On the sensitivity of the diurnal cycle in the Amazon to convective intensity

Abstract

Climate and reanalysis models contain large water and energy budget errors over tropical land related to the misrepresentation of diurnally forced moist convection. Motivated by recent work suggesting that the water and energy budget is influenced by the sensitivity of the convective diurnal cycle to atmospheric state, this study investigates the relationship between convective intensity, the convective diurnal cycle, and atmospheric state in a region of frequent convection—the Amazon. Daily, 3‐hourly satellite observations of top of atmosphere (TOA) fluxes from Clouds and the Earth's Radiant Energy System Ed3a SYN1DEG and precipitation from Tropical Rainfall Measuring Mission 3B42 data sets are collocated with twice daily Integrated Global Radiosonde Archive observations from 2002 to 2012 and hourly flux tower observations. Percentiles of daily minimum outgoing longwave radiation are used to define convective intensity regimes. The results indicate a significant increase in the convective diurnal cycle amplitude with increased convective intensity. The TOA flux diurnal phase exhibits 1–3 h shifts with convective intensity, and precipitation phase is less sensitive. However, the timing of precipitation onset occurs 2–3 h earlier and the duration lasts 3–5 h longer on very convective compared to stable days. While statistically significant changes are found between morning atmospheric state and convective intensity, variations in upper and lower tropospheric humidity exhibit the strongest relationships with convective intensity and diurnal cycle characteristics. Lastly, convective available potential energy (CAPE) is found to vary with convective intensity but does not explain the variations in Amazonian convection, suggesting that a CAPE‐based convective parameterization will not capture the observed behavior without incorporating the sensitivity of convection to column humidity.

Role of Troposphere‐Convection‐Land Coupling in the Southwestern Amazon Precipitation Bias of the Community Earth System Model Version 1 (CESM1)

“…Indeed, the unrealistic diurnal cycle simulated without the entrainment dilution (Rasch et al, ) was one of the motivations for employing the CAPE dilution (Neale et al, ). Observational studies over the Amazon also support a link between convective precipitation and tropospheric moisture (Itterly et al, ; Schiro et al, ), but our result indicates that including entrainment dilution as the link between the tropospheric moisture and convection may not be sufficient to reproduce the observed precipitation over the SW Amazon, particularly during the late afternoon.…”

“…These areas of enhanced convection have spatial scales of 50-100 km, but the rivers themselves are only a few kilometers wide. Other studies found a significant correlation between tropospheric moisture and deep convection (Itterly et al, 2016;Schiro et al, 2016), implying an important role for the entrainment of tropospheric air into rising plumes that is often only crudely represented in global models (Romps, 2010;Sahany et al, 2012;Stirling & Stratton, 2012).…”

Evaluation of the sensitivity of the Amazonian diurnal cycle to convective intensity in reanalyses

“…The TRMM 3B42 product provides 3-hourly 0.25°x0.25° observations of tropical precipitation rates by combining VIRS infrared and microwave retrievals (Kummerow et al 1998 We then apply averaging and compositing techniques to examine the spatial variability of daytime (06:00-21:00 LST) convective DC statistics, with a focus on the diurnal phase in this paper. Precipitation DC statistics are defined only when the 3-hourly average rainrate exceed an experimentally determined convective precipitation threshold defined as 10 mm day -1 (Itterly et al 2016). Convective regimes are defined using percentiles of daily minimum OLR, a proxy for deep (cold) convective cloud tops where lower values indicate more intense convection (Gray and Jacobson 1977).…”

Sensitivity of the Amazonian Convective Diurnal Cycle to Its Environment in Observations and Reanalysis

“…The mean state and variability of the CDC is linked to atmospheric state anomalies on various timescales (e.g., Adams et al, ; Betts et al, , ; Derbyshire et al, ; Dodson & Taylor, ; Itterly et al, ; Lintner et al, ; Taylor, , ; Zelinka & Hartmann, ; Zhang & Klein, ; Zhao et al, ). For example, Zhang and Klein () show robust relationships between preconvective environmental parameters and afternoon deep convection in the U.S. Southern Great Plains indicating that higher convective available potential energy (CAPE) is associated with a later onset time and shorter duration of precipitation, whereas higher humidity above the boundary layer leads to an earlier onset time and longer duration of precipitation due to reduced entrainment of developing cumulus clouds (Derbyshire et al, ; Zhang & Klein, ).…”