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Christopher Taylor

Abstract

This paper investigates the parameterization of rainfall interception by a vegetation canopy in GCMS. Recent land surface schemes have introduced canopy water balance equations, which include sub-grid scale rainfall variability. However, current models do not distinguish between wet and dry areas when solving the set of energy balance equations. An alternative method is set out here that solves one set of surface flux equations for the wet area and one for the dry area. The approximation of using a single surface temperature to determine the surface fluxes is shown to overestimate the wet canopy evaporation from a forest by typically 30%. When implemented in the single column version of the U.K. Meteorological Office GCM, this excessive evaporation leads to significant overprediction of modeled interception loss from Amazonian rain forest.

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Christopher M. Taylor

Abstract

Via its impact on surface fluxes, subseasonal variability in soil moisture has the potential to feed back on regional atmospheric circulations, and thereby rainfall. An understanding of this feedback mechanism in the climate system has been hindered by the lack of observations at an appropriate scale. In this study, passive microwave data at 10.65 GHz from the Tropical Rainfall Measuring Mission satellite are used to identify soil moisture variability during the West African monsoon. A simple model of surface sensible heat flux is developed from these data and is used, alongside atmospheric analyses from the European Centre for Medium-Range Weather Forecasting (ECMWF), to provide a new interpretation of monsoon variability on time scales of the order of 15 days. During active monsoon periods, the data indicate extensive areas of wet soil in the Sahel. The impact of the resulting weak surface heat fluxes is consistent in space and time with low-level variations in atmospheric heating and vorticity, as depicted in the ECMWF analyses. The surface-induced vorticity structure is similar to previously documented intraseasonal variations in the monsoon flow, notably a westward-propagating vortex at low levels. In those earlier studies, the variability in low-level flow was considered to be the critical factor in producing intraseasonal fluctuations in rainfall. The current analysis shows that this vortex can be regarded as an effect of the rainfall (via surface hydrology) as well as a cause.

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Christopher M. Taylor and Thierry Lebel

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This paper examines observational evidence of a positive feedback between the land surface and rainfall in semiarid conditions. The novelty of the work lies in the length scale of study, investigating interactions between soil moisture patterns and deep convection at scales of less than 20 km. The feedback mechanism was proposed in a previous study to explain the development of an anomalous rainfall gradient in the West African Sahel. The aim here is to assess whether such rainfall persistence occurs elsewhere in the region.

Convective-scale rainfall patterns are examined using two years of observations from a dense rain gauge network in southwest Niger. Rainfall differences are analyzed between neighboring gauges separated by 7.5–15 km. Under certain surface conditions, a positive correlation between daily and antecedent rainfall differences is established. These circumstances arise when previous storm patterns have modified local evaporation rates. Rainfall gradients in subsequent events tend to persist, reinforcing soil moisture patterns. The effect appears to be most pronounced in mature, large-scale storms. The widespread occurrence of persistence in the dataset provides strong observational evidence of a surface feedback mechanism, with surface-induced low-level humidity anomalies locally enhancing convection in passing storms. Several rainfall patterns that persist for a month are identified. These patterns are linked to surface processes and the frequency of storm passage.

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Christopher M. Taylor, Frédérique Saïd, and Thierry Lebel

Abstract

The Hydrological Atmospheric Pilot Experiment in the Sahel (HAPEX-Sahel) was designed to investigate land–atmosphere interactions in the semiarid conditions of southwest Niger. During the intensive observation period (IOP) in 1992, a pronounced mesoscale rainfall gradient developed over the Southern Super Site (SSS). Measurements from a high-resolution rain gauge network indicate that over a distance of 9 km, cumulative rainfall in the final 7 weeks of the wet season (31 July–18 September) ranged from 224 mm in the south to 508 mm in the north. The extreme rainfall gradient is not apparent in other years and evolves through persistent local intensification of convection in passing large-scale storms. This paper assesses the influence of the rainfall variability on the surface and atmosphere, and explores the possibility of a land surface feedback on rainfall at this scale.

Soil moisture estimates across the SSS illustrate the importance of rainfall on the water balance and indicate that gradients of soil moisture deficit are likely throughout the IOP. Observations from the three dominant vegetation types reveal the sensitivity of available energy and evaporative fraction to antecedent rainfall. This arises from the high coverage of bare soil and the growth response of Sahelian vegetation to soil moisture. A broad range of evaporation rates are found, while sensible heat fluxes are generally less sensitive to antecedent rainfall. Surface and airborne measurements of temperature and humidity show that rainfall-induced surface variability across the SSS leads to mesoscale gradients in properties of the planetary boundary layer (PBL). On a day with light winds, a thermally induced area of PBL convergence associated with antecedent rainfall conditions is observed.

A surface feedback mechanism has been proposed to explain the organization of rainfall on scales of about 10 km. Typical Sahelian surface conditions generate large anomalies of low-level moist static energy following mesoscale rainfall events. This variability influences the development of individual convective cells within subsequent larger-scale disturbances. The anomalous rainfall pattern at the SSS is linked to typical spatial scales of a convective cell and the preferred direction of travel of Sahelian squall lines. This hypothesis is supported by the temporal variability of the rainfall anomalies. Differences in precipitation across the SSS show a pronounced diurnal cycle in phase with PBL anomalies and are largest during periods when surface variability is high. A case study is also presented from an isolated convective storm over the SSS. This highlights the sensitivity of deep convective instabilities to PBL anomalies of the magnitude that were measured throughout the experiment.

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Cornelia Klein, Francis Nkrumah, Christopher M. Taylor, and Elijah A. Adefisan

Abstract

Mesoscale convective systems (MCSs) are the major source of extreme rainfall over land in the tropics and are expected to intensify with global warming. In the Sahel, changes in surface temperature gradients and associated changes in wind shear have been found to be important for MCS intensification in recent decades. Here we extend that analysis to southern West Africa (SWA) by combining 34 years of cloud-top temperatures with rainfall and reanalysis data. We identify clear trends in intense MCSs since 1983 and their associated atmospheric drivers. We also find a marked annual cycle in the drivers, linked to changes in the convective regime during the progression of the West African monsoon. Before the peak of the first rainy season, we identify a shear regime where increased temperature gradients play a crucial role for MCS intensity trends. From June onward, SWA moves into a less unstable, moist regime during which MCS trends are mainly linked to frequency increase and may be more influenced by total column water vapor. However, during both seasons we find that MCSs with the most intense convection occur in an environment with stronger wind shear, increased low-level humidity, and drier midlevels. Comparing the sensitivity of MCS intensity and peak rainfall to low-level moisture and wind shear conditions preceding events, we find a dominant role for wind shear. We conclude that MCS trends are directly linked to a strengthening of two distinct convective regimes that cause the seasonal change of SWA MCS characteristics. However, the convective environment that ultimately produces the most intense MCSs remains the same.

Open access
Joshua Talib, Christopher M. Taylor, Anmin Duan, and Andrew G. Turner

Abstract

Substantial intraseasonal precipitation variability is observed across the Tibetan Plateau (TP) during boreal summer associated with the subtropical jet location and the Silk Road pattern. Weather station data and satellite observations highlight a sensitivity of soil moisture and surface fluxes to this variability. During rain-free periods of two or more days, skin temperatures are shown to rise as the surface dries, signalling decreased evaporative fraction. Surface fluxes are further enhanced by relatively clear skies. In this study we use an atmospheric reanalysis to assess how this surface flux response across the TP influences local and remote conditions. Increased surface sensible heat flux induced by decreased soil moisture during a regional dry event leads to a deepening of the planetary boundary layer and the development of a heat low. Consistent with previous studies, heat low characteristics exhibit pronounced diurnal variability driven by anomalous daytime surface warming. For example, low-level horizontal winds are weakest during the afternoon and intensify overnight when boundary layer turbulence is minimal. The heat low favors an upper-tropospheric anticyclone that induces an upper-level Rossby wave and leads to negative upper-level temperature anomalies across southern China. The Rossby wave intensifies the upper-level cyclonic circulation across central China, while upper-level negative temperature anomalies across south China extend the west Pacific subtropical high westward. These circulation anomalies influence temperature and precipitation anomalies across much of China. The association between land–atmosphere interactions across the TP, large-scale atmospheric circulation characteristics, and precipitation in East Asia highlights the importance of intraseasonal soil moisture dynamics on the TP.

Open access
Douglas B. Clark, Christopher M. Taylor, and Alan J. Thorpe

Abstract

The surface fluxes of heat and moisture in semiarid regions are sensitive to spatial variability of soil moisture caused by convective rainfall. Under conditions typical of the Sahel, this variability may persist for several days after a storm, during which time it modifies the overlying boundary layer. A model of the land surface is used to quantify the dependence of surface fluxes of heat and moisture on antecedent rainfall amount, time since rainfall, and surface properties. Next, a coupled model of the land and atmosphere is used to characterize the boundary layer variability that results from this surface variability, and its dependence on factors including the length scale of the surface variability. Finally, two- and three-dimensional modeling of squall lines is used to examine the sensitivity of rainfall to boundary layer variability. Boundary layer variability tends to be greater for surface variability on long length scales, but squall-line rainfall shows the strongest response for anomalies on small length scales, comparable to that of the convection. As a result, the feedback between soil moisture and rainfall will be strongest at an intermediate scale.

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Sally L. Lavender, Christopher M. Taylor, and Adrian J. Matthews

Abstract

Recent observational studies have suggested a role for soil moisture and land–atmosphere coupling in the 15-day westward-propagating mode of intraseasonal variability in the West African monsoon. This hypothesis is investigated with a set of three atmospheric general circulation model experiments. 1) When soil moisture is fully coupled with the atmospheric model, the 15-day mode of land–atmosphere variability is clearly identified. Precipitation anomalies lead soil moisture anomalies by 1–2 days, similar to the results from satellite observations. 2) To assess whether soil moisture is merely a passive response to the precipitation, or an active participant in this mode, the atmospheric model is forced with a 15-day westward-propagating cycle of regional soil moisture anomalies based on the fully coupled mode. Through a reduced surface sensible heat flux, the imposed wet soil anomalies induce negative low-level temperature anomalies and increased pressure (a cool high). An anticyclonic circulation then develops around the region of wet soil, enhancing northward moisture advection and precipitation to the west. Hence, in a coupled framework, this soil moisture–forced precipitation response would provide a self-consistent positive feedback on the westward-propagating soil moisture anomaly and implies an active role for soil moisture. 3) In a final sensitivity experiment, soil moisture is again externally prescribed but with all intraseasonal fluctuations suppressed. In the absence of soil moisture variability there are still pronounced surface sensible heat flux variations, likely due to cloud changes, and the 15-day westward-propagating precipitation signal is still present. However, it is not as coherent as in the previous experiments when interaction with soil moisture was permitted. Further examination of the soil moisture forcing experiment in GCM experiment 2 shows that this precipitation mode becomes phase locked to the imposed soil moisture anomalies. Hence, the 15-day westward-propagating mode in the West African monsoon can exist independently of soil moisture; however, soil moisture and land–atmosphere coupling act to feed back on the atmosphere and further enhance and organize it.

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Sonja S. Folwell, Phil P. Harris, and Christopher M. Taylor

Abstract

Soil moisture plays a fundamental role in regulating the summertime surface energy balance across Europe. Understanding the spatial and temporal behavior in soil moisture and its control on evapotranspiration (ET) is critically important and influences heat wave events. Global climate models (GCMs) exhibit a broad range of land responses to soil moisture in regions that lie between wet and dry soil regimes. In situ observations of soil moisture and evaporation are limited in space, and given the spatial heterogeneity of the landscape, are unrepresentative of the GCM gridbox scale. On the other hand, satelliteborne observations of land surface temperature (LST) can provide important information at the larger scale. As a key component of the surface energy balance, LST is used to provide an indirect measure of surface drying across the landscape. To isolate soil moisture constraints on evaporation, time series of clear-sky LST are analyzed during dry spells lasting at least 10 days from March to October. Averaged over thousands of dry spell events across Europe, and accounting for atmospheric temperature variations, regional surface warming of between 0.5 and 0.8 K is observed over the first 10 days of a dry spell. Land surface temperatures are found to be sensitive to antecedent rainfall; stronger dry spell warming rates are observed following relatively wet months, indicative of soil moisture memory effects on the monthly time scale. Furthermore, clear differences in surface warming rate are found between cropland and forest, consistent with contrasting hydrological and aerodynamic properties.

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Martin G. De Kauwe, Christopher M. Taylor, Philip P. Harris, Graham P. Weedon, and Richard. J. Ellis

Abstract

Land–atmosphere feedbacks play an important role in the weather and climate of many semiarid regions. These feedbacks are strongly controlled by how the surface responds to precipitation events, which regulate the return of heat and moisture to the atmosphere. Characteristics of the surface can result in both differing amplitudes and rates of warming following rain. Spectral analysis is used to quantify these surface responses to rainfall events using land surface temperature (LST) derived from Earth observations (EOs). The authors analyzed two mesoscale regions in the Sahel and identified distinct differences in the strength of the short-term (<5 days) spectral variance, notably, a shift toward lower-frequency variability in forest pixels relative to nonforest areas and an increase in amplitude with decreasing vegetation cover. Consistent with these spectral signatures, areas of forest and, to a lesser extent, grassland regions were found to warm up more slowly than sparsely vegetated or barren pixels. The authors applied the same spectral analysis method to simulated LST data from the Joint UK Land Environment Simulator (JULES) land surface model. A reasonable level of agreement was found with the EO spectral analysis for two contrasting land surface regions. However, JULES shows a significant underestimate in the magnitude of the observed response to rain compared to EOs. A sensitivity analysis of the JULES model highlights an unrealistically high level of soil water availability as a key deficiency, which dampens the models response to rainfall events.

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