Abstract. We investigate methods to infer cloud properties such as cloud optical thickness, thermodynamic phase, cloud particle size, and cloud overlap by comparing cloud and clear-sky radiative transfer computations to measurements provided by the Moderate Resolution Imaging Spectroradiometer (MODIS) airborne simulator (MAS). The MAS scanning spectroradiometer was flown on the NASA ER-2 during the Subsonic Aircraft Contrail and Cloud Effects Special Study (SUCCESS) field campaign during April and May 1996. The MAS bands chosen for this study correspond to wavelengths of 0.65, 1.63, 1.90, 2.15, 3.82, 8.52, 11, and 12 gm. Clear-sky absorption due to water vapor, ozone, and other trace gases is calculated using a set of correlated k-distribution routines developed specifically for these MAS bands. Scattering properties (phase function, single-scattering albedo, and extinction cross section) are derived for water droplet clouds using Mie theory. Scattering properties for ice-phase clouds are incorporated for seven cirrus models: cirrostratus, cirrus uncinus, cold cirrus, warm cirrus, and cirrus at temperatures of T = -20øC, -40øC, and -60øC. The cirrus are composed of four crystal types: hexagonal plates, two-dimensional bullet rosettes, hollow columns, and aggregates. Results from comparison of MAS data from a liquid water cloud with theoretical calculations indicate that estimates of optical thickness and particle size are reasonably consistent with one another no matter which spectral bands are used in the analysis. However, comparison of MAS data from a cirrus cloud with theoretical calculations shows consistency in optical thickness but not with particle size among the various band combinations used in the analysis. The methods described in this paper are used in two companion papers to explore techniques to infer cloud thermodynamic phase and cloud overlap.

The potential link between cloud microphysical processes and climate is investigated and theorized. We base our theory on results simulated from a one‐dimensional climate model with an interactive cloud formation and precipitation program. This cloud program includes temperature‐dependent parameterization equations for condensation, evaporation, and precipitation derived from growth equations for water droplets. We show that the cloud liquid water content is directly related to precipitation processes, which are governed by the mean cloud particle radius. In particular, we illustrate that the rate of precipitation generation is directly proportional to the fourth power of this radius. A doubling of CO2 is used as the radiative forcing. If the perturbed mean cloud particle radii for model high, middle, and low clouds are less than the climatological mean values, precipitation decreases because of the presence of smaller cloud particles, leading to an increase in the cloud liquid water content. Cloud solar albedo effects are enhanced, resulting in a reduction of temperature increases due to CO2 doubling (negative feedback). If, however, the perturbed mean cloud particle radii are larger than the climatological mean values, the availability of larger cloud particles would increase precipitation, leading to a decrease in the cloud liquid water content. The temperature increase in the case of CO2 doubling is amplified because of a reduction of cloud solar albedo effects (positive feedback). In the model the particle sizes are not directly related to radiative transfer, but they are indirectly related through precipitation and condensation processes, which determine the cloud liquid water content. We hypothesize that there are uncertainties in cloud microphysical processes and that a possible key to climate stability due to external radiative perturbations is the availability of larger or smaller cloud droplets (in reference to the climatological mean values). Smaller cloud droplets may be produced by additional condensation nuclei over the oceans as a result of greenhouse warming and pollution over land. The existence of larger cloud droplets could be caused by the removal of cloud‐forming nuclei, resulting from enhanced precipitation due to greenhouse perturbations. It is critically important to have a global climatology of the cloud particle radii for various cloud types in the investigation of the role of clouds in climate.

Abstract. The version 6 cloud products of the Atmospheric Infrared Sounder (AIRS) and Advanced Microwave Sounding Unit (AMSU) instrument suite are described. The cloud top temperature, pressure, and height and effective cloud fraction are now reported at the AIRS field-of-view (FOV) resolution. Significant improvements in cloud height assignment over version 5 are shown with FOV-scale comparisons to cloud vertical structure observed by the CloudSat 94 GHz radar and the Cloud-Aerosol LIdar with Orthogonal Polarization (CALIOP). Cloud thermodynamic phase (ice, liquid, and unknown phase), ice cloud effective diameter (D e ), and ice cloud optical thickness (τ ) are derived using an optimal estimation methodology for AIRS FOVs, and global distributions for 2007 are presented. The largest values of τ are found in the storm tracks and near convection in the tropics, while D e is largest on the equatorial side of the midlatitude storm tracks in both hemispheres, and lowest in tropical thin cirrus and the winter polar atmosphere. Over the Maritime Continent the diurnal variability of τ is significantly larger than for the total cloud fraction, ice cloud frequency, and D e , and is anchored to the island archipelago morphology. Important differences are described between northern and southern hemispheric midlatitude cyclones using storm center composites. The infrared-based cloud retrievals of AIRS provide unique, decadal-scale and global observations of clouds over portions of the diurnal and annual cycles, and capture variability within the mesoscale and synoptic scales at all latitudes.

[1] The capability of Weather Research and Forecasting (WRF) model in the simulation of cirrus clouds has been examined, with a focus on the effects of radiative processes and vertical model resolution. We incorporate in WRF a new radiation module, referred to as the Fu-Liou-Gu scheme, which is an improvement and refinement based on the Fu-Liou scheme, particularly in reference to parameterization of the single-scattering properties of ice crystal size and shape. We conducted a number of real-time WRF simulations for cirrus cases that were observed in the coastal and western United States on 29-30 March 2007, and we compared these with available observations from Moderate Resolution Imaging Spectroradiometer (MODIS) and GOES visible and IR images over the same areas. Simulation results show that WRF is capable of generating reasonable cirrus cloud fields and their movement and dissipation processes, especially those associated with the large-scale frontal system. Radiative processes are important in cirrus cloud simulations by affecting the vertical thermal structure and hence convection. The newly implemented radiation module, the Fu-Liou-Gu scheme, has been demonstrated to work well in WRF and can be effectively used for studies related to cirrus cloud formation and evolution and aerosol-cloud-radiation interactions. With the newly implemented radiation scheme, the simulations of cloud cover and cloud and ice water path (CWP and IWP) have been improved for cirrus clouds, with a more consistent comparison with the corresponding MODIS observations in terms of CWP and IWP means and CWP frequency distribution, especially for optically thin cirrus with an improvement of about 20% in simulated mean IWP. The model-simulated ice crystal sizes have also been shown to be comparable to those determined from MODIS cloud products. Finally, we have demonstrated that model vertical resolution plays a significant role in cirrus cloud simulation in terms of altering vertical velocity field and the associated regional circulation.

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The Version 6 cloud products of the Atmospheric Infrared Sounder (AIRS) and Advanced Microwave Sounding Unit (AMSU) instrument suite are described. The cloud top temperature, pressure, and height and effective cloud fraction are now reported at the AIRS field of view (FOV) resolution. Significant improvements in cloud height assignment over Version 5 are shown with pixel-scale comparisons to cloud vertical structure observed by the CloudSat 94 GHz radar and the Cloud-Aerosol LIdar with Orthogonal Polarization (CALIOP). Cloud thermodynamic phase (ice, liquid, and unknown phase), ice cloud effective diameter (D_e), and ice cloud optical thickness (τ) are derived using an optimal estimation methodology for AIRS FOVs, and global distributions for January 2007 are presented. The largest values of τ are found in the storm tracks and near convection in the Tropics, while D_e is largest on the equatorial side of the midlatitude storm tracks in both hemispheres, and lowest in tropical thin cirrus and the winter polar atmosphere. Over the Maritime Continent the diurnal cycle of τ is significantly larger than for the total cloud fraction, ice cloud frequency, and D_e, and is anchored to the island archipelago morphology. Important differences are described between northern and southern hemispheric midlatitude cyclones using storm center composites. The infrared-based cloud retrievals of AIRS provide unique, decadal-scale and global observations of clouds over the diurnal and annual cycles, and captures variability within the mesoscale and synoptic scales at all latitudes

Aircraft, radiosonde, surface-flux, and boundary-layer wind profiler data from the Cooperative Atmosphere Surface Exchange Study's 1997 field project, CASES-97, are combined with synoptic data to study the evolution of the vertically-averaged mixed-layer potential temperature [ ] and mixing-ratio [Q] on three nearly-cloudless days from 1000 CST to 1200 CST (local noon is approximately 1230 CST). This was achieved through examination of the terms in the time-tendency ('budget') equations for [ ] and [Q]. We estimate three of the terms-local time rate of change, vertical flux divergence, and horizontal advection. For the [Q]-budget, vertical flux divergence usually dominates, but horizontal advection is significant on one of the three days. The [Q]-budget balances for two of the three days to within the large experimental error. For the-budget, vertical flux divergence accounts for most of the morning warming, with horizontal advection of secondary importance. The residual in the-budget has the same sign for all three days, indicating that not all the heating is accounted for. We can balance the [ ]-budgets to within experimental error on two of the three days by correcting the vertical-flux divergence for apparent low biases in the flux measurements of one of the aircraft and in the surface fluxes, and accounting for direct heating of the mixed layer by radiative flux divergence allowing for the effects of carbonaceous aerosols. The [ ]-budget with these corrections also balances on the third day if horizontal gradients from synoptic maps are used to estimate the horizontal advection. However, the corrected budget for this day does not balance if the horizontal gradient in the advection term is estimated using CASES-97 aircraft and radiosondes; we suggest that persistent mesoscale circulations led to an overestimate of the horizontal gradient and hence horizontal advection.