Andrew J. Negri

A satellite infrared technique to estimate tropical convective and stratiform rainfall

Abstract This paper describes a new method of estimating both tropical convective precipitation and stratiform precipitation (produced under the anvils of mature and decaying convective systems) from satellite infrared data. The method, denoted CST (Convective-stratiform Technique) locates, in an array of infrared data, all local minima in the brightness temperature field (Tmin. After an empirical screening to eliminate cirrus, these points are assumed to be convective centers. Rainrate and rain area are assigned to each minimum point as a function of its Tmin, based on one-dimensional cloud model results. A stratiform rain algorithm, using a brightness temperature threshold based on the mode temperature of thunderstorm anvils, completes the convective/stratiform rain estimation. Individual CST rain fields wore spatially most similar to the radar for young, isolated storms, and most dissimilar in capturing linear features such as squall lines. Some convective features were missed, while others (generally ...

Rainfall Modification by Major Urban Areas: Observations from Spaceborne Rain Radar on the TRMM Satellite

Data from the Tropical Rainfall Measuring Mission (TRMM) satellite’s precipitation radar (PR) were employed to identify warm-season rainfall (1998‐2000) patterns around Atlanta, Georgia; Montgomery, Alabama; Nashville, Tennessee; and San Antonio, Waco, and Dallas, Texas. Results reveal an average increase of about 28% in monthly rainfall rates within 30‐60 km downwind of the metropolis, with a modest increase of 5.6% over the metropolis. Portions of the downwind area exhibit increases as high as 51%. The percentage changes are relative to an upwind control area. It was also found that maximum rainfall rates in the downwind impact area exceeded the mean value in the upwind control area by 48%‐116%. The maximum value was generally found at an average distance of 39 km from the edge of the urban center or 64 km from the center of the city. Results are consistent with the Metropolitan Meteorological Experiment (METROMEX) studies of St. Louis, Missouri, almost two decades ago and with more recent studies near Atlanta. The study establishes the possibility of utilizing satellite-based rainfall estimates for examining rainfall modification by urban areas on global scales and over longer time periods. Such research has implications for weather forecasting, urban planning, water resource management, and understanding human impact on the environment and climate.

Estimation of Monthly Rainfall over Japan and Surrounding Waters from a Combination of Low-Orbit Microwave and Geosynchronous IR Data

Abstract This paper describes a method to combine geosynchronous IR and low-orbit microwave data to estimate mean monthly rainfall useful for climate studies. The IR data have the advantage of high time resolution (important for rapidly changing precipitation patterns and for the detection of diurnal signals) but lack a strong physical connection between the remotely sensed signal and the surface rainfall. The microwave data provide a stronger relation between the radiance and the rainfall but provide poor time sampling of the rainfall signal. The microwave technique uses the brightness temperature at 37 and 86 GHz from the Special Sensor Microwave/Imager instrument on board the Defense Meteorological Satellite Program (DMSP) satellite to define raining areas over water and land and uses the 86-GHz scattering signal to assign rain rate based on cloud model-microwave calculations. The microwave results are generally good for both individual swaths and monthly totals, except for a glaring underestimation of...

Sampling of the Diurnal Cycle of Precipitation Using TRMM

Abstract The temporal sampling of tropical regions is examined using observations from the Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI) and precipitation radar (PR). It is concluded that PR estimates at any one hour, even using three years of data, are inadequate to describe the diurnal cycle of precipitation over regions smaller than 12°, due to high spatial variability in sampling. It is shown that the optimum period of accumulation is 4 h. Diurnal signatures display half as much sampling error when averaged over 4 h of local time. A similar pattern of sampling variability is found in the TMI data, despite the TMIs wider swath and increased sampling. These results are verified using an orbital model. The sensitivity of the sampling to satellite altitude is presented, as well as sampling patterns at the new TRMM altitude of 402.5 km.

The Impact of Amazonian Deforestation on Dry Season Rainfall

Abstract Many modeling studies have concluded that widespread deforestation of Amazonia would lead to decreased rainfall. Geosynchronous visible and infrared satellite data over southwest Brazil are analyzed with respect to percent cloudiness, and rain estimates are analyzed from both the Tropical Rainfall Measuring Mission and Special Sensor Microwave Imager. The studies conclude that in the dry season, when the effects of the surface are not overwhelmed by synoptic-scale weather disturbances, shallow cumulus cloudiness, deep convective cloudiness, and rainfall occurrence all are larger over the deforested and nonforested (savanna) regions than over areas of dense forest. This paper speculates that this difference is in response to a local circulation initiated by the differential heating of the regions varying forestation. Analysis of the diurnal cycle of cloudiness reveals a shift in the onset of convection toward afternoon hours in the deforested and toward the morning hours in the savanna regions wh...

Regional Rainfall Climatologies Derived from Special Sensor Microwave Imager (SSM/I) Data

Abstract Climatologies of convective precipitation were derived from passive microwave observations from the Special Sensor Microwave Imager using a scattering-based algorithm of Adler et al. Data were aggregated over periods of 3–5 months using data from 4 to 5 years. Data were also stratified by satellite overpass times(primarily 0600 and l800 local time). Four regions [Mexico, Amazonia, western Africa, and the western equatorial Pacific Ocean (TOGA COARE area)] were chosen for their meteorological interest and relative paucity of conventional observations. The strong diurnal variation over Mexico and the southern United States was the most striking aspect of the climatologies. Pronounced morning maxima occurred offshore, often in concavities in the coastline, the result of the increased convergence caused by the coastline shape. The major feature of the evening rain field was a linear-shaped maximum along the western slope of the Sierra Madre Occidental. Topography exerted a strong control on the rainf...

Rain Estimation from Satellites: An Examination of the Griffith-Woodley Technique

Abstract The Griffith-Woodley Technique (GWT) is an approach to estimating precipitation using infrared observations of clouds from geosynchronous satellites. It is examined in three ways: an analysis of the terms in the GWT equations; a case study of infrared imagery portraying convective development over Florida; and the comparison of a simplified equation set and resultant rain maps to results using the GWT. The objective is to determine the dominant factors in the calculation of GWT rain estimates. Analysis of a single days convection over Florida produced a number of significant insights into various terms in the GWT rainfall equations. Due to the definition of clouds by a threshold isotherm (−20°C), the majority of clouds on this day did not go through an idealized life cycle before losing their identity through merger, splitting, etc. As a result, 82% of the clouds had a defined life of 1 h (two images) or less: 64% of the defined clouds were assessed no rain because the empirically derived ratio ...

Contribution of Mesoscale Convective Complexes to Rainfall in Sahelian Africa: Estimates from Geostationary Infrared and Passive Microwave Data

Abstract The contribution of mesoscale convective complexes to the July–September rainfall in Sahelian Africa is assessed using passive microwave data from the Special Sensor Microwave Image and infrared (IR) data from the European Geostationary Meteorological Satellite (Meteosat). A simple precipitation-estimation procedure, which takes advantage of the good time resolution of the IR and the strong relationship between the microwave radiance and rainfall, is developed and applied. The microwave technique uses the 37- and 86-GHz brightness temperatures to define the rain areas and the 86-GHz ice scattering signal to determine the rainfall intensity. The IR cloud shield areas are defined by the 219 K threshold. Regression analyses are used to relate the microwave-derived precipitation characteristics of the system and the IR data closest to the time of the SSM/I observation. These relationships are used to compute the precipitation characteristics of the total set of systems and to determine their monthly ...

A Characterization of Tropical Transient Activity in the CAM3 Atmospheric Hydrologic Cycle

Abstract The Community Atmosphere Model version 3 (CAM3) is the latest generation of a long lineage of general circulation models produced by a collaboration between the National Center for Atmospheric Research (NCAR) and the scientific research community. Many aspects of the hydrological cycle have been changed relative to earlier versions of the model. It is the goal of this paper to document some aspects of the tropical variability of clouds and the hydrologic cycle in CAM3 on time scales shorter than 30 days and to discuss the differences compared to the observed atmosphere and earlier model versions, with a focus on cloud-top brightness temperature, precipitation, and cloud liquid water path. The transient behavior of the model in response to changes in resolution to various numerical methods used to solve the equations for atmospheric dynamics and transport and to the underlying lower boundary condition of sea surface temperature and surface fluxes has been explored. The ratio of stratiform to conve...

A Comparison of the Normalized Difference Vegetation Index and Rainfall for the Amazon and Northeastern Brazil

Abstract This paper presents a comparison of the normalized difference vegetation index (NDVI) and rainfall for the Amazon and northeastern Brazil for the time period of 1988–90. The analysis shows that the NDVI and rainfall are uncorrelated in the Amazon, except in the northernmost part, where the rainfall regime is drier and a savanna type of vegetation is present. In the drier region of northeastern Brazil, the relationship is exponential, with the NDVI showing sensitivity to rainfall within a regime of less than approximately 100 mm per month of rainfall.