Submesoscale Spatiotemporal Variability of North American Monsoon Rainfall over Complex Terrain

Abstract

The authors analyze information from rain gauges, geostationary infrared satellites, and low earth orbiting radar in order to describe and characterize the submesoscale (Ͻ75 km) spatial pattern and temporal dynamics of rainfall in a 50 km ϫ 75 km study area located in Sonora, Mexico, in the periphery of the North American monsoon system core region. The temporal domain spans from 1 July to 31 August 2004, corresponding to one monsoon season. Results reveal that rainfall in the study region is characterized by high spatial and temporal variability, strong diurnal cycles in both frequency and intensity with maxima in the evening hours, and multiscaling behavior in both temporal and spatial fields. The scaling parameters of the spatial rainfall fields exhibit dependence on the rainfall rate at the synoptic scale. The rainfall intensity exhibits a slightly stronger diurnal cycle compared to the rainfall frequency, and the maximum lag time between the two diurnal peaks is within 2.4 h, with earlier peaks observed for rainfall intensity. The time of maximum cold cloud occurrence does not vary with the infrared threshold temperature used (215-235 K), while the amplitude of the diurnal cycle varies in such a way that deep convective cells have stronger diurnal cycles. Furthermore, the results indicate that the diurnal cycle of cold cloud occurrence can be used as a surrogate for some basic features of the diurnal cycle of rainfall. The spatial pattern and temporal dynamics of rainfall are modulated by topographic features and large-scale features (circulation and moisture fields as related to geographical location). As compared to valley areas, mountainous areas are characterized by an earlier diurnal peak, an earlier date of maximum precipitation, closely clustered rainy hours, frequent yet small rainfall events, and less dependence of precipitation accumulation on elevation. As compared to the northern section of the study area, the southern section is characterized by strong convective systems that peak late diurnally. The results of this study are important for understanding the physical processes involved, improving the representation of submesoscale variability in models, downscaling rainfall data from coarse meteorological models to smaller hydrological scales, and interpreting and validating remote sensing rainfall estimates.

Variation of Hydrometeorological Conditions along a Topographic Transect in Northwestern Mexico during the North American Monsoon

“…Nevertheless, further work is required to conclusively verify the soil moisture-vegetation-rainfall feedback mechanism as a means for sustaining the monsoon. One appropriate avenue is to supplement existing instrument networks (e.g., Gochis et al 2004;Gebremichael et al 2007) with measurements of atmospheric conditions and to fill in gaps along well-selected topographic transects. Furthermore, remotely sensed observations of rainfall, water vapor, surface temperature, soil moisture, and vegetation may provide a means for assessing land-atmosphere interaction at the regional scale and the effects of vegetation changes in recycling moisture back to the atmosphere.…”

“…August is an important month for the regional hydroclimatology, as ephemeral stream discharge has been shown to be high and have a low interannual variability (Gochis et al 2006). In addition, this month consists of infrequent, but intense mesoscale storms whose peak activity occurs during the late evening and night (e.g., Gochis et al 2004;Xie et al 2005;Gebremichael et al 2007). Sampling at 30 transect sites (Table 1; labeled from site 1 at the high altitude to site 30 at the low elevation) was performed daily from 3 to 14 August, over local times ranging from 0900 to 1600 LT to coincide with satellite and aircraft data acquisition schedules (SMEX Science Team 2004).…”

“…In particular, the higher-elevation station (site 1 or station 146: 225 mm) received more rainfall as compared to the midelevation site (site 12 or station 132: 189 mm). Gebremichael et al (2007) present additional details on the spatial distribution of rainfall statistics in the domain and identify four rainfall subregions based on spatial coherency (Sierra Aconchi, Cucurpe, Río San Miguel valley, and Río Sonora valley).…”

“…Station 144 is located in the northern valley of the Río San Miguel (799 m), while station 146 is located in the upper transect (1371 m). Comparisons of the rainfall records demonstrate the effect of elevation on rainfall amounts, with the mountain site receiving frequent, small events, while the valley site is dominated by larger, infrequent storms (see Gebremichael et al 2007 for detailed statistical analysis). This is consistent with Gochis et al (2004) who showed that single convection cells were common at high elevations, while mesoscale convective complexes characterize lower elevations.…”

The Applications of Satellite Based Remote Sensing Techniques in the Hydrological Assessment of Mine Water Supply and Management Systems

“…In recent years, remote sensing has become a viable source of data to quantify the state and fluxes of water and energy between the land and the atmosphere (Anagnostou et al 2010;Singh et al 2004;Tang et al 2009). Applications relate to many aspects of water cycles: rainfall (Anagnostou et al 2010;Gebremichael et al 2007;Huffman et al 2007;Kummerow et al 2007;Milewski et al 2009), evapotranspiration (Bastiaanssen et al 1998;Chen et al 2005;Sun et al 2009), soil moisture (Wigneron et al 1998), watershed water balance (Singh et al 2004) and groundwater elevation changes (Katzenstein 2008).…”