Importance of the Low-level Diabatic Heating on the Formation of the Summertime Subtropical Highs:

Sensitivity to Diabatic Heating Data Set

Takafumi Miyasaka, Hisashi Nakamura

Department of Earth and Planetary Science,University of Tokyo, Japan

SLP (July)

hPa

Marine stratus reduces surface insolation and causes radiative cooling.

Equatorward along-shore wind in the eastern flanks of a subtropical high contribute to maintaining lower SST through

Coastal upwelling, Evaporation, andMixing.

OISST (July)

degree C

Lower SST and subsidencefavor the marine stratus.

Low-level cloud (July) ISCCP

%

Previous works for the formation of the summertime subtropical highs

• Rodwell and Hoskins (2001)emphasized the effect of monsoonal convective heatingthrough dry primitive-equation model forced with regional diabatic heating assigned at all vertical levels.

• Shaffrey et al. (2002)supported the conclusion of Rodwell and Hoskins (2001) throughAGCM forced with modified SST, topography and albedo.(Marine stratus is poorly reproduced in their model.)

• Miyasaka and Nakamura (2005)suggested an importance of low-level land-sea heating-cooling couplet through same model as Rodwell and Hoskins.

• sensible heating over dry continent• radiative cooling due to marine stratus

Miyasaka and Nakamura (2005)

sensible heating over dry continentradiative cooling due to marine stratus

Dry primitive-equation model forced with diabatic heating of NCEP-DOE reanalysis.

Diabatic heating is locally assigned only in the lower troposphere.

Diabatic heating in the lower troposphere (0.667<σ <1)

Diabatic heating (20N-50N)σ

60E 60E120W 0E

W m-2

K day-1

SLP response (Pacific) SLP response (Atlantic)

8 hPa 8 hPa

Subtropical highs are formed mainly by the local low-level land-sea heating-cooling couplets.

Miyasaka and Nakamura (2005)

Control run(global topography and global heating at all-levels)

~57% of CNTL (14hPa)~80% of CNTL (10hPa)SLP response (CNTL)

hPa

Using diabatic heating of reanalysis enables us to get realistic heating distribution of each component of diabatic heating (such as radiative heating and vertically diffusive heating) under relatively realistic cloud distribution.

However, diabatic heating may depend on used model parameterizations.

Low-level cloud in July

NCEP-DOE reanalysis

JRA-25 reanalysis

Observation (ISCCP)

60E 60E180E 0EEq

Eq

Eq

60N

60N

60N

Low-level diabatic heating distribution in 2 reanalyses may be different.

• Objectives

– To confirm conclusion of Miyasaka and Nakamura 2005 (the summertime subtropical highs are formed mainly by local low-level land-sea heating-cooling couplets), similar numerical experiments are performed with another diabatic heating data set, JRA-25 reanalysis.

JRA-25 reanalysis

60E 60E180E 0EEq

60N

Model and dataHoskins and Simmons (1975)

– Hydrostatic global primitive-equation model– T42L30– Simplified physical processes

• Newtonian cooling• Rayleigh friction• Horizontal hyper-diffusion• No moist physics• Diabatic heating of NCEP/DOE reanalysis or JRA-25

reanalysis is applied as thermal forcing.（zonally asymmetric component of July climatology for 1979-1998）

– Fixed zonal mean component (July climatology)• NCEP/DOE reanalysis (1979-1998)

– Response; average for 16~25th day

Zonally asymmetric component of diabatic heating vertically integrated (0<σ <1)

NCEP-DOE reanalysis

JRA-25 reanalysis

W m-2

SLP response in control runs and observed SLP zonal asymmetry

Model response to diabatic heating of NCEP-DOE reanalysis

Observation

Model response to diabatic heating of JRA-25 reanalysis

Subtropical highs can be reproduced realistically also by JRA-25 diabatic heating.

hPa

SLP response: 8 hPa~80% of control run (10 hPa)

SLP response: 6 hPa~67% of control run (9 hPa)

NCEP-DOE JRA-25

Diabatic heating (20N-50N) Diabatic heating (20N-50N)

Diabatic heating (0.667<σ <1)Diabatic heating (0.667<σ <1)

W m-2

K day-1

SLP response8 hPa / 14 hPa~57% of control run

SLP response6 hPa / 10 hPa~60% of control runNCEP-DOE JRA-25

Diabatic heating (20N-50N) Diabatic heating (20N-50N)

Diabatic heating (0.667<σ <1)Diabatic heating (0.667<σ <1)

SLP responses to low-level land-sea heating-cooling couplets

North Pacific North Atlantic

South Pacific

South Atlantic

South Indian Ocean

NCEP-DOE 8 hPa response(~80% of CNTL)

8 hPa(~57%)

4 hPa(~80%)

4 hPa(~80%)

3 hPa(~100%)

JRA-25 6 hPa response(~67% of CNTL)

6 hPa(~60%)

3 hPa(~75%)

3 hPa(~75%)

2 hPa(~50%)

All summertime subtropical highs are formed mainly by the low-level land-sea heating-cooling couplets.

Maritime cooling of JRA-25 and its SLP response is weaker than those of NCEP-DOE, while marine stratus is greater in JRA-25 reanalysis than in NCEP-DOE reanalysis

NCEP-DOE JRA-25Low-level cloud in July

Longitude-σ cross-section in the North Pacific (20N-50N)

150W 110W 150W 110W

σ =1

0.667

Radiative heating

NCEP-DOE JRA-25

σ =1

0.667

Cloud cover

%

K day-1

NCEP-DOE JRA-25

Radiative heating

Vertically diffusiveheating

Vertically diffusive heating in JRA-25 offsets radiative cooling significantly.

Longitude-σ cross-section in the North Pacific (20N-50N)

150W 110W 150W 110W

σ =1

0.667

σ =1

0.667

K day-1

Summary

• It is confirmed that the summertime subtropical highs are formed mainly by local low-level land-sea heating-cooling couplets by using diabatic heating of JRA-25, as well as counterpart of NCEP-DOE reanalysis.

• In quantitatively, however, strength of each component of diabatic heating and SLP responses to them are slightly different between two reanalyses.