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Controls of Tropical Pacific Rainfall and SST Bases in Global Climate Models.

紀錄類型:	書目-電子資源 : Monograph/item
正題名/作者:	Controls of Tropical Pacific Rainfall and SST Bases in Global Climate Models./
作者:	Woelfle, Matthew D.
出版者:	Ann Arbor : ProQuest Dissertations & Theses, : 2018,
面頁冊數:	131 p.
附註:	Source: Dissertations Abstracts International, Volume: 80-04, Section: B.
Contained By:	Dissertations Abstracts International80-04B.
標題:	Physical oceanography. -
電子資源:	http://pqdd.sinica.edu.tw/twdaoapp/servlet/advanced?query=10932727
ISBN:	9780438525047

Controls of Tropical Pacific Rainfall and SST Bases in Global Climate Models.
Woelfle, Matthew D.

Controls of Tropical Pacific Rainfall and SST Bases in Global Climate Models. - Ann Arbor : ProQuest Dissertations & Theses, 2018 - 131 p.

Source: Dissertations Abstracts International, Volume: 80-04, Section: B.

Thesis (Ph.D.)--University of Washington, 2018.

This item must not be added to any third party search indexes.

Despite continual improvements in both model physics and resolution, accurate simulation of the tropical Pacific mean state has been an elusive goal for many coupled global climate models (GCMs) for decades. Because temperature and pressure gradients are weak in the deep tropics, small errors in simulating these fields can lead to large circulation biases. Any initial bias may be further amplified through feedbacks involving ocean circulations, cloud radiative forcings, and surface turbulent heat or momentum fluxes. The interrelated nature of these processes in the deep tropics has long complicated our ability to understand and correct the initial source of a given model bias. In this dissertation, we examine two persistent tropical biases within the National Center for Atmospheric Research's Community Earth System Model (CESM): the Pacific cold tongue bias and the double-Intertropical Convergence Zone (ITCZ) bias. The Pacific cold tongue bias refers to the tendency for coupled GCMs to simulate sea surface temperatures (SSTs) which are too cold along the equator in the tropical Pacific. The double-ITCZ bias refers the tendency for coupled GCMs to simulate too much precipitation in a zonal band south of the equator. In Chapter 2, a series of six-month coupled hindcasts show the strength of the rapidly developing Pacific cold tongue bias in CESM version 1 (CESM1) to be sensitive to the convective parameterization employed. In the standard configuration of the model, too strong equatorial surface easterlies drive cooling of up to 1 K in the first two months of coupled integration. In a simulation wherein the deep convective parameterization is disabled, the cold tongue bias intensifies due to an increase in the zonal pressure gradient and associated easterlies. Superparameterized hindcasts show improvements in the cold tongue bias and reduced surface easterlies despite an increase in the zonal pressure gradient. The superparameterized model neglects convective momentum fluxes as the embedded two dimensional cloud resolving models are unable to accurately simulate turbulent momentum flux tendencies. Thus, rather than increasing near-surface wind speeds, the increased zonal pressure gradient drives anomalously strong easterly jet at 1-1.5 km elevation as surface drag effects are incorrectly concentrated in the lowest model levels. A series of sensitivity tests confirm the role of shallow convective momentum transport in determining the low-level zonal wind shear. The simulations presented in this chapter suggest shallow convective momentum fluxes may be an underappreciated mechanism for controlling both the equatorial cold tongue strength and the relationship between the large scale surface pressure gradient and surface easterlies. Despite differences in central Pacific SST of nearly 2 K across these hindcasts, the double-ITCZ bias persists in all model configurations. While the double-ITCZ bias is robust across all simulations presented in Chapter 2, the simulations presented in Chapter 3 show the east Pacific manifestation of the double-ITCZ bias to be greatly improved in the newest version of CESM: CESM version 2 (CESM2). In Chapter 3, we examine the state of the double-ITCZ bias across ten versions of CESM created as part of the development process for CESM2. In CESM1, a warm SSTs bias in the southeast Pacific forces zonal and meridional surface pressure gradients that are favorable for increased convergence and convection in this region. This SST bias is reduced in CESM2 due to an increase in overlying low cloud fraction and a corresponding strengthening of the shortwave cloud forcing (SWCF). Between two model versions with similar configurations but differing ITCZ bias strengths, this cloud change is driven by the removal of the dependence of liquid autoconversion and accretion rates on cloud water variance and by the removal of a secondary condensation scheme. These changes reduce the drizzle production rate in the low liquid clouds of the southeast Pacific which in turn delays their breakup and dissipation. As a result, cloud fraction and SWCF increase to more realistic values in the stratocumulus to trade cumulus transition region. The improvements in SWCF and the double-ITCZ bias persist through subsequent modifications to the liquid microphysics parameterizations. Despite the local improvement in the east Pacific rainfall climatology, neither the Pacific cold tongue bias nor global measures of the double-ITCZ bias show a consistent improvement across the model development process from CESM1 to CESM2.

ISBN: 9780438525047Subjects--Topical Terms:

3168433
Physical oceanography.

Controls of Tropical Pacific Rainfall and SST Bases in Global Climate Models.
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300 $a 131 p.
500 $a Source: Dissertations Abstracts International, Volume: 80-04, Section: B.
500 $a Publisher info.: Dissertation/Thesis.
500 $a Advisor: Bretherton, Christopher S.
502 $a Thesis (Ph.D.)--University of Washington, 2018.
506 $a This item must not be added to any third party search indexes.
506 $a This item must not be sold to any third party vendors.
520 $a Despite continual improvements in both model physics and resolution, accurate simulation of the tropical Pacific mean state has been an elusive goal for many coupled global climate models (GCMs) for decades. Because temperature and pressure gradients are weak in the deep tropics, small errors in simulating these fields can lead to large circulation biases. Any initial bias may be further amplified through feedbacks involving ocean circulations, cloud radiative forcings, and surface turbulent heat or momentum fluxes. The interrelated nature of these processes in the deep tropics has long complicated our ability to understand and correct the initial source of a given model bias. In this dissertation, we examine two persistent tropical biases within the National Center for Atmospheric Research's Community Earth System Model (CESM): the Pacific cold tongue bias and the double-Intertropical Convergence Zone (ITCZ) bias. The Pacific cold tongue bias refers to the tendency for coupled GCMs to simulate sea surface temperatures (SSTs) which are too cold along the equator in the tropical Pacific. The double-ITCZ bias refers the tendency for coupled GCMs to simulate too much precipitation in a zonal band south of the equator. In Chapter 2, a series of six-month coupled hindcasts show the strength of the rapidly developing Pacific cold tongue bias in CESM version 1 (CESM1) to be sensitive to the convective parameterization employed. In the standard configuration of the model, too strong equatorial surface easterlies drive cooling of up to 1 K in the first two months of coupled integration. In a simulation wherein the deep convective parameterization is disabled, the cold tongue bias intensifies due to an increase in the zonal pressure gradient and associated easterlies. Superparameterized hindcasts show improvements in the cold tongue bias and reduced surface easterlies despite an increase in the zonal pressure gradient. The superparameterized model neglects convective momentum fluxes as the embedded two dimensional cloud resolving models are unable to accurately simulate turbulent momentum flux tendencies. Thus, rather than increasing near-surface wind speeds, the increased zonal pressure gradient drives anomalously strong easterly jet at 1-1.5 km elevation as surface drag effects are incorrectly concentrated in the lowest model levels. A series of sensitivity tests confirm the role of shallow convective momentum transport in determining the low-level zonal wind shear. The simulations presented in this chapter suggest shallow convective momentum fluxes may be an underappreciated mechanism for controlling both the equatorial cold tongue strength and the relationship between the large scale surface pressure gradient and surface easterlies. Despite differences in central Pacific SST of nearly 2 K across these hindcasts, the double-ITCZ bias persists in all model configurations. While the double-ITCZ bias is robust across all simulations presented in Chapter 2, the simulations presented in Chapter 3 show the east Pacific manifestation of the double-ITCZ bias to be greatly improved in the newest version of CESM: CESM version 2 (CESM2). In Chapter 3, we examine the state of the double-ITCZ bias across ten versions of CESM created as part of the development process for CESM2. In CESM1, a warm SSTs bias in the southeast Pacific forces zonal and meridional surface pressure gradients that are favorable for increased convergence and convection in this region. This SST bias is reduced in CESM2 due to an increase in overlying low cloud fraction and a corresponding strengthening of the shortwave cloud forcing (SWCF). Between two model versions with similar configurations but differing ITCZ bias strengths, this cloud change is driven by the removal of the dependence of liquid autoconversion and accretion rates on cloud water variance and by the removal of a secondary condensation scheme. These changes reduce the drizzle production rate in the low liquid clouds of the southeast Pacific which in turn delays their breakup and dissipation. As a result, cloud fraction and SWCF increase to more realistic values in the stratocumulus to trade cumulus transition region. The improvements in SWCF and the double-ITCZ bias persist through subsequent modifications to the liquid microphysics parameterizations. Despite the local improvement in the east Pacific rainfall climatology, neither the Pacific cold tongue bias nor global measures of the double-ITCZ bias show a consistent improvement across the model development process from CESM1 to CESM2.
590 $a School code: 0250.
650 4 $a Physical oceanography. $3 3168433
650 4 $a Atmospheric sciences. $3 3168354
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710 2 $a University of Washington. $b Atmospheric Sciences. $3 3174193
773 0 $t Dissertations Abstracts International $g 80-04B.
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