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September 2006 Abstracts

A Climatology of Diurnal and Semidiurnal Surface Wind Variations over the Tropical Pacific Ocean based on the Tropical Atmosphere Ocean Moored Buoy Array

Figure. Hourly wind measurements from 51 moored buoys in the Tropical Atmosphere Ocean array (9°N-8°S, 165°E-95°W) during 1993-2004 are used to document the seasonal and annual mean diurnal and semidiurnal harmonics of surface wind variability over the tropical Pacific Ocean. Averaged over the buoy array, the semidiurnal harmonic is approximately twice as large as the diurnal harmonic for the zonal wind component, while the diurnal harmonic is nearly three times as large as the semidiurnal harmonic for the meridional wind component. The semidiurnal zonal wind harmonic exhibits near-uniform amplitude (0.15 m s-1, except in the eastern equatorial Pacific) and phase (maximum westerlies ~0325/1525 local time) across the basin in all seasons, and is well explained by atmospheric thermal tidal theory. The suppression of the semidiurnal zonal wind signal over the cold surface waters of the eastern equatorial Pacific is associated with enhanced stability of the planetary boundary layer. The diurnal meridional wind harmonic over the western Pacific (165°E-155°W) exhibits a mean amplitude of 0.20 m s-1 and an approximately 12-hour phase difference across the equator which induces a diurnal cycle in equatorial surface wind divergence. The maximum equatorial divergence (convergence) at ~0600 (~1800) local time is coincident with enhanced convergence into (divergence out of) the intertropical convergence zone to the north. In the eastern Pacific (125°-95°W), the diurnal meridional wind cycle exhibits a systematic southward phase progression from 5°N to 8°S that may be associated with gravity waves propagating from diurnally pulsating convection in the intertropical convergence zone.

Corresponding Author:Rei Ueyama
University of Washington, Department of Atmospheric Sciences, Seattle, WA

Authored by Clara Deser
National Center for Atmospheric Research, P.O. Box 3000, Boulder CO 80307

Submitted to Journal of Climate, September 1, 2006

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Atmospheric reanalyses: Results from a workshop highlighting future needs
ECMWF, 19-22 June 2006

Over the past decade, reanalyses of multi-decadal series of past observations, produced using modern versions of the data assimilation systems developed for numerical weather prediction (NWP), have become established as an important and widely utilised resource for the study of atmospheric and oceanic processes and predictability. Reanalyses are also being used increasingly in a wide range of applications that require a record of the state of either the atmosphere or its underlying land and ocean surfaces. Whilst high-resolution operational NWP systems continue to provide good quality analyses for timely study of recent conditions, including climatic extremes, changes made to improve the operational systems introduce inhomogeneities in time series of operational analyses that limit their utility for studies of longer-term climate variability and change. Lower-resolution reanalyses produced using an up-to-date data assimilation system provide products for all but the last few years that are generally superior to those available from the archives of past operational products. The reanalysis products are by design more suitable than their operational counterparts for use in studies of longer-term variability in climate, although they remain susceptible to changes in the observing system that can make accurate depiction of long-term trends problematic.

Authored by Adrian Simmons (corresponding author), Kevin E Trenberth and Sakari Uppala
National Center for Atmospheric Research, email: adrian.simmons@ecmwf.int
Submitted to EOS, September 22, 2006

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Hydroclimatic Trends in the Mississippi River Basin from 1948-2004

The trends of the surface water and energy budget components in the Mississippi River basin from 1948 to 2004 are investigated using a combination of hydrometeorological observations and observation-constrained simulations of the land surface conditions using the latest version of the Community Land Model version 3 (CLM3). The atmospheric forcing data for the CLM3 were constructed by adding the intra-monthly variations from the 6-hourly National Center for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis to observation-based analyses of monthly precipitation, surface air temperature and cloud cover. Our model-based analysis suggests that for the surface water budget, the observed increase in basin-averaged precipitation is compensated by increases in both runoff and evapotranspiration. For the surface energy budget, the decrease of net shortwave radiation associated with observed increases in cloudiness is compensated by decreases in both net longwave radiation and sensible heat flux, while the latent heat flux increases in association with wetter soil conditions. Both the simulated surface water and energy budgets support the view that evapotranspiration has increased in the Mississippi River basin from 1948-2004. Sensitivity experiments show that the precipitation change dominates the evapotranspiration trend, while the temperature and solar radiation changes have only small effects. Large spatial variations within the Mississippi River basin and the contiguous United States are also found. However, the increased evapotranspiration is ubiquitous despite spatial variations in hydrometeorology.

Figure. Basin-averaged trends in the water and energy budget components for the Mississippi River basin. M is the long-term (1948-2004) annual (water-year) mean (in mm for water components and W m for energy components) and b is the annual linear trend during 1948-2004 (in mm century for water components and W m century for energy components, proportional to arrow shaft width). Note that the downward arrow means that the flux increases the trend of dW/dt or G.

Authored by Taotao Qian, Aiguo Dai, and Kevin E. Trenberth
National Center for Atmospheric Research
Corresponding Author: Dr. Taotao Qian NCAR/CGD, P.O. Box 3000 Boulder, Colorado 80307
Email: tqian@ucar.edu, Phone: 303-497-1325
Submitted to Journal of Climate, September 8, 2006, Revised January 17, 2007