Article Type:
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Reply to “Comments on ‘Preconditioning Deep Convection with Cumulus Congestus’”

Cathy Hohenegger Max Planck Institute for Meteorology, Hamburg, Germany

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Bjorn Stevens Max Planck Institute for Meteorology, Hamburg, Germany

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Print Publication:
01 Dec 2013
DOI:
https://doi.org/10.1175/JAS-D-13-0216.1
Page(s):
4155–4156
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Abstract

In his comment, D. M. Schultz asked for clarification concerning (i) the validity of the results of C. Hohenegger and B. Stevens for the development of convection over the midlatitudes and (ii) the exact meaning and computation of the term “moisture convergence.” This reply aims at clarifying these two aspects.

Corresponding author address: Cathy Hohenegger, Max Planck Institute for Meteorology, Bundesstrasse 53, 20146 Hamburg, Germany. E-mail: cathy.hohenegger@zmaw.de

The original article that was the subject of this comment/reply can be found at http://journals.ametsoc.org/doi/abs/10.1175/JAS-D-12-089.1.

Abstract

In his comment, D. M. Schultz asked for clarification concerning (i) the validity of the results of C. Hohenegger and B. Stevens for the development of convection over the midlatitudes and (ii) the exact meaning and computation of the term “moisture convergence.” This reply aims at clarifying these two aspects.

Corresponding author address: Cathy Hohenegger, Max Planck Institute for Meteorology, Bundesstrasse 53, 20146 Hamburg, Germany. E-mail: cathy.hohenegger@zmaw.de

The original article that was the subject of this comment/reply can be found at http://journals.ametsoc.org/doi/abs/10.1175/JAS-D-12-089.1.

Keywords: Convection; Deep convection; Convective clouds
Using satellite observations and large-eddy simulations of the development of deep moist convection over the tropics, Hohenegger and Stevens (2013) argued that the moistening of the atmosphere by congestus clouds is too slow to explain the observed fast transition times, from congestus to cumulonimbus. Schultz (2013) wondered whether this result remains valid for the midlatitudes. Although a full analysis as performed in Hohenegger and Stevens (2013) has not been conducted for the midlatitudes, some insight can be gained based on the results of Zhang and Klein (2010). They studied the transition from shallow to deep convection over the Atmospheric Radiation Measurement (ARM) Climate Research Facility Southern Great Plains (SGP) site during summer. Their analysis can be employed here to estimate τ*, the transition time between congestus and cumulonimbus, as well as τc, the time needed by congestus clouds to moisten the atmosphere. Figure 3d in Zhang and Klein (2010) indicates a τ* of O(1–2 h). Recalling Eqs. (1a)–(1c) of Hohenegger and Stevens (2013), τc can be expressed as
e1a
e1b
e1c
with qυ specific humidity, z height, t time, moisture flux at cloud base, LH latent heat flux, Lυ enthalpy of vaporization (2.5 × 106 J kg−1), and ρ air density at cloud base (assumed as 1 kg m−3).

Based on a visual inspection of Figs. 4 and 6 in Zhang and Klein (2010), LH is set to 300 W m−2 and Δqυ to 1.5 g kg−1. Note that a value of 300 W m−2 is on the upper range of the measured values and strictly speaking only valid between 1200 and 1400 local standard time (LST). We estimate Δqυ as the difference in mixing ratio at 1130 LST between days with shallow and deep convection, whereby the difference is assumed to remain constant above 4 km. With z1 = 500 m and z2 = 5000 m, Eqs. (1a)–(1c) yield τc = 16 h. This is much longer than the observed τ*. Even if it may be argued that 1.5 g kg−1 is too large for the requested amount of moistening as only the moistening by congestus clouds should be considered, a τc of 2 h, on the order of τ*, would imply Δqυ = 0.2 g kg−1. This seems very small. Hence, this analysis suggests that the results of Hohenegger and Stevens (2013) remain valid for the midlatitudes. Zhang and Klein (2010) also noted that moistening of the free troposphere, albeit by shallow convection, is likely too weak to promote deep convection.

Schultz (2013) also asked for clarification concerning the exact meaning of the term “moisture convergence.” In a strict sense only the vertical component of the moisture convergence, [see Eq. (3c) in Hohenegger and Stevens (2013)], was considered in Hohenegger and Stevens (2013). This means that both the advection and the mass convergence term (w is assumed constant in the vertical) in the expression for the horizontal moisture flux convergence, as detailed by Schultz (2013), are zero. In a more practical sense it would be sufficient for any of these terms to be on the order of the estimated vertical moisture convergence to be able to promote a fast-enough transition to deep convection. In particular, a change in moisture advection would affect the tendency of qυ at a point (e.g., Arakawa 2004), whereas the sole presence of a height-varying w would generate a nonzero mass convergence term. This further highlights the importance of the mass convergence term for an ingredients-based forecasting of convective initiation, as emphasized in Banacos and Schultz (2005). We thank Schultz (2013) for bringing this point to our attention.

REFERENCES

  • Arakawa, A., 2004: The cumulus parameterization problem: Past, present, and future. J. Climate, 17, 2493–2525.

  • Banacos, P. C., and D. M. Schultz, 2005: The use of moisture flux convergence in forecasting convective initiation: Historical and operational perspectives. Wea. Forecasting, 20, 351–366.

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  • Hohenegger, C., and B. Stevens, 2013: Preconditioning deep convection with cumulus congestus. J. Atmos. Sci., 70, 448–464.

  • Schultz, D. M., 2013: Comments on “Preconditioning deep convection with cumulus congestus.” J. Atmos. Sci., 70, 3691–3692.

  • Zhang, Y., and S. A. Klein, 2010: Mechanisms affecting the transition from shallow to deep convection over land: Inferences from observations of the diurnal cycle collected at the ARM Southern Great Plains site. J. Atmos. Sci., 67, 2943–2959.

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Journal of the Atmospheric Sciences

  • Arakawa, A., 2004: The cumulus parameterization problem: Past, present, and future. J. Climate, 17, 2493–2525.

  • Banacos, P. C., and D. M. Schultz, 2005: The use of moisture flux convergence in forecasting convective initiation: Historical and operational perspectives. Wea. Forecasting, 20, 351–366.

    • Search Google Scholar
    • Export Citation

  • Hohenegger, C., and B. Stevens, 2013: Preconditioning deep convection with cumulus congestus. J. Atmos. Sci., 70, 448–464.

  • Schultz, D. M., 2013: Comments on “Preconditioning deep convection with cumulus congestus.” J. Atmos. Sci., 70, 3691–3692.

  • Zhang, Y., and S. A. Klein, 2010: Mechanisms affecting the transition from shallow to deep convection over land: Inferences from observations of the diurnal cycle collected at the ARM Southern Great Plains site. J. Atmos. Sci., 67, 2943–2959.

    • Search Google Scholar
    • Export Citation
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