Evolution of precipitation and convective echo top heights observed by TRMM radar over the Indian Ocean during DYNAMO

Abstract

Radar data from the Tropical Rainfall Measuring Mission show the evolution of echo tops of convective elements over the Indian Ocean and Maritime Continent during the Dynamics of the Madden-Julian Oscillation (DYNAMO) field campaign of 2011-2012. Echo top heights exhibited a bimodal distribution wherein cumulonimbi of moderate height constituted a "congestus mode" while vertically extensive cumulonimbus made up a "deep mode." An intraseasonal time scale dominated variability in these modes from October to January over much of the Indian Ocean. Over the Maritime Continent, there was no clear intraseasonal signal in convective echo top heights. Where the intraseasonal oscillation was detected, radar echoes evolved from being dominated by the congestus mode to being characterized by more deep mode convection on time scales of less than 1 week. The areal coverage of congestus echoes began to increase 2-8 days prior to the rise in area of deep echoes. These satellite-derived results confirm that the time scale for convective deepening seen at individual DYNAMO observational sites is consistent with that of convection on the large scale over the Indian Ocean. Intraseasonal variability of zonal wind, temperature, and humidity as depicted by reanalysis is also consistent with that derived from rawinsonde observations during DYNAMO. Thus, the gradual buildup of convection as depicted by recent versions of the "discharge-recharge" hypothesis does not accurately describe evolutions of convection prior to MJO events observed during DYNAMO, although cloud moistening processes may still be relevant on time scales of 1 week or less.

Initiation Processes of the Tropical Intraseasonal Variability Simulated in an Aqua‐Planet Experiment: What is the Intrinsic Mechanism for MJO Onset?

“…We also found that circumnavigating Kelvin waves crucially help determine periods of MJO‐like disturbances, as shown by the KDAMP experiment. Hence, our study is consistent with the observational study of Powell and Houze (), which showed that convective activities associated with the MJO cases observed during the Dynamics of the Madden‐Julian Oscillation (DYNAMO) can be triggered not by the discharge‐recharge mechanism but by equatorial circumnavigation of Kelvin waves. However, we do not rule out the possibility that the discharge‐recharge mechanism operates in reality because MJO‐like disturbances are initiated even without circumnavigating Kelvin waves.…”

Evolution of Precipitation Structure During the November DYNAMO MJO Event: Cloud‐Resolving Model Intercomparison and Cross Validation Using Radar Observations

“…On the other hand, the R/V Mirai (8°S, 80.5°E) radar which is located to the south of the equator, showed nearly opposite precipitation patterns compared to the Revelle radar. Xu and Rutledge (), Powell and Houze (), and Xu and Rutledge () compared ground‐based radars and TRMM PR long‐term observations and noted that the variability observed by the S‐PolKa radar at Addu Atoll and the Revelle C‐band radar were consistent with TRMM PR observations. These radar observations therefore provide a useful, consistent database to validate CRM simulations, which, in turn, allows for an additional investigation into the commonalities and subtle differences between the different regions during the same MJO event.…”

Effect of dry large‐scale vertical motions on initial MJO convective onset

“…The data set and further details about it are available at http://johnson.atmos.colostate.edu/dynamo/products/gridded/index.html. The intraseasonal variability in convection and tropospheric structure observed within spatially limited radar and rawinsonde data sets has been shown to be consistent with the same over a much larger domain spanning much of the equatorial Indian Ocean [ Powell and Houze , ]. They also showed that the time scale required for the distribution of precipitating clouds to transition from moderately deep congestus to deep cumulonimbi prior to MJO convective onset, as observed by S‐PolKa, was consistent with that on the large scale.…”

“…The convective population is mostly suppressed when the large‐scale anomalous vertical motion is downward (red color on curve), and drying occurs throughout most of the troposphere. As the large‐scale anomalous subsidence lessens (light color on curve), more deep convection begins to develop and the lower troposphere moistens over 3–7 days [ Powell and Houze , , ]. Q 2 is negative during the moistening, meaning that moistening occurs via eddy convergence of moisture within congestus clouds and/or by detrainment and subsequent evaporation of cloud condensate.…”

“…Instead, they observed that an increase in depth of convection occurred on a shorter time scale near the beginning of a large‐scale convective event. Powell and Houze [] reached the same conclusions for a larger spatial domain by using satellite‐based radar data. Moreover, several episodes of deep convection, building and decaying on time scales shorter than the MJO time scale, occur during an active period of an MJO [ Zuluaga and Houze , ].…”