Jiangfeng Wei Center for Ocean-Land-Atmosphere Studies Maryland, USA.

Water vapor sources for Yangtze River Valley rainfall: climatology, variability, and

implications for rainfall forecasting

Jiangfeng WeiCenter for Ocean-Land-Atmosphere

StudiesMaryland, USA

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IntroductionYangtze River Valley (YRV) is one of the most

densely populated regions in the world.

Frequent floods caused severe property damage and huge loss of lives.

The decadal precipitation variability over East China is directly related to the changing water vapor transport.

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Vectors: The first EOF of 1979-2010 interannual variation of April-September average vertically integrated moisture flux

Shading: The correlation of the corresponding first principle component time series (PC1) with precipitation at each grid point

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QuestionsWhere does the water vapor for YRV rainfall

come from? What source regions are most important? Can we predict the rainfall using information

on evaporation at the source regions?

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DataThe new NASA reanalysis product Modern

Era Retrospective-analysis for Research and Applications (MERRA) is used.

MERRA was generated with the Goddard Earth Observing System (GEOS-5) atmospheric model and data assimilation system, with a particular emphasis on the hydrologic cycle.

Time range 1979-2010, 2/3°×1/2° horizontal resolution and 72 vertical atmospheric levels.

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Method

For each precipitation event at a grid point, multiple parcels are launched at random humidity-weighed levels.

The parcels are tracked backward in time along the isentropic surfaces until all of its original precipitation is attributed to evaporation but no longer than 15 days.

Quasi-Isentropic Back Trajectory (QIBT) method

Dirmeyer, P. A., and K. L. Brubaker, 1999: Contrasting evaporative moisture sources during the drought of 1988 and the flood of 1993. J. Geophys. Res., 104, 19383–19397.

Dirmeyer, P. A. and K. L. Brubaker, 2007: Characterization of the Global Hydrologic Cycle from a Back-Trajectory Analysis of Atmospheric Water Vapor. J. Hydrometeor., 8, 20–37.

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Arrows: vertically integrated moisture flux

Shading: percentage of evaporative moisture sources supplying rainfall in the box

Major moisture sources vary with time, mostly over land.

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• Most source regions follows the time evolution of rainfall, except the Western Pacific and local YRV.

• Moisture from the Bay of Bengal and Western Pacific shows a seesaw relationship.

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Pentad relationships during June-July 1979-2010

• Recycling ratio is responding passively to the variation of rainfall

• Weak control of local surface condition on rainfall

Color: moisture from YRV that contributes to local rainfall

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Pentad correlation during June-July

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Relationships over Sahel

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EOF1 PC1

Dominant interannual patterns of evaporative sources

Associated time series (PCs) show strong correlations with rainfall

Patterns highlight moisture sources most important for YRV rainfall

Dipole over the Bay of Bengal and Western Pacific (June-September)

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Secondary interannual patterns of evaporative sourcesEOF2 PC2

Increased local recycling after year 2000

Increasing typhoon impact on YRV rainfall

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Interannual correlation between moisture supply from Bay of Bengal and Western Pacific

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Critical evaporative sources for YRV rainfall

Correlation between YRV rainfall and evaporative moisture supply

Percentage of evaporative moisture supply to YRV rainfall

Fraction of local ET that contributes to YRV rainfall

Product of the above three

Product of the first two factors has a good correspondence to EOF1.

Patterns of product are similar to EOF1.

Large correlations (first factor) over ocean, showing the important role of ocean in the circulation.

Results show weak potential of surrounding soil moisture or ET for YRV rainfall forecasts.

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Difference between 6 wettest and 6 driest YRV rainy seasons (June-July)

Arrows: moisture flux

Shading: evaporative moisture supply

Patterns of moisture fluxes and evaporative sources are similar to their EOF1, confirming their dominant impact.

El Niño-like SST pattern and warmer SST over surrounding ocean may be the main reason for the stronger moisture transport to YRV.

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Summary QIBT method can estimate the evaporative moisture sources of

precipitation, which is difficult by only calculating moisture fluxes.

The contributions of major moisture sources and their relative importance show large seasonal variations. The moisture from the Bay of Bengal and the Western Pacific usually compensate each other.

The major direct moisture sources are over YRV and its major moisture transport pathways over land, rather than over the ocean as from the traditional view, but the ocean is important in initiating the moisture transfer.

Over these important land moisture sources, surface evapotranspiration is not controlled by soil wetness and has weak impact on the variability of rainfall. The prediction of YRV rainy season rainfall thus depends more on the knowledge of large-scale circulations and monsoons than land surface conditions.

The local recycling ratio of YRV precipitation and the impact of typhoons both increased after around year 2000.