Abstract
In an attempt to further unravel the process of tropical cyclogenesis, mid-tropospheric relative humidity is examined to weigh its importance in genesis of tropical disturbances. To evaluate its proficiency in remotely-sensing mid-level relative humidity, we compare brightness temperature data from channel 19 of the Advanced Microwave Sounding Unit (AMSU-B) microwave satellite with GPS dropsonde point measurements from the NOAA G-IV hurricane hunter aircraft. A radiative transfer model is used to produce a simulated brightness temperature output from the GPS dropsonde data. To facilitate the interpretation of the results, the brightness temperatures are then converted into layer-averaged relative humidity between 400-700mb. A comparison of data for the period 2001-2009 based upon a co-location of satellite measurements with over 3,000 GPS dropsonde observations reveals close agreement between the two measurements with a positive bias of ~3 % RH and an RMS of close to +10%. The close level of agreement highlights the utility of AMSU-B mid-tropospheric humidity (MTH) values in providing an accurate substitute for spotty point measurements. An analysis of the AMSU-B mid-tropospheric relative humidity is carried out using a database of all NHC tropical systems (including both developing and non-developing invests) in the Atlantic Basin from 2001-2009. A database of Dvorak fixes at 6-hour intervals is used to construct 1,600 km-by-1,600 km storm-centered MTH plots from AMSU-B data. MTH grids are then composited by storm intensity. The analysis indicates that MTH near the storm center is directly correlated to the strength of the system, with more intense storms generally having a higher average MTH. The distribution of MTH around the storm center is shown to be highly asymmetric with drier conditions to the north and west of the storm and wetter conditions to the south and east. This asymmetry is consistent with the expectation that convection within tropical storms locally moistens the mid troposphere (Komaromi 2013). To investigate role of MTH in modulating storm genesis and intensification the storm MTH is examined preceding changes in storm intensity by compositing MTH into both weakening and strengthening storms. Higher water vapor concentrations in the mid-troposphere in the storm environment are proven to be conducive for intensification – particularly in cyclogenesis of NHC invests, and within a 12-hour window preceding intensification. Conversely, low water vapor concentrations are shown to enhance the probability of weakening. Environmental moisture trends are most pronounced when isolating MTH within a 400 km radius of the storm center, although they are still evident when examining an 800 km radius. Trends are also robust in the initial strengthening of a weaker system, as major hurricanes feature pre-existing deep moist convection and changes in their intensity do not show strong dependence on near-storm MTH. The analysis suggests that MTH does influence the likelihood of genesis and intensification and that near real-time monitoring of MTH in developing or existing tropical cyclones could aid intensity forecasts.