Resolution and Dynamical Core Dependence of the Statistics of Atmospheric River events in Community Atmospheric Model (CAM) Samson M. Hagos, L. Ruby Leung, Jian Lu, Qing Yang, Chun Zhao

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Resolution and Dynamical Core Dependence of the
Statistics of Atmospheric River events in
Community Atmospheric Model (CAM)Samson M.
Hagos, L. Ruby Leung, Jian Lu, Qing Yang, Chun
Zhao

• Resolution dependence of sub-tropical westerlies.
  

Introduction
Frequency of AR events
Atmospheric Rivers (ARs) are narrow bands of
intense water vapor transport often found in the
warm sectors of extratropical cyclones. They are
responsible for more than 90 of water vapor
transport to mid-latitudes and cause extreme
precipitation in coastal mountainous regions such
as the West Coast of the US.

• As resolution increases, the sub-tropical
  westerlies shift poleward.
• This effect has been observed in other dycores as
  well (Williamson 2008, Tellus).

• In both real world and aquaplanet simulations AR
  frequency decreases with increasing model
  resolution.
• HOMME dynamical core produces significantly more
  AR events than MPAS-A.

An AR event simulated by the MPAS-A atmospheric
model. Vertically integrated precipitable water
content is shown.
After accounting for the effects of this poleward
shift of the westerlies by allowing the threshold
latitude to depend on the latitude of 0m/s mean
zonal wind at 800hPa,
Sensitivity to precipitable water content and
profile

• This pole-ward shift explains most of the
  resolution dependence of the frequency of AR
  events.

Objectives

• The role of tropical diabatic heating variability

• As a step towards accurate modeling of the
  frequency of these extreme events and its
  response to climate variations including global
  warming, this study aims to
• examine the sensitivity of the frequency of
  these events to dynamical core and resolution of
  a global model in real world and aqua-planet
  frameworks.
• estimate the relative contribution of tropical
  variability to circulations associated with AR
  events.

Precipitation and stream function anomalies
composite is constructed by shifting all the AR
events to the same longitude (120oW), averaging,
and then subtracting the 5-year climatology.

• Increasing the model resolution decreases the
  vertically integrated PW but it increases the
  fraction below 800hPa.

CT25K

• Equatorially centered heating introduces a pair
  of cyclonic circulations to the northeast and
  southeast. Such heating is found to contribute
  about 30 of the stream function associated with
  AR events.

After accounting for the effects of precipitable
water content and its profile by adjusting the
thresholds for vertically integrated PW and its
fraction below 800hPa and recalculating the
frequency of AR events
Definition of ARs

• Summary
• The latitudinal shift of sub-tropical explains
  the decrease in the frequency of AR events with
  the increase in model resolution.
• Equatorially centered heating anomalies
  contribute significantly to circulations
  associated with AR events.

• The resolution dependence of the frequency of AR
  events still exists, but dycore dependence is
  somewhat reduced.
• Therefore, the dependence on resolution is
  primarily of dynamical origin.

samson.hagos_at_pnnl.gov, 509-375-7395
Regional and Global Climate Modeling Program