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Current-climate downscaling at the National Center for Atmospheric
Research (NCAR)
Thomas Warner, Scott Swerdlin, Yubao Liu and Daran Rife
National Center for Atmospheric Research, Boulder, Colorado, USA; warner@ucar.edu
1. Introduction
Mesoscale analyses of current climates can be used
for many purposes, including optimal siting of
wind-energy farms and airports, calculating the
most probable direction of the transport of
hazardous material at some future date and time,
and scheduling the time and season for events that
require specific meteorological conditions. To
construct such climatologies for the many areas of
the world where there are few routine fourdimensional
(4D) observations of the atmosphere,
NCAR has developed a Climate Four-Dimensional
Data Assimilation (CFDDA) system. The CFDDA
system uses the Penn State University – National
Center for Atmospheric Research Mesoscale Model
Version 5 (MM5), and the Weather Research and
Forecasting (WRF) model, wherein Newtonianrelaxation
terms in the prognostic equations
continually nudge the model solution toward
surface, radiosonde, aircraft, and satellite-based
observations. This is one of the few current-climate
downscaling systems that relaxes the model solution
to observations rather than to gridded analyses of
observations.
The CFDDA system is able to generate a 4D
description of the diurnal and seasonal evolution of
regional atmospheric processes, with a focus on the
boundary layer. Unlike point measurements, the
gridded fields define coherent multi-dimensional
realizations of complete physical systems. Not only
does the CFDDA system define mean values of
variables as a function of season and time of day,
extremes are also estimated, and example days are
produced. See Hahmann et al. (2007) for an early
application of CFDDA.
2. Example applications of CFDDA
The specific application of CFDDA determines the
configuration of the model. For limited-area
applications, lateral-boundary conditions are defined
from the NCEP-NCAR or NCEP-DOE reanalyses.
For situations where high-resolution analyses are
required globally, a global version of WRF or MM5
is used.
As an example of a limited-area application of
CFDDA, Fig. 1 shows a map of the probability that
30-m above-ground-level (AGL) winds will exceed
10 m s -1 in the month of February in southern
Europe, where such an analysis would be valuable
for wind-energy prospecting. These statistics are
based on a 20-year downscaling from the NCEP-
NCAR Reanalysis Project (NNRP) global data set.
Figure 1. CFDDA-generated map of the
probability that the 30-m AGL winds will exceed 10
m s -1 in the month of February, for southern Europe
(based on a 20 year data-assimilation period).
In a global application of CFDDA, a 21-year
reanalysis has been produced, with hourly output,
using a 40-km grid increment. A composite-mesh
technique was used with MM5, wherein twin polar
stereographic grids, one centered over each pole, are
integrated separately, then joined to form a seamless
global analysis. The solutions are constrained by
both the driving NCEP-DOE Reanalysis (an
improved version of the NNRP) and the assimilated
surface and radiosonde observations. The largescale
climates of CFDDA and the NCEP-DOE
Reanalysis remain synchronized, while the
mesoscale model and observations simultaneously
define the small-scale features. Figure 2 shows
example output from the model, and illustrates the
ability of CFDDA to represent complex atmospheric
structures across a spectrum of scales, as well as
climate extremes. The infrared satellite image is
shown with the CFDDA analysis for 0230 UTC 5
January 2001, when tropical cyclone Ando was
active over the Indian Ocean. The CFDDA-analyzed
cyclone has a remarkable resemblance to that
observed. Also reasonably represented is the diurnal
pattern of moist convection over Africa, as well as
the wave patterns in the higher latitudes. In terms of
cyclone Ando, CFDDA produced a storm whose
structure remained highly similar to that observed
throughout its life cycle, and the simulated storm