Relationships between radiation, clouds, and convection during DYNAMO

Abstract

The relationships between radiation, clouds, and convection on an intraseasonal time scale are examined with data taken during the Dynamics of the Madden‐Julian Oscillation (MJO) field campaign. Specifically, column‐net, as well as vertical profiles of radiative heating rates, computed over Gan Island in the central Indian Ocean (IO) are used along with an objective analysis of large‐scale fields to examine three MJO events that occurred during the 3 month period (October to December 2011) over this region. Longwave (LW) and shortwave radiative heating rates exhibit tilted structures, reflecting radiative effects associated with the prevalence of shallow cumulus during the dry, suppressed MJO phase followed by increasing deep convection leading into the active phase. As the convection builds going into the MJO active phase, there are increasingly top‐heavy anomalous radiative heating rates while the column‐net radiative cooling rate <Qr> progressively decreases. Temporal fluctuations in the cloud radiative forcing, being quite sensitive to changes in high cloudiness, are dominated by LW effects with an intraseasonal variation of ~0.4–0.6 K/d. While both the water vapor and cloud fields are inextricably linked, it appears that the tilted radiative structures are more related to water vapor effects. The intraseasonal variation of column‐net radiative heating <Qr> enhances the convective signal in the mean by ~20% with a minimum in this enhancement ~10 days prior to peak MJO rainfall and maximum ~7 days after. This suggests that as MJO convective envelope weakens over the central IO, cloud‐radiative feedbacks help maintain the mature MJO as it moves eastward.

Moisture Distributions in Tropical Cold Pools From Equatorial Indian Ocean Observations and Cloud‐Resolving Simulations

“…The five examples described here support the idea that drier midtropospheres support stronger cold pools. Within the larger statistical sample of Figure , these cases support the idea that environments with intermediate column‐integrated moisture values can sustain strong cold pools and are consistent with the observation made within Ciesielski et al () that periods with increasing rainfall tend to possess dry midlevels.…”

“…Cold pools are hypothesized to be a key process within shallow‐to‐deep convection transitions, such as the 7–14 October and 11–15 November 2011 time periods outlined in Figure (Rowe & Houze, ; Ruppert & Johnson, ). Dry midlevel tropospheres coexist with increasing column‐integrated moisture and significant rainfall during these times (Ciesielski et al, ; Zhang et al, ). The convectively suppressed time periods at the two sites coincide statistically with MJO phases 4–8 (Gottschalck et al, 2013).…”

The Two Diurnal Modes of Tropical Upward Motion

“…This effect may indeed manifest in the ITCZ and Hadley cell (Ackerman et al, ; Ciesielski et al, ; Webster & Stephens, ). While variations in latent heating are undoubtedly much larger in magnitude than those of radiative heating in the low‐middle‐troposphere, this is not strictly the case in the upper‐cloud region where SW ∗ maximizes (Ciesielski et al, ; Johnson et al, ; Johnson et al, ; Schumacher et al, ).…”

A Survey of Precipitation-Induced Atmospheric Cold Pools over Oceans and Their Interactions with the Larger-Scale Environment

“…Atmospheric cold pools over oceans are receiving attention for their ability to reorganize the mesoscale cloud distributions and potentially facilitate the transitions from high-to low-albedo shallow cloud cover. In the Tropics, the expansion of the spatial and height distribution of convection by cold pools may facilitate transitions from shallow to deep convective regimes, and therefore the eastward propagation of the Madden-Julian oscillation (MJO) into moistening environments (Rowe and Houze 2015;Feng et al 2015;Ruppert and Johnson 2015;Schlemmer and Hohenegger 2016;Hannah et al 2016;Ciesielski et al 2017).…”