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Jaime Garatuza-Payan, Rachel T. Pinker, and W. James Shuttleworth

Abstract

The first stage in a program of research to develop a regional model capable of describing the hydrology of semiarid areas of northwest Mexico and southwest United States, using remotely sensed data, is described in this paper. Finescale information on cloud cover is required to provide the radiation forcing for making simple, near-real-time estimates of daytime evaporation in hydrologic models, and frequent satellite observations have the potential to document cloud variability at high spatial and temporal resolutions. In this study, the operational framework for obtaining information on cloud cover was developed and applied, using hourly sampled, 1-km resolution GOES-7 data as received in real time in Obregon, Mexico. These satellite data were collected and analyzed from 1 July 1993 to 31 July 1994 for an approximately 106 km2 rectangular area in northwest Mexico. An efficient method was devised to provide clear-sky radiance images for the study area, at 4 km × 4 km resolution, and updated at monthly intervals, by applying thresholds indexed to the locally appropriate clear-sky radiance, thereby allowing for spatial and temporal changes in surface conditions. Manual image inspection and comparison with ground-based measurements of cloud cover and surface solar radiation provided reassurance that the high-resolution cloud-screening algorithm gave satisfactory results.

This algorithm was applied to investigate the effects of temporal sampling frequency on estimates of daytime-average cloud cover and to document aspects of the cloud characteristics for the study area. The high-resolution algorithm proved to be efficient and reliable and bodes well for its future use in providing high-resolution estimates of surface solar radiation for use in a hydrologic model. Monthly clear-sky composite images were consistently generated, showing little evidence of contamination by persistent clouds, and tracked the seasonal evolution in surface radiance. Comparison with ground-based measurements gave confidence in the credibility of the satellite estimates and revealed weaknesses in the Campbell–Stokes solarimeter. The seasonal evolution of spatial patterns of cloud and its diurnal cycle were investigated. The average cloudiness for the study area is 0.25, with a substantial annual variation from 0.19 in April to 0.40 in December. Persistent cloudy conditions throughout the year were detected over the Pacific Ocean west of Baja California. The derived high-resolution cloud estimates, when compared with similar estimates from the International Satellite Cloud Climatology Project (ISCCP D1), were about half those obtained with the low-resolution data, indicating that, in this complex study area where land and water boundaries are in close proximity, low-resolution satellite observations of clouds may not be able to depict the true cloud cover.

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David J. Gochis, Alejandro Jimenez, Christopher J. Watts, Jaime Garatuza-Payan, and W. James Shuttleworth

Abstract

Analyses of rainfall characteristics and their linkage to physiographic features are made from the North American monsoon experiment (NAME) Event Rain Gauge Network (NERN) in northwest Mexico. The findings are based on the network configuration for the 2002 and 2003 warm seasons. Despite the relatively short record used, a clearer structure of core-region monsoon rainfall is beginning to emerge. In agreement with earlier, coarser-scale studies, the seasonal precipitation maximum overlies the western slope of the Sierra Madre Occidental but does not strictly parallel a particular elevation band. It is shown that the distance to the Gulf of California and, potentially, the configuration of the terrain profile may also play an important role in determining where the axis of maximum precipitation lies. The diurnal cycles of precipitation frequency and intensity are shown to have distinct relationships to terrain elevation that are qualitatively similar to those observed over the Front Range of the Rocky Mountains in the central-western United States. The relationship between precipitation and gulf surge events occurring during the summer of 2003 is also explored.

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David J. Gochis, Juan-Carlos Leal, W. James Shuttleworth, Christopher J. Watts, and Jaime Garatuza-Payan

Abstract

The purpose of this note is to present preliminary findings from a new event-based surface rain gauge network in the region of northwest Mexico. This region is characterized as semiarid, owing the largest percentage of its annual rainfall to summer convective systems, which are diurnal in nature. Although the existing surface network and satellite-derived precipitation products have clarified some features of convective activity over the core region of the North American monsoon (NAM), a detailed examination of the spatial and temporal structure of such activity has been prohibited by the lack of a surface observation network with adequate temporal and spatial resolution. Specifically, the current network of sparsely spaced climate stations has inhibited a detailed diagnosis of the timing, intensity, and duration of convective rainfall in general, and of the topography–rainfall relationship in particular. In this note, a brief overview of the network and present preliminary analyses from the first monitoring season, summer 2002, is provided. It is shown that the diurnal cycle of precipitation varies with elevation in a way that is consistent with a hypothesis that convective events organize and, occasionally, propagate from high terrain onto lower-elevation plains, but more conclusive statements will require expansion of the network and increased record length. It is also emphasized from these studies that it is essential to evaluate wet-day statistics or rainfall intensities from precipitating periods in parallel, with comparable all-day statistics, when conducting hydrometeorological analyses in semiarid convective regimes where precipitation is infrequent and highly localized.

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David J. Gochis, Christopher J. Watts, Jaime Garatuza-Payan, and Julio Cesar-Rodriguez

Abstract

Detailed information on the spatial and temporal characteristics of precipitation intensity from the mountainous region of northwest Mexico has, until recently, been lacking. As part of the 2004 North American Monsoon Experiment (NAME) enhanced observing period (EOP) surface rain gauge networks along with weather radar and orbiting satellites were employed to observe precipitation in a manner heretofore unprecedented for this semiarid region. The NAME Event Rain gauge Network (NERN), which has been in operation since 2002, contributed to this effort. Building on previous work, this paper presents analyses on the spatial and temporal characteristics of precipitation intensity as observed by NERN gauges. Analyses from the 2004 EOP are compared with the 2002–04 period and with long-term gauge observations. It was found that total precipitation from July to August of 2004 was similar in spatial extent and magnitude to the long-term average, though substantially wetter than 2003. Statistical analyses of precipitation intensity data from the NERN reveal that large precipitation events at hourly and daily time scales are restricted to coastal and low-elevation areas west of the Sierra Madre Occidental. At 10-min time scales, maximum intensity values equal to those at low elevations could be observed at higher elevations though they were comparatively infrequent. It is also shown that the inclusion of NERN observations in existing operational analyses helps to correct significant biases, which, on the seasonal time scale, are of similar magnitude as the interannual variability in precipitation in key headwater regions of northwest Mexico.

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Christopher J. Watts, Russell L. Scott, Jaime Garatuza-Payan, Julio C. Rodriguez, John H. Prueger, William P. Kustas, and Michael Douglas

Abstract

The vegetation in the core region of the North American monsoon (NAM) system changes dramatically after the onset of the summer rains so that large changes may be expected in the surface fluxes of radiation, heat, and moisture. Most of this region lies in the rugged terrain of western Mexico and very few measurements of these fluxes have been made in the past. Surface energy balance measurements were made at seven sites in Sonora, Mexico, and Arizona during the intensive observation period (IOP) of the North American Monsoon Experiment (NAME) in summer 2004 to better understand how land surface vegetation change alters energy flux partitioning. Satellite data were used to obtain time series for vegetation indices and land surface temperature for these sites. The results were analyzed to contrast conditions before the onset of the monsoon with those afterward. As expected, precipitation during the 2004 monsoon was highly variable from site to site, but it fell in greater quantities at the more southern sites. Likewise, large changes in the vegetation index were observed, especially for the subtropical sites in Sonora. However, the changes in the broadband albedo were very small, which was rather surprising. The surface net radiation was consistent with the previous observations, being largest for surfaces that are transpiring and cool, and smallest for surfaces that are dry and hot. The largest evaporation rates were observed for the subtropical forest and riparian vegetation sites. The evaporative fraction for the forest site was highly correlated with its vegetation index, except during the dry spell in August. This period was clearly detected in the land surface temperature data, which rose steadily in this period to a maximum at its end.

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Monsoon Region Climate Applications

Integrating Climate Science with Regional Planning and Policy

Andrea J. Ray, Gregg M. Garfin, Luis Brito-Castillo, Miguel Cortez-Vázquez, Henry F. Diaz, Jaime Garatuza-Payán, David Gochis, René Lobato-Sánchez, Robert Varady, and Chris Watts
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