Christopher E. Holloway and J. David Neelin
J. Atmos. Sci., 66, 1665-1683, 2009.
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© Copyright 2009 by the American Meteorological Society.
Abstract. The vertical structure of the relationship between water vapor and precipitation is analyzed in five years of radiosonde and precipitation gauge data from the Nauru Atmospheric Radiation Measurement (ARM) site. The first vertical principal component of specific humidity is very highly correlated with column water vapor (CWV) and has a maximum of both total and fractional variance captured in the lower free troposphere (around 800 hPa). Moisture profiles conditionally averaged on precipitation show a strong association between rainfall and moisture variability in the free troposphere, and little boundary layer variability. A sharp pick-up in precipitation occurs near a critical value of CWV, confirming satellite-based studies. A lag-lead analysis suggests it is unlikely that the increase in water vapor is just a result of the falling precipitation. To investigate mechanisms for the CWV/precipitation relationship, entraining plume buoyancy is examined in sonde data and simplified cases. Higher CWV results in progressively greater plume buoyancies for several mixing schemes, notably upper-tropospheric buoyancy that can yield deep convection. All other things equal, higher values of lower-tropospheric humidity, via entrainment, play a major role in this buoyancy increase. A small but significant increase in subcloud layer moisture (and thus equivalent potential temperature) with increasing CWV also contributes to buoyancy. Entrainment based on mass flux increase through a deep lower-tropospheric layer yields a relatively even weighting for the impact on mid-tropospheric buoyancy of all lower levels, explaining the association of CWV and buoyancy available for deep convection.
Citation. Holloway, C. E. and J. D. Neelin, 2009: Moisture vertical structure and tropical deep convection. J. Atmos. Sci., 66, 1665-1683. doi: 10.1175/2008JAS2806.1
Acknowledgements. This work was supported in part by National Science Foundation ATM-0082529, National Oceanic and Atmospheric Administration NA05OAR4311134. JDN acknowledges the John Simon Guggenheim Memorial Foundation and the National Center for Atmospheric Research for sabbatical support. CEH was supported by NASA Earth System Science Fellowship Grant NNX06AF83H. We thank K. Emanuel, M. Hughes, B. Lesht, B. Medeiros, R. Neale, L. Nuijens, B. Stevens, J. Tribbia, X. Wu, and G. Zhang for discussions and J. Meyerson for graphical assistance.