Below-canopy microclimate during tropical storm Isabel in Baltimore, MD

Measurements for a study of soil temperatures and moisture in and near Baltimore as part of the USDA Forest Service contribution to a Long Term Ecological Research (LTER) site, the Baltimore Ecosystem Study (BES), are providing an opportunity to observe the influence of urban structures on wind and air temperature below urban canopies. In this poster presentation, hourly average wind speed, gust wind speed, and air temperature at four below-canopy points are compared to wind speed and temperature in the open at the BES primary weather station and at the Baltimore-Washington International (BWI) Airport during 15 days that included the passage of Tropical Storm Isabel. Below-canopy measurements are at about people height (2 m for wind, and 1.5 m for air temperature). Comparisons are also made to air temperature at the National Weather Service Downtown Baltimore (DMH) weather station. All of the measurement sites except DMH are outside the city of Baltimore. The BES primary station (BES Primary) is near Owning Mills, MD, 19 km to the northwest of DMH. One of the below-canopy sites (Woods) is in a small forest 0.5 km north of BES Primary. BWI is 14 km south-southwest of DMH, and the other three sites are about 16 km northeast of DMH in an area known as Cub Hill. One of these (Tower) is near the base of a former fire lookout tower in an area that can now be categorized as residential with remnant forest. The other below-canopy sites are in a neighborhood of single-family homes on large lots (Residential) and in a complex of 2- and 3-story apartment buildings (Apartments). Elevations range from 6 m at DMH to 156 m at BES Primary. All of the below-canopy sites except for DMH were decidedly influenced by trees. Within 1 km, GIS analysis of tree cover showed that average tree cover varied from <1% at DMH to 45% at the Woods site. (The Woods station was only 50 m from an open agricultural field.) During tropical storm Isabel, gust winds at the 10-m-height at BWI reached 24 m/s, while average speeds were about 17 m/s at 10 m, which, assuming neutral thermal stratification, would translate to about 12 m/s at the 2-m height. Gust winds at BES Primary, which was in the middle of a large pasture field, reached 15 m/s, and hourly average wind speed was 8 m/s, while the other below-canopy sites had gusts of 9 m/s or less, and average wind speeds of 3 m/s or less. At these sites, trees rather than buildings were probably most responsible for the large reductions in wind speed. The intense mixing during the storm evened out air temperature differences between sites to about 1°C. At other times, the generally warmest site was just south of the central business district of Baltimore at DMH, although the Apartment site sometimes registered maximum daily temperatures about 1°C higher than DMH, probably owing largely to radiation errors at the Apartment station, which has a Gill-type, naturally ventilated shield. The Apartment site did have the highest impervious cover (24% within 1 km) after DMH (56%). The BES Primary station, which had an accurate, platinum, fan-aspirated temperature sensor, had maximum temperatures 1.5 to 2°C cooler than DMH, and minimum temperatures as much as 6°C cooler than DMH. The coolest site was the Woods, which was generally 1 to 2°C cooler than the open BES Primary site both day and night. The Woods site had minimum temperatures as much as 7.5°C cooler than DMH. The more developed sites registered minimum temperatures about 4°C cooler than DMH. The largely graphical analysis reported here is preliminary to the addition of 3 more weather stations and more analytical modeling of urban influences on wind and air temperature. Hemispherical photographs help to characterize the sites in the presentation and will provide solar and sky radiation inputs for modeling.