Diurnal precipitation variability over the Gulf Stream and the Kuroshio

Shoshiro Minobe and Shogo Takabeyashi(Hokkaido University, Sapporo, Japan)

Outline Introduction Global View Gulf Stream region Kuroshio region Summary, schematics & discussion

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Introduction 1/3

• A challenge of air-sea interaction studies is to understand how high-frequency variability (weather) interact with low-frequency phenomena (climate).

• A number of studies are conducted for synoptic disturbances.

• Today, I would like to show that another interesting high-frequency phenomenon, diurnal variability, plays important role in air-sea interaction over WBCs …

• as implicitly mentioned by Minobe et al. (2008).

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Introduction 2/3

• They showed

Minobe et al. (2008 Nature)

Occurrence frequency of daytime satellite-derived OLR levels lower than 160 W m-

2 (colour).

This implies that the Gulf Stream’ influence is not clear in nighttime.

So, diurnal variability should occur.

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Introduction 3/3

Kikuchi and Wang (2007)

Actually, diurnal variability of precipitations are seen in previous studies, but they focus their attentions on tropics and did not mention about mid-latitude air-sea interaction nor WBCs.

So, we explorer how diurnal cycles are related to WBCs.

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Data

• Satellite precipitation products– GSMaP MVK v. 5 as the main precipitation data

• 0.1x0.1, hourly, March 2000 to December 2010• Microwave + IR satellite (movement vector & brightness) with Kalman filter

– TRMM 3B42 v7 for check• 0.25x0.25, 3 hourly, analysis period is 2000-2010• Microwave (at available points) + IR (otherwise) temporal & spatial

heterogeneity • OLR for high clouds

– NASA/GEWEX Surface Radiation Budget (SRB) project ver. 3.1, – 1x1, 3 hourly– analysis period is 2000-2007 (data end at 2007 Dec.)– Occurrence rate of OLR<160 W/m2 as occurrence frequency of high

clouds as Minobe et al. (2008)

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Methods

• Harmonic analysis of diurnal climatology in each month and each season.

• We also define relative amplitude (RA)– RA amp/mean– For a sinusoidal diurnal

climatology, 0<RA<1.– But for a delta-function like

diurnal climatology, RA can be as large as 2.

mean×RA

mean×(1-RA)

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Background information: Precipitation & Evaporation

Global distribution of diurnal precipitation amplitudes (GSMaP-MVK)

• Diurnal precipitation amplitudes are outstanding over the Gulf Stream and the Kuroshio poleward of 25N/S in boreal winter.

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25N

25S

25N

25S

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Global distribution:GSMaP vs TRMM comparison

• GSMaP MVK & TRMM 3B42 show essentially the same structures, but TRMM amplitudes are weaker globally and more fragment in the Kuroshio region.

25N

25S

25N

25S

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Relative amp. & phase, JJA. (GSMaP MVK)

• 50-80% of relative amp. in GS and Kuroshio regions.

• Morning maxima in the GS and Kuroshio regions, as widely known for tropical oceans.

Relative amp.= amp./(average rain rate)

Phase=Time of max. in Local Solar Time (UTC+lon./15deg)

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GULF STREAM REGION

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Diurnal amp. & average, June to August

• Amplitudes are large over the Gulf Stream just before & after the separation from the coast.

• Large amplitudes are shifted westward compared with large average precipitation.

• Strongest in July with north & south poles.

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Relative amplitudes in July

• Relative amplitudes are 50-80% over strong amplitude regions, but high relative amplitudes are not limited to those regions nor over GS.

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72-70W, 35-37N (peak at 4LST)78-76W, 31-33N (peak at 8LST)

Diurnal evolution in July

• Over the Gulf Stream, peaks occur from early to late morning.

• The peak precipitation at the north pole leads the south pole by 4 hours.

• Over land, peak is in late afternoon (out of phase relation is essential?).

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Land & Ocean interaction?

• Lag-correlations of raw hourly precipitations onto area-averaged raw precipitation time series (2x2 box).

Diurnal modulation of correlations is evident.

No land-ocean out-of-phase relation.

A weak land-ocean relation suggests diurnal variations on the land can influence those over the ocean (with 12-18 hour lag), but not vice versa.

(land) (ocean)

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High clouds frequency (OLR<160W/m2)

• Diurnal cycle is evident also in occurrence frequency of high clouds.

• Relative amplitudes are 60-100%, larger than those in precipitations (50-80%). – This means that diurnal cycle is more dominant in high clouds than

in precipitations, – and further suggests convective precipitation is the main body of

diurnal precipitations rather than largescale precipitation.

Amplitude, July Relative Amplitude, July

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KUROSHIO REGION

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Diurnal amp. & average, May to Jul

• Large month to month change.

• June maximum in the East China Sea over the Kuroshio (west of Ryukyu island) and south of Kyushu.

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Relative amplitudes: Kuroshio & GS comparison

• Much larger averages (Baiu-Maiyu rainband) smaller relative amp.

• High relative amplitudes are limited to the Kuroshio in the East China Sea and not found in the adjacent Pacific Ocean.

Kuroshio

GS0.24 0.5

0.20.2

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Diurnal evolution in June

• Precipitations maxima occur south of Kyushu and west of Ryukyu islands with roughly the simultaneous peak time (LST 9-10).

130-131E, 30-31N & 128-129E, 28-29N

Note scale is double of that for the GS.

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High clouds occurrence rate

• The amplitudes are large around Ryukyu islands, with 8-10% similar to those over the Gulf Stream.

• The relative amplitude just above only 60%, again much weaker than that over the Gulf Stream (60-100%).

• High relative amplitudes are also limited to just above the Kuroshio compared with amplitudes, as seen in precipitations.

Amplitude, June Relative Amplitude, June

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SUMMARY & DISCUSSION

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Summary

Gulf Stream region• Diurnal precipitations just

after and before the separation in JJA.

• Early morning (4-8 LST) peaks with NE leading.

• High relative amp (50-80%) not limited to GS

Kuroshio region• Diurnal precipitations in

the East China Sea (not in extension) in June.

• Late morning (9-10 LST) simultaneous peaks,

• Moderate relative amp (40-70%) just over the Kuroshio,

• High cloud occurrence has stronger relative amplitudes than precipitations, indicative of convective precipitation is the main body of diurnal precipitations.

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Schematics

Gulf Stream

Diurnally modulated convections

Gulf Stream region

Kuroshio

Large-scale Baiu-Meiyu rainband

Diurnally modulated convections

Kuroshio region

Diurnal precipitations are not limited to the Gulf Stream, which gives stronger mean precipitations and thus diurnal amplitudes.

Diurnal precipitations are limited to the Kuroshio, and are embedded in large-scale Baiu-Meiyu rainband, which does not have strong diurnal cycles.

NESW phase propagation

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Discussion 1: relation to deep heating mode.

• Seasonal and spatial distribution of diurnal precipitations and high clouds generally coincide with distribution of deep heating atmospheric response mode, proposed by Minobe et al. (2010).

• The deep heating mode is prominent in summer, and is similar to tropical atmospheric variations. The other shallow heating mode is strong in winter.

• Sasaki Minobe et al. (2012) showed that deep heating mode in this region is the strongest in June, the middle of Baiu-Meiyu season.

• Consequently, diurnal modulation of deep convection associated with SST fronts is an essential aspect of the deep heating mode.

TotalConvective latentLargescale latentSensible

upward winds at 300 hPa, JJA

Heating rate, JJA

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Discussion 2: mechanism 1/2, diurnal SST?

• Diurnal SST variability may not be important, as atmospheric models (IFS & NICAM) forced with SST without diurnal cycle are good. – Dirmeyer et al. (2012) mentioned oceanic diurnal precipitations off

Carolinas, but never link them with oceanic SSTs nor currents.

Dirmeyer et al (2012)

Ocean masked for RA<0.5

phase amp

mm/d

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Discussion 2: mechanism 2/2

• There are a number of hypotheses for diurnal precipitations over the (tropical) oceans (see review by Young and Smith, 2006). Maybe the following two mechanisms can be important over WBCs– Static radiation convection: enhanced nighttime cloud-top radiative

cooling thermally destabilizes the upper cloud, thus increasing convection and precipitation. High SST over WBCs modulate the stability.

– Static radiation convection with enhanced moistening: radiative cooling for disturbed convective conditions increases humidities, thus amplifying condensation and precipitation. WBCs provide larger humidity.

• To identify the mechanism, numerical model analysis should be useful.

• Probably we can improve of atmospheric models by understanding the difference between good (NICAM, IFM) and not good (AFES, MRI-NHM) models.

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Final words

• Diurnal modulation of deep convections connects …– the ocean and the atmosphere, – and also weather and climate,

• and provide new possibilities of studies for air-sea interaction over WBCs.