***************************************************************************** ***************************************************************************** ***** THIS IS A HELP FILE FOR THE FORTRAN SUBROUTINE CONVECT ***** ***** VERSION 3.20 ***** ***** December 1, 1996 ***** ***************************************************************************** ***************************************************************************** ----------------------------------------------------------------------------- ----- TO BE SURE THAT YOU ARE ON THE MAILING LIST FOR UPDATES, NEWS, _____ ----- ETC., PLEASE SEND EMAIL TO emanuel@texmex.mit.edu STATING THAT _____ ----- YOU INTEND TO USE THIS SOFTWARE _____ ----------------------------------------------------------------------------- CONVECT accepts the one-dimensional input arrays T,Q,QS,P, and PH and returns the arrays FT and FQ as well as the scalars PRECIP, WD, TPRIME AND QPRIME THE FLOATING POINT SCALAR CBMF AND THE INTEGER SCALAR NCOUNT MUST BE INITIALIZED TO ZERO AT THE BEGINNING OF A TIME INTEGRATION AND MUST BE "REMEMBERED" BETWEEN CALLS TO CONVECT AT THE SAME HORIZONTAL GRID POINT. That is, the calling program must return the same values of CBMF and NCOUNT that it received from CONVECT at the same horizontal position at the previous time step. The first element of each array represents the LOWEST model level (i.e. near the surface). The input arrays are returned unaltered, UNLESS the dry adiabatic adjustment part of the routine is performed, in which case T and Q are adjusted to represent well mixed profiles in superadiabatic regions, with a small increment at the lowest level to represent the surface layer. Here are the definitions of the input arrays. With the exception of PH, they are dimensioned ND; PH is dimensioned ND+1: T: Temperature in degrees Kelvin. Q: Specific humidity (NOT mixing ratio) in gm/gm (NOT gm/Kg). QS: Saturation specific humidity in gm/gm. P: Pressure in millibars. PH: Pressure (in millibars) at the half levels; i.e. halfway in between the levels at which P,T,R and RS are defined. The first PH level is BELOW (i.e. at a higher pressure than) the first P level. One also needs to specify: ND: The dimension of the arrays T,R,RS,P,FT and FQ. PH is dimensioned ND+1. NL: The maximum number of model levels you wish to have checked for moist convection. NL MUST be less than or equal to ND-1. One should never expect moist convection to penetrate above NL. DELT: The model's time step, in seconds. On output one receives: FT: The tendency of ACTUAL (not potential!) temperature in degrees/second. FQ: The tendency of specific humidity (NOT mixing ratio) in (gm/gm)/second. PRECIP: The rate of convective precipitation, in mm/day. WD: A convective downdraft surface velocity scale, likewise for use in the calling program's formulation of surface fluxes. TPRIME: A convective downdraft surface temperature perturbation for use in the surface flux formulation. QPRIME: A convective downdraft surface specific humidity perturbation for use in the surface flux formulation. ***************************************************************************** * Note: Suppose the surface flux formulation has the form * * C|V|(x* - x) * * where C is the exchange coefficient, |V| is a surface wind speed scale, * * x* is the surface value of temperature or specific humidity and x is the* * boundary layer value of the temperature or specific humidity. When this * * is averaged over an area that contains downdraft outflows the it can * * be shown that the result of this averaging is given by * * C[V2](x* - xm) - C[V2 - V1]x'. * * Here V1 is defined as SQRT[u^2 + v^2 + w*^2], where u and v are * * the x and y components of the mean flow velocity and w* is a dry * * convective velocity scale; V2 is defined as SQRT[u^2+v^2+w*^2+wd^2], * * where wd is the moist convective downdraft velocity scale; xm is the * * mean value of x over the area in question and x' is the departure of * * x in convective outflows from its mean value. In the output of CONVECT, * * wd is given by WD, and x' is given by TPRIME or QPRIME, in the case * * of temperature and specific humidity, respectively. It was found that * * the contribution of these modifications to the surface fluxes is not * * negligible in the TOGA/COARE single column tests. * ********************************************************************************** IFLAG: A flag that denotes the following: VALUE INTERPRETATION ----- -------------- 0 No moist convection; atmosphere is not unstable, or surface temperature is less than 250 K or surface specific humidity is non-positive. 1 Moist convection occurs. 2 No moist convection: lifted condensation level is above the 200 mb level. 3 No moist convection: cloud base is higher then the level NL-1. 4 Moist convection occurs, but a CFL condition on the subsidence warming is violated. This does not cause the scheme to terminate. ------------------------------------------------------------------------------- GENERAL USAGE: CONVECT is designed for use in a time-marching model; it should NOT be used for single time step evaluations. The routine takes several time steps to "spin up" the convective mass fluxes toward equilibrium with the forcing. In the event one desires the convective tendencies associated with a single sounding, CONVECT should be initialized with that sounding and run for enough time steps that the tendencies have relaxed back to zero; the average of the convective tendencies would then be the desired result. This is not recommended, however, and may not yield results that are independent of the time step and other numerical parameters. CONVECT should be called at least every 20 minutes of integration. Longer intervals may result in noisy distributions of FT and FQ. ------------------------------------------------------------------------------- NOTES: 1. At the beginning of the subroutine there is a parameter NA which ---- should in general be set equal to ND+1 or greater. 2. All the adjustable parameters appear near the beginning of the subroutine. The values provided have been optimized using the December, 1992, forcing values deduced from the TOGA/COARE sounding array. 3. The parameters ALPHA and DAMP control the smoothness of the mass fluxes in time and may be altered to achieve a smoother integration. ALPHA and DAMP control the rate of approach to quasi-equilibrium. PPMIN is the minimum pressure below which the atmosphere is not checked for unstable parcel origin. Convection can only originate at the FIRST level above the surface at which the air is unstable to upward displacement. CONVECT can represent elevated convection, but not if the boundary layer is also unstable in the same column. 4. Thermodynamic constants are defined at the beginning of the routine and again at the beginning of the subroutine TLIFT. These should be altered, if necessary, to be consistent with those of the calling program. The value of the gravitational acceleration is also defined here. 5. This version differs in several important respects from that described in the report from the MIT Center for Global Change Science. 6. Many modifications have been made to the scheme as described in J. Atmos. Sci., 1991, pgs. 2313-2335. These will be described in a forthcoming paper. 7. Please report any problems to Kerry Emanuel, Room 54-1620, M.I.T., Cambridge, MA 02139 EMAIL address: emanuel@texmex.mit.edu (Internet) telephone: (617) 253-2462 Fax: (617) 253-6208 Thanks. -------------------------------------------------------------------------------