A Conceptual Model for the Hydrometeor Structure of Mesoscale Convective Systems during the Active Stage of the MJO

The NCAR S-PolKa radar is a dual-wavelength, dual-polarimetric radar that was deployed for three months (October – January) on Addu Atoll in the Maldives during the Dynamics of the MJO (DYNAMO) and ARM MJO Investigation Experiment (AMIE) field experiments in 2011-2012. Three episodes of the Madden-Julian Oscillation (MJO) were successfully captured by S-PolKa and the active stage of each MJO was characterized by an increase in mesoscale organization. The high spatial and temporal resolution of S-PolKa's dual-polarimetric data provided a unique opportunity to investigate the spatial variation of hydrometeors within the precipitating clouds of the mesoscale convective systems (MCSs) present during the active MJO periods. The particle identification algorithm (PID) of Vivekanandan et al. (1999) has been applied and used to identify the dominant type of hydrometeor in spatial bins of 150 m. By compositing the location of each hydrometeor type with respect to convective and stratiform overturning layers of the types identified by Moncrieff (1992) and Kingsmill and Houze (1999), we present a conceptual model for the structure of hydrometeors in MCSs during the active stage of an MJO. Additionally, we use the polarimetric data to suggest which microphysical processes are associated with these hydrometeor structures. While convective updraft cores were characterized by moderate rain, the heaviest rain occurred just downwind of the updraft core, possibly due to differential sedimentation. Above the melting level, dry aggregates existed within the core and extended to the base of the divergence signature, marking the top of the convective updraft. Also, riming produced narrow zones of graupel just downwind of the updraft core. However, these zones of graupel were relatively shallow and only extended 1.5-2 km above the freezing level. Small ice crystals surrounded the region of dry aggregates and scattered areas of horizontally oriented ice particles existed along the edges of the convective echo. These horizontally oriented ice particles were possibly associated with entrainment induced evaporation.

In October and November, during the increase of convective activity of the MJO, S-PolKa detected non-squall MCSs that were maintained by new convection entering the stratiform regions of pre-existing MCSs. This behavior of the convection occurred when the low-level zonal winds were weak and directional shear was strong. However, in December when the low-level zonal winds were strong and the directional shear was weak, squall MCSs with a leading-line and trailing stratiform structure were observed.

Despite these large-scale structural differences, these two types of MCSs had similar hydrometeor organization relative to the airflow when a mid-level inflow was identifiable in the stratiform region. Ice particles in stratiform regions generally occurred in a layered configuration independent of the environment and mesoscale dynamics of the MCSs. Within the upper portions of the stratiform regions small ice crystals dominated. Below the relatively large region of small crystals, aggregation occurred and a smaller region of dry aggregates was identified. Finally, near the freezing level these aggregates began to melt and a band of wet aggregates was identified. This band of wet aggregates became fragmented in the anvil. Numerous brightbands, whose intensity exceeded 40 dBZ, were characterized by a thin band of graupel/rimed aggregates located just above the wet aggregates. We are currently investigating if these hydrometeors were graupel that resulted from riming within the stratiform updraft or if these particles were very large aggregates that were melting. Isolated regions of horizontally oriented ice particles were located near echo top and scattered in the stratiform. While the horizontally oriented ice crystals near echo top were likely associated with entrainment induced evaporation, horizontally oriented ice particles in the interior of the echo likely represented regions of dendrite formation. While light rain was most common in stratiform regions, the rain intensity systematically decreased towards the anvil possible due to evaporation or differential sedimentation. Additionally, coalescence sometimes occurred near the surface in these lightly raining regions. Heavy rain only occurred in stratiform regions close to where the mid-level inflow intersected the surface, suggesting that these cases had characteristics that were more convective.

Particle identification algorithms based on dual-polarization radar are difficult to verify and have not been previously used along the equator. We evaluate the uncertainty in our classifications by investigation how alternative particle identification algorithms classify these data. Despite these inherent uncertainties, the systematic organization identified by these PIDs is notable, lending confidence to the validity of our conceptual diagram. This conceptual model presents a means to qualitatively compare model output to radar observations.