Moisture vertical structure, column water vapor, and tropical deep convection
Holloway, C. and Neelin, J. D. (2009) Moisture vertical structure, column water vapor, and tropical deep convection. Journal of the Atmospheric Sciences, 66 (6). pp. 1665-1683. ISSN 1520-0469
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To link to this item DOI: 10.1175/2008JAS2806.1
The vertical structure of the relationship between water vapor and precipitation is analyzed in 5 yr 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 pickup 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. For several different mixing schemes, higher CWV results in progressively greater plume buoyancies, particularly in the upper troposphere, indicating conditions favorable for deep convection. All other things being 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 with increasing CWV also contributes to buoyancy. Entrainment coefﬁcients inversely proportional to distance from the surface, associated with mass ﬂux increase through a deep lower-tropospheric layer, appear promising. These yield a relatively even weighting through the lower troposphere for the contribution of environmental water vapor to midtropospheric buoyancy, explaining the association of CWV and buoyancy available for deep convection.