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  <h2>Fwd: Cloud Feedbacks Paper submitted to JAMES</h2>
  <span style="color:#337799"><b>From</b> <a href="mailto:pblossey@u.washington.edu">
  Peter Blossey</a></span><br />
  <span style="color:#337799"><b>Date</b> August 4, 2008</span><p></p>

Dear CMMAP Low Cloud Feedbacks group,<p></p>

Matt Wyant, Chris Bretherton and I have submitted a paper on our<br />
recent CMMAP work that analyzes the subtropical low cloud response of<br />
SP-CAM to a Cess-style climate perturbation and develops a column<br />
analogue in which the response can be studied using a CRM/LES (or SCM)<br />
in a single-column framework.<p></p>

The paper has been submitted to JAMES and is up on their open<br />
discussion site (JAMES-D) here:<p></p>

<a href="http://www.cmmap.org/ojs/index.php/JAMES-D/article/view/6">
http://www.cmmap.org/ojs/index.php/JAMES-D/article/view/6</a><p></p>

We would welcome your comments, either directly through email or on<br />
the JAMES-D site.<p></p>

Title and abstract is appended below.<p></p>

Peter<p></p>

==============================================<br />
Understanding Subtropical Low Cloud Response to a Warmer Climate in a<br />
Superparameterized Climate Model<br />
Matthew C Wyant, Christopher S Bretherton, Peter N Blossey<p></p>

Abstract<p></p>

The subtropical low cloud response to a climate with SST uniformly<br />
warmed by 2 K is analyzed in the SP-CAM superparameterized climate<br />
model, in which each grid column is replaced by a two-dimensional<br />
cloud-resolving model (CRM). Intriguingly, SP-CAM shows substantial<br />
low cloud increases over the subtropical oceans in the warmer climate.<br />
The paper aims to understand the mechanism for these increases, and to<br />
test their sensitivity to the coarse CRM resolution (4 km horizontal,<br />
30 vertical levels). The approaches presented also apply to other<br />
global climate models or warming scenarios.<br />
The subtropical low cloud increase is analyzed by sorting grid-column<br />
months of the climate model into composite cloud regimes using<br />
percentile ranges of lower tropospheric stability (LTS). LTS is<br />
observed to be well correlated to subtropical low cloud amount and<br />
boundary layer vertical structure. The low cloud increase is<br />
attributed to boundary-layer destabilization due to increased<br />
clear-sky radiative cooling in the warmer climate. This drives more<br />
shallow cumulus convection and a moister boundary layer, inducing<br />
cloud increases and further increasing the radiative cooling.<br />
The SP-CAM resolution sensitivity is tested with a new CRM analogue to<br />
an SP-CAM composite cloud regime. The CRM is run to steady state using<br />
composite advective tendencies, winds, and sea-surface temperature<br />
from SP-CAM control and +2 K climates. A new 'weak temperature<br />
gradient' algorithm based on an idealized form of gravity wave<br />
adjustment is used to adjust vertical motion in the column to keep the<br />
simulated virtual temperature profile consistent with the<br />
corresponding SP-CAM composite profile. Humidity is also slowly<br />
relaxed toward the SP-CAM composite above the boundary layer. With<br />
SP-CAM grid resolution, the CRM shows +2 K low cloud increases similar<br />
to SP-CAM. With fine grid resolution, the CRM-simulated low cloud<br />
fraction and its increase in a warmer climate are much smaller. Hence,<br />
the negative low cloud feedbacks in SP-CAM may be exaggerated by<br />
under-resolution of cloud-topped boundary layers.<p></p>

  <br /><hr />

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