The impact of radiative heating and cooling on marine stratocumulus dynamics.

摘要

We investigate the impact of radiative heating on the dynamics of the stratocumulus-topped boundary layer (STBL). Radiative heating computations through one-dimensional static cloudy model atmospheres show us that both longwave and shortwave radiative heating are sensitive to droplet concentration (Nd) and liquid water path (LWP) when LWP is low (LWP 20 g m-2). For higher LWPs, longwave radiative heating is not sensitive to Nd or LWP while shortwave radiative heating continues is sensitive to both quantities.;We used large-eddy simulation to study the STBL dynamical response to radiative heating. Nocturnal LESs of the STBL are sensitive to N d when LWP is low and the free tropospheric air is dry. Entrainment and longwave radiative cooling lead to lower cloud fractions when Nd is high as compared to when it is low. These low cloud fractions are associated with less longwave radiative cooling and weaker STBL circulations, and entrainment is able to suppress cloud growth. In contrast, when Nd is low and cloud fractions are not as low, longwave radiative cooling is large enough to support stronger STBL circulations and the cloud layer grows against entrainment. We suggest that accounting for changes in longwave radiative heating with droplet concentration is important in simulating low level liquid water clouds. When LWP is not low, changes in drizzle strength with Nd mitigate differences in nocturnal STBL dynamics owing to changes in longwave radiative heating with Nd. The dependence of longwave radiative heating on Nd is not as significant for these LWPs.;Daytime simulations of the STBL revealed that shortwave radiative heating affects the STBL primarily through increasing thermodynamic stability and this effect increases as solar zenith angle (theta) decreases. This increase in stability is associated with decreased LWP, slower entrainment, weakened circulations and strengthen decoupling of the cloud layer from the sub-cloud layer. When LWP is not low, this decoupling is stronger for high Ndbecause shortwave absorption is stronger and drizzle is weak. For low Nd a stronger drizzle process may aid in partially re-coupling the cloud and sub-cloud layers together through generation of conditional instability in the sub-cloud layer. For low LWP, increased shortwave warming also leads to reduced longwave cooling. This reduction in longwave cooling leads to even weaker circulations and stronger decoupling of the cloud layer from the sub-cloud than for STBLs with higher LWP. Because entrainment is more vigorous and circulations are weaker for high Nd, low LWP clouds in the STBL are more likely to dissipate over the diurnal cycle when Nd is high as compared to when Nd is low.;Radiative in the simulations described above was computed using the Independent Column Approximation (ICA). We tested the impact on STBL dynamics of using the ICA in shortwave radiative heating rate computation by coupling a three-dimensional Monte Carlo shortwave radiative transfer solver to our LES. Preliminary results show that the use of the ICA for shortwave radiative heating computation has a minimal impact on STBL dynamics.