The interaction of deep convection with the general circulation in Titan's atmosphere. Part 1: Cloud Resolving Simulations

Abstract

The deep convective cloud-environment feedback loop is likely important to Titan's global methane, energy, and momentum cycles, just as it is for Earth's global water, energy, and momentum budgets. General circulation models of Titan's atmosphere are unable to explicitly simulate deep convection and must instead parameterize the impact of this important subgrid-scale phenomenon on the model-resolved atmospheric state. The goal of this study is to better quantify through cloud resolving modeling the effects of deep convective methane storms on their environment and to feed that information forward to improve parameterizations in global models. Dozens of atmospheric profiles unstable with respect to deep moist convection are extracted from the global Titan Atmospheric Model (TAM) and used to initialize the cloud-resolving Titan Regional Atmospheric Modeling System (TRAMS). Mean profiles of heating/cooling and moistening/drying of the large-scale environment in TRAMS indicate that Titan's deep convection forces the environment in a manner analogous to Earth: Large-scale subsidence of the environmental air warms and dries the environment, but clouds can also moisten the environment through the detrainment and evaporation of condensate near cloud top. Relative humidity profiles and characteristic convective time scales are derived to guide the tuning of the deep convective parameterization implemented in TAM, as described in a companion paper. The triggering of convection, the dry convective mixing of the planetary boundary layer, and the entrainment of environmental air into rising air parcels are found to be critical to determining whether a deep convective cloud will form. Only profiles with relatively large convective available potential energy (CAPE) and well mixed planetary boundary layers with high relative humidity were found to produce storms. Environments with low level thermal inversions and planetary boundary layers with low relative humidity or rapidly decreasing moisture with height failed to generate deep convection in TRAMS despite positive CAPE.

Keywords: Titan; Titan Atmosphere; Titan Clouds; Titan Model.

Figure 1.

Cloud forcing of the environment and the closed feedback loop between deep moist convection and the environment. The storm updraft feeds off of convective available potential energy generated by a warm, moist planetary boundary layer and a modest temperature lapse rate in the free atmosphere. Compensating subsidence (not the cloud) warms and dries the cloud-free atmosphere, which reduces CAPE. Boundary layer moisture is transported to the upper cloud where it detrains and moistens the environment. The storm downdraft cools and typically dries (in an absolute sense) the boundary layer. The combined drying of the PBL and warming of troposphere through compensating subsidence reduces CAPE.

Figure 2.

Thermodynamic diagram for simulation TRAMS-39 (cf. Table 2) with the unperturbed initial sounding (left) with LCL at 1.2 km, LFC at 6.1 km, and a CAPE of 332 J/kg, and for a perturbed warm bubble sounding (right) with LCL at 2.1 km, LFC at 4.8 km, and CAPE of 462 J/kg. See supplementary material for a detailed description of interpreting the diagram.

Figure 3.

Initial potential temperature and virtual potential temperature profiles in the lower atmosphere of TRAMS-39. The warm bubble depth is ~200 m.

Figure 4.

TRAMS vertical velocity (shaded) and CH₄ vapor mixing ratio (contoured) at t=6 hours for TRAMS-39. The rising thermal associated with the initial 1 K perturbation is at x=500 km and is evidenced by positive vertical velocity. Entrainment of dry air lowered the bubble buoyancy and it failed to reach the LCL.

Figure 5a.

Sounding for TRAMS-14. A rising surface parcel cannot reach the high LFC due to a strong thermal inversion. Compare with a similar sounding in Fig. 2. The rapidly drying atmosphere with height further hinders cloud development.

Figure 5b.

Sounding for TRAMS-29. Although the low-level superadiabatic temperature profile is conducive to rising motion, the dry atmosphere puts the LCL at relatively high levels. Clouds are further inhibited by the more stable profile above the superadiabatic layer.

Figure 5c.

Sounding for TRAMS-33. The superadiabatic low level lapse rate is conducive to a rising parcel, and the surface is relatively moist, but entrainment of air from the much drier atmosphere above pushes the parcel LCL and LFC to unattainable altitudes

Figure 6

TRAMS-1 sounding (left) and convective cloud after 8 hours (right) A deep, nearly adiabatic layer extends up to the LCL/LFC with no inversion. The surface and low-level air is moist, which minimizes the impact of dry air entrainment on a rising parcel. During the mature phase of the storms (typically after ~6 to 9 hours of integration), updrafts are 5 to 10 m/s with weaker downdrafts and cloud tops in excess of 20 km (~500 hPa). Cloud condensate (g/kg) is shaded and vertical velocity is contoured. The approximate cloud boundary is shown by the thick blue line.

Figure 7.

The sounding for TRAMS-9 (left), the resulting towering cumulus after 5 hours (center), and the vapor mixing ratio profile at initialization and after 5 hours (right). The sounding is similar to that shown in Fig. 6, but the air is a little drier above the surface. Entrainment pushes the parcel LCL and LFC upward such that by the time the LFC is reached, the parcel is barely warmer than then the environment. CAPE is present for a non-entraining parcel, but for an entraining parcel, the parcel buoyancy is quickly reduced to zero and vertical development of the cloud ceases at ~7 km above the ground. The neutral to slightly unstable boundary layer also triggers mixing, which further dries the air near the surface and prohibits further development.

Figure 8.

Environmental moistening (left), heating (center), and relative humidity (right) for TRAMS-1 as a function of time. Curves are at 2-hour increments beginning at 2 hours after initialization.

Figure 9.

Same as Fig. 8, but for the average of all deep convective events. The same patterns of heating/cooling and moistening/drying found for TRAMS-1 are present in the convective ensemble average.

Figure 10.

The decrease in average environmental CAPE as a function of time within the 300 km surrounding the storm (left). Normalized CAPE (middle) indicates that environmental CAPE decreases roughly exponentially with an e-folding time between 3 and 8 hours with an average close to 6 hours. Most cases have residual, upper level CAPE, as shown for TRAMS-9 with 44 J/kg after convection ceases (right).

Figure 11.

SkewT-LogP for TRAMS-42 with initial superadiabatic layer (left) and barely resolved PBL mixing at a great distance from the initial bubble and convective activity after 1 hour (middle; vertical velocity cm/s shaded). Mixing reduces the surface parcel CAPE by cooling and drying the surface parcel (right).

Figure 12.

Left: Rainfall distribution for each storm. Right: Total domain precipitation as a function of time for each storm.

Figure 13.

Momentum. Left: initial zonal wind profiles for the different storms. Right: Average composite change in wind as a function of time.