Observed Trends in Northeast Brazil Mean and Extreme Precipitation
Brant LiebmannNOAA Earth System Research Laboratory
and University of ColoradoBoulder, Colorado, USA
Leila M. V. CarvalhoUniversity of California at Santa Barbara
Ileana Bladé University of Barcelona, Barcelona, Spain
Dave Allured NOAA Earth System Research Laboratory
and University of Colorado
Charles JonesUniversity of California at Santa Barbara
Station Data:Agência Nacional de Águas (Brasil)
FUNCEME (Ceará, Brasil)0.5 degree grid
Trend in mm/year
Change = Trend * (Length of Segment)
(mm/length)
0.5 degree grid 1.0 degree grid
Changes in Daily Extreme Events
• Most previous studies have used arbitrary threshold to define extreme daily value (e.g., 95th percentile)
- Does not weigh huge event more than one barely above threshold
In present study, we compute a time series of daily squared values from gridded data (not variance)
Problems:- Not clear whether squared value is appropriate amplifier- Sensitive to errors in data, particularly accumulated values
Advantages:- Threshold for ‘extreme’ is not arbitrary- Weighting increases for larger values
Summary
• Annual Average Precipitation Varies by a Factor of 5 Across Northeast Brazil
• Most of Domain Has Summer Maximum; East Coast Maximum in Winter
• Trends Are Sensitive to Range of Dates Used in Calculation
• Index Computed of Squared Daily Values in Order to Emphasize Large Events
• Often, Changes in Squares Mimic Changes in Mean
• In Some Locations, Mean Precipitation has Decreased While ‘Extremes’ have Increased
