Vernon E. Kousky

Simulations and Seasonal Prediction of the Asian Summer Monsoon in the NCEP Climate Forecast System

Abstract Analysis of the retrospective ensemble predictions (hindcasts) of the NCEP Climate Forecast System (CFS) indicates that the model successfully simulates many major features of the Asian summer monsoon including the climatology and interannual variability of major precipitation centers and atmospheric circulation systems. The model captures the onset of the monsoon better than the retreat of the monsoon, and it simulates the seasonal march of monsoon rainfall over Southeast Asia more realistically than that over South Asia. The CFS predicts the major dynamical monsoon indices and monsoon precipitation patterns several months in advance. It also depicts the interactive oceanic–atmospheric processes associated with the precipitation anomalies reasonably well at different time leads. Overall, the skill of monsoon prediction by the CFS mainly comes from the impact of El Nino–Southern Oscillation (ENSO). The CFS produces weaker-than-observed large-scale monsoon circulation, due partially to the cold bi...

An Improved Gridded Historical Daily Precipitation Analysis for Brazil

Abstract A gauge-only precipitation data quality control and analysis system has been developed for monitoring precipitation at NOAA’s Climate Prediction Center (CPC). Over the past 10 yr the system has been used to develop and deliver many different precipitation products over the United States, Mexico, and Central and South America. Here the authors describe how the system has been applied to develop improved gridded daily precipitation analyses over Brazil. Consistent with previous studies, comparisons between the the gridded analyses and station observations reveal fewer dry days, a greater number of low precipitation days, and fewer extreme precipitation events in the gridded analyses. Even though the gridded analysis system reduces the number of dry days and increases the number of wet days, there is still a good correlation between time series of the gridpoint precipitation values and observations. Retrospective analyses are important for computing basic statistics such as mean daily/monthly rainfa...

South Asian High and Asian-Pacific-American Climate Teleconnection

Growing evidence indicates that the Asian monsoon plays an important role in affecting the weather and climate outside of Asia. However, this active role of the monsoon has not been demonstrated as thoroughly as has the variability of the monsoon caused by various impacting factors such as sea surface temperature and land surface. This study investigates the relationship between the Asian monsoon and the climate anomalies in the Asian-Pacific-American (APA) sector. A hypothesis is tested that the variability of the upper-tropospheric South Asian high (SAH), which is closely associated with the overall heating of the large-scale Asian monsoon, is linked to changes in the subtropical western Pacific high (SWPH), the mid-Pacific trough, and the Mexican high. The changes in these circulation systems cause variability in surface temperature and precipitation in the APA region. A stronger SAH is accompanied by a stronger and more extensive SWPH. The enlargement of the SWPH weakens the mid-Pacific trough. As a result, the southern portion of the Mexican high becomes stronger. These changes are associated with changes in atmospheric teleconnections, precipitation, and surface temperature throughout the APA region. When the SAH is stronger, precipitation increases in southern Asia, decreases over the Pacific Ocean, and increases over the Central America. Precipitation also increases over Australia and central Africa and decreases in the Mediterranean region. While the signals in surface temperature are weak over the tropical land portion, they are apparent in the mid latitudes and over the eastern Pacific Ocean.

An Examination of Precipitation in Observations and Model Forecasts during NAME with Emphasis on the Diurnal Cycle

Abstract Summertime rainfall over the United States and Mexico is examined and is compared with forecasts from operational numerical prediction models. In particular, the distribution of rainfall amounts is examined and the diurnal cycle of rainfall is investigated and compared with the model forecasts. This study focuses on a 35-day period (12 July–15 August 2004) that occurred amid the North American Monsoon Experiment (NAME) field campaign. Three-hour precipitation forecasts from the numerical models were validated against satellite-derived estimates of rainfall that were adjusted by daily rain gauge data to remove bias from the remotely sensed estimates. The model forecasts that are evaluated are for the 36–60-h period after the model initial run time so that the effects of updated observational data are reduced substantially and a more direct evaluation of the model precipitation parameterization can be accomplished. The main findings of this study show that the effective spatial resolution of the mo...

Intercomparison of Daily Precipitation Statistics over the United States in Observations and in NCEP Reanalysis Products

Abstract A comparison of the statistics of daily precipitation over the conterminous United States is carried out using gridded station data and three generations of reanalysis products in use at the National Centers for Environmental Prediction (NCEP). The reanalysis products are the NCEP–NCAR reanalysis (Kalnay et al.), the NCEP–Department of Energy (DOE) reanalysis (Kanamitsu et al.), and the NCEP Climate Forecast System (CFS) reanalysis (Saha et al.). Several simple measures are used to characterize relationships between the observations and the reanalysis products, including bias, precipitation probability, variance, and correlation. Seasonality is accounted for by examining these measures for four nonoverlapping seasons, using daily data in each case. Relationships between daily precipitation and El Nino–Southern Oscillation (ENSO) phase are also considered. It is shown that the CFS reanalysis represents a clear improvement over the earlier reanalysis products, though significant biases remain. Comp...

Extreme Precipitation Events in the South-Central United States during May and June 2010: Historical Perspective, Role of ENSO, and Trends

AbstractAn analysis of extreme daily precipitation events that occurred in the south-central United States during May and June 2010 is carried out using gridded station data and reanalysis products in use at the National Centers for Environmental Prediction (NCEP). Various aspects of the daily extremes are examined from a climate perspective using a 62-yr (1948–2010) period of record, including their historical ranking, common circulation features, moisture plumes, and the possible influence of ENSO. The analysis also considers how the frequency and intensity of daily extremes is changing in the United States. Each of the 2010 flash flood events examined here was associated with historic daily rainfall totals. Several of the events had meteorological conditions in common at upper and lower levels of the atmosphere, and all of the events fit well into an existing classification scheme for heavy precipitation events associated with flash flooding. Each case exhibited characteristics of the “Maya Express” fl...

Variations of U.S. Regional Precipitation and Simulations by the NCEP CFS: Focus on the Southwest

Abstract Variations of U.S. regional precipitation in both observations and free-run experiments with the NCEP Climate Forecast System (CFS) are investigated. The seasonality of precipitation over the continental United States and the time–frequency characteristics of precipitation over the Southwest (SW) are the focus. The differences in precipitation variation among different model resolutions are also analyzed. The spatial distribution of U.S. precipitation is characterized by high values over the East and the West Coasts, especially over the Gulf Coast and southeast states, and low values elsewhere except over the SW in summer. A large annual cycle of precipitation occurs over the SW, northern plains, and the West Coast. Overall, the CFS captures the above features reasonably well, except for the SW. However, it overestimates the precipitation over the western United States, except the SW in summer, and underestimates the precipitation over the central South, except in springtime. It also overestimate...

Comparison of Daily Precipitation Statistics for the United States in Observations and in the NCEP Climate Forecast System

Abstract An intercomparison of the statistics of daily precipitation within seasonal climate over the conterminous United States is carried out using gridded station data and output from the NCEP Climate Forecast System (CFS). Differences in the occurrence of daily precipitation between the observations and a set of CFS reforecasts are examined as a function of forecast lead time for 1982–2005. Difference patterns show considerable evolution depending on season and lead time, with positive biases in CFS at most locations and leads except along the southern tier of states during the spring and summer months. An examination of differences in daily precipitation statistics by ENSO phase and in the frequencies of wet and dry spells is also conducted using a longer period of gridded daily station data (1948–2006) and a pair of 100-yr CFS coupled simulations. These comparisons expose additional details of the regional and seasonal dependence of the bias in the CFS simulations and reforecasts over the contermino...

Outgoing longwave radiation biases and their impacts on empirical orthogonal function modes of interannual variability in the tropics

The effects of different equator-crossing times on the outgoing longwave radiation (OLR) data record are examined. A simple procedure proposed by Kousky and Kayano (1994) is used to reduce these effects. Large-scale interannual modes of OLR within the tropics are determined by performing empirical orthogonal function analyses on OLR anomalies and on bias-corrected OLR anomalies. Both analyses result in two physically meaningful leading modes, which describe different large-scale anomalous features of the OLR related to the southern oscillation. A mode related to changes in the satellite observing system is found only in the analysis for the uncorrected OLR anomalies. Evidence is presented indicating that estimates of the mean daily OLR are affected by changes in the equator-crossing times due either to the replacement of satellites or to long-term drifts from the orbital plane during the lifetime of individual satellites.

Zonally symmetric and asymmetric features of the tropospheric Madden-Julian oscillation

The evolving intraseasonal (IS) modes of the zonally symmetric (ZS) part (the latitudinal profile) of the 200-hPa stream function have been determined by performing extended empirical orthogonal function (EEOF) analyses for the northern hemisphere (NH) summer and winter. In each analysis the first EEOF mode describes an oscillation with a period of about 50 days. The prominent feature of the upper tropospheric stream function latitudinal profiles, for both seasons, is the propagation of the largest loadings in both hemispheres from near the equator toward higher latitudes. This propagation is more evident during the NH winter and throughout the year in the southern hemisphere (SH). In the NH the poleward propagation of the loadings is more conspicuous within the equator and the 30°N latitudinal band. The associated patterns for the zonally asymmetric (ZA) part of the sea level pressure and the 850- and 200-hPa zonal winds have also been determined. These patterns feature, in general, the largest correlations in the tropics and a large-scale zonal wavenumber one structure propagating continuously eastward around the globe with a 50-day period. The ZA patterns for certain variables show strong seasonal dependence. The 850-hPa ZA zonal wind patterns feature the largest correlations approximately along the climatological position of the Intertropical Convergence Zone, whereas the largest correlations for the upper level ZA zonal wind patterns are found near the equator, mainly in the winter hemisphere.