On the Sensitivity of the Diurnal Cycle in the Amazon to Convective Intensity

Abstract:

The sensitivity of the diurnal cycle to convective intensity is investigated for the wet season (DJF) and dry season (JJA) in the Amazon region. Model output reveals large water and energy budget errors in tropical rainforests, arising from a misrepresentation of the diurnal cycle of the complex processes inherent to diurnally forced moist convection. Daily, 3-hourly satellite observations of CERES Ed3a SYN1DEG TOA fluxes and 3-hourly TRMM 3B42 precipitation rate from 2002-2012 are split into regimes of convective intensity using percentile definitions for both daily minimum OLR and daily maximum precipitation rate to define regimes. These satellite-defined regimes are then co-located with convective parameters calculated from radiosonde observations. Diurnal statistics from satellite include: phase, amplitude, precipitation onset, precipitation duration and diurnal mean. The diurnal phase of outgoing longwave radiation (OLR) and longwave cloud forcing (LWCF) occurs several hours earlier on convective days compared to stable days, however, climatological precipitation phase is less sensitive to convective intensity, occurring between 1-4PM local time for all regimes and 1-2 hours later on very convective days, which is related to longer duration precipitation events from increased humidity. Diurnal convection in the Amazon is strongly related to 8AM values of both dynamic and thermodynamic variables, most of which are related to: the background moisture content of the troposphere, the stability of the lower troposphere, convective inhibition (CIN) and wind speed and direction in the column. Morning values of CIN, lifted condensation level (LCL), level of free convection (LFC) and equilibrium level (EL) are lower in DJF than JJA, and lower on very convective days than stable days for all stations. Higher background humidity is related to longer duration precipitation events (r-values between 0.4-0.6, depending on station and season), earlier phases and onset times of clouds and precipitation (r-values ranging from 0.2-0.5) and larger diurnal amplitudes and diurnal mean values of precipitation and OLR with r-values as high as 0.7 in JJA between 8AM upper tropospheric humidity (UTH) and convective intensity.