Precipitation Feedback Research Articles

Feedbacks of Vegetation on Summertime Climate Variability over the North American Grasslands. Part II: A Coupled Stochastic Model

Abstract A coupled linear model is derived to describe interactions between anomalous precipitation and vegetation over the North American Grasslands. The model is based on biohydrological characteristics in the semiarid environment and has components to describe the water-related vegetation variability, the long-term balance of soil moisture, and the local soil–moisture–precipitation feedbacks. Analyses show that the model captures the observed vegetation dynamics and characteristics of precipitation variability during summer over the region of interest. It demonstrates that vegetation has a preferred frequency response to precipitation forcing and has intrinsic oscillatory variability at time scales of about 8 months. When coupled to the atmospheric fields, such vegetation signals tend to enhance the magnitudes of precipitation variability at interannual or longer time scales but damp them at time scales shorter than 4 months; the oscillatory variability of precipitation at the growing season time scale (i.e., the 8-month period) is also enhanced. Similar resonance and oscillation characteristics are identified in the power spectra of observed precipitation datasets. The model results are also verified using Monte Carlo experiments.

Feedbacks of Vegetation on Summertime Climate Variability over the North American Grasslands. Part I: Statistical Analysis

Abstract Feedbacks of vegetation on summertime climate variability over the North American Grasslands are analyzed using the statistical technique of Granger causality. Results indicate that normalized difference vegetation index (NDVI) anomalies early in the growing season have a statistically measurable effect on precipitation and surface temperature later in summer. In particular, higher means and/or decreasing trends of NDVI anomalies tend to be followed by lower rainfall but higher temperatures during July through September. These results suggest that initially enhanced vegetation may deplete soil moisture faster than normal and thereby induce drier and warmer climate anomalies via the strong soil moisture–precipitation coupling in these regions. Consistent with this soil moisture–precipitation feedback mechanism, interactions between temperature and precipitation anomalies in this region indicate that moister and cooler conditions are also related to increases in precipitation during the preceding months. Because vegetation responds to soil moisture variations, interactions between vegetation and precipitation generate oscillations in NDVI anomalies at growing season time scales, which are identified in the temporal and the spectral characteristics of the precipitation–NDVI system. Spectral analysis of the precipitation–NDVI system also indicates that 1) long-term interactions (i.e., interannual and longer time scales) between the two anomalies tend to enhance one another, 2) short-term interactions (less than 2 months) tend to damp one another, and 3) intermediary-period interactions (4–8 months) are oscillatory. Together, these results support the hypothesis that vegetation may influence summertime climate variability via the land–atmosphere hydrological cycles over these semiarid grasslands.

Inferred bimodality in the distribution of soil moisture at Big Meadows, Shenandoah National Park, Virginia

Recent work has shown that distinct steady‐state modes of soil moisture frequency can occur resulting in soils being preferentially in either dry states or wet states. Bimodal distributions of summer soil moisture can be due to precipitation feedback, to variations in climate from early summer to late summer, or to interannual rainfall variability. We examined the frequency distribution in the Palmer Drought Severity Index at Big Meadows, which is a relatively flat, open area in the Blue Ridge Mountains of Virginia. We found that a pronounced bimodality exists not only during the summer months, but also during the remainder of the year. Furthermore, our analyses show that the bimodality in the distribution exists for the month of August alone, suggesting that there are intrinsically dry states and wet states and that the bimodality is not merely a result of climate differences from early to late summer.

Characterization of land-surface precipitation feedback regimes with remote sensing

A simple methodology for characterizing the land surface's response to precipitation is proposed based on the average behavior of coarse resolution remote sensing data. Feedback regimes are designated based on the temporal correlations between the vegetation and precipitation and the surface temperature and precipitation. The different feedback regimes are linked to the relative importance of vegetation and soil moisture in determining land–atmosphere interactions. The resultant feedback regimes are well localized spatially. In addition, the temporal dynamics are assessed in terms of lagged covariances and phase–space plots. The two dominant feedback regimes are distinguishable by the vegetation–precipitation correlation, implying that vegetative control may be the dominant factor in influencing surface-precipitation feedbacks. These zones are not easily distinguished in the NDVI-surface temperature correlations, but exhibit different behavior in their NDVI-precipitation and surface temperature–precipitation correlations. In addition, these zones can be distinguished in phase plots of both temperature and NDVI. These results suggest a methodology for quantifying the rate of the hydrological cycle in different regions of the globe as well as identifying areas where human induced land cover change may have the most effect transitioning between different feedback zones, and therefore altering the rate of the hydrologic cycle.

Weak Land–Atmosphere Coupling Strength in HadAM3: The Role of Soil Moisture Variability

Abstract A recent model intercomparison, the Global Land–Atmosphere Coupling Experiment (GLACE), showed that there is a wide range of land–atmosphere coupling strengths, or the degree that soil moisture affects the generation of precipitation, amongst current atmospheric general circulation models (AGCMs). Coupling strength in the Hadley Centre atmosphere model (HadAM3) is among the weakest of all AGCMs considered in GLACE. Reasons for the weak HadAM3 coupling strength are sought here. In particular, the impact of pervasive saturated soil conditions and low soil moisture variability on coupling strength is assessed. It is found that when the soil model is modified to reduce the occurrence of soil moisture saturation and to encourage soil moisture variability, the soil moisture–precipitation feedback remains weak, even though the relationship between soil moisture and evaporation is strengthened. Composites of the diurnal cycle, constructed relative to soil moisture, indicate that the model can simulate key differences in boundary layer development over wet versus dry soils. In particular, the influence of wet or dry soil on the diurnal cycles of Bowen ratio, boundary layer height, and total heat flux are largely consistent with the observed influence of soil moisture on these properties. However, despite what appears to be successful simulation of these key aspects of the indirect soil moisture–precipitation feedback, the model does not capture observed differences for wet and dry soils in the daily accumulation of boundary layer moist static energy, a crucial feature of the feedback mechanism.

Understanding Hydrometeorology Using Global Models

The land surface coupling, a crucial element of the climate system, is explored in the recent 40-yr European Centre for Medium–Range Forecasts (ECMWF) reanalysis (ERA-40) model. In seasonal forecasts for the Northern Hemisphere summer, initialized with idealized soil moisture fields, the ERA-40 model has a large evaporation–precipitation feedback over the continents, and the memory of initial soil moisture is longest at high northern latitudes. Thirty years of hourly data from the ERA-40 reanalysis are averaged over the Madeira, Red–Arkansas, and Athabasca River basins. Although the model fully resolves the diurnal cycle and has an interactive prognostic cloud field, the transitions in the boundary layer climate over land can be mapped with remarkable precision by the daily mean state and daily flux averages. The coupling to cloud processes plays an essential role in the surface and boundary layer equilibrium. Soil moisture, cloud base, cloud cover, radiation fields, and evaporative fraction are coupled quite tightly on daily time scales. The long wave flux control by cloud-base height and cloud cover is particularly strong across all basins. Evaporation can be regarded as being determined somewhat indirectly by the dependence of net radiation on cloud cover and cloud base, and sensible heat flux on subcloud-layer processes. Cloud and boundary layer processes and the land surface components of a model must be evaluated as a tightly coupled system, not as independent components. This analysis provides a new framework for comparing global models with each other, and for evaluating them against observations.

Soil moisture–precipitation feedback on the North American monsoon system in the MM5‐OSU model

AbstractIn this study, the Pennsylvania State University/National Center for Atmospheric Research fifth generation Mesoscale Model (MM5) linked to the Oregon State University (OSU) land‐surface scheme, is used to assess the strength of soil moisture–precipitation feedback in the region of influence of the North American monsoon (NAM). Two control simulations are made with external forcing taken from the National Centers for Environmental Prediction re‐analysis, and with a nested horizontal resolution of 30 km, for the period 1 June to 30 September in wetter than average (1999) and drier than average (2000) monsoon seasons. These two model runs are then repeated with a prescribed precipitation rate anomaly in July over the entire NAM region, and comparisons made between atmospheric and land‐surface states in the two control runs and the two runs with anomalous precipitation. The results show that size and importance of soil moisture–precipitation feedbacks in the NAM region have substantial interannual variability, and that the resulting behaviour has a strong dependency on the intensity of the prescribed precipitation anomaly. It is also shown that a marked precipitation anomaly in the NAM region results in modified soil moisture, rainfall, and surface temperature, which persist for about one month, and that a precipitation anomaly within the NAM region not only has an impact on soil moisture locally, but also causes a remote, downwind soil moisture anomaly one month later. Analysis of the modelled response to the soil moisture anomaly indicates that not only land–atmosphere interactions, but also the large‐scale atmospheric circulation act together to determine the modified precipitation and soil moisture fields in the NAM system. Copyright © 2004 Royal Meteorological Society

The Stabilization of the Thermohaline Circulation by the Temperature–Precipitation Feedback

Abstract The meridional freshwater flux in the atmosphere, and hence the strength of the hydrological cycle, undergoes variations on glacial–interglacial as well as on some shorter timescales. A significant portion of these changes to the hydrological cycle are due to the temperature–precipitation feedback according to which there is more precipitation over the higher latitudes during warm periods when the moisture holding capacity of the atmosphere is higher. It is proposed here that this feedback may play an important role in determining the stability of the thermohaline circulation (THC). The THC stability to different parameterizations of the meridional atmospheric freshwater flux is therefore investigated using a simple box model of the ocean, atmosphere, and sea ice. It is demonstrated that parameterizations that are consistent with the temperature–precipitation feedback, and hence with the observed variations of the hydrological cycle during glacial–interglacial cycles, stabilize the THC for a wide range of forcing parameters.

The Soil–Precipitation Feedback: A Process Study with a Regional Climate Model

Month-long integrations with a regional climate model covering Europe and the Northern Atlantic are utilized to study the sensitivity of the summertime European precipitation climate with respect to the continental-scale soil moisture content. Experiments are conducted for July 1990 and 1993. For each of the two months, the control experiment with the initial soil water distribution derived from the operational ECMWF analysis is compared against two sensitivity experiments with dry and wet initial soil moisture distributions. The results demonstrate that summertime European precipitation climate in a belt ∼1000 km wide between the wet Atlantic and the dry Mediterranean climate heavily depends upon the soil moisture content. In this belt, changes in monthly mean precipitation amount to about half of the changes in mean evapotranspiration. Budget analysis of water substance over selected subdomains demonstrate that the simulated sensitivity cannot be interpreted with the classical recycling mechanism, that is, the surplus of precipitation that falls over wet (as compared to dry) soils does not directly derive from evapotranspiration. Rather, the surplus of precipitation primarily originates from water vapor extracted from the ambient atmospheric flow. Thus, the soil–precipitation feedback must rely on some indirect mechanism, whereby wet soils increase the efficiency of convective precipitation processes. In order to isolate the physical mechanisms underlying the soil–precipitation feedback, a detailed analysis including an investigation of the mean diurnal cycle throughout the integration period is performed. The key elements of the feedback are the following. First, wet soils (small Bowen ratios) imply the buildup of a comparatively shallow boundary layer. The surface fluxes of heat and moisture are thus concentrated into a comparatively small volume of air, leading to the buildup of high values of low-level moist entropy, thereby providing a source of convective instability. Second, the level of free convection is lowered in the wet experiment, thus facilitating the release of convective instability. Third, the net shortwave absorption at the soil decreases in the wet experiment (as a result of increased cloud cover), but this effect is overpowered by the decrease in net longwave emission (as a result of decreased emission, increased cloud backscatter, and increased water vapor greenhouse effect). Thus the net radiative energy flux is larger in the wet experiments, thereby increasing the moist entropy fluxes into the boundary layer. These three processes act in concert to increase the potential for convective activity.