean precipitation between ENSO phases is large have greater shifts in the extreme tail of the PDF. The ENSO-related response of moisture flux convergence is similar to that of precipitation. ENSO-related shifts in the precipitation PDF do not appear to have a strong relationship to the shifts in precipitable water.

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Min Wen, Song Yang, Wayne Higgins, and Renhe Zhang

Abstract

During the boreal summer (June–August), vigorous convection appears over the eastern Pacific, southern Mexico, and northern South America, and oscillates on a distinct time scale of 10–20 days. Extended empirical orthogonal function (EEOF) analysis shows that the quasi-biweekly oscillation (QBWO) of the convection has two major modes: a west–east-orientated mode (WEM) and a north–south-orientated mode (NSM). The WEM, which is explained by the first two EEOF modes, originates over the eastern Atlantic, propagates westward along 15°N, and enhances over the Caribbean Sea before disappearing over the central Pacific. The NSM, explained by the third and fourth EEOF modes, originates over the western Pacific, moves eastward, and strengthens over the eastern Pacific. It shifts northward after arriving over the Caribbean Sea. Both modes have notable seasonal dependence, with the WEM more active in July and August and the NSM more active in June or earlier.

The two distinct QBWO modes are linked to different rainfall patterns over the United States and Mexico. When the WEM is active in July and August, wet conditions occur over the southern central United States and dry conditions appear to the north. When the NSM is active in June, northern Mexico, the southwestern United States, the Missouri basin, and the northern Great Lakes are drier than normal, while southern Mexico and the eastern United States are wetter than normal. Significant variations in atmospheric circulation are found to be associated with the interannual variability of the NSM activity in June. However, these variations may not necessarily result from QBWO but, rather, provide a background for QBWO activity instead. In July and August, the association of QBWO with the precipitation pattern over North America may sometimes be related to hurricane activity.

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Viviane B. S. Silva, Vernon E. Kousky, and R. Wayne Higgins

Abstract

In this study, the authors document the extent to which the precipitation statistics of the new CFS reanalysis (CFSR) represent an improvement over the earlier reanalyses: the NCEP–NCAR reanalysis (R1) and the NCEP–DOE Second Atmospheric Model Intercomparison Project (AMIP-II) reanalysis (R2). An intercomparison between the CFSR, R1, R2, and observations over South America was made for the period 1979–2006. The CFSR shows notable improvements in the large-scale precipitation patterns compared with the previous reanalyses (R1 and R2). In spite of these improvements, the CFSR has substantial biases in intensity and frequency of occurrence of rainfall events. Over west-central Brazil, the core region of the South American monsoon system (SAMS), the CFSR displays a dry bias during the onset phase of the SAMS wet season and a wet bias during the peak and decay phases of the SAMS wet season. The CFSR also displays a dry bias along the South American coast near the mouth of the Amazon and along the east coast of northeastern Brazil. A wet bias exists in all seasons over southeast Brazil and over the Andes Mountains.

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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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Baoxiang Pan, Kuolin Hsu, Amir AghaKouchak, Soroosh Sorooshian, and Wayne Higgins

Abstract

Precipitation variability significantly influences the heavily populated West Coast of the United States, raising the need for reliable predictions. We investigate the region’s short- to extended-range precipitation prediction skill using the hindcast database of the Subseasonal-to-Seasonal Prediction Project (S2S). The prediction skill–lead time relationship is evaluated, using both deterministic and probabilistic skill scores. Results show that the S2S models display advantageous deterministic skill at week 1. For week 2, prediction is useful for the best-performing model, with a Pearson correlation coefficient larger than 0.6. Beyond week 2, predictions generally provide little useful deterministic skill. Sources of extended-range predictability are investigated, focusing on El Niño–Southern Oscillation (ENSO) and the Madden–Julian oscillation (MJO). We found that periods of heavy precipitation associated with ENSO are more predictable at the extended range period. During El Niño years, Southern California tends to receive more precipitation in late winter, and most models show better extended-range prediction skill. On the contrary, during La Niña years Oregon tends to receive more precipitation in winter, with most models showing better extended-range skill. We believe the excessive precipitation and improved extended-range prediction skill are caused by the meridional shift of baroclinic systems as modulated by ENSO. Through examining precipitation anomalies conditioned on the MJO, we verified that active MJO events systematically modulate the area’s precipitation distribution. Our results show that most models do not represent the MJO or its associated teleconnections, especially at phases 3–4. However, some models exhibit enhanced extended-range prediction skills under active MJO conditions.

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Emily J. Becker, Ernesto Hugo Berbery, and R. Wayne Higgins

Abstract

This study examines the characteristics of cold-season (November–March) daily precipitation over the contiguous United States during active periods of the Madden–Julian oscillation (MJO). A large response in the precipitation rate anomaly is found over the eastern United States when MJO-related enhanced tropical convection is moving through the far western to central Pacific (conventionally known as phases 5, 6, and 7 of the MJO). Positive anomalies occur in the region of the eastern Mississippi River basin, and negative anomalies occur in the Southeast. The relative stability of this pattern throughout the three phases suggests that they can be considered together. During phases 5–7, the central United States has a daily precipitation rate between 110% and 150% of normal, while the precipitation rate over much of Florida is less than 70% of normal. Much of the lower Mississippi River basin region receives somewhat more frequent daily precipitation during MJO phases 5–7, but a greater increase is found in the daily precipitation intensity, suggesting more intense storms. On the other hand, Florida has substantially fewer daily precipitation events, with a smaller decrease in the intensity.

To understand the atmospheric mechanisms related to the above shifts in daily precipitation, elements of the atmospheric circulation were examined. Positive moisture flux convergence anomalies, which have been linked to increased precipitation rate and intensity, are found in the region of increased precipitation rate during MJO phases 5–7. During those phases, the North American jet stream is shifted northward, likely leading to a higher incidence of storms over the lower Mississippi River basin and fewer storms over Florida. This is supported by the fact that the storm track also shows increased activity over the central United States during MJO phases 5–7.

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Charles Jones, Leila M. V. Carvalho, R. Wayne Higgins, Duane E. Waliser, and J-K. E. Schemm

Abstract

Tropical intraseasonal convective anomalies (TICAs) play a significant role in the coupled ocean–atmosphere system and the Madden–Julian oscillation (MJO) is the primary mode of this variability. This study describes statistical forecast models of intraseasonal variations. Twenty-four years of outgoing longwave radiation (OLR) and zonal components of the wind at 200 (U200) and 850 hPa (U850) are used. The models use the principal components (PCs) of combined EOF analysis of 20–90-day anomalies of OLR, U200, and U850 data. Forecast models are developed for each lead time from 1 to 10 pentads and for winter and summer seasons separately. The forecast models use a combination of the five most recent pentad values of the first five PCs of the combined EOF of (OLR, U200, U850) to predict the future values of a given PCK (k = 1, 5). The spatial structures are obtained by reconstructing the fields of OLR, U200, and U850 using the forecasts of PCK (k = 1, 5) and the associated EOFs. Verification with independent winter and summer data indicates useful forecasts of the first five PCs extending up to five pentads of lead time. The verification against 20–90-day anomalies indicates useful forecasts of the reconstructed fields of OLR, U200, and U850 extending up to four pentads of lead time over most of the Tropics. Furthermore, the statistical models provide useful forecasts of U200 and U850 intraseasonal anomalies up to two to three pentads of lead times in portions of the North Pacific region.

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