Mechanisms of convective cloud organization by cold pools over tropical warm ocean during the AMIE/DYNAMO field campaign

Abstract

This paper investigates the mechanisms of convective cloud organization by precipitationdriven cold pools over the warm tropical Indian Ocean during the 2011 Atmospheric Radiation Measurement (ARM) Madden-Julian Oscillation (MJO) Investigation Experiment/Dynamics of the MJO (AMIE/DYNAMO) field campaign. A high-resolution regional model simulation is performed using the Weather Research and Forecasting model during the transition from suppressed to active phases of the November 2011 MJO. The simulated cold pool lifetimes, spatial extent, and thermodynamic properties agree well with the radar and ship-borne observations from the field campaign. The thermodynamic and dynamic structures of the outflow boundaries of isolated and intersecting cold pools in the simulation and the associated secondary cloud populations are examined. Intersecting cold pools last more than twice as long, are twice as large, 41% more intense (measured with buoyancy), and 62% deeper than isolated cold pools. Consequently, intersecting cold pools trigger 73% more convection than do isolated ones. This is due to stronger outflows that enhance secondary updraft velocities by up to 45%. However, cold pool-triggered convective clouds grow into deep convection not because of the stronger secondary updrafts at cloud base, but rather due to closer spacing (aggregation) between clouds and larger cloud clusters that form along the cold pool boundaries when they intersect. The close spacing of large clouds moistens the local environment and reduces entrainment drying, increasing the probability that the clouds further develop into deep convection. Implications for the design of future convective parameterization with cold poolmodulated entrainment rates are discussed.

Tropical continental downdraft characteristics: mesoscale systems versus unorganized convection

“…Data from the DOEBrazil Green Ocean Amazon (GoAmazon) campaign Martin et al, 2016) provide an unprecedented opportunity to evaluate downdraft characteristics in the Amazon with sufficiently large data sets for quantifying robust statistical relationships describing leading order processes for the first time. Relationships explored previously, primarily in tropical oceanic (Barnes and Garstang, 1982;Feng et al, 2015;de Szoke et al, 2017) or mid-latitude regions (Charba, 1974;Engerer et al, 2008), such as time composites of wind and thermodynamic quantities relative to downdraft precipitation, are also revisited and compared to our findings over the Amazon. Downdrafts in MCSs and isolated cells are compared to inform decisions concerning their unified or separate treatment in next generation models.…”

“…Out to 25 km, the statistics are very similar to those observed using in situ precipitation. Theoretical (Romps and Jevanjee, 2016), modeling (Tompkins, 2001;Feng et al, 2015), and observational (Feng et al, 2015) studies have all examined typical sizes of cold pools, which can be on the order of 25 km in diameter for any one cell. Cold pools can combine, however, to form a larger, coherent mesoscale-sized cold pool (radius of 50 km or greater), as is commonly associated with mesoscale convective systems (Fujita, 1959;Johnson and Hamilton, 1988).…”

Relationship between the direction of diurnal rainfall migration and the ambient wind over the Southern Sumatra Island

“…Some previous studies suggest that cold pool dynamics are important to the redevelopment of rainfall systems over the ocean [Sakurai et al, 2009[Sakurai et al, , 2011Fujita et al, 2010]. Cold pools, however, are associated with intense downdrafts produced in the convective core [Feng et al, 2015]. Although the stratiform rainfall areas dominate the convective areas over the ocean west of Sumatra Island, convective rainfall coexists (Figures 3e-3h), which might be critical to maintain systems over the ocean [Moteki et al, 2008].…”

Quantifying the impact of sub-grid surface wind variability on sea salt and dust emissions in CAM5

“…Mesoscale atmospheric processes such as topographic gravity waves and moist convection are also important contributors to wind variability. Gusty winds can be generated by topographic gravity waves near mountain downslopes (Durran, 1990), or in thunderstorm outflows over land and ocean (Mahoney, 1988) where strong downdrafts (cold pools) occur in association with strong precipitation events (Zeng et al, 2002;Feng et al, 2015). Jabouille et al (1996) showed that wind gusts generated by convective outflow can significantly enhance the surface heat fluxes.…”