Clim Dyn DOI 10.1007/s00382-015-2593-2
Impact of moisture flux convergence and soil moisture on precipitation: a case study for the southern United States with implications for the globe Jiangfeng Wei1 · Hua Su1 · Zong‑Liang Yang1
Received: 24 December 2014 / Accepted: 1 April 2015 © Springer-Verlag Berlin Heidelberg 2015
Abstract Interactions between soil moisture, evapotranspiration (ET), atmospheric moisture fluxes and precipitation are complex. It is difficult to attribute the variations of one variable to another. In this study, we investigate the influence of atmospheric moisture fluxes and land surface soil moisture on local precipitation, with a focus on the southern United States (U.S.), a region with a strong humidity gradient and intense moisture fluxes. Experiments with the Weather Research and Forecasting model show that the variation of moisture flux convergence (MFC) is more important than that of soil moisture for precipitation variation over the southern U.S. Further analyses decompose the precipitation change into several contributing factors and show that MFC affects precipitation both directly through changing moisture inflow (wet areas) and indirectly by changing the precipitation efficiency (transitional zones). Soil moisture affects precipitation mainly by changing the precipitation efficiency, and secondly through direct surface ET contribution. The greatest soil moisture effects are over transitional zones. MFC is more important for the probability of heavier rainfall; soil moisture has much weaker impact on rainfall probability and its roles are similar for the probability of intermediate-to-heavy rainfall (>10 mm day−1). Although MFC is more important than soil moisture for precipitation over most regions, the impact of soil moisture could be large over certain transitional regions. At the submonthly time scale, the African Sahel appears to be the only major region where soil
* Jiangfeng Wei
[email protected] 1
Center for Integrated Earth System Science, Jackson School of Geosciences, University of Texas at Austin, 2275 Speedway C9000, Austin, TX 78712, USA
moisture has a greater impact than MFC on precipitation. This study provides guidance to understanding and further investigation of the roles of local land surface processes and large-scale circulations on precipitation. Keywords Southern U.S. · WRF · Soil moisture · Moisture flux convergence · Land–atmosphere interaction
1 Introduction Water vapor for precipitation comes from both local evapotranspiration (ET) and remote moisture transport. The contribution of local ET to local precipitation is called precipitation recycling, and its amplitude (called recycling ratio) is closely related to local land surface processes (Eltahir and Bras 1996; Trenberth 1999; Wei et al. 2012). Moreover, rain-producing weather systems usually draw in moisture from distances about 3–5 times the radius of precipitation region (Trenberth et al. 2003). Therefore, remote moisture may also be very important for precipitation. Past studies from Global Land–Atmosphere Coupling Experiments (GLACE; Koster et al. 2004, 2006) have identified regions where soil moisture may have a strong impact on precipitation, but how strong are the soil moisture effects compared to other effects is not clear. Modeling studies for Europe (Schär et al. 1999) and India (Asharaf et al. 2012) found that the soil moisture anomalies affect precipitation mainly through an indirect process, in which precipitation efficiency (the percentage of moisture entering an atmospheric column that falls as precipitation) is altered so that precipitation is easier or more difficult to occur, and the direct moisture contribution from land surface ET is relatively small. Based on highly constrained reanalysis data, Findell et al. (2011)
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(a)
(b)
(c)
(d)
Fig. 1 1979–2012 May–September average a precipitation from PREC/L, b ET and c MFC from ERA-Interim, and d monthly anomaly correlation between precipitation and MFC in the domain of WRF
model simulations. Units of a–c: mm day−1. Seasonal cycles were removed before calculating the correlation. Red box encloses the southern U.S. study region
discovered that high evaporation enhances the probability of afternoon rainfall over eastern U.S. and Mexico, but its impact on rainfall intensity is weak, a result similar to that found by D’Odorico and Porporato (2004) over Illinois. Ruiz-Barradas and Nigam (2005, 2013) found that remote moisture sources are more important than local ET for precipitation variability over the U.S. Great Plains, and argued against the strong soil moisture-precipitation feedback found in some modeling studies (e.g., Koster et al. 2004). Li et al. (2013) showed similar findings for the Southeastern U.S. Lately, Su et al. (2014) found that the impact of soil moisture on precipitation over the Southern Great Plains is closely related to the large-scale circulations. It is evident that there are complex impacts on precipitation from both local and remote sources. However, there is still no clear understanding of the relative importance of local land surface and large-scale atmospheric processes (such as moisture transport) to precipitation, and of how their interactions may shape precipitation,
together with the climatic and regional dependency of these interactions. The southern United States (U.S.) is a major center of economic activity near the Gulf of Mexico. This region is strongly affected by the moisture transport from the oceans and is vulnerable to hurricanes, severe thunderstorms, floods, and droughts. Precipitation in this region is related to regional and large-scale processes, including the Great Plains low-level jet (GPLLJ), North Atlantic subtropical high (NASH), North Atlantic Oscillation (NAO), and El Niño–Southern Oscillation (ENSO) (Weaver and Nigam 2008; Weaver et al. 2009; Cook et al. 2008; Schubert et al. 2009; Li et al. 2011). Also, the climate of this region has a strong gradient from very dry in the west to very wet in the east (Fig. 1a). Both model simulations (Koster et al. 2004) and reanalysis data (Wei and Dirmeyer 2012; Mei and Wang 2012) have shown that in the transitional climate over central Texas, soil moisture has a relatively strong impact on precipitation. Therefore, precipitation over the
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Impact of moisture flux convergence and soil moisture on precipitation...
southern U.S. may be affected by both atmospheric moisture transport and soil moisture, whose roles under different climates in this unique region are interesting to study. Interactions between soil moisture, ET, moisture fluxes and precipitation are complex (e.g., Dirmeyer et al. 2009; Wei et al. 2012, 2013). Rather than being a completely external factor that affects precipitation, the moisture fluxes may respond to precipitation changes, especially during deep convection (Zeng and Neelin 1999). The soil moisture change could also impact the large-scale moisture fluxes, especially in the monsoon regions where the wind and moisture fluxes are strongly affected by the land-sea thermal contrast (Douville et al. 2001; Asharaf et al. 2012). It is difficult, in observations, to separate the impacts of soil moisture and moisture fluxes on precipitation. In this study, well-controlled experiments with a regional climate model were designed to explore their respective impacts over the southern U.S. The results from model simulations are further examined by reanalysis data at regional and global scales.
2 Datasets and initial analyses 2.1 Datasets We used 1979–2012 reanalysis datasets from the North American Regional Reanalysis (NARR; Mesinger et al. 2006), the European Centre for Medium-Range Weather Forecasts (ECMWF) Interim Reanalysis (ERA-Interim) (Dee et al. 2011), and the Modern Era Retrospectiveanalysis for Research and Applications (MERRA) (Rienecker et al. 2011). The use of several different datasets can account for the potential uncertainties in the reanalysis products. Observation-based NOAA PRECipitation REConstruction over Land (PREC/L) monthly precipitation product at 0.5° × 0.5° resolution (Chen et al. 2002) and CPC Unified gauge-based analysis of global daily precipitation at 0.5° × 0.5° (Xie et al. 2007; Chen et al. 2008) are also used for model evaluation. The two different precipitation data and the precipitation from NARR are all observation-based and their differences are very small; their usage in this study is based on their temporal and spatial resolution and their consistency with the reanalysis data (NARR precipitation is usually used with other NARR data). 2.2 Initial analyses To investigate the impact of moisture fluxes, we used the metric of the vertically integrated moisture flux convergence (MFC). MFC is an important component of the atmospheric water balance equation
∂Q = E − P + MFC, ∂t
(1)
where Q is the amount of water vapor in the atmospheric column, t is time, E is ET, and P is precipitation. MFC is calculated as
1 MFC = − ∇ · g
Ps
qV� dp,
(2)
0
where g is the gravitational acceleration, Ps is surface pressure, q is specific humidity, V is wind vector, and p is pressure. In this study, we used the MFC output from NARR and ERA-Interim, which were calculated by the reanalysis models at each time step and then averaged. The land surface water balance equation is
∂S = P − E − R, ∂t
(3)
where S is soil moisture, and R is runoff. As the longterm mean of atmospheric moisture change is small (∂Q/∂t ≈ 0), if there is no long-term trend in the soil moisture (∂S/∂t = 0), the mean MFC is very close to the mean runoff for the long term (Eqs. 1, 3). MFC has more direct effects on available moisture in an area than moisture fluxes (qV ), which only measures the moisture passing through an area. Because of its close relationship with precipitation, MFC has been used for precipitation parameterization and forecasting (Banacos and Schultz 2005). For the long term, MFC is balanced by the precipitation and ET MFC ≈ P − E . Figure 1 shows that, for the May–September warm seasons, MFC or P − E are mostly negative over U.S. continent. This is consistent with an observation-based analysis (Fig. 7 in Swenson and Wahr 2006). In cold seasons, P − E and MFC are mostly positive (not shown). MFC has very large temporal variabilities—it is positive before and during precipitation events and negative after precipitation events when the water vapor from follow-on ET is transported out of the domain. Therefore, the value of MFC is strongly affected by the timescale. The monthly anomaly correlation between MFC and precipitation shows some resemblance to the pattern of precipitation climatology, with stronger positive correlation over regions with higher precipitation (Fig. 1d). This indicates that MFC may be more important for precipitation in wetter regions, while in drier regions other factors, such as local surface conditions, may have a strong impact. This needs further investigation. Note that the effects of MFC and soil moisture (or ET) on precipitation are not simple hydrological processes that can be calculated with the water balance equation. Their effects also involve complex thermodynamics such as the atmospheric stability change. Some clues may be found
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by detailed lead-lag analysis of the components of water or energy cycles (e.g., Findell et al. 2011), but the signals are usually mixed with all kind of atmospheric variabilities, making attribution difficult (e.g., Wei et al. 2008). Model simulations are often used to separate the different effects (like in GLACE). For the southern U.S. region (105°W–85°W, 28.5°N–36.5°N; Fig. 1) and May–September 1979–2012, we performed a multivariate Empirical Orthogonal Function (MV-EOF) analysis for MFC based on NARR dataset. MV-EOF analysis separates dominant coupled modes of two or more variables (e.g., Jin et al. 2014). Each variable was standardized by its mean standard deviation in the domain so that the variance of the different variables was comparable, and each variable maintains its own spatial distribution of variance in the analysis. Figure 2 shows that the entire region has a generally consistent monthly variability of MFC and precipitation, which represents a third of the total variance. This means that, in general, the entire region is temporally consistent regarding MFC and precipitation. Therefore, for this period, we can select 16 months with the greatest MFC and 16 with the least MFC (divergent moisture) for further analysis (Table 1). Figure 3 shows that the NASH retreats eastward during the high MFC months compared with the low MFC months. When regressing the 850 hPa wind and SST on the PC1, a cyclone appears over the southern U.S. and some weak sea surface cooling appears over the western central Atlantic, the Caribbean Sea, and the Gulf of Mexico (Fig. 3). Cooler ocean and land (because of more precipitation) may squeeze the NASH eastward. Enhanced wind flow from the Caribbean and western north Atlantic to the southern U.S. are also shown. There is no significant relationship between MFC and ENSO during this period. PC1 has a significant negative correlation with the strength of GPLLJ (−0.26; p
