The Urban Moisture Climate by DOUGLAS L. SISTERSON and RICHARD A. DIRKS, graduate research assistant and professor, Department of Atmospheric Science, University of Wyoming, Laramie, Wyo. Sisterson is now with the Atmospheric Physics Section, Argonne National Labora- tory, Argonne, Ill. ABSTRACT.-Data collected on 26 July 1974 as a part of project METROMEX in St. Louis show the three-dimensional structure of the urban moisture field. Mesoscale dry regions at the urban surface, corresponding to large residential and light industrial land-use characterization, were responsible for a reduction in specific humidity in the urban mixing layer. Anthropogenic sources and sinks of water vapor were estimated and found to be insignificant for summer daytime mesoscale influences on the atmospheric water balance of the urban area. The effects of re- duced moisture in the urban area are directly related to the urban heat island, visibility, fog, precipitation, and human comfort in our cities. HUMANLIFE PATTERNS have reduction in latent heat energy, may changed radically in the last century also account for anomalous precipitation with the increasing migration from patterns (Terjung et al. 1971; Boatman rural to urban areas. The increased 1974). On the local scale uncertainty magnitude of climatic anomalies ac- has been expressed whether the overall companying urban-industrial expansion water contribution from the city is re- are well documented by the urban heat duced compared to rural areas since island and are further evident in the effects on visibility, fog, moisture, and much water vapor is released by com- precipitation. These factors are im- bustion, industry, and cooling operations. portant to human comfort, land-use Project METROMEX (METROpoli- planning, and engineering considera- tan Meteorological Experiment) was tions. initiated in 1971 to statistically and The moisture budget appears to be a physically evaluate the possible modifi- key contributor to urban climate ano- cation of weather and climate by an malies. Seasonal surface-moisture ano- urban-industrial complex. Recent studies malies have been noted in large urban by the University of Wyoming in Proj- areas. The greatest urban-rural humid- ect METROMEX focused on the prob- ity differences are during summer after- lem of urban moisture (Sisterson 1975). noons when the urban dry island is Data were acquired from the instru- greatest in magnitude. Summer night- mented Wyoming research aircraft and time differences in urban-rural absolute instrumented meteorological weather humidity patterns suggest that the city mobiles, along with surface networks is drier when the nocturnal heat island operated by cooperating groups (Illinois is strongest. Wintertime studies, how- State Water Survey, St. Louis City and ever, show that the urban area is more County Air Pollution Agencies). moist than its rural counterpart. In- Three aspects of urban moisture were tensification of the urban heat island has investigated : (1) relative magnitudes been directly related to less absolute of anthropogenic sources and sinks, (2) moisture (Wood 1971) and reduced eva- the three-dimensional structure of the poration (Myrup 1969). A reduction in moisture field, and (3) evaluation of urban moisture, with a corresponding the moisture budget components. SOURCES AND SINKS OF MOISTURE precipitation days, so the precipitation component was neglected. Particulate Estimates of automobile and in- mass concentrations of 10OPgm m-3 dustrial sources of water vapor were (0.5pm size) have been reported for a compared to ~otentialevaporation cal- highly polluted day in Los Angeles, culations. The industries chosen in this California. In the extreme case, all study warranted investigation because particles are assumed to be hygroscopic of their size and location in the St. Louis salt particles at a relative humidity of area. The results of water vapor pro- 90 percent. For this case, the maximum duction rates (gm hr-') for anthropo- change in specific humidity in the urban genic sources and potential evaporation area (30 km by 30 km by 1.5 km) would for the urban area are shown in table 1. be only 2.75 x 10-4 gm kgm-I. This Even if only a fraction of the radiation amount is insignificant and cannot be energy is used for evaporation, it is ap- detected by moisture measurement. parent that potential evaporation (for a Investigation of photochemical re- 30-km by 30-km surface area) is several actions in the urban atmosphere was orders of magnitude greater than an- limited to the formation of sulfuric acid, thropogenic moisture sources. However, the most common and prolific photo- industrial sources are essentially point chemical pollutant. In the extreme case, sources of water vapor. If the potential all sulfur dioxide was assumed to react evaporation is considerd for an area with water vapor to form sulfuric acid. only 3 km by 3 km, the rate of water In all cases, the water vapor necessary vapor production approaches that of to convert all the SO, to HISO, was at the major industrial sources. Thus, least one order of magnitude less than during the daylight hours, anthropo- the water vapor emissions from the genic sources do not appear to be of industry. consequence except on the local scale. Sulfuric acid is also deliquescent at At nighttime, when potential evapora- high concentrations. The excess water tion is greatly reduced and mixing is vapor between industrial source and re- limited, anthropogenic sources are in- action requirements was used to dilute deed a relevant factor in the urban sulfuric acid concentrations in the in- water balance, so that the urban atmo- dustrial plumes. The Labadie power sphere at night may be more moist than plant plume showed the greatest poten- the rural atmosphere. tial concentrations of sulfuric acid. Only Hygroscopic aerosols, photochemical if the environmental relative humidity reactions, and precipitation are major is greater than 94 percent could sulfuric moisture sinks for an urban atmosphere. acid from the Labadie plant absorb In this study, cases were chosen on non- ambient water vapor--again assuming Table I.-Urban area sources of water vapor Classification Source Water vapor (gm hr-I) Auto Automobile Refineries Amoco Shell Power Plants Meramec Portage des Sioux Labadie Cement Alfa Missouri Portland Steel Granite City Nature Potential Evaporation -- all SO, converted to H,SO,. The photo- served on a number of fair summer chemical reaction of sulfuric dioxide and days. Low-level winds on this day were water vapor is therefore clearly not a northerly with speeds less than 5 m significant factor in the extraction of sec-l. Specific humidity and potential water vapor from the urban atmo- temperature patterns are shown for the sphere on the mesoscale, and in general, surface at 1600 CDT (fig. 1). Dry re- the deliquescense of sulfuric acid is gions exist along the northern, southern, insignificant. and western edges of St. Louis, cor- responding to major residential and ST. LOUIS CASE STUDY- light industrial land-use characteriza- 26 JULY 1974 tions (fig. 2). Heat islands are apparent north of the city and along the southern Atmospheric moisture patterns are in- edge. Because these analyses are based fluenced by advection, turbulent mixing, upon individual stations, local effects and local sources and sinks of moisture. influence station readings, and small In the METROMEX field studies, air- regions of low moisture and high temp- craft transects were chosen so that eratures are apparent (Chandler 1967, unique air columns were successively Kopec 1973). Weather mobile data also intercepted at two flight levels as they show as much as 2 gm kgm-I and a 1C passed over the urban area. The flight variation in moisture and temperature, pattern thus minimized advection. Hori- respectively, over distances of 1 km. zontal turbulent mixing was found to be Patterns of specific humidity and insignificant and downward entrainment potential temperature are shown for of dry air into the mixing layer was 450 m MSL (300 m AGL) in figure 3. A initially assumed insignificant with the single widespread dry region exists presence of a subsidence inversion. along the southern edge of St. Louis, Because surface data were acquired with tongues of dry air extending along from fixed stations, the time rate of the eastern and western edges of the change of surface moisture measure- metropolitan area. A single heat island ments included advection. Calculations is also apparent and corresponds to the showed that surface moisture advection dry region. Both humidity and temp- on the mesoscale was insignificant. Local erature patterns are reasonably well advection, however, was not assessable correlated with those on the surface. and may be important. The patterns of specific humidity and Temperature and moisture compar- potential temperature patterns at 900 m ison between corrected weather mobiles, MSL (fig. 4) correlate well with those at hygrothermographs, and telemetry sta- 450 m MSL. Both dry regions and heat tions at locations where data from all fields are more widespread at the upper three uniquely different sources could be level. compared, showed consistent results. A From data acquired at the three levels similar consistence was observed be- between 1530 and 1630 CDT, three- tween aircraft and radiosonde measure- dimensional analyses of the moisture ments in the free atmosphere. It is fields in the mixing layer were con- important to recognize that the charac- structed. Isopleth analysis of the specific teristic of the moisture anomalies de- humidity is shown for 10.2 gm kgm-I scribed in this section were consistent (fig. 5) and 11.8 gm kgm-I (fig. 6). A among four independent types of in- plume-like structure with an hourglass struments. configuration is evident. The drier sur- Data from 26 July 1974 for 1530 to face air is apparently mixed vertically 1630 CDT are presented to illustrate through the mixing layer. Measure- iiioisture patterns that have been ob- ments of the wind field convergence also Figure I.-Specific humidity (gm kgm-1) and potential temperature (OK) patterns for the surface at 1600 CDT.