Simple Multicloud Models for the Diurnal Cycle of Tropical Precipitation. Part I: Formulation and the Case of the Tropical Oceans

Abstract

The variation of tropical precipitation due to the diurnal cycle of solar heating is examined here in the context of two simple models for tropical convection. The models utilize three cloud types-congestus, deep, and stratiform-that are believed to characterize organized tropical convection and are based on the two first baroclinic modes of vertical structure plus a boundary layer mode. The two models differ mainly in the way they treat the boundary layer dynamics. The first one is purely thermodynamical and is reduced to a single equation for the equivalent potential temperature u e connecting the boundary layer to the upper troposphere through downdrafts and to the surface through evaporation while the second uses full bulk boundary layer (FBBL) dynamics with a careful separation between sensible and latent heat fluxes and parameterization of nonprecipitating shallow cumulus. It turns out that in the case of the precipitation over the ocean where the Bowen ratio is small, both models yield a qualitatively similar solution, characterized by an overnight initiation and early morning peak in precipitation consistent with observations. The modeled diurnal cycle of precipitation over the ocean is divided into four cyclic phases: 1) a CAPE (re)generation phase characterized by the enhancement of the boundary layer u e and moisture fluxes during midday and early afternoon that is followed by 2) a (re)moistening phase dominated by congestus heating during the late afternoon and moistening from downdrafts (due to detrainment of shallow cumulus, specifically in the FBBL model) and radiative cooling that lasts until midnight. 3) Deep convection is initiated around midnight when the midtroposphere is sufficiently moist and cool and (re)establishes the precipitation level near its radiative convective equilibrium (1 K day 21 ) and then 4) peaks with sunrise at 0600 LST to yield a precipitation maximum of roughly 2 K day 21 at around 0900 LST that dries the troposphere and consumes CAPE and closes the cycle.

Cloud-radiation feedback and atmosphere-ocean coupling in a stochastic multicloud model

“…The value of the ocean layer evaporation constant, eo is determined by RCE conditions, and the surface temperature variable T s is an anomaly from equilibrium conditions (i.e., it takes the value of zero in RCE). The sensible heat flux will be ignored, since its contribution to the energy budget is small compared to the shortwave radiation and latent heat fluxes (Peters and Bretherton, 2005;Sobel et al, 2004;Kikuchi and Wang, 2008;Frenkel et al, 2010). In their model for Walker circulation, Sobel et al (2004) used a similar mixed-layer ocean model.…”

“…In addition to an enhanced representation of clouds, the framework is simple enough to allow semianalytic solutions. In particular these authors were able to study stability and bifurcations of the solutions attributable to the diurnal surface fluxes (Frenkel et al, 2010). Despite its apparent simplicity, the multicloud model is very successful in capturing most of the Wheeler-Kiladis-Takayabu spectrum of convectively coupled waves (Takayabu, 1994;Wheeler and Kiladis, 1999) in terms of linear wave theory (KM06a, KM08a, Han and Khouider (2010)) and nonlinear organization of large-scale envelopes mimicking cross-scale interactions of the Madden-Julian oscillation (MJO) and convectively coupled waves (KM07; KM08b; Majda et al, 2007), in the idealized context of a simple two-baroclinic modes model employed here.…”

Climate science in the tropics: waves, vortices and PDEs

“…The diurnal cycle of precipitation in the multicloud model over both land and ocean is studied by Frenkel et al [119,120]. Both the land and the ocean regimes of the diurnal cycle are correctly predicted by the multicloud model [119,120]. The interactions of the periodic solar forcing and the internal modes of atmospheric variability is very complex and remains one of the most challenging problems in atmospheric science.…”

Physical processes controlling the diurnal cycle of convective storms in the Western Ghats

“…It is known that the CSs initiate over the ocean in the morning (0830–1130 LT) and propagate eastward to upslope regions to mountain peak (1130–1430 LT), where it reaches the maximum at about 1430–1730 LT, and then move to leeward side 37 . The diurnal variability in CSs may be due to the diurnal cycle of solar radiation and moisture 15 , 46 , air–sea interactions 47 , 48 , or large-scale atmospheric conditions 10 , 14 . To better understand what drives the diurnal cycle in CSs occurrence over the WG, this sub-section presents the diurnal variability of large-scale atmospheric conditions like temperature, moisture, and winds etc.…”