Tuesday, 13 January 2009: 8:45 AM
Dynamically-downscaled global reanalyses to resolve mesoscale variability in precipitation statistics and their association with large scale circulations
Room 127BC (Phoenix Convention Center)
A 21-year period of the NNRP2 reanalyses has been dynamically downscaled to 40 km resolution and hourly output rate to assess intra-seasonal to inter-annual variations in mesoscale variability in low-level winds and precipitation. The dynamical downscaling is performed using a modified version of NCAR/RAL's RTFDDA system run in hind-cast mode. At low levels, the analyses are steered toward the surface observations while higher up radiosonde data constrains the analyses. The nudging is performed using Newtonian relaxation which is accomplished by using a set of height- and time-dependent nudging terms in the prognostic equations for temperature, moisture and wind. In addition, above 3 km, the simulations are nudged toward the NNRP2 large-scale pattern (known as “analysis nudging”). The magnitude and vertical distribution of the analysis nudging coefficients were chosen by performing a sensitivity study aimed toward preserving spectral energy at smaller scales while keeping the large-scale analyses close to those in the NNRP2 reanalyses. The goal of this approach was to preserve the large-scale pattern provided by the NNRP2 reanalysis while allowing the dynamically downscaling to “spin-up” mesoscale structures. It will be shown that inter-annual variability in precipitation, such as variations in the precipitation pattern associated with ENSO is well captured. At the same time, extreme events, such as tropical cyclones (and associated rainbands) are also reproduced in the analyses. The statistical representation of precipitation in disparate regions of the globe will be assessed using GPCP and CMORPH datasets over their period of record. The presentation will focus on the inter-annual variability of extreme variations that have occurred between 1985 and 2005. This new reanalysis dataset was developed for used by resource planners (e.g., outdoor recreation planning, construction, civil engineering, hazardous release planning, etc) or also as initial conditions for finer scale reanalyses – for example to study the climatology of precipitation event for a particular catchment.