ALBERT

Description: ABSTRACT Researchers have come to differing conclusions about multi-decadal changes in the position of the western ridge of the Bermuda High during the summer. This communication examines trends for 72 periods within the entire period of record (1948–2012), while also comparing variances in the Western Bermuda High Index (WBHI) and the latitudinal position of the western ridge. The western ridge has not moved significantly eastward or westward over either 1948–2012 or the past 60 years. The western ridge underwent a major westward shift from 1976 to 1977, but it did not stay in that western position during subsequent decades. Over the past 30–40 years, the western ridge has moved significantly eastwards and southwards. Finally, the WBHI, rather than the latitude of the western ridge, has had a definitive increase in variance over time and is the likely cause of increased summer rainfall variability in the southeastern United States.

Description: This paper assesses the variability and trends in summer-season rainfall from 1948 to 2009 for the Atlanta, Georgia region. The rainfall variables are total rainfall, frequency of rainfall days, and frequency of heavy-rainfall days. The main methods involve classifying daily 500-hPa geopotential height fields into synoptic types, determining the rainfall characteristics of the synoptic types, testing for significant temporal trends in rainfall, middle-troposphere circulation, lower-troposphere circulation, and atmospheric humidity, and using multiple linear regression to determine the impact of circulation and humidity variables on inter-annual variations in the rainfall variables. There were a total of eight synoptic types: the wet types involved troughing across or to the immediate west of the Atlanta region, while the dry types involved either an anticyclone across or to the immediate west of the region. The rainfall variables and two lower-troposphere circulation indices, the Bermuda High Index (BHI) and the Western Bermuda High Index (WBHI), had significant positive trends in variance over time. Among the three rainfall variables, only the frequency of rainfall days had a significant trend: the periods 1976–2009 and 1977–2009 had significant positive trends in rainfall days. The BHI had a significant positive trend from the 1970s to 2009, and the western ridge of the Bermuda High moved significantly southeastward from approximately the mid-1970s to 2009. Atmospheric humidity (i.e. 850-hPa specific humidity, 500-hPa specific humidity, and precipitable water) over the region had significant positive trends during most periods, with all humidity variables having significant increases from the 1970s to 2009. Increased interannual variability in the WBHI appears to be the cause of the increased variance in rainfall variables. An increase in atmospheric humidity, which is actually a global phenomenon, appears to be the principal cause of the increase in rainfall days during the past three decades. Copyright © 2012 Royal Meteorological Society

Description: This paper assesses the variability and trends in summer-season rainfall from 1948 to 2009 for the Atlanta, Georgia region. The rainfall variables are total rainfall, frequency of rainfall days, and frequency of heavy-rainfall days. The main methods involve classifying daily 500-hPa geopotential height fields into synoptic types, determining the rainfall characteristics of the synoptic types, testing for significant temporal trends in rainfall, middle-troposphere circulation, lower-troposphere circulation, and atmospheric humidity, and using multiple linear regression to determine the impact of circulation and humidity variables on inter-annual variations in the rainfall variables. There were a total of eight synoptic types: the wet types involved troughing across or to the immediate west of the Atlanta region, while the dry types involved either an anticyclone across or to the immediate west of the region. The rainfall variables and two lower-troposphere circulation indices, the Bermuda High Index (BHI) and the Western Bermuda High Index (WBHI), had significant positive trends in variance over time. Among the three rainfall variables, only the frequency of rainfall days had a significant trend: the periods 1976–2009 and 1977–2009 had significant positive trends in rainfall days. The BHI had a significant positive trend from the 1970s to 2009, and the western ridge of the Bermuda High moved significantly southeastward from approximately the mid-1970s to 2009. Atmospheric humidity (i.e. 850-hPa specific humidity, 500-hPa specific humidity, and precipitable water) over the region had significant positive trends during most periods, with all humidity variables having significant increases from the 1970s to 2009. Increased interannual variability in the WBHI appears to be the cause of the increased variance in rainfall variables. An increase in atmospheric humidity, which is actually a global phenomenon, appears to be the principal cause of the increase in rainfall days during the past three decades. Copyright © 2012 Royal Meteorological Society

Description: Out of four long‐term satellite‐based rainfall products (ARC2, CHIRPS, PERSIANN, and TARCAT), CHIRPS is the only product that can be considered sufficiently accurate at estimating seasonal rainfall totals throughout most of the region. TARCAT is the most homogeneous product, while ARC2, CHIRPS, and PERSIANN had significant negative change points that caused a drying bias over the 1983–2016 period. Homogeneity‐adjusted rainfall for the four products indicates that western Uganda has experienced a significant increase in annual rainfall over the period. Long‐term trends in equatorial African rainfall have proven difficult to determine because of a dearth in ground‐measured rainfall data. Multiple, satellite‐based products now provide daily rainfall estimates from 1983 to the present at relatively fine spatial resolutions, but in order to assess trends in rainfall, they must be validated alongside ground‐based measurements. The purpose of this paper is twofold: (a) to assess the accuracy of four rainfall products covering the past several decades in western Uganda; and (b) to ascertain recent, multi‐decadal trends in annual rainfall for the region. The four products are African Rainfall Climatology Version 2 (ARC2), Climate Hazards Group InfraRed Precipitation with Stations (CHIRPS), Precipitation Estimation from Remotely Sensed Information using Artificial Neural Networks–Climate Data Record (PERSIANN‐CDR), and TAMSAT African Rainfall Climatology And Timeseries (TARCAT). The bias and accuracy of 10‐day, monthly, and seasonal rainfall totals of the four products were assessed using approximately 10 years of data from 10 rain gauges. The homogeneity of the products over multiple time periods was assessed using change‐point analysis. The accuracy of the four products increased with an increase in temporal scale, and CHIRPS was the only product that could be considered sufficiently accurate at estimating seasonal rainfall totals throughout most of the region. TARCAT tended to underestimate totals, and ARC2 and PERSIANN were in general the least accurate products. Only annual rainfall estimates from CHIRPS and TARCAT were significantly correlated with ground‐measured rainfall totals. TARCAT was the most homogeneous product, while ARC2, CHIRPS, and PERSIANN had significant negative change points that caused a drying bias over the 1983–2016 period. After adjusting the satellite‐based rainfall estimates based on the timing and magnitude of the change points, annual rainfall totals derived from all four products indicated that western Uganda experienced significantly increasing rainfall from 1983 to 2016.

Description: The purpose of this study was to assess trends in the intensity of the North American monsoon anticyclone over multiple decades from 1948 to 2010 during July and August, with a focus on the Lower Colorado River Basin (LCRB). The methodology included a 500 hPa geopotential-height regionalization of the monsoon-anticyclone domain (i.e. a large portion of the western United States and northern Mexico), a typing of 500 hPa circulation patterns over the LCRB, and an examination of multi-decadal trends as well as inter-epochal differences in geopotential heights and frequencies of synoptic types. Three regions (i.e. Northwest, Northeast, and South) were revealed that differed based on inter-annual variations in 500 hPa geopotential heights. The Northwest and South regions had significant increases in geopotential heights from 1948–1978 to 1980–2010. The synoptic types reflected the location of the monsoon anticyclone over the LCRB. The monsoon anticyclone intensified primarily over the northwestern region, which includes the LCRB, since the mid- to late 1970s. The anticyclone has thus been expanding over the LCRB. The anticyclone has been in a northern position, specifically a north-central position, over the basin more frequently over the past 30 years; conversely, the anticyclone has been in southern and eastern positions over the basin less frequently. Copyright © 2012 Royal Meteorological Society

Description: Abstract While long‐term rainfall trends and related atmospheric dynamics have been researched over the past several decades across equatorial Africa, little is known about rainfall in western Uganda, a transition zone in the middle of the continent. Using satellite‐derived rainfall and reanalysis data from 1983‐2017, this study examines atmospheric characteristics of seasons and multi‐decadal trends in rainfall. Most of the region has a biannual rainfall regime (i.e., the first rains within March‐May and the second rains during August‐November). Ascending (descending) air and increased (decreased) specific humidity are observed over western Uganda during the rainy (dry) seasons. Southeasterly air‐parcel back‐trajectories are common throughout western Uganda at all times except for the first dry season (i.e., December‐February). For all seasons, wet days in western Uganda are characterized by increases in ascending air and specific humidity in addition to westerly‐flow anomalies. Wet days in most seasons also have a disproportionately high frequency of westerly back‐trajectories extending over the Congo Basin. These Congo westerlies are associated with more vertical ascent and a more humid middle troposphere compared to the other trajectories. Rainy seasons, especially the first rains, have gotten longer and wetter throughout western Uganda. The duration of the first rains increased by about one month over the 35 years; in turn, the rainfall total increased by approximately 70%. Rainfall also has increased for climatological seasons, with the exception being December‐February. Increases in middle‐troposphere specific humidity and vertical ascent over time provide support for the wetting trends derived from the satellite‐derived rainfall data.