Characteristics of satellite derived Latent Heating variability associated with the MJO over the Indian and West Pacific Ocean

Previous studies have shown that the latent heating profiles in convective and stratiform regions are fundamentally different. The particular combination of convective and stratiform convective elements occurring in a region determines the overall latent heating profile. In this study, we investigate how the latent heating profile of the MJO is affected by different the forms of convection seen by satellite radar. While convective regions are characterized by heating in the lower-mid troposphere, heating in the upper troposphere and cooling in the lower troposphere characterize stratiform regions. However, convective regions can be further classified as shallow, deep, or wide. These three types of convective have different latent heating profiles and the current study investigates how these types of convection affect the variability in latent heating observed during the Madden-Julian Oscillation (MJO) in the central Indian Ocean and West Pacific Ocean. In order to isolate these different types of convection we use the methodology of Barnes and Houze (2013), who analyzed data from the TRMM Precipitation Radar (TRMM PR) in the central Indian and West Pacific Oceans to identify different forms of convection, including shallow isolated echoes and several categories of extreme convective features, referred to as deep convective cores, wide convective cores, and broad stratiform regions. Shallow isolated echoes are echoes whose top is at least 1 km below the climatology freezing level and are separate from other, deeper convection. Deep convective cores are contiguous three-dimensional echo objects exceeding 30 dBZ in intensity and 8 km in altitude. Wide convective cores (WC) are contiguous three-dimensional echoes >30 dBZ over areas of at least 800 km2. Broad stratiform regions (BS) are stratiform echoes extending contiguously over at least 50,000 km2. These extreme echo entities are not mutually exclusive; two or more may occur within a single convective cloud system. Latent heating profiles are estimated using the spectral latent heating (SLH) dataset, which is based on information derived from the TRMM PR to compute the appropriate latent heating profile from look-up tables that are based on numerical simulations of tropical convection observed during TOGA COARE (Shige et al. 2009).

In general, in the tropical zone where the MJO manifests most strongly, the heating associated with the rainfall from the extreme convective features (deep convective cores, wide convective cores, and broad stratiform regions) maximizes at an altitude of 6-9 km during the active phase of the MJO. This behavior is largely attributable to the increased occurrence of mesoscale convective systems containing wide convective cores and broad stratiform regions during these phases.

Departures from this general behavior are seen when the central Indian Ocean and West Pacific sectors are considered separately. These departures occur mainly below 6 km and are attributable to the relative frequency of occurrence of convection in the form of wide convective cores. In the central Indian Ocean, frequent wide convective cores produce substantially more heating at approximately 5 km during all phases of the MJO than in the West Pacific. The magnitude of heating from broad stratiform regions and wide convective cores is comparable during active MJO phases in the central Indian Ocean. Additionally, deep convective cores never dominate the latent heating profile associated with these extreme mesoscale features in the central Indian Ocean. The greater amount of low-level heating by extreme forms of convection in the central Indian Ocean is thus due to the greater presence of wide convective cores in that region.

In contrast, in the West Pacific Ocean, heating associated with wide convective cores rarely dominates over heating from deep convective cores or broad stratiform regions. During the active phase, the latent heating from both deep and wide convective cores is relatively minor compared that produced by broad stratiform regions. Thus, there is relatively minor low-level heating from extreme mesoscale features during the active phases of the MJO in the West Pacific Ocean.

These differences between MJO convective manifestations in the central Indian Ocean and West Pacific are consistent with results from Barnes and Houze (2013), who showed that wide convective cores are more prevalent than deep convective cores in the central Indian Ocean but deep convective cores are more common than wide convective cores in the west Pacific Ocean.

In addition to examining the heating by extreme convective features alone, we further investigate what fraction of the total latent heating by MJO convection is attributable to each of the forms of extreme convection and how that fraction changes with phase of the MJO and tropical longitudinal zone. The results from this study indicates that the different forms taken by convection in different regions affected by the MJO lead to variations in the shape of the latent heating profile from region to region in the tropical latitude belt affected most strongly by the MJO. This study shows that while stratiform regions behave generally the same from region to region, the different forms taken by the extreme convective elements lead to different heating profiles as the MJO progresses eastward. This result is consistent with the conclusions of Houze (1989), who suggested that despite the importance of heating by stratiform components of convective cloud systems, the regional variability of net heating profiles was likely determined by differences in the forms taken by the convective components as opposed to the stratiform components.