Tropical Humidity Structure Inferred from a Microwave Radiometer during DYNAMO

In the Tropics, convection is sensitive to small variations in humidity, especially within the free-troposphere. Radiosondes, although they provide high vertical resolution profiles, do not resolve variations in humidity fields at high temporal and spatial resolution well. Such variations are valuable for understanding the relationship of convection to tropical humidity at small time and space scales, such as at convectively-induced cold pools and as part of the shallow-to-deep cloud transition. Microwave radiometers (MWR), are able to profile and horizontally-scan autonomously and output measurements frequently (~1 Hz). To date, few assessments of microwave humidity profiling in the Tropics have been undertaken. Here, we evaluate the ability of a MWR to observe the tropical humidity structure using four months of data from the equatorial Indian Ocean, at Gan Island, collected from University of Miami’s 22-30 GHz radiometer during the Dynamics of Madden-Julian Oscillation (DYNAMO) field campaign. Column-integrated liquid water paths and water vapor paths, and water vapor mixing ratio profiles, are physically retrieved using an iterative Gauss-Newton optimal-estimation approach. This has the advantage that the information content and uncertainty analyses are directly computed during the retrieval process. After careful calibration of the instrument, the MWR-retrieved water vapor paths agree well with those from radiosondes. We first reassure that the MWR captures previously-documented large-scale moisture fluctuations, such as a 5-day lower-tropospheric drying period priori to the active phase of MJO. The analyses of the humidity information content are then extended to newer analyses, including compositing vertically-resolved humidity profiles to identified cold pool events, and assessing the variability in integrated water vapor content in vertically-resolved layers as a function of MJO phases. Recommendations for future field-campaign strategies of deploying MWR will also be given.