Effect of Urban Heat Island on Minimum and Maximum Temperatures of a Metropolitan Area in Central Iowa

With an 18.3 percent increase in population from 557,918 people in 2000 to 659,779 people in 2011, the Des Moines Metropolitan Area (DSMA) is one of the fastest growing metropolitan areas in the country. This growth in population has caused an increase in man-made surfaces, buildings and streets, and a reduction of natural surfaces, grasslands and fields. These surface changes have led to an alteration of nighttime longwave radiative cooling, and retention of daytime shortwave radiation. The density increase of available weather stations in and around the DSMA in the past decade allows for the measurement of the effect of urbanization on diurnal variations in temperature. Previous studies in urban climate have been more concerned with the effect an urban heat island has on downwind precipitation and less on its effect on timing of diurnal variations of other meteorological variables such as temperature.

Trends in monthly and seasonal minimum and maximum temperatures around the DSMA are examined. Temperatures from 2002 to 2011 were collected from ASOS, AWOS and RWIS stations within 50 miles of downtown Des Moines to determine differences in minimum and maximum temperatures and differences of timing of diurnal variations between stations. Maximum temperatures will gradually decrease as the distance from the Des Moines city center increases but not with less magnitude than minimum temperatures. These minimum and maximum temperature trends will lead to a smaller diurnal range for stations closer to Des Moines than those further away. Daily minimum temperatures are expected to occur earlier in Des Moines with maximum temperatures occurring later when compared to surrounding areas.

Preliminary results between the ASOS station at the Des Moines International Airport (DSM), well within the metropolitan area, and the AWOS station at the Boone Municipal Airport (BNW), 33 miles to the northwest of Des Moines, are promising. The winter month mean minimum temperature difference between DSM and BNW is -10ºC and -9ºC below the freezing point, respectively. The winter month mean maximum temperature difference is 1.5ºC and 0ºC. This trend continues through spring and summer, but is not significant in autumn. The impact of this trend on the diurnal variations of temperature and timing of daily maximum and minimum temperature in different seasons will be reported.