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title: Report Series

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<h1>Report 31: Seasonal Characteristics of Precipitation over the United

States in AMIP Simulations

<ul><b>Boyle,</b> James S.

<br>February 1996, 30 pp.&nbsp;

<hr><font size=-1>The monthly mean precipitation patterns of the Atmospheric

Model Intercom parison Project (AMIP) decadal simulations over the US and

adjoining oceans are intercompared. A simple harmonic analysis of the 12

month seasonal mean precipitation values and a principal component(PC)

analysis of the 120 monthly values were carried out. Emphasis is placed

on the basic seasonal variation for three subregions, the Eastern US, Central

US and West Coast US. The results indicate the following:</font>

<br>&nbsp;

<ul><font size=-1>There are rather severe problems for almost all the models

in capturing the seasonal variation of the precipitation over the Eastern

US. The models typically overemphasize the Summer / Spring rainfall amounts.

The PC analysis indicates that many of the models tend to extend the precipitation

regime typical of the Central US too far to the east, resulting in a precipitation

maxima occurring in the summer for the Eastern region.</font>

<p><font size=-1>The seasonal variation of the west coast is handled with

the greatest fidelity. This result cuts across all the models and may be

attributable to the fact the SST forcing is specified and common to all

the simulations. The common SST forcing is apparently a dominant factor

in determining this region's precipitation climatology.</font>

<p><font size=-1>On the space scales of the regions selected, there is

little consistent evidence that points to any specific model feature as

a predictor of model performance. None of the obvious candidates such as

horizontal resolution, convective closure schemes or land surface schemes

are reliable discriminators of a model's ability to simulate precipitation.</font>

<p><font size=-1>For one smaller sub-region centered over Arizona, chosen

because of the dominance of the semiannual cycle, there is evidence that

increased horizontal resolution has an effect. For this intermountain region

the higher resolution models as a whole do better than the low resolution

models. However, even in this case there is enough variation amongst the

individual simulations as to obscure the conclusion that increased horizontal

resolution is a necessary or sufficient quality to produce a reliable simulation.</font>

<p><font size=-1>The models tend to have less interannual variation than

the observations with more variance being explained by the leading ( annual

cycle ) PC, while the observations have a less peaked spectrum.</font>

<p><font size=-1>The models consistently overestimate the precipitation

in the spring and early summer in all regions. This might indicate a common

failing of all the convective schemes in dealing with extratropical convective

instability that is endemic to this time of year.</font>

<br>&nbsp;</ul>

<font size=-1>It would appear that the models of the generation represented

by the AMIP in tegrations would not be suitable for direct coupling to

a watershed disaggregation scheme even on a seasonal basis. The results

indicate that there is substantial uncertainty in the distribution of precipitation

throughout the year as simulated by most of these models. <a href="pdf/31.pdf">(pdf

file)</a></font>

<p><font size=-1>UCRL-MI-123395</font></p>