ESRL Global Systems Division

OAR Contributes to Improved Understanding of Precipitation in the Amazon

ESRL scientists participated in a three and one half year study of how convective precipitation events occur in the Equatorial Amazon Rain Forest. Despite the fact that the processes that create towering thunderstorms are multifaceted and complex, the time to transition from shallow cumulus fields to precipitating cumulonimbus is quite uniform. This transition, traditionally difficult to model, is detectable in high temporal resolution GPS-Met water vapor measurements as a water vapor convergence time scale.

A recent study published in Geophysical Research Letters and highlighted as an American Geophysical Union Research Spotlight Article shows that it takes about four hours of water vapor convergence to transition from shallow to deep convection, and this happens regardless of the time of year or the time of the day. Prior to this shallow-to-deep transition where water vapor convergence dominates, local evaporation appears dominant in the process which moistens the atmosphere prior to deep convection. With the assistance of OAR, scientists at the National Institute for Research in the Amazon in Manaus, Brazil (Instituto Nacional de Pesquisas da Amazônia, INPA) established a continuously operating GPS station at INPA for the purpose of monitoring atmospheric water vapor variability at high temporal resolution over an extended period for the first time in an equatorial region.

The observations were transmitted to the Forecast Applications Branch (FAB) of ESRL's Global Systems Division, where they were processed in near-real time, along with all other GPS-Met observations in the U.S. and around the world. The water vapor values and ancillary meteorological observations were then provided to Dr. David Adams, the Principal Investigator and lead author in this study, and his students for analysis. Throughout the study, FAB scientists participated in the interpretation of these observations and writing the paper, as well as providing technical and scientific advice and training to Dr. Adams and his associates.

Examination of the GPS-Met data revealed that the convective process actually occurs in two distinct steps in the Amazon. The first is a period that starts on average about 12 hours prior to precipitation when localized evaporation dominates over water vapor convergence, and the second (i.e., the shallow-to-deep convective transition) starts about four hours prior to precipitation. Each has a time scale that is manifested in the GPS precipitable water vapor time series. The question is whether the "water vapor convergence time scale" introduced in the GRL publication has a counterpart at mid-latitudes, and if so, can this information be exploited to improve the lead time and reliability of NOAA's goal of warn on forecast?

Reference: Adams, D.K. (Centro de Ciencias de la Atmósfera, UNAM, Mexico), S.I. Gutman, K.L. Holub, and D.S. Pereira, 2013: GNSS observations of deep convective time scales in the Amazon. Geophys. Res. Lett., 40, 1-6, doi:10.1002/grl.50573.

Contact information
Name: Seth I Gutman
Tel: 303-497-7031