Refinement, Validation and Application of Cloud-Radiation Parameterization in a GCM [report]

Dr. Graeme L. Stephens
2009 unpublished
The research performed under this award was conducted along 3 related fronts: (i) Refinement and assessment of parameterizations of sub-grid scale radiative transport in GCMs. (ii) Diagnostic studies that use ARM observations of clouds and convection in an effort to understand the effects of moist convection on its environment, including how convection influences clouds and radiation. This aspect focuses on developing and testing methodologies designed to use ARM data more effectively for use
more » ... atmospheric models, both at the cloud resolving model scale and the global climate model scale. (iii) Use (i) and (ii) in combination with both models and observations of varying complexity to study key radiation feedback Our work toward these objectives thus involved three corresponding efforts. First, novel diagnostic techniques were developed and applied to ARM observations to understand and characterize the effects of moist convection on the dynamical and thermodynamical environment in which it occurs. Second, an in house GCM radiative transfer algorithm (BUGSrad) was employed along with an optimal estimation cloud retrieval algorithm to evaluate the ability to reproduce cloudy-sky radiative flux observations. Assessments using a range of GCMs with various moist convective parameterizations to evaluate the fidelity with which the parameterizations reproduce key observable features of the environment were also started in the final year of this award. The third study area involved the study of cloud radiation feedbacks and we examined these in both cloud resolving and global climate models. Radiation studies 2.1 Assessing two-stream radiative fluxes derived from optimal estimation cloud properties Wood, N. B., G. L. Stephens and R. T. Austin, 2007: Assessment of two-stream radiative fluxes derived from optimal estimation cloud properties. J. Atmos. Sci, in preparation. Austin, R.T., and G.L. Stephens, 2008: Improved retrieval of stratus cloud microphysical parameters using millimeter-wave radar and visible optical depth: 1. Algorithm and synthetic analysis. J. Geophys. Res., in journal review. Austin, R.T., S. Miller, Y. Min, and G.L. Stephens and N. Wood, 2008: Improved retrieval of stratus cloud microphysical parameters using millimeter-wave radar and visible optical depth: II. Application and Evaluation. J. Geophys. Res., in journal review. This study took results of a cloud property retrieval scheme (Austin and Stephens, 2001 , later Austin et al., 2009 a and b, and also Austin et al., 2009) and combined these with the BUGSrad radiative transfer algorithm to derive radiative fluxes that were compared to measured fluxes. This approach was used as a way of evaluating the optical property parameterizations and other aspects of BUGSrad. In general, this assessment concludes that longwave fluxes at the surface are well reproduced for both liquid and ice cloud cases. Shortwave fluxes at the surface are also well reproduced for all liquid cases. For ice cloud cases, however, biases and rms differences between observed and modeled shortwave fluxes were 18%-26% of observations. The differences in results between the ice and liquid water cloud cases emphasize some particular issues regarding both aerosols and spatial sampling in the application of this technique. The principal differences between the ice and liquid cloud cases arise from two factors. First, the ice clouds are generally much lower in optical depth than the liquid clouds (less than 10 for the ice cloud cases and from 20 to 100 for the liquid cloud cases). Because of the smaller optical depths, the fluxes for the ice cases are much more sensitive to optical depth errors than are those for the liquid cloud cases. Additionally, these lower optical depths make the fluxes, particularly the downwelling shortwave, sensitive to aerosol optical depth. A combined retrieval product (Min et al., 2004), which produces aerosol and cloud optical depths which together are consistent with the observed shortwave flux gave aerosol optical depths that resulted in significant changes to the modeled downwelling shortwave fluxes. However, in cases in which the retrieved cloud optical depths were forced to match observations, the addition of this aerosol extinction slightly worsened biases in the calculated downwelling surface shortwave flux. Broader Relevance of this result: BUGSrad is the radiation scheme that is used by the CloudSat product (2b-FLXHR, L'Ecuyer et al., 2008: Impact of clouds on atmospheric heating based on the R04 CloudSat fluxes and heating rates data set. ) to produce radiative fluxes and heating rates by clouds as observed by CloudSat and CALIPSO. Assessing the performance of such schemes is central to the validation of such products and thus provides a way of spreading the influence and relevance of the ARM observations globally. The scheme has also been applied to CloudSat data to simulate top-of-atmosphere fluxes and comparisons to CERES fluxes that show an important level of agreement. largely gone unused for this purpose. Extension of this analysis to 5 years of Manus data Haynes, J. and G. L. Stephens, 2009: The properties of tropical convection derived over Manus, J. Geophys. Res., in preparation. The analysis reported in Stephens and Wood (2007) has subsequently been extended to five years of TWP Manus data. This study made use of the MMCR data from the ARM site at Manus Island. Profiles of radar reflectivity were grouped together according to the phase of the MJO as determined from satellite daily mean OLR observations. The structure of clouds and precipitation observed by the radar during various phases of the MJO were then studied and matched to the surface meteorology observations. Profiles were classified according to two parameters, echo top height and height of the 10 dBZ echo, which is taken to coincide with the maximum vertical extent that hydrometeors are present in the cloud. The progression of cloud cover from 14 days before the MJO event to 14 days after was documented. A chief discovery in this study was that multi-layered cloud systems are very common at the Manus site, most frequently occurring during the peak of the MJO event. Overall, when rain was observed, multiple cloud layers were also observed approximately 35% of the time. Similarly, about 35% of the total precipitation at the ARM site was associated with these multi-layer clouds. One of the most common occurrences of multi-layer clouds at Manus is a shallow cloud with an overlying mid-level cloud layer between 5 and 10 km. Shallow and mid-level clouds are also commonly associated with an overlying cirrus deck, particularly before the OLR minimum associated with the MJO (for example, when a shallow or mid-level cloud is present, cirrus is also present about 30% of the time). The prevalence of multiple layered clouds, especially when precipitation is occurring, suggests that latent heating of the atmosphere during rain events in the West Pacific may be more complicated that the simple convective and stratiform heating modes that are often envisioned. The presence of multiple layers can also complicate passive remote sensing-based cloud retrievals, many of which have coarse vertical resolution. Broader Relevance of this Result: This study ties the observations of MMCR to the broader properties of atmospheric state and confirms the statistical relevance of the Stephens and wood (2007) findings. Studies of this type are important as they illustrate ways to extend information content of the MMCR to broader applications 3.3 Trimodal structure of convection Posselt, D.J., S.C. van den Heever, and G.L. Stephens, 2008: Trimodal cloudiness and tropical stable layers in simulations of radiative convective equilibrium. Geophys. Res. Lett., 35, L08802, doi:10.1029/2007GL033029. This study attempts to provide a more mechanistic explanation for the observed trimodal character of convection that is evident in the data of the previous two research efforts. The study examines the tropical environment at radiative convective equilibrium using a large-domain cloud system resolving numerical model. As in observed studies of convectively active periods over warm tropical oceans (in particular the tropical western Pacific), we find a trimodal cloud struc ture that is closely associated with the presence of three distinct stable layers, including a prominent stable layer located near the zerodegree Celsius level. In addition, the simulation exhibits three separate large-scale zonal over-turning circulations, with two of these circulations located above the trade wind inversion and separated by the freezing level stable layer. Simulation results suggest that at equilibrium, this stable layer can be maintained by subsidence in the presence of longwave radiative cooling above the zero-degree level. Broader Relevance of this Result: This study offers a clear explanation of an observed feature of convection that has a number of implications as above. Cloud-Radiative Feedback studies Global scale controls on precipitation Stephens, G.L., and T.D. Ellis, 2008: Controls of Global-Mean Precipitation Increases in Global Warming GCM Experiments. J. Clim., 21, 6141-6155. This research began with an extensive review of the cloud-climate feedback problem (Stephens, 2005: Cloud Feedbacks in the Climate System: A Critical Review. J. Clim., 18, 237-273) and subsequently extended as part of the Ph.D research of Dr. Todd Ellis (graduated, Summer 2009). This research provides entirely new and fundamental insights into precipitation changes in global warming and highlights entirely new perspectives on cloud radiation feedbacks in climate as they relate to global precipitation. The study of Stephens and Ellis (2008) represents part of this work. This study of water vapor, precipitation, and radiative energy budget changes, evident in the 1% increase in carbon dioxide until doubled (1pctto2x) scenarios of global climate models (GCMs), was included in the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment report (AR4). Ensemble mean differences between initial and doubled carbon dioxide model states exhibit moistening at a rate greater than would be predicted by the Clausius-Clapeyron relationship. The robust features of the small relative humidity changes in the lower atmosphere correspond well with changes in model precipitation. However, the model observed sensitivity of column water vapor to changes in boundary layer temperature is more than three times greater than the sensitivity of precipitation to temperature changes. A simple measure of precipitation efficiency, defined as the ratio of these sensitivities, is introduced to show that the changes in Broader Significance of this Research: The fundamental feedbacks between convection and radiation are poorly understood. This is most apparent in the failure of current global models to adequately represent the diurnal cycle of convection. This study elucidates the key radiation interactions that affect the organization of convection over time scales of 20+ days and thus highlights mechanisms that are likely to be important to such phenomena as the Madden Julian Oscillation and possibly other modes of variability. It shows how the radiative heating by high clouds in particular, is an important process that regulates tropical convection. Again this study underscores the fundamental importance of cloud radiative heating on critical feedbacks and thus demonstrates the importance of ARM's activities on heating rate. .3 Radiative feedbacks and shallow convection Pakula, L. and G.L. Stephens, 2008: The Role of Radiation in Influencing Tropical Cloud Distributions in a Radiative-Convective Equilibrium Cloud-Resolving Model, J. Atmos. Sci., 66, 62-76. The effect of radiation on tropical cloud distributions was investigated in the context of large domain (20,000 km) Radiative-Convective Equilibrium Cloud Resolving Model (RCE-CRM) experiments. Observations show that actively convecting regions are associated with deep moisture profiles, reduced low-level static stability and cloud structures varying from shallow cumulus clouds to congestus clouds and deep convection. These experiments reproduced such structures. Radiation is found to produce a positive feedback that promotes more shallow clouds (tops between 1.5-5km) for a Sea Surface Temperature (SST) of 300K. The complicated effects of the water vapor continuum cause a low-level (2km) cooling maxima that destabilizes the atmosphere below 2km and stabilizes the atmosphere in the 2-4.5km layer. This stabilization layer promotes cloud detrainment and prevents the immediate formation of deep convection, resulting in more low-levels clouds in this 2-4.5km layer. This enhanced cloud water detrainment results in larger relative humidity values that appear to permit more recipitation efficient deep convection once it forms. This radiative feedback is sensitive o the absolute moisture path and conjectured to thus, be sensitive to SST. p t Broader Significance of Research: This research illustrates the central importance of shallow convection as a source of moisture to the atmosphere and illustrates how radiation controls the amount of such convection in simple RCE states. Kay, J.E., T. L'Ecuyer, A. Gettelman, G.L. Stephens and C. O'Dell, 2008: The contribution of cloud and radiation anomalies to the 2007 Arctic sea ice extent minimum. Geophys. Res. Lett., 35, L08503, doi:10.1029 Reduced cloudiness and enhanced downwelling shortwave radiation was shown to be associated with the unprecedented 2007 Arctic sea ice loss. Over the Western Arctic, where significant sea ice loss occurred, total cloud cover estimated from spaceborne radar and lidar data was 16% lower during the 2007 melt season than during the 2006 melt season. The clearer skies led to downwelling shortwave (longwave) radiative fluxes increases of +32 Wm-2 (-4 Wm-2) from 2006 to 2007. Over three months, these radiation anomalies alone could melt 0.3 m of ice or warm the surface ocean by 2.4 K. Reduced humidity and increased air temperatures associated with an anti-cyclonic circulation pattern explain the reduced cloudiness. Surface observations show that the 2007 cloudiness is anomalous in the recent past, but not unprecedented. Thus, in a warmer world with thinner ice, we suggest that natural cloud and circulation variability is an increasingly important control on sea ice extent. Radiatve forcing of summertime sea ice Broader Significance of this Research: This study showed how satellite and ground-based observations could be used to determine that decreased cloudiness and increased downwelling shortwave radiation are associated with the record-breaking 2007 sea ice extent minimum. While the 2007 cloud reductions are anomalous in the recent past, a 62-year record of cloudiness from Barrow hints that the 2007 cloudiness was not unprecedented. Thus, our results suggest that when sea ice is vulnerably thin, natural year-to-year variations in the summertime atmospheric circulation and associated changes in clouds and shortwave radiation can play an increasingly large role in modulating sea ice extent. Thus cloud radiation feedbacks are likely to be a significant to understanding future sea ice change. Papers and other Products Delivered ABSTRACT Observations by Johnson et al. depict regions of active tropical convection as possessing increased relative humidity through a deep layer and reduced low-level static stability when compared to nonconvecting regions. Shallow cumulus clouds, congestus clouds, and deep convection all coexist within these convecting regions. This investigation explores the effect that radiation might have on the tropical cloud distributions by using large-domain (20 000 km) radiative-convective equilibrium cloud-resolving model (RCE-CRM) experiments that reproduce similar moisture, stability, and cloud structures to those observed. Radiation is found to significantly increase the amount of shallow and intermediate-level clouds (tops between 1.5 and 5 km) by increasing low-level stability and thus promoting additional low-level cloud detrainment. The mechanism by which radiation stabilizes the low levels is found to differ between convectively suppressed and active regions. In convectively suppressed regions, strong relative humidity gradients within the trade inversion layer produce a low-level cooling maximum that further stabilizes the stable layer, much as proposed by Mapes and Zuidema. In convectively active regions, sufficiently moist columns with no relative humidity gradients are also found to produce a low-level cooling maximum that stabilizes the lower levels. This cooling maximum is due to the complicated effects of the water vapor continuum and is sensitive to the absolute moisture path. Because of the dependence on absolute moisture, the radiative enhancement of shallow and intermediate-level clouds in convectively active regions is potentially sensitive to SSTs.
doi:10.2172/951779 fatcat:o4bwnls42zdl5ezzygdjmofnoa