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  • View in gallery

    Three subregions of Europe used in this study and their mean summer (June, July, and August) precipitation (mm day−1) SP, Spain (37.5°–43.5°N, 9°W–0°); CE, central Europe (46.5°–52.5°N, 3°–12°E); and BA, Balkans (43.5°–49.5N°, 18°–27°E).

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    Precipitation minus evaporation (mm day−1) and the vectors of the vertically integrated moisture transport (kg ms−1) for the mean of JJA calculated for the period 1979–2001.

  • View in gallery

    Area sizes of the eight nested regions.

  • View in gallery

    Recycling ratio as a function of area size for the summer (JJA).

  • View in gallery

    Time series (July 1998) of precipitation (mm day−1) at several forecast times averaged over central Europe.

  • View in gallery

    Time series of the water vapor balance terms (mm day−1) averaged over (a) central Europe, (b) the Balkans, and (c) Spain for 1979–2001: P, precipitation; E, evaporation; ∂w/t, change in atmospheric water balance storage; divQ, water vapor flux divergence; and res, residual.

  • View in gallery

    Time series of the mean summer (JJA) recycling ratio (rr) and area-averaged precipitation divided into convective precipitation (cp), large-scale precipitation (lsp), and recycled precipitation (Pr) for the three subregions: (a) central Europe, (b) the Balkans, and (c) Spain. Note that the precipitation scale in (c) ranges only from 0 to 1 mm day−1.

  • View in gallery

    Comparison of area-averaged monthly mean (JJA) hydrological data for (a)–(d) central Europe, (e)–(h) Balkans, and (i)–(l) Spain: (a), (e), (i) rr vs PE; (b), (f), (j) Pr vs PE; (c), (g), (k) rr vs the magnitude of the water vapor flux transport; and (d), (h), (i) Pr vs the magnitude of the water vapor flux transport. Shading depicts evaporation (mm day−1).

  • View in gallery

    Precipitation minus evaporation (mm day−1) and horizontal transport (kg ms−1) as in Fig. 2 but for the summers (JJA) of (a) 1987 and (b) 1995.

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Precipitation Recycling: Moisture Sources over Europe using ERA-40 Data

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  • 1 Department of Hydrology and Geo-Environmental Sciences, VU University, Amsterdam, Netherlands
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Abstract

Atmospheric moisture within a region is supplied by both local evaporation and advected from external sources. The contribution of local evaporation in a region to the precipitation in the same region is defined as “precipitation recycling.” Precipitation recycling helps in defining the role of land–atmosphere interactions in regional climate. A dynamic precipitation recycling model, which includes the moisture storage term, has been applied to calculate summer variability of the precipitation recycling over Europe based on 40-yr European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-40) data. Time series for three subregions in Europe (central Europe, the Balkans, and Spain) are obtained to analyze the variability in recycling and to compare the potential in the subregions for interactions between land surface and atmospheric processes. In addition, the recycled precipitation and recycling ratios are linked to several components of the water vapor balance equation [precipitation, evaporation, precipitation minus evaporation (PE), and moisture transport]. It is found that precipitation recycling is large in dry summers for central Europe, while the opposite is true for the Balkans. Large precipitation recycling is determined in relation with weak moisture transport and high evaporation rates in central Europe. This occurs for dry summers. For the Balkans, precipitation recycling is large in wet summers when moisture transport is weak, and PE and evaporation are large. Here, the recycling process intensifies the hydrological cycle due to a positive feedback via convective precipitation and therefore the amount of recycled precipitation is larger. For Spain, recycling is also larger when moisture transport is weak, but other correlations are not found. For regions such as central Europe in dry summers and the Balkans in wet summers, which are susceptible to land–atmosphere interactions, future climate and/or land use can have an impact on the regional climate conditions due to changes in evaporation.

Corresponding author address: B. Bisselink, Hydrology and Geo-Environmental Sciences, VU University, Boelelaan 1085, 1081 HV Amsterdam, Netherlands. Email: berny.bisselink@falw.vu.nl

Abstract

Atmospheric moisture within a region is supplied by both local evaporation and advected from external sources. The contribution of local evaporation in a region to the precipitation in the same region is defined as “precipitation recycling.” Precipitation recycling helps in defining the role of land–atmosphere interactions in regional climate. A dynamic precipitation recycling model, which includes the moisture storage term, has been applied to calculate summer variability of the precipitation recycling over Europe based on 40-yr European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-40) data. Time series for three subregions in Europe (central Europe, the Balkans, and Spain) are obtained to analyze the variability in recycling and to compare the potential in the subregions for interactions between land surface and atmospheric processes. In addition, the recycled precipitation and recycling ratios are linked to several components of the water vapor balance equation [precipitation, evaporation, precipitation minus evaporation (PE), and moisture transport]. It is found that precipitation recycling is large in dry summers for central Europe, while the opposite is true for the Balkans. Large precipitation recycling is determined in relation with weak moisture transport and high evaporation rates in central Europe. This occurs for dry summers. For the Balkans, precipitation recycling is large in wet summers when moisture transport is weak, and PE and evaporation are large. Here, the recycling process intensifies the hydrological cycle due to a positive feedback via convective precipitation and therefore the amount of recycled precipitation is larger. For Spain, recycling is also larger when moisture transport is weak, but other correlations are not found. For regions such as central Europe in dry summers and the Balkans in wet summers, which are susceptible to land–atmosphere interactions, future climate and/or land use can have an impact on the regional climate conditions due to changes in evaporation.

Corresponding author address: B. Bisselink, Hydrology and Geo-Environmental Sciences, VU University, Boelelaan 1085, 1081 HV Amsterdam, Netherlands. Email: berny.bisselink@falw.vu.nl

1. Introduction

The origin of precipitation in a region can generally be divided into two main sources: local evaporation and externally advected moisture. Precipitation originating from local evaporation is then referred to as “recycled precipitation” (Dominguez et al. 2006). Soil moisture may influence the generation of precipitation in a region through a feedback loop involving evaporation from the land. These feedback mechanisms are very important components for the land surface–atmosphere system. Precipitation recycling can be described with a recycling ratio that essentially shows the contribution of local evaporation to local precipitation. Precipitation in regions with a large “recycling ratio” is potentially susceptible to the underlying causes, such as in land cover and land use (Eltahir and Bras 1996).

Several precipitation recycling studies have been performed to define the role of land surface–atmosphere interactions. A considerable amount of these studies is based on the simple one-dimensional recycling model derived by Budyko (1974) and on the atmospheric moisture balance in a region (Brubaker et al. 1993; Eltahir and Bras 1994; Schär et al. 1999; Trenberth 1999; Bosilovich and Schubert 2001). Brubaker et al. (1993) extended Budyko’s model into two dimensions, and Trenberth (1999) and Schär et al. (1999) used this model for their studies of global recycling and for the analysis of recycling in Europe. Eltahir and Bras (1994) developed a bulk diagnostic recycling model to estimate the seasonal and spatial distributions of recycling. Bosilovich and Schubert (2001) used this model to calculate precipitation recycling over the central United States.

The results of the different analytical models depend strongly on the region size. The recycling ratio increases if the region becomes larger, because of the larger possibility that an evaporative particle in a region precipitates in the same region. Trenberth (1999) investigated recycling globally for annual means at length scales of 500 and 1000 km. The global annual mean recycling they found is 9.6% for the 500-km scale and 16.8% for the 1000-km scale. A more detailed description of the methods and the results of the analytical models are given in a review of precipitation recycling (Eltahir and Bras 1996).

All of these analytical models depend on two crucial assumptions that limit their calculations to a monthly or yearly time scale. The first assumption is that moisture from local evaporation is well mixed with the advected moisture (well-mixed atmosphere). The second assumption is that the change in atmospheric water vapor storage is neglected, because of the small contribution of the storage term compared to the other terms of the water vapor balance equation at a monthly or longer time scale. As Δt grows, the contribution of the storage term is less significant. Zangvil et al. (2004) introduce a model that circumvents these restrictions and therefore can be used at a daily scale. The drawback of this model is that it can only be used for days that have similar large-scale moisture characteristics. Their results show a clear need to analyze shorter time scales. To deal with all these restrictions, Dominguez et al. (2006) developed a new dynamical precipitation recycling model including the moisture storage term. With the new dynamical recycling model, it is allowed to minimize Δt and to calculate recycling at the daily scale. This is a useful tool for relating precipitation recycling to daily meteorological processes. This new model produces the same spatial pattern as the models that neglect the moisture storage term, but it does predict higher recycling ratios. Although the moisture storage term is small when compared to the advection term at a monthly scale, it appears to make a significant contribution in the recycling process (Dominguez et al. 2006).

Many of the land–atmosphere interactions and recycling studies have been performed for the United States or in the tropics but not for the European continent. Schär et al. (1999) conducted several experiments to study the summertime soil moisture–precipitation feedback mechanism over Europe. They found that the soil–precipitation feedback must rely on some indirect mechanism, whereby wet soils increase the potential for convective activity. Koster et al. (2004) calculate the strength of the land–atmosphere feedback with 12 models, expressed as a coupling factor. They found with the 12-model average that most regions with high land–atmosphere coupling or “hotspots” are found in the transient zones, which are either not too dry or too humid, such as the Sahel. There is, however, considerable scatter to be found between the 12 models.

In addition to the direct soil moisture–precipitation feedback loop, there are also some recent studies investigating the soil–temperature feedback loops (Schär et al. 2004; Seneviratne et al. 2006). In a future climate, the temperature variability is likely to be increasing in summer (Schär et al. 2004). Soil–moisture–temperature interactions in central and eastern Europe appear also to increase the variability in summer temperature due to the potential northward shift of climate zones in a future climate with increasing greenhouse gas concentrations (Seneviratne et al. 2006). Thus, transient zones with significant variability in wet and dry conditions are important regions for land–atmosphere feedbacks in both precipitation (Koster et al. 2004) and temperature (Seneviratne et al. 2006) coupling. However, most of these scenario or model studies are focused on an interannual time scale year after year, and therefore feedback coupling can be different at seasonal time series.

In a future climate and/or with land-use change, the regional atmospheric water cycle can also be affected due to changes in evaporation. It is therefore important to investigate which areas are sensitive to this type of feedback. In this study we apply the model of Dominguez et al. (2006) to determine which areas in Europe are susceptible to land–atmosphere interactions by calculating a dynamic recycling ratio, which investigates this feedback at all relevant meteorological time scales.

The paper is organized as follows. Section 2 presents the data used in this study. In section 3 a brief description of the terms of the water balance—precipitation, evaporation, and the water vapor flux fields—is given to describe the most important moisture sources and sinks for Europe. Section 4 gives a description of the new precipitation recycling model and we discuss the model’s limitations. The results and interpretations of the land–atmosphere interactions are finally presented in section 5 with conclusions in section 6.

2. Data

We use the 40-yr European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-40; Uppala et al. 2005). ERA-40 is an analysis system, incorporating surface, upper-air, and satellite observations, which has a temporal resolution of 4 times per day: 0000, 0600, 1200, and 1800 UTC. In terms of land surface, ERA-40 uses a surface scheme with four layers in the soil (Viterbo and Beljaars 1995; van den Hurk et al. 2000). More detailed information about the physical parameterizations can be found in Gregory et al. (2000). The 23-yr period from 1979 to 2001 is used to calculate each term of the vertically integrated water vapor balance equation.

The surface data, precipitation, and evaporation are 6-hourly forecast fields available on a 1.125° latitude × 1.125° longitude grid. The atmospheric data, the wind components, and specific humidity are analysis fields provided on a 1.5° latitude × 1.5° longitude grid with 60 model levels in the vertical. Mass-weighted vertical integrations are calculated through all 60 model levels. Three subregions with the same area size (0.5 × 106 km2) are defined on the basis of their position in different climate zones (Fig. 1). The Balkan region is one of the wettest areas in summer in Europe. Spain has a dry summer climate and the central Europe region has a temperate climate caused by warm westerly winds from the North Sea. For these three regions recycling ratios are calculated at monthly and seasonal scales. The three summer months (June, July, and August) are selected for this study because of the relative high evaporation rate in the summer.

3. Moisture sources and sinks

The spatial distribution of the precipitation minus evaporation rate and the vertically integrated water vapor flux averaged for June–August (JJA) are shown in Fig. 2. This figure indicates the locations of the main sinks and sources of atmospheric water vapor. It appears that the waters near the west coast of Africa are a strong moisture source for the atmospheric branch of the hydrological cycle. This is due to the high evaporation rates over the warm oceanic waters at 25°N (not shown). Because of the high precipitation rates due to high tropical cyclone activity, the western tropical Atlantic Ocean acts like a moisture sink. The European continent behaves as a source of atmospheric water vapor to the atmosphere, except for the mountainous areas of the Alps, the Balkan region, and the Kaukasus, which are moisture sinks due to orographic rainfall.

Clearly recognizable are the anticyclonic circulations in the subtropics such as the Azores anticyclone. Because of this, there is an eastward moisture transport by the mainly westerly winds on the midlatitudes. Over the evaporating oceans much moisture is absorbed by the atmosphere and transported to the European continent. Farther to the north the moisture transport becomes weaker.

4. Precipitation recycling

a. Model description

In this study, we have applied the Dominguez et al. (2006) dynamical precipitation recycling model to calculate a local recycling ratio ρ, which is defined as the ratio of precipitation in a grid cell that originated from evaporation within a region to the total precipitation in that cell:
i1525-7541-9-5-1073-e1
where Pri is the precipitation of recycled origin in a grid cell and Pi is the total precipitation in a grid cell. The precipitation recycling model is derived from the vertically integrated water vapor balance equation:
i1525-7541-9-5-1073-e2
where P is precipitation and E is evaporation. The first term on the left-hand side represents the time derivative of the precipitable water (w). The second term expresses the horizontal divergence of the water vapor flux, where Qλ and Qϕ are the vertically integrated moisture flux vector components in longitude–latitude direction:
i1525-7541-9-5-1073-e3
i1525-7541-9-5-1073-e4
i1525-7541-9-5-1073-e5
where q, p, g, u, and υ represent the specific humidity, atmospheric pressure, acceleration due to gravity, horizontal zonal wind speed, and horizontal meridional wind speed, respectively. The integral extends from the surface (p = ps) to the top of the atmosphere (pt = 0.1 hPa). Calculations of w, Qλ, and Qϕ are performed 4 times a day.
Equations (1)(5) represent the classic set of equations used to calculate recycling. However, when using the dynamic recycling ratio calculation developed by Dominguez et al. (2006), we are effectively following the atmospheric moisture along the paths defined by the zonal and meridional moisture fluxes. A new coordinate system (χ = xut, ξ = yυt, τ = t) for the local recycling ratio R(χ, ξ, τ), evaporation ɛ(χ, ξ, τ), and precipitable water ω(χ, ξ, τ) enables us to follow these paths of the moisture flow backward in time. Time-averaged fields are used as input for the calculation of the recycling ratios. In this work, we use monthly averaged fields, because monthly values of the recycling ratios are calculated. We calculate the ratio of moisture from evaporative origin to total moisture within the column throughout the trajectory at every 6-h time step of the reanalysis data, and integrate it from the time the column enters the region until the water precipitates:
i1525-7541-9-5-1073-e6
The advantage of this new model is that it explicitly incorporates the moisture storage term. Thus, the only assumption we have to make is of a well-mixed atmosphere. The value of R can be transformed back again into the original coordinates and then the value of the local recycling ratio ρ can be determined. To calculate the regional recycling ratio, the grid-based approach of Eltahir and Bras (1994) is applied. The regional recycling ratio rr is then a function of the local recycling ratio ρ:
i1525-7541-9-5-1073-e7
where ΔAi is an area consisting of n grid cells. We now have a tool to perform temporal and spatial analyses of the process of precipitation recycling. A more detailed description of the dynamical precipitation recycling model can be found in Dominguez et al. (2006).

b. Performance of the precipitation recycling model

1) Area size

In the next sections, we will discuss the performance of this recycling model with a view toward discovering its main limitations. As we have noticed before, one of the complications in determining the recycling ratio is in defining the size of the area of interest. According to Dominguez et al. (2006), the recycling ratio has a logarithmic relationship with the spatial scale for the United States. To express the recycling ratio as a function of the spatial scale for the European continent, which has different climate dynamics, the recycling model is used for eight nested regions of different sizes. In Fig. 3, the smallest region is approximately 1.5 × 105 km2 and the largest region is approximately 5 × 106 km2. The analysis is centered over an area east of Poland to eliminate the effects of moisture sources from the sea (which is not limited by the precipitation) as much as possible. Figure 4 shows the average summer (JJA) recycling ratio obtained from the 23-yr ERA-40 reanalysis data as a function of the spatial scale. In general, the recycling ratio ranges from 0 to 1 and depends on the scale of the domain, but the increase in the recycling ratio is not linear, but rather will level off at larger scales. Thus, the recycling ratio also has a logarithmic relationship with the spatial scale for the European continent.

Dominguez et al. (2006) used a region size of approximately 1 × 106 km2, which is comparable with the study areas in the United States in the studies of Brubaker et al. (1993) and Eltahir and Bras (1994). In this study, the area size is half of this value.

2) Spinup

The forecast fields of the ERA-40 data are available in 6-, 12-, 24-, and 36-h forecasts. Although the spinup effects on the hydrological cycle of the ERA-40 data are reduced in comparison to ERA-15, this still has an impact on the global water cycle (Hagemann et al. 2002). Precipitation in the 6-h ERA-40 forecasts tends to be lower than for other forecast times due to decreasing spinup problems as the forecast range increases. Forecasts in several time windows are averaged and compared over central Europe for July 1998 in Fig. 5. In general, the 0–12- and the 12–24-h forecasts are similar. The 0–6-h forecast of precipitation tends to miss the peaks compared to the 0–12- and 12–24-h forecasts (Fig. 5). On the other hand, the 0–6-h forecast is not continuously lower in time compared with the higher forecast ranges. The mean monthly value of the precipitation rate is even larger for the 0–6-h forecast than for the 0–12-h forecast. In this work, we calculate monthly values of the recycling ratio and therefore it is expected that the monthly means of the 0–6-, 0–12-, and 12–24-h precipitation rates will be quite similar. So, for the calculation of the recycling ratio we used the data of the 6-hourly forecasts, because the 6-h time step gives a more realistic backward trajectory of a moisture parcel than do larger time steps.

3) Balance closure

For the determination of the recycling ratios, the terms of the water vapor balance are calculated. The water vapor balance equation [Eq. (2)] derived from the ERA-40 dataset has closure problems due to increments in the data assimilation and therefore must be written as
i1525-7541-9-5-1073-e8
where res is a residual term. Figure 6 shows the terms of the moisture budget including the residual term at monthly and seasonal scales. Evaporation is a major term in the water balance for all three regions. The evaporation is larger than the precipitation, which is common for areas in this climate region in the summer months. The divergence term is smaller than the evaporation term for all three regions, suggesting that a part of the moisture source for precipitation must come from recycled evaporation.

The most remarkable feature in the figure is the large negative residual for central Europe (Fig. 6a) and Spain (Fig. 6c), and the positive residual for the Balkans (Fig. 6b). A negative residual suggests moisture accumulation in the atmosphere that is removed by the data assimilation (Draper and Mills 2005), while a positive residual indicates a dry atmosphere. It is expected that the residual term for the 12–24-h forecast is smaller than for the 0–6-h forecast, because of the generally higher precipitation rates in the 12–24-h forecast (Fig. 5). According to Kanamitsu and Saha (1996), this is an inherent systematic error in budget closure studies when using assimilated data. The storage term (∂w/t) is very low for all three regions, which is to be expected on monthly or seasonal scales.

Due to the restrictions discussed above, the recycling ratio remains an approximate value and some caution is necessary when interpreting the results (Trenberth 1999). Nevertheless, the calculated recycling ratios do reflect land–atmosphere interactions in the hydrological cycle and we expect that the restrictions do not cause drastic changes in the recycling ratios patterns.

5. Results: Precipitation recycling over Europe

Figure 7 shows the mean summer recycling ratios (rr) and recycled precipitation [Pr; see Eq. (1)]. Recycled precipitation is precipitation from local evaporation (Dominguez et al. 2006). The area-averaged precipitation rates are divided into large-scale and convective precipitation (lsp and cp, respectively). It is noted that the three subregions have different origins for their precipitation sources. For central Europe, the large-scale precipitation contribution is larger than the convective precipitation, which implies that most of the summer precipitation is originating from large-scale systems from the ocean. For the Balkans, the major sources for summer precipitation are convective systems. For Spain, the distinction between large-scale and convective precipitation is not that obvious.

The dynamic recycling ratios of the three subregions range from 0.05 to 0.3. For central Europe (Fig. 7a), most of the recycling ratios are lower than 0.1, with the exception of the three peak years: 1983, 1995, and 1997. For the summers of 1983 and 1995, the precipitation was below average and it is emphasized that in summers with higher precipitation than average, for example, 1979 and 1987, the recycling ratios are exceptionally low. Thus, precipitation recycling is important in sustaining precipitation mostly in dry summers with little large-scale precipitation. Consequences of high recycling ratios in dry summers are the low absolute values of the recycled precipitation due to the lack of total precipitation. For the Balkans (Fig. 7b), the recycling ratios are higher than 0.1 with three peaks between 0.25 and 0.3. These peaks (1992, 1995, and 1999) are in years with precipitation that is above the climatological average. In these years the amount of convective precipitation is one of the largest in the time series. Thus, precipitation recycling can be important in wet summers with a considerable amount of precipitation of convective origin. For Spain (Fig. 7c), the recycling ratio fluctuates between 0.1 and 0.2, but it is more difficult to link the recycling ratio to the precipitation type. Because Spain is much drier than central Europe and the Balkans, the values of the recycled precipitation are very low.

To understand the conditions under which increased precipitation recycling originates, Fig. 8 shows scatterplots of the recycling and two important factors of the water vapor balance equation: precipitation minus evaporation (PE) and the water vapor flux associated with moisture transport. Each point is a monthly mean for June–August, and the shading indicates high (dark) and low (light) evaporation rates. For central Europe, both the recycling ratio (Fig. 8a) and the recycled precipitation (Fig. 8b) appear to have no relationship with PE, but the recycling ratio shows a clear relationship with evaporation. The recycling ratio decreases with increasing water vapor flux and is larger for higher values of evaporation (Fig. 8c). Recycled precipitation also decreases with increasing water vapor flux, but appears to be less dependent on evaporation (Fig. 8d). For the Balkans, the recycling ratio has a low correlation with both PE and evaporation (Fig. 8e). The recycled precipitation increases with increasing PE (Fig. 8f). The recycling ratio increases for decreasing moisture transport, and it increases for higher values of evaporation (Fig. 8g). The recycled precipitation is also dependent on moisture transport and evaporation, but there is considerably more scatter for this region compared to the others (Fig. 8h). For Spain, both the recycling ratio (Fig. 8i) and the recycled precipitation (Fig. 8j) appear to have no dependency on either PE or evaporation. Most months are clustered around low values of the recycling and PE. The recycling ratio decreases with increasing water vapor flux, but has no correlation with evaporation (Fig. 8k). Both small and larger values for the water vapor flux result in low values for the recycled precipitation. There are a few outliers with a value higher than 0.1 for the recycled precipitation due to the relativly high precipitation rate in these months (Fig. 8l).

Summarizing, it appears that the degree of horizontal moisture transport is an important factor for recycling. In general the recycling ratios and the recycled precipitation are low at high values of the water vapor flux. For central Europe, large values of the water vapor flux result in more large-scale systems, which suppress the precipitation recycling in summer. Therefore, for precipitation recycling in central Europe to be important, the horizontal water vapor transport must be low. This corresponds to generally dry summers. This also means that the absolute amount of recycled precipitation is low because of the reduction in the total precipitation. These results are very similar to those of Bosilovich and Schubert (2001). They computed precipitation recycling by focusing on the central United States. They found in the dry summer of 1988 a large recycling ratio as a result of the lack of moisture transport.

To give an example of the differences in circulation patterns between a dry and a wet summer, we show the spatial distribution of the precipitation minus evaporation fields and the mean summer moisture transport for 1987 in Fig. 9a and 1995 in Fig. 9b. The summer of 1987 corresponds to a wet summer and 1995 corresponds to a dry summer for central Europe. These wet and dry summers differ in the magnitudes of the moisture transports. In 1987, there is a large amount of moisture transport to central Europe and the recycling ratio has one of the lowest values (0.05) in the time series (Fig. 7a). For 1995, the moisture transport is very low and the recycling ratio has a peak of 0.20. For the Balkans, small amounts of moisture transport, high PE, and evaporation rates are indicators of strong positive land–atmosphere feedbacks. This corresponds to generally wet summers when convective precipitation is the dominant precipitation source. Figure 9 illustrates this again for the dry year of 1987 and the wet year of 1995 over the Balkan region. The moisture transport in 1987 is much larger than in 1995. Moreover, in 1995 the Balkans is a moisture sink (P > E). This implies that the availability of surface moisture is large. The recycling ratio is 0.10 in 1987 and 0.30 in 1995 (Fig. 7b). This confirms our expectation that local evaporation contributes more to the precipitation amount in wet years than in dry years. For Spain the picture is more complex. The recycling ratio is 0.11 in 1987 and 0.22 in 1995 (Fig. 7c). Although there are indications that large values of the moisture transport suppress the recycling ratio, the circulation patterns in the summers of 1987 and 1995 are almost similar. In summer, the atmospheric circulation over the Iberian Peninsula is dominated by the Azores anticyclone, but it is suggested that precipitation has a very local (convective) character that is independent from the circulation pattern (Trigo and DaCamara 2000). The total precipitation shows very low values and therefore the recycled precipitation is nearly zero.

6. Concluding remarks

In this study, the dynamical precipitation recycling model of Dominguez et al. (2006) has been successfully applied to produce recycling ratios for three regions in Europe. Twenty-three years’ worth of data from the ERA-40 reanalysis dataset were used to estimate the terms of the water vapor balance equation as an input for the recycling model. It is a well-known feature that the ERA-40’s 6-h precipitation forecast yields less precipitation than observation datasets (Hagemann et al. 2002). However, the use of the ERA-40 precipitation data has hardly any consequences on the calculated recycling ratio. The regional recycling ratio will be a little higher than would be obtained using datasets based on observed precipitation. The most immediate consequence of the slightly higher recycling ratio and lower precipitation values is the lower absolute values of the recycled precipitation. This will not affect our conclusions.

We applied a new dynamic recycling model that does not neglect the moisture storage, and therefore the model gives a more realistic presentation of the feedback processes. Additionally, the new model can be linked to meteorological processes on a daily time scale. However, the new model remains scale dependent and therefore results must be interpreted cautiously. Nevertheless, the precipitation recycling model is a good quantitative tool to use in finding out which and when areas are susceptible for land–atmosphere interactions. Dominguez et al. (2006) compared the new dynamical recycling model with the classic models of Eltahir and Bras (1994) and Brubaker et al. (1993). The new recycling model shows comparable spatial and temporal variabilities, indicating good performance.

Recycling is an important process in understanding the hydrological cycle. Results indicate that in general lateral moisture transport is a key factor for the existence of local precipitation from local evaporation. Large amounts of moisture transport into Europe will suppress the recycling, but for each of the three subregions in Europe this will occur in different manners:

  • For central Europe, local evaporation contributes more to precipitation in dry summers. The moisture transport is weak, which minimizes the advected moisture from external sources (and suppresses the arrival of large-scale systems). High evaporation rates in combination with a weak moisture transport have a positive effect on the recycling ratio. The consequences of high recycling ratios in a dry summer are low total precipitation and the limitation of evaporation (P < E), which results in low absolute values of recycled precipitation.
  • For the Balkans, precipitation recycling is large in wet summers when convective precipitation is the dominant precipitation source. This occurs when moisture transport is weak and the evaporation and PE are large. Thus, the recycling process intensifies the hydrological cycle due to a positive feedback through the generation of convective precipitation. In other words, the combination of weak moisture transport and the availability of surface moisture is important for the existence of recycled precipitation from local evaporation. Therefore, the regional hydrological cycle of the Balkans may be particularly susceptible to either future climate change or changes in land cover.
  • For Spain, recycling is large when moisture transport is weak. Evaporation and PE do not appear to be correlated with the recycling. Moreover, the total precipitation amount in summer is very small and has a very local (convective) character. Therefore, it is hard to conclude what contribution recycling makes to the hydrological cycle.

In general, the European continent has the potential for precipitation recycling. This is in good agreement with the findings of Schär et al. (1999) that summertime European precipitation is highly sensitive to the soil moisture content. However, moisture transport is the dominant factor most of the time and suppresses the precipitation recycling. In summers in which the moisture transport is not dominant, evaporation is a limiting factor for the occurrence of precipitation of recycled origin. Thus, feedback processes over land can be important in dry periods when the limitation of evaporation causes high sensible heat fluxes. This process can be important in central Europe for sustaining a high pressure cell over the European continent, such as during the heat wave of 2003. Seneviratne et al. (2006) found that soil moisture feedback plays a crucial role in the European summer heat wave (temperature variability) caused by global warming. Farther inland, a different physical mechanism drives the precipitation recycling. For the Balkans, convection is the dominant precipitation source and evaporation is not limited. In these areas precipitation recycling makes a significant contribution to the local hydrological cycle.

Acknowledgments

This work was carried out within the framework of the ACER project under the Dutch National Research Program “Climate Changes Spatial Planning.”

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Fig. 1.
Fig. 1.

Three subregions of Europe used in this study and their mean summer (June, July, and August) precipitation (mm day−1) SP, Spain (37.5°–43.5°N, 9°W–0°); CE, central Europe (46.5°–52.5°N, 3°–12°E); and BA, Balkans (43.5°–49.5N°, 18°–27°E).

Citation: Journal of Hydrometeorology 9, 5; 10.1175/2008JHM962.1

Fig. 2.
Fig. 2.

Precipitation minus evaporation (mm day−1) and the vectors of the vertically integrated moisture transport (kg ms−1) for the mean of JJA calculated for the period 1979–2001.

Citation: Journal of Hydrometeorology 9, 5; 10.1175/2008JHM962.1

Fig. 3.
Fig. 3.

Area sizes of the eight nested regions.

Citation: Journal of Hydrometeorology 9, 5; 10.1175/2008JHM962.1

Fig. 4.
Fig. 4.

Recycling ratio as a function of area size for the summer (JJA).

Citation: Journal of Hydrometeorology 9, 5; 10.1175/2008JHM962.1

Fig. 5.
Fig. 5.

Time series (July 1998) of precipitation (mm day−1) at several forecast times averaged over central Europe.

Citation: Journal of Hydrometeorology 9, 5; 10.1175/2008JHM962.1

Fig. 6.
Fig. 6.

Time series of the water vapor balance terms (mm day−1) averaged over (a) central Europe, (b) the Balkans, and (c) Spain for 1979–2001: P, precipitation; E, evaporation; ∂w/t, change in atmospheric water balance storage; divQ, water vapor flux divergence; and res, residual.

Citation: Journal of Hydrometeorology 9, 5; 10.1175/2008JHM962.1

Fig. 7.
Fig. 7.

Time series of the mean summer (JJA) recycling ratio (rr) and area-averaged precipitation divided into convective precipitation (cp), large-scale precipitation (lsp), and recycled precipitation (Pr) for the three subregions: (a) central Europe, (b) the Balkans, and (c) Spain. Note that the precipitation scale in (c) ranges only from 0 to 1 mm day−1.

Citation: Journal of Hydrometeorology 9, 5; 10.1175/2008JHM962.1

Fig. 8.
Fig. 8.

Comparison of area-averaged monthly mean (JJA) hydrological data for (a)–(d) central Europe, (e)–(h) Balkans, and (i)–(l) Spain: (a), (e), (i) rr vs PE; (b), (f), (j) Pr vs PE; (c), (g), (k) rr vs the magnitude of the water vapor flux transport; and (d), (h), (i) Pr vs the magnitude of the water vapor flux transport. Shading depicts evaporation (mm day−1).

Citation: Journal of Hydrometeorology 9, 5; 10.1175/2008JHM962.1

Fig. 9.
Fig. 9.

Precipitation minus evaporation (mm day−1) and horizontal transport (kg ms−1) as in Fig. 2 but for the summers (JJA) of (a) 1987 and (b) 1995.

Citation: Journal of Hydrometeorology 9, 5; 10.1175/2008JHM962.1

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