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---
layout: default
title: Report Series
---
<head>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
<meta name="Author" content="Anna McCravy">
<meta name="GENERATOR" content="Mozilla/4.51 [en] (WinNT; U) [Netscape]">
<title>rpt28</title>
</head>
<body text="#000000" bgcolor="#FFFFFF" link="#0000FF" vlink="#0000FF" alink="#FF0000">
<h1>Report 28: Interannual Tropical Rainfall Variability in
General Circulation Model Simulations Associated with the Atmospheric Model
Intercomparison Project </h1>
<ul><b>Sperber,</b> K. R. and T. N. Palmer
<br>November 1995, 79 pp.
<hr><font size=-1>The interannual variability and Reproducibility of rainfall
over the Indian subcontinent, the Sahel and the Nordeste region of Brazil
have been evaluated from the suite of AMIP simulations. Rainfall variations
over the Nordeste region are the most readily captured owing to the intimate
link with Pacific and Atlantic SSTs. The precipitation variations over
the Indian subcontinent and the Sahel are relatively less well captured
by the models respectively. Additionally, an Indian monsoon wind shear
index was calculated for each model. The models are generally more adept
at simulating the variability of the wind shear index than that associated
with the rainfall over this region, indicating that the models exhibit
greater fidelity at capturing the large-scale dynamical fluctuations. For
each region improved skill scores and enhanced Reproducibility result for
those models that qualitatively simulate the observed rainfall/SST correlation
pattern which is dominated by an ENSO teleconnection in the Pacific Ocean.
Accordingly for this subset of the models, the enhancement in skill and
Reproducibility occurred mainly during years of strong El Niño and
La Niña conditions.</font>
<p><font size=-1>A suite of six ECMWF AMIP runs (differing only in their
initial conditions) have also been examined. The Indian monsoon rainfall
exhibits a consistent response during 1987 and 1988, while during other
years differences are simply not very predictable, possibly because of
internal chaotic dynamics that are associated with intraseasonal monsoon
fluctuations and/or land surface process interactions. In this case the
Reproducibility is poor indicating that the average intramodel spread is
greater than the temporal variability of the ensemble mean. For the Sahel
and the Nordeste the Reproducibility increases to 2.4 and 9.0 respectively
indicating a robust response to the boundary conditions for this model.</font>
<p><font size=-1>Five SUNY/NCAR Genesis low resolution initial condition
realizations were also examined. In this model the Nordeste rainfall also
exhibited the largest Reproducibility, however for all regions the Reproducibility
was smaller than that for the ECMWF model. The relationship of the all-India
and Sahel rainfall/SST teleconnections with horizontal resolution, convective
closure and numerics has been evaluated. Models with resolution >=T42 performed
more poorly than lower resolution models. The higher resolution models
were predominantly spectral. At low resolution, spectral vs. gridpoint
numerics performed with nearly equal verisimilitude. At low resolution,
moisture convergence closure was slightly preferable to other closure techniques.
At high resolution, the models that utilized a moisture convergence criteria
performed very poorly suggesting that moisture convergence may be inappropriate
for models with horizontal resolution >=T42. <a href="pdf/28.pdf">(pdf
file)</a></font></ul>
<p><font size=-1>UCRL-MI-123395</font></p>