11/6/2024 URBAN CLIMATOLOGY VI. Precipitation in urban areas Paper to read Urban ClimaiE 51 (2023) 1O1É0B ELSEVIER Contents listí available at 5en ce Direct Urban Climate journal homepage: www.efsevier.com/focate/LJCfip Urban effects on precipitation; Do the diversity of research strategies and urban characteristics preclude general conclusions? Morgane Lalonde , Ludovic Oudin Ly Sophie Bastin: *-Sarbtmne Urd^inzi C~;R~, FPfC, L\:IR _'l3T7T", Fcriz. France 1LATMOS/JFSL, ZT'.'SQ l>:V. ■i'iiU Zr-.i-5::leJ} Scrbanne Urnvaaset CNRS, Guytmcourt, France https://is.muni.cz/auth/el/sci/podzim2024/ZA311 /um/67875456/06 Urba n effects on precipitation.pdf 1 6.1 Urban precipitation Conceptual model Modification of precipitation regime in urban environment; a general model adopted from http://www. ucar. edu/communications/staffnotes/0603/cities. shtml) Conceptual model II Calm Conditions - Strong UHl Convergence Precip Maximum over Urban Center Strong Regional Winds Upwind Divergence Lateral/Downwind convergence Precip Minimum over City Lateral and Downwind Precip Maximum Weak Regional Winds - UHl Convergence Maximum Precip all Advected to Downwind Urban Edge Urban Morphology and Size Significant to Spatio-Temporal Patterns of Convergence and Heating After Formation Aerosols Impact Precipitation Efficiency (xr y, 1) and Lightning Other cross-cutting factors to consider: Bifurcation-thermodynamic dome or physical barrier dome? How does urban moisture content (lack thereof) and heat island affect local storm dynamics? Seasonality? Diurnal effects? Topography? Towards a conceptualization of the urban rainfall effect. Source: Original figure courtesy of R. Bornstein as adapted and presented in Shepherd (2013). Urban precipitation https://kids.frontiersin.org/articles/10.3389/frym.2018.00038 • Precipitation is not continuous in time and space • It is hard to separate urban influence from others (position, relief, ...) • Closely related to meteorology and climatology of clouds • Precipitation regime si modified by wind direction • There can be different effects on convective precipitation and atmospheric fronts (advection systems) • It is not clear whether urban environments initialize new precipitation events or whether they just intensify existing precipitation • Empirical studies sometimes show contradictory results Typical features of urban climate Table U2 Urban climate effects for a mid-latitude city with about I million inhabitants (values for summer unless otherwise noted) Variable Char Magnitude/comments Turbulence intensity Wind speed Wind direction UV radiation Solar radiation Infrared input Visibility Evaporation Cotivective heat flux Heat storage Air temperature Humidity üioud Fog 'capitation Snow Total hundcrstorms Greater Decreased Increased Altered Much less Less Greater Reduced Less Greater Greater Warmer Drier Mats moist More haze More els /lore or les 10-50% 5-30% at 10 m in strong flow In weak flow with heal island I—10 degrees 25-90% 1-25% 5^H)% About 50% About 50% About 200% 1-3°C per 100 years: 1-3T annual mean up to 12°C hourly mean Summer daytime Summer night, all day winter Tn and downwind of city Especially in lee of city Depends on aerosol and surroundings Some turns to rain To the lee of rather than in city (Landsberg 1981) Urban precipitation • Precipitation anomaly in La Porte, USA (Changnon 1968) • METROMEX project (St. Louis, 1971-1975) • Most studies proved that precipitation totals in cities and in their leeward side are 5-15% higher compared to rural areas • The summer is the time of maximum urban effect on precipitation (in other seasons effects may be quite different) • Some studies show no local effect on precipitation or even deficits in precipitation that accompany urbanization Urban precipitation Precipitation regime in urban areas is modified due to combination of three different effects: • thermal effect related to UHI formation • mechanical effect related to higher roughness • pollution effect related to more condensation nuclei Processes of cloud formation, moisture sources, local wind systems as well as geography (topography) play an important role Station measurements are insufficient, it must be accompanied with remotely sensed systems (satellite imagery, radar, lidar, ceilometer, etc.) 6.2 Thermal effect on urban precipitation Warmer urban climate due to UHI formation positively impacts convection that is supported with numerous processes in urban environment (Shepherd, 2005): Hcflil^Mfl clfEfiJatlun -;-1 Ileal island circulation ■ i m m 11 Clouds • urban heat island-induced convection • higher instability of urban atmosphere • sensible heat flux enhancement Consequences: • Stronger convection in summer -> more showers and thunderstorms) • The urban heat island can induce or modify local flow/circulation • Due to UHI there is lower proportion of precipitation in the form of snow • Higher roughness -> lower velocity of atmospheric fronts -> more precipitation) • Higher roughness -> higher intensity of turbulence -> more precipitation • Larger urban surface roughness can disrupt or bifurcate precipitating convective systems formed outside cities while passing over the cities. • Urban-modified precipitating systems can either increase or decrease precipitation over and/or downwind of cities. Mechanical effect on urban precipitation Polution effect on urban precipitation • The availability of more cloud condensation nuclei in urban areas primarily influence formation of clouds • Aerosol number, size, type and chemical properties initiate processes that may enhance, suppress or delay cloud formation and precipitation occurrence • Different pollution sources generate particles of different chemical properties w.r.t. condensation processes and cloud formation • More dust from cars fuel, industry and quarrying which contribute to the hydroscopic nuclei making them larger • Cloud cover may also often be the result of smog, a mixture of fog and smoke (low-lying clouds). • Ice particles of anthropogenic origin -> condensation nuclei for stratus clouds -> more frequent light snowfall in city 6.3 Clouds • There tends to be more cloud cover over urban areas; cities receive thicker and up to ten per cent more frequent cloud cover than that compared to rural areas. • The reason for this is because there is more convection caused by higher temperatures and a larger number of condensation nuclei • The increase or decrease in amount of cloud cover can directly impact the precipitation levels in urban areas • Intensity, frequency and length of fogs are much greater in urban areas particularly under anticyclone conditions. • For example, Kew in the middle suburbs of London has 79 hours of very dense fog, with visibility being less than 40metres. Whereas, London Airport on the outer suburbs has only 46 hours, and south east England (the mean of 7 weather stations) has 20 hours. • This shows that further away from the urban areas of a city, towards rural areas, fog density decreases. Obviously, the larger the city and the greater the quantity of urban structures and materials the greater the impacts of these microclimatic changes. Clouds „Invigoraton effect" of aerosols on clouds in moist climate Cr0winS Mature n,„,„.,._ Crowing Dissipating —* Direction of airflow * Ice and snow crystals d Graupel or small hail » Raindrop Dissipating • Large cloud droplet ■ Small cloud droplet ■ Smaller cloud droplet ■ Aerosol particles Low concentration of aerosols, few cloud droplets, more large drops Rapid precipitation formation More condensation nuclei leads to greater number of small droplets Precipitation suppression in the first phases O More liquid in cumulus clouds Enhanced cloud development and more intense precipitation Convective cloud development in clean (top) and poluted (bottom) atmosphere (Rosenfeld et a. 2008) 6.4 Urban precipitation at atmospheric fronts • Cloud systems crossing the city show the disruption of a frontal system passing over the city and the reshaping of the frontal system after crossing it. • Moving thunderstorms with strong regional flows tend to bifurcate and move around the city due to the urban barrier effect in the New York City area Schematic of low-level airflow over and around an urban area due to changes in surface roughness. (Cotton and Pielke 1995) • Larger surface roughness in a city than in its surrounding rural area causes air approaching the city to slow down near the upwind city boundary and/or over the city. • In addition, the air approaching a city tends to divert around it and the diverted air can converge on the downwind side of the city, yielding upward motion there. Atmospheric fronts vs. strong convection Overall U-i^-' Source: www.nature.com/articles/s41598-019-42494-2.pd-f The bars indicate the sample standard deviation for the precipitation change, and circles correspond to the mean change in precipitation location. On average, urban areas and the surrounding region experienced precipitation increases. The largest signal was prominently in the downwind region of the city and experienced the highest rainfall change: 18% increase on average, (a range of 14 to 22% with one standard deviation). The distance over which these changes occurred (mostly increases in rainfall) is approximately 52 km downwind, and about 31 to 41 km upwind._ 6.5 Strong convection and thunderstorms Source: https://jipr.springeropen.com/articles/10.1186/s43065-020-00003-0 • Convective thunderstorms were initiated in urban heat island-induced convergence zones. • The rapid growth of moving storms passing over cities was observed in some major urban areas, such as in the London area (Atkinson, 1971) and the Chicago area • It is not confirmed that roughness alone can initiate moist convection (or the roughness can play an important role in initiating moist convection) • There is a possibility that updrafts produced by high-rise buildings in highly built-up urban areas can initiate moist convection Strong convection and thunderstorms • Higher temperatures of urban areas mean that the likelihood of thunderstorms is increased by 25%. • Thunderstorms develop especially in hot humid air and are accompanied by numerous extreme phenomena (downpours, hail, lightning and thunder). • They are particularly common in the late afternoon when heat energy has had the dnance to build up in the atmosphere. Unevenness in precipitation distribution ©iMas express Moto: If your garden is flooded at times or, conversely, everything dries up, it does not necessarily mean that it is raining a lot or not enough. It can also mean that it rains very unevenly. .....Ilni Jill i .1.1 llil Erratic rainfall patterns? Human impact to blame https://www.newindianexpress.com/xplore/2024/Jul/31/ Daily precipitation totals for Brno area in spring 2010 and 2011 representing two seasons with even and uneven precipitation distribution as an example (a); their corresponding Lorenz curves and Gini index (b) and pdf estimates (c) Unevenness in precipitation distribution The mean annual precipitation total in an urban area and the number of days with less than 5 mm of rainfall can both be between 5-15% greater than in rural areas. This means that cities get a larger amount of dry days, yet have more rainfall when they do have rain. This happens because of convection currents which are generated by the higher temperatures, and due to an increased amount of microscopic condensation nuclei. Decrease in light precipitation frequency has been reported in many regions and urbanization has largely contributed to the observed downward trend in it. scientific reports open Observed decrease in light precipitation in part due to urbanization Light and heavy urban precipitation Raindrop Concentration Decrease *•*,*..•••". * Dropping Distance Increase 1 " ■* ____ • * * i 1» 4T1 i i Hi ■ Evaporation I ncrease f Cold Wet Aerosol I Rain Droplet • Cloud Droplet ■ — - Condensate Layer - — - Boundary Layer b. pinion] _I ^ I [Condensation Height) Cloud Amount] Urban Relative Humidity) + f Droplet Size ) i i 1 1 1 Re-evaporation] I i Decrease Increase https://www.nature.com/articles/s41598-022-07897-8 6.6 Water runoff in urban environment precipiuuon Forested Urban Pndpiution Source: https:/Awww.melbournewater.com.au/sites/default/files/forested- urban-stormwater.jpg 6.7 Observational evidence (precipitation regime in Brno) Network of stations in Brno area and its temporal development in the period 1890-2012: a) period of measurements; b) number of stations One can characterize rain variability: in its amount (totals) in precipitation frequency in its intensity in its seasonal distribution Long term variability of precipitation in Brno 800 200 1800 1820 1840 1860 1880 1900 1920 1940 1960 1980 2000 Fluctuation of mean annual precipitation totals in Brno in the period 1803-2010; annual values smoothed with Gaussian filter for 20 years are in bold and horizontal line is the mean for 1961-2010 reference period Comparison of mean annual variation of precipitation at selected stations in Brno in the period 1961-2010 Kninicky Lesna ■ Troubsko Turany Zabovresky I II III IV V VI VII VIII IX X XI XII Number of rainy days Variability in annual number of days with precipitation totals > 0.1, 1.0, 10.0 and 20.0 mm including linear trends at Turany station, period 1961-2010 Number of rainy days 11,0 mm 1961 1971 1981 1991 2001 í 10,0 mm 1961 1971 1981 1991 2001 S. 20,0 mm 19S1 1971 19S1 1991 2001 1961 1971 1981 1991 2001 Variability in annual number of days with precipitation totals > 0.1, 1.0, 10.0 and 20.0 mm including linear trends at Turany station in the period 1961- 2010 Maximum daily precipitation totals Maximum daily precipitation totals and assessment of mean return periods at selected stations in Brno in the period 1961-2010 stanice max. denní úhrn srážek datum výskytu doba opakování (roky) Brno-Kníníčky Brno-Tuřany Brno-Žabovřesky Troubsko 100 80 60 40 20 0 Tuřany 1961 1971 1981 1991 2001 1961 1971 1981 1991 2001 Variability of maximum daily precipitation totals including linear trends at two stations in Brno in the period 1961-2010 Conclusion: there are several signs that the precipitation regime become more extreme ^9999799999999^ Analysis of precipitation anomalies W (NW, SW) advection 3ii?rnnn 3 f. room ^fijuuou Linear trend in daily rainfall totals (left) and positive (blue) and negative (red) deviations of daily rainfall totals from their linear trend (b) in the Brno area and its surroundings for W winds R, Conclusion: There is a signal indicating an increase in precipitation totals on the leeward side of the city of Brno, which may be related to the urban climate effect Mean prec anomalies for individual stations and for three station groups Urban Precipitation Radar Observations Weather Radar (Ground located) Frequency of the above-average maximum radar reflectivity in Brno region composed from 26 situations with extreme convection at Turany station in the period 2000-2007 Spaceborne Rain Radar on the TRMM Satellite Mean Reference Wind Direction Houston Area with TRMM PR at 0.5 Degree Coordinate System and with Coordinate System Gauge Locations Mean annual distribution of the Tropical Rainfall Measuring Mission (TRMM)-derived rainfall rates from January 1998 to May 2002 (excluding August 2001). The oval is the approximate Houston urban zone. The vector indicates the mean annual 700-hPa wind direction over the Houston area. The pentagon-shaped box is the downwind urban-impacted region, and the rectangular box is the upwind control region, [after Shepherd and Burian (2003).]_ 6.8 Final remarks and questions Urban precipitation and Global warming projections • Higher probability of occurrence of short-term extreme precipitation totals and flash floods • Longer periods without any precipitation, higher probability of drought occurrence • Non-uniform precipitation distribution during the year 1. What are the main impacts of changed precipitation regime on people living in cities? 2. How we can define extremity of precipitation regime? 3. What is the role of other factors such as relief, position, land use etc.? 4. How can be negative effects mitigated in urban-planning design?