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The Role of Cloud Heating Within the Atmosphere on the High Cloud Amount and Top-Of-Atmosphere Cloud Radiative Effect Bryce E

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The Role of Cloud Heating Within the Atmosphere on the High Cloud Amount and Top-Of-Atmosphere Cloud Radiative Effect Bryce E JAMES, VOL. ???, XXXX, DOI:10.1002/, The role of cloud heating within the atmosphere on the high cloud amount and top-of-atmosphere cloud radiative effect Bryce E. Harrop,1 Dennis L. Hartmann,1 Key Points. ◦ ACRE both enhances high cloud amount through direct destabilization of the cloud layer and reduces high cloud amount by stabilizing the troposphere to deep convection ◦ ACRE produces more high, optically thin clouds relative to high, optically thick clouds, increases TOA CRE Abstract. The effect of cloud radiation interactions on cloud properties is examined in the context of a limited domain cloud resolving model. The atmospheric cloud radia- tive effect (ACRE) influences the areal extent of tropical high clouds in two distinct ways. The first is through direct radiative destabilization of the elevated cloud layers, mostly as a result of radiation heating the cloud bottom and cooling the cloud top. The sec- ond effect is radiative stabilization, whereby cloud radiative heating of the atmospheric column stabilizes the atmosphere to deep convection. In limited area domain simulations, the stabilizing effect is the dominant role of the cloud radiative heating, thus reducing the cloud cover in simulations where ACRE is included compared to those where it is removed. Cloud radiative heating increases cloud amount, decreases cloud optical depth, and increases cloud top pressure. The combination of these effect increases the top-of- atmosphere cloud radiative effect. In mock-Walker circulation experiments, the decrease in high cloud amount owing to radiative stabilization tends to cancel out the increase in high cloud amount owing to the destabilization within the cloud layer. The changes in cloud optical depth and cloud top pressure, however, are similar to those produced in the limited area domain simulations. 1. Introduction Chen and Cotton, 1988; Lilly, 1988; Fu et al., 1995; Mace et al., 2006]. Ackerman et al. [1988] showed that the heat- Clouds have a strong impact on the radiative budget of ing profile of anvil clouds can vary by as much as 200 K/day the atmosphere. They influence the amount of shortwave from the bottom to the top of the anvil cloud. Powell et al. and longwave radiation absorbed by the atmosphere and [2012] showed that the heating and cooling at cloud bot- surface. The effect of clouds on the energy balance of the tom and top, respectively, is greatest for the thickest anvils. atmosphere is quite different from their effect at the sur- Fu et al. [1995] showed that a simulated squall line system face or the top of the atmosphere. On average, clouds cool run with interactive cloud radiation produced greater high the surface by reflecting shortwave radiation, and may heat cloud cover than the same squall line system run without it by increasing the downward flux of longwave radiation. interactive cloud radiation. They also noted, however, that The effect of clouds on the atmospheric energy budget, how- the cloud radiative heating would begin to stabilize the at- ever, is less straightforward. Because the tropospheric lapse mosphere to deep convection if they continued their experi- rate is positive, clouds can either warm or cool the atmo- ments beyond a few hours. Similar cloud expansion can be sphere, depending on their vertical structure. Low clouds simulated for tropical tropopause layer cirrus as shown by can cool the atmosphere by increasing the downward emis- Durran et al. [2009] and Dinh et al. [2010]. sion of longwave radiation, while high clouds warm it by As already noted, ACRE is positive in tropical convective decreasing the upward emission of longwave radiation [see, regions [Slingo and Slingo, 1988], and thus, the upper tro- for example, Slingo and Slingo, 1988]. Over regions of deep posphere is warmed by radiative heating within the clouds convection within the tropics, cloud-radiation interactions [see also, Slingo and Slingo, 1991]. Sherwood et al. [1994] warm the atmosphere [Slingo and Slingo, 1988]. found that removing the cloud-radiation interactions in a The cloud-radiative heating within the atmosphere (the general circulation model reduced the cloud cover over the difference in the all-sky and clear-sky radiative heating pro- tropical warm pool. It is unlikely, however, that their model files) is termed the Atmospheric Cloud Radiative Effect setup was capable of resolving the within cloud destabiliza- (ACRE). It has been shown that ACRE can be used to tion process, and it is unclear whether the radiative heating drive more realistic tropical circulations [Hartmann et al., gradient between cloud bottom and top would sustain clouds 1984; Tian and Ramanathan, 2003]. It has also been shown without this mixing. Thus it is difficult to assess the rela- that ACRE feeds back on the clouds through destabiliza- tive importance of both the cloud-layer destabilization and tion of the cloud layer by radiation [Ackerman et al., 1988; the atmospheric stability change from the Sherwood et al. [1994] experiments. Lebsock et al. [2010] showed a strong correlation be- 1Department of Atmospheric Sciences, University of tween tropical mean convection and cloud radiative fields on Washington, Seattle, WA, USA. daily timescales. They showed that tropical mean precipita- tion correlates positively with reflected shortwave and neg- atively with outgoing longwave radiation. They suggested a Copyright 2016 by the American Geophysical Union. \radiative-convective cloud feedback," wherein precipitation 1 X - 2 HARROP AND HARTMANN.: THE ROLE OF CLOUD HEATING enhances clouds, which decrease the radiative cooling, and We first perform a suite of simulations with a horizon- hence reduce precipitation. In this paper, we include an ad- tal grid of 96 km x 96 km with 1 km resolution. In these ditional pathway in which the radiation changes associated small-domain simulations, we use a uniform fixed sea surface with clouds feeds back on the clouds directly. temperature for the lower boundary condition. In the real To summarize, cloud-radiation interactions have been tropical warm pool, energy is diverged through atmospheric shown to be important for destabilizing anvil clouds in trop- motions that are not present in these small-domain simula- ical deep convective systems. It has also been suggested that tions. Thus, we perform a similar set of simulations within the net heating by these clouds will stabilize the atmosphere a mock-Walker experiment. The mock-Walker simulations and alter deep convection, which will change other compo- are run on a 6144 km x 32 km domain with 4 km resolu- nents of the tropical system such as the OLR and the pre- tion. We tested the sensitivity of the cloud areal extent and cipitation. Much of the prior work centers on case studies ACRE profiles to resolution in the small domain and found of individual cloud systems and their interaction with radi- little change between 1 km, 2 km, 3 km, or 4 km horizontal ation [e.g. Fu et al., 1995; Durran et al., 2009; Dinh et al., resolutions, so we do not expect the change from 1 km to 2010]. Other studies have examined the change in cloud 4 km horizontal resolution to be a major difference between amount to the heating of the atmosphere by ACRE [e.g. the two modeling setups. For the mock-Walker simulations, Sherwood et al., 1994]. To our knowledge, however, ours is the sea surface temperature (SST) is again held fixed, but the first study to examine both of these effects together to in these experiments, we introduce a sinusoidal SST pattern quantify the net impact of cloud-radiation interactions on with a peak-to-peak amplitude of 4 K. the tropical clouds. We run the model both with the cloud-radiation interac- We have designed a series of equilibrium cloud resolving tions included (T1C1) and with the cloud-radiation inter- model experiments as a means of exploring this problem, actions removed (T0C0). When the cloud-radiation inter- and with which we can examine the role of cloud radiative actions are removed, the clear-sky radiative heating rates heating on tropical convection. Equilibrium experiments al- are used to update the energy tendency terms, making the low us to collect statistics from numerous cloud life cycles clouds invisible to both shortwave and longwave radiation. instead of relying on drawing conclusions from a single cloud To separate the cloud changes associated with the direct radiative heating of clouds from those associated with the system. We hope to address several questions in this paper: stability effect, we run another pair of simulations. In the (1) how does cloud fraction respond to ACRE? (2) How first, we still use the clear-sky fluxes to update the energy does the top-of-atmosphere cloud radiative effect respond budget, but we additionally add in the domain mean average to ACRE? (3) Does a large-scale circulation mitigate the ACRE from the T1C1 experiment. In this case, the domain stability effect of the cloud radiative heating? and (4) What mean radiative cooling profile remains the same between are the implications for cloud feedbacks? this experiment and the T1C1 experiment, but the direct In this paper, we confirm that the cloud fraction increases cloud-radiation interactions are removed, so that only the owing to an increase in turbulent kinetic energy driven by stability effect is included. We refer to this experiment as cloud-radiation interactions. We also show, however, that T1C0. In the second experiment, ACRE is included, but we the stability changes over the depth of the troposphere dom- remove its domain mean profile at each time step. Again, inate the cloud fraction by stabilizing the atmosphere to this method has the desired effect of maintaining a radiative deep convection and limiting both the cloud fraction and cooling profile that matches that of the T0C0 experiment, the precipitation.
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