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Abstract
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.
Abstract
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.
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.
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.
Abstract
A set of nighttime tethered balloon and kite measurements from the central Sahel (15.2°N, 1.3°W) in August 2005 were acquired and analyzed. A composite of all the nights’ data was produced using boundary layer height to normalize measured altitudes. The observations showed some typical characteristics of nocturnal boundary layer development, notably a strong inversion after sunset and the formation of a low-level nocturnal jet later in the night. On most nights, the sampled jet did not change direction significantly during the night.
The boundary layer thermodynamic structure displayed some variations from one night to the next. This was investigated using two contrasting case studies from the period. In one of these case studies (18 August 2005), the low-level wind direction changed significantly during the night. This change was captured well by two large-scale models, suggesting that the large-scale dynamics had a significant impact on boundary layer winds on this night. For both case studies, the models tended to underestimate near-surface wind speeds during the night, which is a feature that may lead to an underestimation of moisture flux northward by models.
Abstract
A set of nighttime tethered balloon and kite measurements from the central Sahel (15.2°N, 1.3°W) in August 2005 were acquired and analyzed. A composite of all the nights’ data was produced using boundary layer height to normalize measured altitudes. The observations showed some typical characteristics of nocturnal boundary layer development, notably a strong inversion after sunset and the formation of a low-level nocturnal jet later in the night. On most nights, the sampled jet did not change direction significantly during the night.
The boundary layer thermodynamic structure displayed some variations from one night to the next. This was investigated using two contrasting case studies from the period. In one of these case studies (18 August 2005), the low-level wind direction changed significantly during the night. This change was captured well by two large-scale models, suggesting that the large-scale dynamics had a significant impact on boundary layer winds on this night. For both case studies, the models tended to underestimate near-surface wind speeds during the night, which is a feature that may lead to an underestimation of moisture flux northward by models.
