Urban Induced Precipitation Changes:Implications for Regional PlanningErica Betts

April 24, 2008

Scientists vs. Planners

Scientists Producers of scientific knowledge

Planners Linking knowledge to action

Urbanization – Climate Feedbacks Urban Heat Island (UHI) Land use changes – climate feedbacks

Surface albedo changes Changes in latent vs. sensible heat flux

Boundary layer changes due to building height Coceal and Belcher – urban canopy modeling efforts

Precipitation changes Urban areas cause surface winds to decrease under

significant synoptic flow and increase under weak synoptic flow due to surface roughness

Implications/Importance

Urban areas account for less than 2% of the Earth’s land surface Feedbacks more a local and regional issue rather

than global More than 50% of the Earth’s population lives

in these urbanized areas

Various scales linking urban environments to the environmental system [modified after Oke ( 1987)] urban canopy layer (UCL) urbanboundary layer (UBL).

Urban-Induced Precipitation Changes Long held realization that change from

naturally vegetated to urban environments results in temperature changes

Precipitation changes have been witnessed as well but harder to analyze until later part of this century

Many questions remain concerning what factors/mechanisms drive these precipitation changes and/or their relative importance.

Shepherd, J. M., et al. (2002), Rainfall Modification by Major Urban Areas: Observations from Spaceborne Rain Radar on the TRMM Satellite, Journal of Applied Meteorology, 41, 689-701.

19.5% increase in precipitationdownwind of Atlanta when comparedwith upwind control. During warmseasons 1998-2000

Urban-induced changes in natural precipitation likely due to: Atmospheric destabilization due to UHI-thermal

perturbation of the boundary layer Increased low-level convergence due to surface roughness Modification of microphysical and dynamic processes by

the addition of cloud condensation nuclei Modification of low-level atmospheric moisture content by

additions from urban industrial sources and/or irrigation practices

Bifurcated or diverting of precipitation systems by the urban canopy or related processes

Urban-Induced Precipitation Changes

http://www.gsfc.nasa.gov/gsfc/earth/pictures/20020613urbanrain/Urban%20Heat%20Island.mpg

Urban Heat Island

UHI-induced precipitation events have been found to occur when atmosphere is only marginally unstable Too stable, no convection occurs Too unstable, widespread convection occurs

regardless of urban influence

QSW + QLW + QSH + QLE + QG + QA = 0

Typical rural and urban surface energy balance. The values are in units ofkW h m2 day−1 (courtesy of R. Sass, Rice University, online at http://www.ruf.rice.edu/∼sass/UHI.html).

Shepherd, J. M. (2005), A Review of Current Investigation or Urban-induced rainfall and recommendations for the future, Earth Interactions, 9(12), 1-27.

QSW = net shortwave irradianceQLW = net longwave irradianceQSH = surface sensible heat fluxQLE = latent turbulent heat fluxQA = anthropogenic heat inputQG = ground heat conduction

Wind speed, cloudiness, and relativehumidity are negatively correlated withthe maximum UHI intensity

Cloud Condensation Nuclei

Increased precipitation downwind of urban areas due to the influx of CCN

Increased CCN from pollution creates a greater number of small droplets but less coalescence and drop formation. Droplets do not become large enough to fall as rain.

(left) Satellite-retrieved median effective radius of particles near the top of deep convective clouds at various stages of their vertical development, as a function of the cloud-top temperature, which serves as a surrogate for cloud-top height. The effective radius is the ratio of the integral of the third moment (r3) of the radius, weighted with the number concentration at that radius, to its second moment (r2). This is shown for clouds forming in polluted (solid lines) and pristine air (broken lines). The red lines denoted by “INDOEX polluted” are for data along a track that runs from southwestern India into the Indian Ocean. The blue lines are for tracks over urban southeastern Australia. Violet lines are for Thailand premonsoon clouds with suppressed coalescence. Green lines are for biomass smoke over the Amazon. Black lines are for desert dust over Israel. The vertical green line denotes the 14-μm precipitation threshold radius (following Ramanathan et al. 2001). (right) TRMM satellite data near Australia showing rainfall suppression by biomass burning and smoke (yellow). Area 3 is characterized by no aerosol contamination and rainfall (white) (following Rosenfeld 2000).

Questions for Planners How much does UHI-induced precipitation vary from

natural precipitation patterns (quantity, severity) Impacts on built environment

At what level of density or development do these impacts start to be felt? Do they continue to increase? Are they uniform across climate regions? At what size or density will a city have to begin addressing

possible impacts from UHI or urban influenced precipitation changes?

At what point in the future will impacts stop being purely a local phenomenon? Megaregions

UHI-initiated precipitation events plotted by location of storm center overlaid on counties, limited-access highways, and a buffer around the highways.

Dixon, P. G., and T. L. Mote (2003), Patterns and Causes of Atlanta's Urban Heat Island-Initiated Precipitation, Journal of Applied Meteorology, 42(9), 1273.

Questions of Interest to Climate Scientists Most studies focus on one city.

Comparisons across a number of cities may help in determining the relative impact of various processes or factors influencing UHI-induced precipitation

Identification of magnitude of precipitation changes Historical or trend analysis of regional precipitation

patterns may be enhanced by a treatment of change attributed to UHI-effects as opposed to larger climate changes

Proposed Research Dixon and Mote (2003) developed

methodology for isolating UHI-induced precipitation events

Incorporated spatial synoptic classification method Based on surface based observations of

Temperature dew point Wind Pressure cloud cover

Dixon and Mote (2003), found that UHI-induced precipitation soundings showed much higher dew points (as much as 5°C) below 550 hPa than average days.

Airmass analysis showed that these events were more frequent under the most humid air masses rather than the ones with the greatest UHI-intensities.

The difference in sounding values (temperature and dewpoint) between the average UHI-induced precipitation event and the total study period.

Proposed Research

Stone, B., Jr. (2007), Urban and rural temperature trends in proximity to large US cities: 1951-2000, International Journal of Climatology, 27, 1801-1807.