Sensitivity of moist convection to environmental humidity

As part of the EUROCS (EUROpean Cloud Systems study) project, cloud-resolving model (CRM) simulations and parallel single-column model (SCM) tests of the sensitivity of moist atmospheric convection to midtropospheric humidity are presented. This sensitivity is broadly supported by observations and some previous model studies, but is still poorly quantified. Mixing between clouds and environment is a key mechanism, central to many of the fundamental differences between convection schemes. Here, we define an idealized quasi-steady ‘testbed’, in which the large-scale environment is assumed to adjust the local mean profiles on a timescale of one hour. We then test sensitivity to the target profiles at heights above 2 km. Two independent CRMs agree reasonably well in their response to the different background profiles and both show strong deep precipitating convection in the more moist cases, but only shallow convection in the driest case. The CRM results also appear to be numerically robust. All the SCMs, most of which are one-dimensional versions of global climate models (GCMs), show sensitivity to humidity but differ in various ways from the CRMs. Some of the SCMs are improved in the light of these comparisons, with GCM improvements documented elsewhere. © Crown copyright, 2004.

[1]  Hugh Swann.  Evaluation of the mass‐flux approach to parametrizing deep convection , 2001 .

[2]  A. Brown,et al.  A similarity hypothesis for shallow‐cumulus transports , 1999 .

[3]  M. Doutriaux-Boucher,et al.  Evaluation of cloud thermodynamic phase parametrizations in the LMDZ GCM by using POLDER satellite data , 2004 .

[4]  J. Kain,et al.  A One-Dimensional Entraining/Detraining Plume Model and Its Application in Convective Parameterization , 1990 .

[5]  J. Ridout.  Sensitivity of tropical Pacific convection to dry layers at mid- to upper levels: Simulation and parameterization tests , 2002 .

[6]  Adrian M. Tompkins,et al.  Organization of Tropical Convection in Low Vertical Wind Shears: The Role of Cold Pools , 2001 .

[7]  Audrey B. Wolf,et al.  Intercomparison and evaluation of cumulus parametrizations under summertime midlatitude continental conditions , 2001 .

[8]  P. Rowntree,et al.  A Mass Flux Convection Scheme with Representation of Cloud Ensemble Characteristics and Stability-Dependent Closure , 1990 .

[9]  David B. Parsons,et al.  Recovery Processes and Factors Limiting Cloud-Top Height following the Arrival of a Dry Intrusion Observed during TOGA COARE , 2002 .

[10]  J. Grandpeix,et al.  Improved mixing representation in Emanuel's convection scheme , 2004 .

[11]  Jorgen B. Jensen,et al.  A Study of the Source of Entrained Air in Montana Cumuli , 1988 .

[12]  David J. Raymond,et al.  Instability and large‐scale circulations in a two‐column model of the tropical troposphere , 2000 .

[13]  Hugh Swann,et al.  Sensitivity to the representation of precipitating ice in CRM simulations of deep convection , 1998 .

[14]  Christopher S. Bretherton,et al.  Modeling Tropical Precipitation in a Single Column , 2000 .

[15]  Jean-Christophe Golaz,et al.  Large‐eddy simulation of the diurnal cycle of shallow cumulus convection over land , 2002 .

[16]  J. Royer,et al.  A statistical cloud scheme for use in an AGCM , 1993 .

[17]  Brian E. Mapes,et al.  Bimodality in tropical water vapour , 2003 .

[18]  P. Bougeault,et al.  A Simple Parameterization of the Large-Scale Effects of Cumulus Convection , 1985 .

[19]  K. Emanuel.  A Scheme for Representing Cumulus Convection in Large-Scale Models , 1991 .

[20]  Minghua Zhang,et al.  An intercomparison of cloud‐resolving models with the atmospheric radiation measurement summer 1997 intensive observation period data , 2002 .

[21]  Véronique Ducrocq,et al.  Simulation of an Observed Squall Line with a Meso-Beta-Scale Hydrostatic Model , 1995 .

[22]  P. Mason,et al.  Large-Eddy Simulation of the Convective Atmospheric Boundary Layer , 1989 .

[23]  Adrian M. Tompkins,et al.  Organization of Tropical Convection in Low Vertical Wind Shears: The Role of Water Vapor , 2001 .

[24]  David A. Randall,et al.  Alternative methods for specification of observed forcing in single-column models and cloud system models , 1999 .

[25]  A. Arakawa,et al.  Interaction of a Cumulus Cloud Ensemble with the Large-Scale Environment, Part I , 1974 .

[26]  E. Bazile,et al.  A mass‐flux convection scheme for regional and global models , 2001 .

[27]  Wojciech W. Grabowski,et al.  MJO-like Coherent Structures: Sensitivity Simulations Using the Cloud-Resolving Convection Parameterization (CRCP) , 2003 .

[28]  D. Raymond,et al.  Thermodynamic control of tropical rainfall , 2000 .

[29]  David A. Randall,et al.  Similarity of deep continental cumulus convection as revealed by a three-dimensional cloud-resolving model , 2002 .

[30]  M. Tiedtke.  A Comprehensive Mass Flux Scheme for Cumulus Parameterization in Large-Scale Models , 1989 .

[31]  Leo J. Donner,et al.  A Cumulus Parameterization Including Mass Fluxes, Vertical Momentum Dynamics, and Mesoscale Effects , 1993 .

[32]  Jon Petch,et al.  Sensitivity studies using a cloud‐resolving model simulation of the tropical west Pacific , 2001 .

[33]  A. J. Waters,et al.  Images in Weather Forecasting a Practical Guide for Interpreting Satellite and Radar Imagery , 1995 .