J. Marshall Shepherd

A Review of Current Investigations of Urban-Induced Rainfall and Recommendations for the Future

Precipitation is a key link in the global water cycle and a proxy for changing climate; therefore, proper assessment of the urban envi- ronments impact on precipitation (land use, aerosols, thermal properties) will be increasingly important in ongoing climate diagnostics and prediction, Glob- al Water and Energy Cycle (GWEC) analysis and modeling, weather forecast- ing, freshwater resource management, urban planning-design, and land- atmosphere-ocean interface processes. These facts are particularly critical if current projections for global urban growth are accurate. The goal of this paper is to provide a concise review of recent (1990-present) studies related to how the urban environment affects precipitation. In addition to providing a synopsis of current work, recent findings are placed in context with historical investigations such as Metropolitan Meteorological Experiment (METROMEX) studies. Both observational and modeling studies of urban- induced rainfall are discussed. Additionally, a discussion of the relative roles of urban dynamic and microphysical (e.g., aerosol) processes is presented. The paper closes with a set of recommendations for what observations and capa- bilities are needed in the future to advance our understanding of the processes.

Detection of Urban-Induced Rainfall Anomalies in a Major Coastal City

There is increasing evidence that large coastal cities, like Houston, Texas, can influence weather through complex urban land use- weather-climate feedbacks. Recent work in the literature establishes the ex- istence of enhanced lightning activity over and downwind of Houston. Since lightning is a signature of convection in the atmosphere, it would seem rea- sonable that urbanized Houston would also impact the distribution of rainfall. This paper presents results using data from the worlds first satellite-based precipitation radar (PR) aboard the Tropical Rainfall Measuring Mission (TRMM) and ground-based rain gauges to quantify rainfall anomalies that we hypothesize to be linked to extensive urbanization in the Houston area. It is one of the first rigorous efforts to quantify an urban-induced rainfall anomaly near a major U.S. coastal city and one of the first applications of space-borne radar data to the problem. Quantitative results reveal the presence of annual and warm season rainfall anomalies over and downwind of Houston. Several hypotheses have surfaced to explain how the sea breeze, coastline curvature,

International Global Precipitation Measurement (GPM) Program and Mission: An Overview

Eric A. Smith , Ghassem Asrar , Yoji Furuhama , Amnon Ginati , Christian Kummerow , Vincenzo Levizzani , Alberto Mugnai , Kenji Nakamura , Robert Adler , Vincent Casse , Mary Cleave , Michele Debois , John Durning , Jared Entin , Paul Houser , Toshio Iguchi , Ramesh Kakar , Jack Kaye , Masahiro Kojima , Dennis Lettenmaier , Michael Luther , Amita Mehta , Pierre Morel , Tetsuo Nakazawa , Steven Neeck , Ken’ichi Okamoto , Riko Oki , Garudachar Raju , Marshall Shepherd , Erich Stocker , Jacques Testud , and Eric Wood 19

An Investigation of Warm-Season Spatial Rainfall Variability in Oklahoma City: Possible Linkages to Urbanization and Prevailing Wind

Abstract This study used 9 yr (1998–2006) of warm-season (June–September) mean daily cumulative rainfall data from both the Tropical Rainfall Measuring Mission (TRMM) Multisatellite Precipitation Analysis and rain gauge stations to examine spatial variability in warm-season rainfall events around Oklahoma City (OKC). It was hypothesized that with warm-season rainfall variability, under weakly forced conditions, a rainfall anomaly would be present in climatological downwind areas of OKC. Results from both satellite and gauge-based analyses revealed that the north-northeastern (NNE) regions of the metropolitan OKC area were statistically wetter than other regions. Climatological sounding and reanalysis data revealed that, on average, the NNE area of OKC was the climatologically downwind region, confirming that precipitation modification by the urban environment may be more dominant than agricultural/topographic influences on weakly forced days. The study also established that satellite precipitation estimat...

The Impact of Urbanization on Current and Future Coastal Precipitation: A Case Study for Houston

The approach of this study was to determine, theoretically, what impact current and future urban land use in the coastal city of Houston, Texas has on the space and time evolution of precipitation on a ‘typical’ summer day. Regional model simulations of a case study for 25 July 2001 were applied to investigate possible effects of urban land cover on precipitation development. Simulations in which Houston urban land cover was included resolved rain cells associated with the sea breeze front and a possible urban circulation on the northwest fringe of the city. Simulations without urban land cover did not capture the initiation and full intensity of the ‘hypothesized’ urban-induced rain cell. The response is given the terminology the ‘urban rainfall effect’ or URE. An urban growth model (UrbanSim) was used to project the urban land-cover growth of Houston, Texas from 1992 to 2025. A regional atmospheric-land surface model was then run with the 2025 urban land-cover scenario. Though we used a somewhat theoretical treatment, our results show the sensitivity of the atmosphere to urban land cover and illustrate how atmosphere — land interactions can affect cloud and precipitation processes. Two urban-induced features, convergence zones along the inner fringe of the city and an urban low-pressure perturbation, appear to be important factors that lead to enhanced rain clouds independently or in conjunction with the sea breeze. Simulations without the city (NOURBAN) produced less cumulative rainfall in the west-northwest Houston area than simulations with the city represented (URBAN). Future urban land-cover growth projected by UrbanSim (URBAN2025) led to a more expansive area of rainfall, owing to the extended urban boundary and increased secondary outflow activity. This suggests that the future urban land cover might lead to temporal and spatial precipitation variability in coastal urban microclimates. It was beyond the scope of the analysis to conduct an extensive sensitivity analysis of cause — effect relationships, though the experiments provide some clues as to why the rainfall evolution differs. This research demonstrates a novel application of urban planning and weather — climate models. It also raises viable questions concerning future planning strategies in urban environments in consideration of hydroclimate changes.

The influence of large dams on surrounding climate and precipitation patterns

Understanding the forcings exerted by large dams on local climate is key to establishing if artificial reservoirs inadvertently modify precipitation patterns in impounded river basins. Using a 30 year record of reanalysis data, the spatial gradients of atmospheric variables related to precipitation formation are identified around the reservoir shoreline for 92 large dams of North America. Our study reports that large dams influence local climate most in Mediterranean, arid and semi-arid climates, while for humid climates the influence is least. During the growing season, large dams in Mediterranean climates increase CAPE 2-3 times near the reservoir compared to the non-growing season. Clear spatial gradients of CAPE, specific humidity and surface evaporation are also observed around the fringes between the reservoir shoreline and further from these dams. Because of the increasing correlation observed between higher percentile of rain and CAPE, our findings point to the possibility of storm intensification in impounded basins of the Mediterranean and arid climates of the United States.

INCLUSION OF URBAN LANDSCAPE IN A CLIMATE MODEL How Can Satellite Data Help

rban regions, which cover only approximately 0.2% of the earths’ land surface, contain about half of the human population (UNPD 2001). Modeling urban weather and climate is critical for human welfare, but has been hampered for at least two reasons: i) no urban landscape has been included in global and regional climate models (GCMs and RCMs, respectively), and ii) detailed information on urban characteristics is hard to obtain. With the advance of satellite observations, adding urban schemes into climate models in order to scale pro-jections of global/regional climate to urban areas becomes essential. Inclusion of urbanized landscape into climate models was discussed in depth at the fall American Geophysical Union (AGU) meeting of 2003 in the session entitled “Human-induced climate variations linked to urbanization: From observations to modeling,” which took place on 12 December 2003 in San Francisco, California (most of the presenta-tions of this session can be found online at www.atmos.umd.edu/~mjin/AGU03urban.html). The following notes summarize what is known and what needs to be advanced on this topic.In a GCM and RCM, land physical processes are simulated in a land surface model, which is coupled with the atmosphere model through exchanges of heat fluxes, water, and momentum. Currently, an urban classification is not included in any major GCM/RCM land surface model [e.g., the second National Center for Atmospheric Research (NCAR) Community Land Model (CLM2), National Aero-nautics and Space Administration (NASA) Global Modeling and Assimilation Office (GMAO) unified land surface model, Biosphere–Atmosphere Transfer Scheme (BATS), simple Biosphere model, version 2 (SIB2), etc.]. This exclusion makes GCMs/RCMs in-adequate for realistically simulating urban modifica-tions to climate.The same land surface model can be coupled to a GCM or RCM. For example, the NCAR CLM is coupled to both the NCAR community atmosphere

The Contribution of Mesoscale Convective Complexes to Rainfall across Subtropical South America

This study uses a database consisting of 330 austral warm-season (October‐May) mesoscale convective complexes (MCCs) during 1998‐2007 to determine the contribution of MCCs to rainfall across subtropical South America (SSA). A unique precipitation analysis is conducted using Tropical Rainfall Measuring Mission (TRMM) 3B42 version 6 data. The average MCC produces 15.7 mm of rainfall across 381 000 km 2 , withavolumeof7.0km 3 . MCCsin SSAhavethelargestprecipitation areascomparedtoNorthAmericanand African systems. MCCs accounted for 15%‐21% of the total rainfall across portions of northern Argentina and Paraguay during 1998‐2007. However, MCCs account for larger fractions of the total precipitation when analyzed on monthly and warm-season time scales. Widespread MCC rainfall contributions of 11%‐20% were observed in all months. MCCs accounted for 20%‐30% of the total rainfall between November and February, and 30%‐50% in December, primarily across northern Argentina and Paraguay. MCCs also produced 25%‐66% of the total rainfall across portions of west-central Argentina. Similar MCC rainfall contributionswereobservedduringwarmseasons.AnMCCimpactfactor(MIF)wasdevelopedtodetermine the overall impact of MCC rainfall on warm-season precipitation anomalies. Results show that the greatest impactson precipitation anomaliesfromMCCrainfallwerelocatednearthecenteroftheLa Platabasin.This study demonstrates that MCCs in SSA produce widespread precipitation that contributes substantially to the total rainfall across the region.

Rainfall Morphology in Florida Convergence Zones: A Numerical Study

Abstract Central Florida is the ideal test laboratory for studying convergence zone–induced convection. The region regularly experiences sea-breeze fronts and rainfall-induced outflow boundaries. The focus of this study is convection associated with the commonly occurring convergence zone established by the interaction of the sea-breeze front and an outflow boundary. Previous studies have investigated mechanisms primarily affecting storm initiation by such convergence zones. Few have focused on rainfall morphology, yet these storms contribute a significant amount of precipitation to the annual rainfall budget. Low-level convergence and midtropospheric moisture have been shown to be correlated with rainfall amounts in Florida. Using 2D and 3D numerical simulations, the roles of low-level convergence and midtropospheric moisture in rainfall evolution are examined. The results indicate that area- and time-averaged, vertical moisture flux (VMF) at the sea-breeze front–outflow convergence zone is directly and ...

Integration of Lidar Data into a Coupled Mesoscale–Land Surface Model: A Theoretical Assessment of Sensitivity of Urban–Coastal Mesoscale Circulations to Urban Canopy Parameters

AbstractUrban–coastal circulations affect urban weather, dispersion and transport of pollutants and contaminants, and climate. Proper characterization and prediction of thermodynamic and dynamic processes in such environments are warranted. A new generation of observation and modeling systems is enabling unprecedented characterization of the three-dimensionality of the urban environment, including morphological parameters. Urban areas of Houston, Texas, are classified according to lidar-measured building heights and assigned typical urban land surface parameters appropriate to each classification. The lidar data were degraded from 1 m to the model resolution (1 km) with the goal of evaluating the impact of degraded resolution urban canopy parameters (UCPs) and three-dimensionality on the coastal–urban mesoscale circulations in comparison to typical two-dimensional urban slab approaches. The study revealed complex interactions between the sea breeze and urban heat island and offers a novel diagnostic tool,...