Rainfall Regimes and the Surface Energy Balance in the Southeastern United States

RAINFALL REGIMES AND THE SURFACE ENERGY BALANCE IN THE SOUTHEASTERN UNITED STATES John L. Bedford The climate of a place may be characterized by the processes through which radiation, heat, and moisture are received and dissipated at the earth's surface. Collectively, these processes result in an area's surface energy balance or budget. The radiation balance refers to exchanges of solar and infrared radiation, the algebraic sum of which is the net radiation. The heat balance reflects how energy available from net radiation is dissipated by heating the air, evaporating moisture , and heating the soil, i.e., respectively, the sensible, latent, and soil heat fluxes. The moisture balance accounts for precipitation in terms of évapotranspiration, runoff, and changes in soil moisture. Variations in the surface energy balance are caused in part by the geographic and topographic position of a place and its surface conditions , and in part by the sequence of weather events occurring at the place which modifies the energy exchanges. Geiger, Budyko, and Sellers have each published widely reproduced graphs showing mean monthly values of the surface heat balance. (1) Graphs such as these can be used to examine the energy environments of climatic stations in different regions and to provide further insight into the fundamental processes which characterize regional climates. A layman observing world or national maps of climatic regions may conclude that the southeastern United States is a broad, rather homogeneous climatic region, i.e., the Humid Subtropical (Cfa) region of the Koeppen system. Within the Southeast, however, the differential effects of latitude, elevation, and continentality upon temperatures and upon amounts and seasonality of precipitation result in substantial regional variations in surface energy budgets. Furthermore, because of the large amount of energy transferred by évapotranspiration, one might expect that variations in energy budgets are related to regional variations in precipitation. Dr. Bedford is Assistant Professor of Geography at the University of North Carolina at Charlotte in Charlotte, JV.C. 28223. Vol. XVI, No. 2 99 PRECIPITATION-ENERGY BALANCE RELATIONSHIPS. Energy balance climatology focuses on fundamental energy transfers at the earth's surface and the factors that control these exchanges. Precipitation is a direct component of the moisture balance, but because it is associated with other weather elements several indirect relations between precipitation and energy balances can be expressed: 1) Changes in cloud types or cloud coverage during periods of precipitation may affect shortwave and longwave radiation exchanges. 2) Both cool surfaces and high humidity reduce longwave radiative losses to space. 3 ) A wet period can alter the heat balance by reducing surface or air temperatures which affect all three heat balance components because they are a function of near surface temperature gradients. 4) The latent heat flux is reduced when increasing humidity lowers the rate of évapotranspiration. 5) The thermal conductivity and heat capacity of soil increase with increasing soil moisture. 6) Evapotranspiration rates and moisture surpluses are a function of soil moisture which is related to current and antecedent precipitation. Because of the above relationships it is essential that seasonal and regional variations of precipitation be considered in energy balance studies. RAINFALL REGIMES. The climate of the United States may be characterized by regional differences in the seasonal distribution of precipitation. Trewartha has recognized seven regional rainfall types in Anglo-America. (2) Such variations in rainfall regimes result largely from seasonal and regional variations in atmospheric circulation patterns. Several regional differences in annual rainfall regimes exist in the southeastern states ( Figure 1 ) . Large areas within Texas and Oklahoma have a bimodal precipitation distribution with maxima occurring in late spring and in late summer (Texas subtype of region 3 in Figure 1). A large central portion of the Southeast (region 5) has winter or spring concentrations—sometimes called a Tennesseetype regime. Along the Gulf and Atlantic coasts (region 6) a summer maximum prevails. Graphs of mean annual variations in precipitation during 1951-70 for six southeastern stations are indicative of variations in the region's rainfall regimes (Figure 2). Atlanta's spring and summer maxima represent a transition between the Tennessee, Gulf, and CarolinasVirginia regimes. Greensboro, with a weak summer maximum, is 100 Southeastern Geographer REGIONAL RAINFALL TYPES Oklahoma City Rock FALL 4 OHIO VALLEY...