We present 'Reverse Skew-T', a tool that allows users to direct a physically inspired simulation of layered clouds. To achieve this, we extend existing models for cloud simulation and provide a graphical user interface for providing important simulation parameters to our system

Anderson, Eike F.

McLoughlin, L.

English

Bournemouth University Research Online

Reverse Skew-T – A Cloudmaking Tool for CGLeigh McLoughlin1 and Eike Falk Anderson11The National Centre for Computer Animation, Bournemouth University, United KingdomAbstractWe present ‘Reverse Skew-T’, a tool that allows users to direct a physically inspired simulation of layered clouds.To achieve this, we extend existing models for cloud simulation and provide a graphical user interface for providingimportant simulation parameters to our system.Categories and Subject Descriptors (according to ACM CCS): I.3.7 [Computer Graphics]: Three-DimensionalGraphics and Realism—Animation; I.6.8 [Simulation and Modeling]: Types of Simulation—Visual1. IntroductionClouds in the sky are an important aspect of natural out-door scenes. Pure physically based cloud simulations, how-ever,are notoriously difficult to control. The challenge then isfor any solution to be computationally lightweight and ableto be user directed in some measure while at the same timeproducing believable visual results.2. BackgroundThere exist a number of physically-based approaches forcloud simulation [MDN02, HBSL03], for which improve-ments have mostly focused on the fluid dynamics solutionwhile the cloud physics model has largely remained un-changed. Most works incorporate the thermodynamic effectsof phase transitions and assume that the atmospheric temper-ature falls with height according to a fixed lapse rate.A certain amount of user direction for cloud control hasbeen presented by Dobashi et al. [DKNY08], where usersdetermine a desired shape for the cloud height and then theirsystem adjusts the latent heat release and vapour levels toforce the cloud into this shape. This technique is able toachieve a relatively fine level of control and could comple-ment our work.3. Directable Cloud SimulationExisting cloud simulation solutions in computer graph-ics employ a fixed-rate for the simulation’s background-atmospheric temperature profile. Our inclusion of an arbi-trary temperature profile can lead to more visually inter-Figure 1: Using our ‘Reverse Skew-T’ tool to adjust the tem-perature profile (left) to give a cloud formation (right).esting and controllable results. To achieve this, our cloudphysics model uses the following equation adapted fromc© The Eurographics Association 2013.Published in EG 2013 - Posters (editors: M. Chover and A. A. de Sousa); Proceedings of Eurographics 2013, the 34th Annual Conference of the European Association for Computer Graphics; The definitive version is available at http://diglib.eg.org/; DOI: 10.2312/conf/EG2013/posters/017-018prprint1L. McLoughlin & E.F. Anderson / Reverse Skew-TRogers and Yau [RY89]:FB = gB = g(TT ′− (1+q))where g is the gravitational acceleration vector, T is the at-mospheric parcel temperature, T ′ is the ambient environ-ment temperature taken from our background atmosphericdescription and q is the parcel’s total mixing ratio of all con-densates (all water categories except vapour).Figure 2: Our ’reverse’ skew-T tool. The user clicks forcloud base and cloud height, the tool automatically gener-ates the background temperature profile and calculates themoisture and temperature seed levels required to producethis cloud.Our work addresses the task of controlling the ambient en-vironment temperature value T ′. Whereas previous CG ap-proaches have used a fixed function that defines T ′ as a func-tion of height, i.e. the simulation’s ‘lapse-rate’, our systemadds a level of directability by allowing users to define thisvertical temperature profile manually. This use of a user de-fined function allows the cloud simulation to generate morevaried results, including layered clouds (Figure 3). To sim-plify user input – the provision of a list of numbers is not in-tuitive and rather difficult to control or predict – some formof graphical user interface is needed.Meteorologists use the Skew-T log(P) graph (commonlyreferred to as just a “skew-T”) thermodynamic chart[AWS90] to plot atmospheric profiles showing how temper-ature varies with height, i.e. to visually present this samedata that users are meant to provide to our system. Inspiredby this analysis tool we have developed the ‘Reverse Skew-T’ user interface (Figure 1) as a tool for specifying verticaltemperature profiles for cloud simulation. Using the skew-T,specifying a cloud base and a cloud height becomes a rela-tively straight-forward problem for users of our system (Fig-ure 2). From this, the atmospheric temperature profile andthe required temperature and moisture of a source that willgenerate the desired cloud can be automatically determinedby our system.Figure 3: Layering effects achieved by manually adjustingthe temperature profile with our tool. Cloud formation is en-hanced in regions where the profile is to the left of the moistadiabat (the thick pink line in the Skew-T view), rising fromthe lifted condensation level (LCL – the point above whichthe cloud will start forming). Cloud formation is reduced inregions to the right of the curve. The example shown resultsin three layered regions, of which the mid-height one is thebest defined.4. Discussion and Future WorkWe have presented a cloud simulation user interface that al-lows users to visually define a background atmospheric de-scription using temperature profiles that can be arbitrarily setand changed rather than being confined to a specific lapse-rate. The level of control that our system offers is a substan-tial improvement on previous work, allowing us to generatemore varied results than are achievable with existing sys-tems, but we believe that the control is still rudimentary andcan be improved further, which we hope to address in thefuture.References[AWS90] AIR WEATHER SERVICE SCOTT AFB I.: The Use ofthe Skew T, Log P Diagram in Analysis and Forecasting. Revi-sion. Defense Technical Information Center, 1990. 2[DKNY08] DOBASHI Y., KUSUMOTO K., NISHITA T., YA-MAMOTO T.: Feedback control of cumuliform cloud formationbased on computational fluid dynamics. In ACM SIGGRAPH2008 (2008), pp. 1–8. 1[HBSL03] HARRIS M. J., BAXTER W. V., SCHEUERMANNT., LASTRA A.: Simulation of cloud dynamics on graph-ics hardware. In HWWS ’03: Proceedings of the ACM SIG-GRAPH/EUROGRAPHICS conference on Graphics hardware(2003), pp. 92–101. 1[MDN02] MIYAZAKI R., DOBASHI Y., NISHITA T.: Simulationof cumuliform clouds based on computational fluid dynamics.In Proceedings of EUROGRAPHICS 2002 Short Presentations(2002), pp. 405–410. 1[RY89] ROGERS R. R., YAU M. K.: A Short Course in CloudPhysics, third ed. International Series in Natural Philosophy. But-terworth Heinemann, Cambridge, UK, 1989. 2© The Eurographics Association 2013.repri t2

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