Sensitivity to precipitable water content and profile Sensitivity to precipitable water content and profile

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

CT25K

samson.hagos@pnnl.gov, 509-375-7395Regional and Global Climate Modeling Program

Resolution dependence of sub-tropical westerlies. Resolution dependence of sub-tropical westerlies. IntroductionAtmospheric 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 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.

An AR event simulated by the MPAS-A atmospheric model. Vertically integrated precipitable water content is shown.

Objectives

Definition of ARs

Frequency of AR events

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.

Increasing the model resolution decreases the vertically

integrated PW but it increases the fraction below 800hPa.

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.

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;

As resolution increases, the

sub-tropical westerlies shift

poleward.

This effect has been observed

in other dycores as well

(Williamson 2008, Tellus).

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,

This pole-ward shift explains most of the resolution

dependence of the frequency of AR events.

The role of tropical diabatic heating variabilityThe role of tropical diabatic heating variability

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.

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.

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.