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On the origin of continental precipitation

Geophysical Research Letters

July 2010
37(13)

DOI:10.1029/2010GL043712

Authors:

Luis Gimeno

University of Vigo

Anita Drumond

University of Vigo

Raquel Nieto

University of Vigo

Ricardo M Trigo

University of Lisbon

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About 9 out of 10 liters of water evaporated from the oceans every year precipitates back onto oceans. However, the remaining 10% that get transported to continents play an irreplaceable role feeding the land branch of the hydrological cycle. Here we use an objective 3-D Lagrangian model (FLEXPART) to detect major oceanic moisture source areas and the associated continental regions significantly influenced by each moisture source. Our results reveal a highly asymmetrical supply of oceanic moisture to the continents, with the Northern Atlantic subtropical ocean source impacting the continents considerably more than the large Southern Indian and North Pacific sources. Also, the small Mediterranean Sea and Red Sea basins are important moisture sources for relatively large land areas. The Indian subcontinent receives moisture from six different major oceanic source regions. Future changes in meteorological conditions over the oceanic moisture source regions may have an impact on water availability for many river basins.

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rticl e

On the origin of continental precipitation

Luis Gimeno,

Anita Drumond,

Raquel Nieto,

Ricardo M. Trigo,

2,3

and Andreas Stohl

Received 22 April 2010; revised 2 June 2010; accepted 7 June 2010; published 7 July 2010.

[1] About 9 ou t of 10 liters of water evaporated from the

oceans e very year precipitates back on to oceans. However,

the remaining 10% that get transported to continents play an

irreplaceable role feeding the land branch of the hydrological

cycle. Here we use an objective 3‐D L agrangian model

(FLEXPART) to detect major oceanic moisture source

areas and the associated continental regions significantly

influenced by each moisture source. Our results reveal a

highly asymmetrical sup ply of oceani c moistur e to the

continents, with the Northern Atlantic subtropical ocean

source impacting the continents considerably more than the

large Southern Indian and North Pacific sources. Also, the

small Medite rranean Sea and Red Sea ba sins are important

moisture source s for relatively la rge land areas. The Indian

subcontinent receives moisture from six different major

oceanic source re gions. Future changes in meteorological

conditions over the oceanic moisture source regions may

have an impact on water availability for many river basins.

Citation: Gimeno, L., A. Drumond, R. Nieto, R. M. Trigo, and

A. Stohl (2010), On the origin of continental precipitation, Geophys.

Res. Lett., 37, L13804, doi:10.1029/2010GL043712.

1. Introduction

[2] The global hydrological cycle is supplied annually

with circa 500 000 km

of water evaporated from the

Earth’s surface, with the bulk of this volume evaporating

from the oceanic surface (86%) and only 14% from con-

tinents [Oki, 2005]. The vast majority of the water evapo-

rated from the oceans (90%) precipitates back onto oceans

while the remaining 10% is transported to continents where

it precipitates. About two thirds of the latter are recycled

over the continents and only one‐third runs off directly

to the ocean. Ultimately, despite the continental recycling

component, all water used by available to land ecosystems

and human socio‐economic activities has its origins in the

oceans.

[

3] In this context, the relentless upward trend in tem-

perature observed in recent decades, that is expected to

continue towards a warmer world, may pose an additional

burden on the reliability of moisture sources in the future.

Modeling studies suggest that the high sensitivity to tem-

perature of saturation vapor pressure will result in increases

of evaporation and precipitation leading to an exacerbation

of the water cycle [ Held and Soden, 2006]. The volume of

water evaporating will depend largely on changes of sea and

air temperatures and winds over major moisture source re-

gions and these changes are bound to influence specific

regions over continents.

[

4] Thus, identification of regions particularly vulnerable

to changes in the hydrological cycle requires locating all

oceanic moisture sources and, additionally, to pin down

where exactly water evaporating from these sources pre-

cipitates over land. While major oceanic source sectors have

been relatively well identified recently [Trenberth and

Guillemot, 1998], their contribution towards precipitation

over continental land masses has not been equally well

established.

[

5] Here we used the 3‐D Lagrangian transport model

FLEXPART based on meteorological analysis data and a

moisture tracking scheme to identify where continental

regions are affected by precipitation originating f rom spe-

cific oc eanic regions. Several such methods have recen tly

been dev eloped by Stohl and James [20 04, 2005] (the one

used in our study), Sodemann et al. [2008a, 2008b] and

Dirmeyer an d Brubaker [2007] to diagnose the net water

vapor changes along a la rge number of bac k trajectories to

infer the moisture sources for precipitation falling in a target

region. Recently, these methodshavebeenusedbysome

of us to identify and quantify t he moisture sources i n dif-

ferent climatic regions such as the Sahel [Nieto et al.,2006],

CentralBrazilandLaPlataBasin[Drumond et al.,2008],

the Antarctic [Sodemann and Stohl, 2009] or the Ib erian

Peninsula [Gimeno et al., 2010].

2. Methods

2.1. Method to Identify the Main Moisture Source

Regions

[

6] Moisture source regions are defined as maxima of

vertically integrated moisture flux divergence (i.e., E‐P)

[Trenberth and Guillemot, 1998]. The vertically integrated

moisture transport is defined as 1/g

qvdp,whereg is

the acceleration due to gravity, q is the specific humidity,

is the surface press ure, and v is the horizontal wind

vector. ECMWF re‐analysis (ERA‐40) data on a 2.5° × 2.5°

grid was used to compute f lux dive rgences for th e 44‐year

long period spanning from January 1958 to December 2001.

Figure 1 shows the annual mean vertically integrated

moisture flux divergence field, where values higher than

250 mm/yr are shown in gray scale , and the interv al

between the isolines is 250 mm/yr. The areas inside the red

contour lines indicate the spatial extent of all major mois-

ture sourc e regions used i n the forward integra tions. These

source regions were defined based on the threshold of

750 mm/yr for the oceanic sources (Figure 1, top) and

500 mm/yr for the land sources (Figure 1 (middle) and 1

(bottom)). As a supplement for Fig ure 1 we have inc luded

in the auxiliary material (Figures S1 and S2) global dis-

EPhysLab, Facultade de Ciencias, Universidade de Vigo, Ourense,

Spain.

CGUL, IDL, University of Lisbon, Lisbon, Portugal.

Departamento de Engenharias, Universidade Lusófona, Lisbon,

Portugal.

Norwegian Institute for Air Research, Kjeller, Norway.

0094‐8276/10/2010GL043712

GEOPHYSICAL RESEARCH LETTERS, VOL. 37, L13804, doi:10.1029/2010GL043712, 2010

L13804 1of7

tributions of continental precipitation (Che n et a l., 20 02) fo r

annual and seaso nal basis.

2.2. Lagrangian Approach to Quantify

the Contribution of Each Moisture Source Region

to the Continental Precipitation

[

7] We make use of the metho d developed by Stohl and

James [2004, 2005], which relies on the Lagrangian parti-

cle dispersion model FLEXPART [Stohl et al., 2005]. Using

this model, the atmosphere is divided homogeneously into a

large number of virtual particles which have a constant

mass and then these particles are advected by the model

using three‐dimensional (3‐D) operat ional ECMWF [White,

2002] winds as well as superimposed stochastic turbulent

and convective motions. The particle positions and specific

humidity (q) are recorded every 6 hours. The increases

(evaporation, e) and decreases (precipitation, p)inmoisture

along the trajectory can be calculated from changes in (q)

with time e − p = m

,wherem is the mass of each particle.

Figure 1. Climatological (top) annual, (middle) JJA and (bottom) DJF vertically integrated moisture flux divergence (mm/

yr). Values higher than 250 mm/yr are in gray scale, with an interval between isolines of 250 mm/yr. Areas inside the red

contour lines indicate the regions considered as moisture sources in the forward integrations. The areas were defined based

on the threshold of 750 mm/yr for the oceanic sources (CORALS, Coral Sea; NPAC, North Pacific; SPAC, South Pacific;

MEXCAR, Mexico Caribbean; NATL, North Atlantic; SATL, South Atlantic; ARAB, Arabian Sea; ZAN, Zanzibar Cur-

rent; AGU, Agulhas Current; IND, Indian Ocean) and 500 mm/yr for the land sources (WAF, Winter Africa; WSA, Winter

South America; SAHEL, Sahel). Two boxes were also defined using the physical boundaries of oceanic b asins (MED,

REDS). Data: ERA40 (1958–2001).

Auxiliary materials are available in the HTML. doi:10.1029/

2010GL043712.

GIMENO ET AL.: ORIGIN OF CONTINENTAL PRECIPITATION L13804L13804

2of7

When adding ( e − p) for all th e particles in the atmosph eric

column over an area, we c an obtain (E‐P), where the surface

freshwater flux (E) is the e vaporation and (P) is the pre-

cipitation rate per unit area. The method can also track (E‐P)

from any specific region backwards or forwards in time

along the trajectories, allowing to diagnose the relation-

ships between net moisture source and net moisture sink

regions. Full details of the method and its limitations are

described by Stohl a nd James [2004, 2005].

[

8] In the work reported here we used the tracks of 1.3

million particles over a 5‐year period (2000–2004), com-

puted using ECMWF operational analyses available every

6 hours (00, 06, 12 and 18 UTC) plus short‐term forecasts

available at intermediate times (3, 9, 15, 21 UTC) at a 1° × 1°

resolution in latitude and longitude on 60 vertical levels.

[

9] A database of trajectories (position and q interpolated

from ECMWF data) emanating from each source region

identified in Figure 1 was constructed. We traced (E‐P) for-

wards from each source region limiting the transport time

to 10 days, which is the average time that water vapour resides

in the atmosphere [Numaguti, 1999], assessing the location

of the most important sinks of moisture associated to each

Figure 2. DJF fields of (E‐P) integrated over 10 days for the period 2000–2004 calculated by forward tracking from the

moisture sources indicated by the pink lines and identified on the left bottom of each plot. Only negative values are plotted

and they were scaled by the different factors indicated in each plot in order to use the same colour bar.

GIMENO ET AL.: ORIGIN OF CONTINENTAL PRECIPITATION L13804L13804

3of7

source. Results were analyzed for the full annual period as

well as split into the four seasons defined as DJF, MAM, JJA

and SON.

3. Results

[10] Major sources of moisture can be identified as large

regions characterized with high values of vertically integrated

moisture flux divergence [Trenberth and Guillemot, 1998],

which is equivalent to net evaporation (E) minus precipitation

(P). We tracked more than one million virtual particles from

the identified source regions to the landmasses where most of

that water precipitates in the 10 days following evaporation.

Results are shown in Figures 2 and 3 for DJF and JJA

respectively, while results for transition seasons are shown in

Figures S3 and S4. At a global scale most of the evaporated

water has its origin in tropical and subtropical oceanic areas

(Figure 1). Seasonally, tropical continental areas over

America and Africa contribute, too. It could be that the

continental sources (mainly the Sahel moisture source) are

coming at least partially from analysis increments in soil

moisture. It should be stressed that most water evaporating

from the oceanic source regions precipitates back onto oceans

and only a minor, but vital, fraction precipitates over land. To

help readers to visualize the role of the atmospheric circula-

tion in the transport of moisture we have added maps of

Figure 3. The same as Figure 2, but for JJA.

GIMENO ET AL.: ORIGIN OF CONTINENTAL PRECIPITATION L13804L13804

4of7

surface wind for annual and seasonal basis in the electronic

online material (Figures S5 and S6).

[

11] The North Atlantic source (NATL) is the area pro-

viding the most moisture for precipitation over continents

among the regions defined in Figure 1. It provides moisture

for precipitation in Eastern North America, Central America

and Northern South America during JJA but it extends its

influence during DJF providing moisture also to Europe,

Northern Africa and central South America. These results

reflect the main physical mechanisms transporting moisture

to Europe during winter, the warm conveyor belt [Eckhardt

et al. , 2004] and the systems of low level jets in America:

Great Plains low level jet [Song et al., 2005], Caribbean

low level jet [Amador, 2008] and South America low level

jet [Marengo et al., 2004]. The importance of this source

has been well documented in previous analysis for Central

America [Durán‐Quesada et al., 2010] Central South

America [Drumond et al., 2008] and Europe [Gimeno et

al., 2 010]. The other source over the North Atlantic, the

Mexico‐Caribbean one (MEXCAR) does not provide mois-

ture to the South American low level jet and plays a relatively

minor role for the European continental moisture during

winter since most of the moisture falls over the North

Atlantic, so its influence is limited to Eastern North America

and Central America. The South Atlantic source (SATL) is

the second largest source of moisture for precipitation over

continents in the world, being the main source region for

precipitation in Eastern South America with the exception of

NE Brazil where flux diffluence associated to the transition of

the ITCZ complicates the moisture fluxes (A. Drumond et al.,

Lagrangian identification of the main sources of moisture

affecting northeastern Brazil during its pre‐rainy and rainy

seasons, submitted to PLoS ONE, 2010). The latitudinal

seasonal propagation of the ITCZ is clearly apparent in the

Figure 4. (a) Moisture source regions identified as maxima of vertically integrated moisture flux divergence in the period

1958–2001. The regions were defined based on the threshold of 750 mm/yr (500 mm/yr) for the oceanic (land) sources.

(b) (E‐P) contour of −0.05 × 10

−2

mm/day during JJA and (c) (E‐P) contour of − 0.05 × 10

−2

mm/day during DJF. (d) As

Figure 4c but expanded for Indian region. Contour colors in Figure 4b–4d correspond to the color scheme used in Figure 4a.

GIMENO ET AL.: ORIGIN OF CONTINENTAL PRECIPITATION L13804L13804

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extension to the North during JJA of the pattern of E‐P over

Northern South America and in the influence of this source

during JJA for the precipitation over the Sahel [Nieto et al.,

2006].

[

12] Among the three Pacific sources, two of them, the

North Pacific source (NPAC) and the South Pacific source

(SPAC) provide moisture for precipitation over the latitu-

dinal extremes of the American continent, with a stronger

influence in the respective hemispheric winter. As com-

mented for Europe in respect to NATL the physical mecha-

nism responsible for this transport is the feeding of moisture

to extra‐tropical cyclones by the Warm Conveyor Belt sys-

tems [Koster, 1986]. During the Northern Hemisphere winter

the Eastern coast of North America receives moisture for

precipitation from the Atlantic (NATL and MEXCAR) and

the Pacific (NPAC), however the amount of moisture

received from the former is about 100 times higher than the

moisture received from the latter. The other Pacific source

(CORALS) is located in the Southern Hemisphere along the

eastern coast of Australia and it is the major source over

Oceania continent along the year.

[

13] The structure of Indian Ocean sources is much more

complex. During DJF the four sources, (IND, embracing

most of the oceanic areas between Australia and south

Africa; AGU, located over the Agulhas current; ZAN, sit-

uated over the Zanzibar Current and ARAB, placed on the

Arabian Sea) provide moisture for surrounding continental

areas of the African continent and the Arabian Peninsula;

however during the Southern Hemisphere winter (JJA) the

monsoon circulations transforms the four areas into sources

of moisture for precipitation falling over the Indian Peninsula

[Annamalai et al., 1999]. This unique situation implicates

that the Indian subcontinent receives moisture from 6 dif-

ferent major source regions during the Southern Hemisphere

winter, including the four I ndic ones plus th e Red Sea

(REDS) an d a co ntinental source over tropical Southern

Africa (WAF).

[

14] Despite their relatively small size the two inner Seas,

i.e., the Mediterranean (MED) and the Red Seas (REDS)

play an important role at a much larger scale. During the

Northern Hemisphere winter (DJF), both sources supply

moisture for precipitation on continental areas placed to

their Northeast. D uring JJA the Mediterranean provides

moisture to its surroundings extending to Northe rn Europe

while the Red Sea provides m oisture to the remote area

over the Indian Peninsula as commented before.

[

15] Although in a secondary way (after precipitation plus

further evaporation) there are two other important moisture

regions during the Southern Hemisphere winter (JJA), one

placed i n the tropical South A frica (WAF) and the other

over the Amazon (WSA) and one during the Nort hern

Hemisphere winter (DJF) over the Sahel (SAHEL). Their

evaporationissohighthatthey could be considered as

continental S eas [Dirmeyer and Brubake r, 2007]. They

provide moisture for most of the precipitation over impor-

tant areas of t he world s uch as the Parana, Orinoc o or Co ngo

river basins.

4. Summary and Discussion

[16] In this study we use an objective 3‐D Lagrangian

model (FLEXPART) to detect major oceanic moisture source

areas and to identify the associated continental regions sig-

nificantly influenced by each moisture source. To summarize

our results, Figure 4 illustrates the source regions (Figure 4a)

and compares their relative importance on the origin of

precipitation on continental landmasses by showing a single

contour line (E‐P=−0.05 × 10

−2

mm/day) for JJA (Figure 4b)

and DJF (Figure 4c).

[

17] Our results illustrate the highly asymmetrical role

of major oceanic moisture sources over continents. The

Northern Atlantic subtropical ocean source provides mois-

ture for precipitation over vast geographical areas in winter

(from Mexico to large parts of Eurasia), whereas the influ-

ence of other large oceanic sources is confined to much

smaller continental areas (e.g., Southern Indian and the North

Pacific oceans). The small enclosed Red Sea source provides

vast amounts of moisture that precipitate between the Gulf of

Guinea and Indochina (JJA) and the African great lakes and

Asia (DJF). Likewise vast continental areas lack appreciable

direct water transport from any major source regions, usually

corresponding to some of the most arid inland regions (e.g.,

inner Asian continent).

[

18] Our analysis further emphasizes that some land masses

obtain moisture from only one or two sources located in the

same hemisphere (e.g., Northern Europe or Eastern North

America), while others receive moisture from both hemi-

spheres with large seasonal variations (e.g., Northern South

America). Finally, the continental areas characterized by mon-

soon regimes (India, tropical Africa and the great lakes region)

benefit from a large number of source regions (Figure 4d)

which indicates the complex nature of precipitation.

[

19] Although further study on changes in water source

due to climate change is necessary it is obvious that changes

in the atmospheric circulation in a changing climate will

result in changes in circulation between source and sink

redirecting moisture in a different way. Those continental

regions receiving moisture from only one or two source

region(s) may be exposed more strongly to changes in the

water cycle due to a changing climate than regions that draw

on multiple moisture sources.

[

20] Acknowledgme nts. Authors tha nks the Spanish Ministry of

Science and FEDER for funding this res earch through the project MSM.

A. Stohl was supported by the Norwegian Research Council in the frame-

work of the WATER‐SIP project.

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The role of trees in watershed hydrology is governed by many environmental factors along with their inherent characteristics and not surprisingly has generated into diverse debates in the literature. Herein, this state-of-the-art review provides an opportunity to propose a conceptual model for understanding the role of trees in watershed hydrology and examine the conditions under which they can be an element that increases or decreases water supply in a watershed hydrology. To achieve this goal, this review addressed the interaction of forest cover with climatic conditions, soil types, infiltration, siltation and erosion, water availability, and the diversity of their ecological features. The novelty of the proposed conceptual model highlights that tree species and densities, climate, precipitation, type of aquifer, and topography are important factors affecting the relationships between trees and water availability. This suggests that forests can be used as a nature-based solution for conserving and managing natural resources, including water, soil and air. To sum up, forests can reduce people’s imprint, thanks to their role in improving water and air quality, conserving soil, and other ecosystem services. The outcomes of this study should be valuable for decision-makers when investing in reforestation in a watershed hydrology.

Interactions Between Forest Cover and Watershed Hydrology: A Conceptual Meta-Analysis

Article

Full-text available

Nov 2024

The role of trees in watershed hydrology is governed by many environmental factors along with their inherent characteristics and not surprisingly has generated diverse debates in the literature. Herein, this conceptual meta-analysis provides an opportunity to propose a conceptual model for understanding the role of trees in watershed hydrology and examine the conditions under which they can be an element that increases or decreases water supply in a watershed. To achieve this goal, this conceptual meta-analysis addressed the interaction of forest cover with climatic con ditions, soil types, infiltration, siltation and erosion, water availability, and the diversity of ecologi cal features. The novelty of the proposed conceptual model highlights that tree species and densi ties, climate, precipitation, type of aquifer, and topography are important factors affecting the rela tionships between trees and water availability. This suggests that forests can be used as a nature based solution for conserving and managing natural resources, including water, soil, and air. To sum up, forests can reduce people’s footprint, thanks to their role in improving water and air qual ity, conserving soil, and other ecosystem services. The outcomes of this study should be valuable for decision-makers in understanding the types of forests that can be used in an area, following an approach of environmental sustainability and conservation aiming at restoring hydrological ser vices, mitigating the costs of environmental services, promoting sustainable land use, managing water resources, and preserving and restoring soil water availability (SWA) when investing in re forestation for watershed hydrology, which is important for the human population and other activities.

Sea Surface Salinity Provides Subseasonal Predictability for Forecasts of Opportunity of U.S. Summertime Precipitation

Article

Full-text available

Mar 2025

Global water cycling plays a fundamental role in the climate system, with the majority of terrestrial water ultimately sourced from the ocean. As oceanic moisture evaporates, it leaves a signature on sea surface salinity, allowing the salinity fields to be a predictor of terrestrial precipitation. This research is among the first in the published literature to assess the role of sea surface salinity for predictions on low‐skill summertime subseasonal timescales for terrestrial precipitation predictions. Neural networks are trained with the CESM2 Large Ensemble using North Atlantic salinity anomalies to quantify predictability of U.S. Midwest summertime heavy rainfall events at 0‐ to 56‐day leads. Using explainable artificial intelligence, salinity anomalies in the Caribbean Sea and Gulf of Mexico are shown to provide skill for subseasonal forecasts of opportunity. Further, to independently validate the CESM2‐based findings, a moisture‐tracking algorithm applied to ERA5 reanalysis data demonstrates that the regions of evaporation identified by neural networks directly provide moisture that precipitates in the Midwest.

Could Tropical Cyclone Expansion Boost Migration of Temperate Trees to Boreal Forests?

Article

Full-text available

Apr 2025

The poleward expansion of tropical cyclones (TCs) inevitably triggers unprecedented ecological consequences for cool‐temperate and boreal forests, including shifts in species distribution, global carbon dynamics, or forest policies. However, our current understanding of the impact of TCs' expansion into new regions is limited and lacks attention by both, the media and research community, compared to TCs' impact on (sub‐) tropical forests. Shifts in TC activity are expected to pose a considerable threat to extensive areas globally under climate change. Nevertheless, we suggest that TCs should not only be perceived as destructive weather phenomena but also as a vehicle (i) facilitating the migration of temperate species to southern boreal forests and (ii) mitigating the impact of climate change on forest ecosystems. Hence, it is vital to establish globally coherent long‐term and large‐scale research to capture unique ongoing (and currently overlooked) ecological processes induced by TC expansion, which may lead to a complex unprecedented forest transition dynamic.

Comparative Analysis of Continuous and Reinitialized Dynamical Downscaling in the North Atlantic and Surrounding Continents

Preprint

Full-text available

Mar 2025

General Circulation Models provide comprehensive climate projections but are limited by coarse spatial resolution. To address this issue, Regional Climate Models are used for higher-resolution simulations, particularly to assess regional climate change impacts. This process, called dynamical downscaling, typically involves continuous simulations over a selected period. Alternatively, multiple reinitialized simulations over shorter intervals can be employed to minimize error accumulation and reduce computation time through parallel processing. However, this approach may hinder the development of certain atmospheric phenomena. In this study, the Weather Research and Forecasting model was used for continuous and daily reinitialized dynamical downscaling. Simulations were driven by ERA5 and Coupled Model Intercomparison Project Phase 6 data at 1° and 1.25° spatial resolution, respectively, covering 115° W–40° E in longitude and 20° N–60° N latitude, and downscaled to 20-km resolution. The results were evaluated against ERA5 data at 0.25° resolution to assess accuracy. The analysis focused on wind speed, temperature, humidity, precipitation, surface pressure, and solar radiation. Overall, both techniques demonstrated good to excellent correlation with the reference data. However, neither method reliably captured wind speed nor surface pressure in mountainous areas. In ERA5-driven simulations, the reinitialized technique performed better than the continuous for air temperature and humidity in coastal regions, whereas the continuous approach showed a slight advantage in estimating solar radiation across all surfaces. For CMIP6-driven simulations, both downscaling techniques produced similar results, except for solar radiation and over land, where the continuous method demonstrated marginally better performance. Considering the significantly lower computational cost of the reinitialized method – approximately 30 times less in this study – it is recommended as the preferred approach when its performance is comparable to or better than that of the continuous method.

Characteristics and Evolutions of the Atmospheric River in the “23.7” Extreme Rainstorm Event in Hebei Province of China in July 2023

Article

Mar 2025
J HYDROMETEOROL

An extreme rainstorm occurred in Hebei Province, China, from July 29 to August 2, 2023. In this study, the rainfall observation data and reanalysis data are used to diagnose the evolution characteristics of the atmospheric river (AR) in this rainstorm case. The water vapor (WV) sources and transport paths are also analyzed by using the Lagrangian trajectory tracking model (namely the Flexible Particle dispersion model) and the Weather Research and Forecasting (WRF) model. The results indicate that the low-latitude AR exists throughout this rainstorm process, and it is mainly located in the Arabian Sea, the Bay of Bengal and the South China Sea. This AR event in Hebei is the result of the circulations of Typhoon Doksuri and Typhoon Khanun transporting WV to Hebei. The AR develops the most vigorously below 500 hPa and is often accompanied by a strong low-level jet. The convergence in the left front of its exit region, along with the divergence south of the upper-level westerly jet entrance, enhances the dynamic conditions that sustain the low-level shear line. Combined with the topographic uplifting effect from the Taihang and Yanshan Mountains, the coupling of upper-level jet and low-level jet provides a good circulation background for the ascending motion over the precipitation area. The WV source tracking shows that the target air parcels that substantially contribute to the heavy rainfall in Hebei come from Typhoons Doksuri and Khanun, which greatly strengthen the AR intensity and prolong the AR path. The sensitivity experiments based on the WRF model further verify the extension of the AR by Typhoons Doksuri and Khanun, as well as the fact that the AR is blocked by the Taihang and Yanshan Mountains while moving northward, forcing the WV to uplift and thus triggering the extreme rainstorm.

Heavy summer rainfall in southeastern Australia

Article

Full-text available

Feb 2025
Q J ROY METEOR SOC

In the austral summer, parts of southeastern Australia are prone to heavy rainfall that causes major riverine flooding and fatalities. Easterly flow associated with an anticyclone in the Tasman Sea, large moisture transports from the Coral Sea, and upper tropospheric cyclonic disturbances all contribute to these heavy rainfall episodes. However, questions regarding their synoptic dynamics remain, including which of these ingredients are the most critical. These questions are addressed by comparing composite pressure and moisture fields of heavy rainfall days over selected regions with non‐heavy rainfall days that have a similar synoptic pattern. A synoptic climatology is constructed for this purpose by k k ‐means cluster analysis of 500 hPa geopotential height anomalies from the European Centre for Medium‐range Weather Forecasts Reanalysis v5, for all December to March days over a period of 40 years. Heavy rainfall days in the wettest clusters have negative 500 hPa geopotential height anomalies immediately west of the affected region that are stronger on average than those of non‐heavy rainfall days. Their accompanying distributions of surface pressure, precipitable water, and vertical motion are consistent with cyclonic baroclinic development and are preceded by anticyclonic Rossby wave breaking. Heavy rainfall days also show an increased frequency of blocking near 150∘

15{0}^{\circ }

E; however, this peaks 1–2 days after the onset of heavy rain. Regional rainfall in these clusters shows strong sensitivity to lower pressure immediately westward but little sensitivity to high pressure in the Tasman Sea until after the commencement of rain. A companion study using the same cluster analysis illustrated the link between anticyclonic Rossby wave breaking and heatwaves in southeastern Australia. These latest results highlight the upper cyclonic anomalies that often form on the equatorward flank of anticyclonic Rossby wave breaking as the key ingredient separating days with a favourable synoptic‐scale pattern of surface high pressure into those that rain heavily and those that do not.

Climatology of extreme precipitation spells induced by cloudburst-like events during the Indian Summer Monsoon

Article

Full-text available

Dec 2024

This study enhances existing understanding of extreme precipitation spells induced by cloudburst-like (EPS-CBL) events in India, emphasizing climatology and geographical distribution often overlooked by traditional observations. EPS-CBL is defined as continuous rainfall exceeding 200 mm/day and intermittent extreme rates above 30 mm/hour or the 99.9th percentile threshold, differing from definitions proposed by the IMD and other studies. Our findings reveal significant biases in various precipitation products compared to IMD data. CMORPH consistently outperforms other datasets by capturing more extreme events and showing significant rising trends in regions influenced by orographic effects, such as the Himalayan foothills and the Western Ghats. Although IMERG aligns well with IMD overall, it exhibits variability in extreme events, while IMDAA tends to underestimate these extremes, especially in complex terrains. Analysis of EPS-CBL trends from 2000 to 2022 highlights regional differences across datasets. Both CMORPH and IMERG show an increase in EPS-CBL events in the hilly region, while IMDAA indicates a decline. Understanding EPS-CBL climatology provides valuable insights for modeling studies exploring the underlying mechanisms of these events.

A new look at Earth’s water and energy with SWOT

Article

Jan 2025

A Lagrangian identification of major sources of Sahel moisture

Article

Full-text available

Sep 2006

The drying trend in the Sahelian region from the 1950s to the 1990s was a hotspot of decadal-scale changes in the climate of the 20th century. However, the sources of moisture in this region have been poorly studied. Motivated by the excellent skills of a new Lagrangian method of diagnosis for identifying the sources of moisture over a region (Stohl and James, 2004, 2005), this study examines the main sources over the Sahel. The method computes budgets of evaporation minus precipitation by calculating changes in the specific humidity along the trajectories. We tracked the air masses residing over the Sahel over a period of five years (2000-2004). Recycling was identified as the major source of moisture over the Sahel. Two additional sources of moisture reaching Sahel have been identified here, namely; 1) a band in the North Atlantic stretching between the Sahel and Iberia, and 2) the entire Mediterranean basin, and the nearby Red Sea.

A 15Year Climatology of Warm Conveyor Belts

Article

Full-text available

Jan 2004

This study presents the first climatology of so-called warm conveyor belts (WCBs), strongly ascending moist airstreams in extratropical cyclones that, on the time scale of 2 days, rise from the boundary layer to the upper troposphere. The climatology was constructed by using 15 yr (1979 93) of reanalysis data and calculating 355 million trajectories starting daily from a 1° × 1° global grid at 500 m above ground level (AGL). WCBs were defined as those trajectories that, during a period of 2 days, traveled northeastward and ascended by at least 60% of the zonally and climatologically averaged tropopause height. The mean specific humidity at WCB starting points in different regions varies from 7 to 12 g kg-1. This moisture is almost entirely precipitated out, leading to an increase of potential temperature of 15 22 K along a WCB trajectory. Over the course of 3 days, a WCB trajectory produces, on average, about four (six) times as much precipitation as a global (extratropical) average trajectory starting from 500 m AGL. WCB starting points are most frequently located between approximately 25° and 45°N and between about 20° and 45°S. In the Northern Hemisphere (NH), there are two distinct frequency maxima east of North America and east of Asia, whereas there is much less zonal variability in the Southern Hemisphere (SH). In the NH, WCBs are almost an order of magnitude more frequent in January than in July, whereas in the SH the seasonal variation is much weaker. In order to study the relationship between WCBs and cyclones, an independent cyclone climatology was used. Most of the WCBs were found in the vicinity of a cyclone center, whereas the reverse comparison revealed that cyclones are normally accompanied by a strong WCB only in the NH winter. In the SH, this is not the case throughout the year. Particularly around Antarctica, where cyclones are globally most frequent, practically no strong WCBs are found. These cyclones are less influenced by diabatic processes and, thus, they are associated with fewer high clouds and less precipitation than cyclones in other regions. In winter, there is a highly significant correlation between the North Atlantic Oscillation (NAO) and the WCB distribution in the North Atlantic: In months with a high NAO index, WCBs are about 12% more frequent and their outflow occurs about 10° latitude farther north and 20° longitude farther east than in months with a low NAO index. The differences in the WCB inflow regions are relatively small between the two NAO phases. During high phases of the Southern Oscillation, WCBs occur more (less) frequent around Australia (in the South Atlantic).

Where Does the Iberian Peninsula Moisture Come From? An Answer Based on a Lagrangian Approach

Article

Full-text available

Apr 2010

This study investigated the main sources of moisture in the atmosphere over the Iberian Peninsula (IP) at annual and seasonal scales using FLEXPART, a powerful new 3D Lagrangian diagnosis method that identifies the humidity contributions to the moisture budget of a region. This method can identify moisture sources at lower cost and with greater accuracy than standard isotopic content methods. The results are based on back-tracking analysis of all air masses residing over the IP in the 5-yr period from 2000 to 2004. The results show that the two most important moisture source regions affecting the IP are in a tropical-subtropical North Atlantic corridor that extends from the Gulf of Mexico to the IP, and the IP itself and the surrounding Mediterranean. The importance of these two source areas varies throughout the year, and also with respect to different climatic regions inside the IP. The former source region is the dominant moisture source for the entire IP during winter and in western regions throughout the year, whereas the latter source region dominates the moisture supply to the IP in summer and in the eastern Mediterranean region of the IP throughout the year. The results also demonstrate that winter precipitation in the IP is influenced by both atmospheric instability that forces air masses to rise, and the supply of moisture from the tropical-subtropical North Atlantic corridor on a daily scale and a seasonal basis. Thus, a combination of high (low) moisture supply from the North Atlantic corridor and high (low) atmospheric instability appears to be responsible for the most recent wet (dry) winter in the IP.

Correction to “Moisture sources for Central America: Identification of moisture sources using a Lagrangian analysis technique”

Article

Full-text available

Mar 2010

We herein present an analysis of the sources of atmospheric moisture for Central America using a Lagrangian technique. The results of backward and forward moisture tracking analysis using the FLEXPART model has enabled the identification of the main sources of moisture that reach Central America, as well as an evaluation of their spatial evolution during their passage toward the region of interest. Data from the European Center for Medium-Range Weather Forecasts (ECMWF) for a 5 year period (2000-2004) were used as input for the FLEXPART model. The applied method reproduces the variations in the location of the Intertropical Convergence Zone (ITCZ) over the study area very well. The primary source of moisture for Central America is identified over the Caribbean Sea, and a secondary source appears to exist near the equatorial Pacific region. The dominance of the Caribbean Sea region as a source of moisture for this region is clear, as is the importance of the Caribbean Low-Level Jet (CLLJ) as the principal transport mechanism. These characteristics are confirmed by inspection of the moisture transport patterns and their seasonal behavior.

Global sources of local precipitation as determined by the NASA/GISS GCM.

Article

Jan 1986

R. Koster

The origin of water precipitating in different geographic regions is investigated with the NASA/GISS GCM. Water evaporating from various source regions is 'tagged' and then followed as a tracer in four model simulations, one for each season. The contributions of source region evaporations to simulated rainfall at specific locations is tabulated. The results show that in the summer, water vapor for midlatitude and high latitude precipitation tends to be recycled locally, whereas low latitude continental precipitation is more dependent on oceanic moisture sources. -Authors

Characterization of the Global Hydrologic Cycle from a Back-Trajectory Analysis of Atmospheric Water Vapor

Article

Feb 2007

Regional precipitation recycling may constitute a feedback mechanism affecting soil moisture memory and the persistence of anomalously dry or wet states. Bulk methods, which estimate recycling based on time-averaged variables, have been applied on a global basis, but these methods may underestimate recy- cling by neglecting the effects of correlated transients. A back-trajectory method identifies the evaporative sources of vapor contributing to precipitation events by tracing air motion backward in time through the analysis grid of a data-assimilating numerical model. The back-trajectory method has been applied to several large regions; in this paper it is extended to all global land areas for 1979-2003. Meteorological information (wind vectors, humidity, surface pressure, and evaporation) are taken from the NCEP-Depart- ment of Energy (DOE) reanalysis, and a hybrid 3-hourly precipitation dataset is produced to establish the termini of the trajectories. The effect of grid size on the recycling fraction is removed using an empirical power-law relationship; this allows comparison among any land areas on a latitude-longitude grid. Recy- cling ratios are computed on a monthly basis for a 25-yr period. The annual and seasonal averages are consistent with previous estimates in terms of spatial patterns, but the trajectory method generally gives higher estimates of recycling than a bulk method, using compatible spatial scales. High northern latitude regions show the largest amplitude in the annual cycle of recycling, with maxima in late spring/early summer. Amplitudes in arid regions are small in absolute terms, but large relative to their mean values. Regions with strong interannual variability in recycling do not correspond directly to regions with strong intra-annual variability. The average recycling ratio at a spatial scale of 105 km2 for all land areas of the globe is 4.5%; on a global basis, recycling shows a weak positive trend over the 25 yr, driven largely by increases at high northern latitudes.

Global Land Precipitation: A 50-yr Monthly Analysis Based on Gauge Observations

Article

Jun 2002

ABSTRACT This paper describes the initial work,toward the production of monthly,global (land and ocean) analyses of precipitation for an extended period from 1948 to the present. Called the precipitation reconstruction (PREC), the global analyses are defined by interpolation of gauge observations over land (PREC/L) and by EOF recon- struction of historical observations over ocean (PREC/O). This paper documents,the creation of the land com- ponent of the analyses (PREC/L) on a 2.58 latitude/longitude grid for 1948‐2000. These analyses are derived from gauge,observations from over 17 000 stations collected in the Global Historical Climatology Network (GHCN), version 2, and the Climate Anomaly Monitoring System (CAMS) datasets. To determine the most suitable objective analysis procedure for gridding, the analyses generated by four published objective analysis techniques [those of Cressman, Barnes, and Shepard, and the optimal interpolation (OI) method of Gandin] were compared. The evaluation demonstrated,two crucial points: 1) better results are obtained when,interpolating anomalies rather than the precipitation totals, and 2) the OI analysis procedure provided the most accurate and stable analyses among the four algorithms that were tested. Based on these results, the OI technique was used to create monthly,gridded analyses of precipitation over the global land areas for the 53-yr period from 1948 to 2000. In addition, some diagnostic investigations of the seasonal and interannual variability of large-scale precipitation over the global land areas are presented. The mean,distribution and annual cycle of precipitation observed in the PREC/L showed good agreement with those in several published gauge-based datasets, and the anomaly,patterns associated with ENSO resemble,those found in previous studies. The gauge-based dataset (PREC/L) will be updated on a quasi-real-time basis and is available online (ftp.ncep.noaa.gov/pub/precip/50- yr).

Robust Response of the Hydrological Cycle to Global Warming

Article

Dec 2008

Using the climate change experiments generated for the Fourth Assessment of the Intergovernmental Panel on Climate Change, this study examines some aspects of the changes in the hydrological cycle that are robust across the models. These responses include the decrease in convective mass fluxes, the increase in horizontal moisture transport, the associated enhancement of the pattern of evaporation minus precipitation and its temporal variance, and the decrease in the horizontal sensible heat transport in the extratropics. A surprising finding is that a robust decrease in extratropical sensible heat transport is found only in the equilibrium climate response, as estimated in slab ocean responses to the doubling of CO2, and not in transient climate change scenarios. All of these robust responses are consequences of the increase in lower-tropospheric water vapor.

Origin and recycling processes of precipitating water over the Eurasian continent: Experiments using an atmospheric general circulation model

Article

Jan 1999

Atusi Numaguti

By using an atmospheric general circulation model, origin and transport processes of water in the atmosphere-land system are examined. The water vapor and the land-surface water in the model are tagged according to the geographical regions of water input (evaporation) and are separately treated. The results are examined focusing on the water cycle over the Eurasian continent. In the winter season, most of the precipitating water over the Eurasian continent is supplied by evaporation from the oceans. On the other hand, the precipitating water in the summer is mostly supplied by the evaporation from the continental surface, indicating active recycling of water between the atmosphere and the land in this season. Considering that the water in the continental surface should be supplied from the oceans sometime before, the history of the water from its origin (evaporation from the oceans) is examined, by separately treating the components of the soil water and the snow according to the geographical regions of the origin. Two additional types of tracers are included in order to determine the timescale of the transport and the frequency of the recycling between the atmosphere and the land. The results show that the main origin of water in the northern part of the Eurasian continent is the Northern Atlantic Ocean and that in the southern part is the Northern Indian Ocean. In the southern part the mean age of the precipitating water since its origin is 1 month or shorter, and the mean count of recycling is less than one, indicating that the water coming directly from the oceans by atmospheric transport is dominant. In the northern inland part in summer, however, the mean age is 3 months or longer, and the mean count of recycling is above two. These results suggest that a significant portion of the precipitating water in inland Eurasia in the summer originates in the Atlantic Ocean in the previous winter and is transported eastward with a few recycling cycles between the atmosphere and the continental surface.

The mean evolution and variability of the Asian sum-mer monsoon: Comparison of ECMWF and NCEP-NCAR r

Article