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Ken Takahashi, Ph. D.
Thermotidal and land-heating forcing of the diurnal cycle of oceanic surface winds in the eastern tropical Pacific*
Reunión LMI DISCOH, 29 de marzo, 2012IMARPE, Callao
* Geophysical Research Letters, 39, L04805, doi:10.1029/2011GL050692, 2012
“Upsidence wave”: Diurnal cycle in vertical velocity(wave forced by heating of the Andes, Garreaud & Muñoz, 2004)
Rahn & Garreaud, 2010
Diurnal cycle in surface winds in
the tropical Pacific
Data: TAO/TRITONBased in diurnal harmonic
Ueyama & Deser, 2008
01 Local time
04 Local time
07 Local time
10 Local time
13 Local time
Linear theory of the sea breeze(Rotunno, 1983)
|Latitude|> 30° : Coastal trapping< 30° : Wave propagation=30° : Singular
MM5 v3 simulation (Δx=120 km)(Gayno-Seaman, Grell, CCM2)BC: NNRP Oct. 2008 mean conditions, fixed SST
Surface wind diurnal variation(6 am minus 6 pm, local time, m/s)
MM5 v3
Takahashi, 2012
Simulated diurnal cycle in surface
wind and sea level pressure (SLP) (in local time)
MM5 v3 Δx=120 km(GS, Grell, CCM2)
Takahashi, 2012
Migratory diurnal thermal
tide
Observational estimate
Lieberman & Leovy, 1995
Model with tropospheric solar absorption
Tidal component in SLP (6 UTC)
SOLAR RADIATIONTERRESTRIAL RADIATION
Hartmann 1994High absorptionAtm
osph
eric
abs
orpti
on (%
) Visible Near IR Infrared (IR)
Atmospheric absorption
Absorción atmosféricaG
ases
inve
rnad
ero
Radiación solar Radiación terrestre (infrarrojo)
Hartmann, 1994
H2O
CO2
O2, O3
CH4
N2O
El vapor de agua es el responsable principal de la absorción de radiación solar en la atmósfera
ExperimentsControl
Diurnal land heating suppressed*
Absorption of solar radiation (near-IR) by atmospheric water vapor suppressed**
Modeled diurnal amplitude of• Land surface temperature• SLP
*Land-slab layer heat capacity and relaxation time-scale multiplied by 100.** Corresponding absorption coefficients set to zero in radiation code.
b) + c)
Takahashi, 2012
Modeled diurnal variations in SLP and surface wind
Control Thermal tide Extended sea breeze
x
x
x
Solar “speed” = 462 m/s
~60
m/s
Takahashi, 2012
Zonal mean thermal tide
Colors:Temperature (°C)Contours: Pressure (hPa)Vectors: (v,w) (m/s, cm/s)
MM5 v3 Δx=120 km(GS, Grell, CCM2)
Takahashi, 2012
Low pressure
Radiative air heating
West
East
Sun
Equatorward surface wind
High pressureNot to scale
Migratory atmospheric thermal tide
Diurnal cycle off Ica (central-southern coast of Peru)7 pm LT 11 pm 3 am
7 am 11 am 3 pmMaximumWind (m/s) at
40 m above surface
Takahashi et al, in preparation
Diurnal anomalies7 pm LT 11 pm 3 am
7 am 11 am 3 pm
Wind (m/s) at 40 m above surface
Takahashi et al, in preparation
Conclusions
• The diurnal cycle of surface winds has a substantial large-scale contribution (not only “sea breeze”)
• Atmospheric shortwave absorption by water vapor is an important forcing of this diurnal cycle (thermal tides)
• Diurnal land-heating produces an important contribution to the diurnal cycle within 2000 km from the coast.