Properties Of High Clouds Research Articles

Satellite Observed Sensitivity of Tropical Clouds and Moisture to Sea Surface Temperature on Various Time and Space Scales: 1. Focus on High Level Cloud Situations Over Ocean

AbstractThis study examines variations in high cloud properties and relative humidity (RH) with sea surface temperature (SST) over tropical oceans (30°N–30°S) using spaceborne lidar and microwave radiometer observations. The mean values over the tropics indicate that middle‐tropospheric RH increases, high cloud covers decrease and cloud altitudes rise with SST. These signatures are consistent with the hypotheses proposed in the literature. The analysis of this same data set but at the scale of local processes shows different behaviors for SSTs <302 K and SSTs >302 K. Between 299 and 302 K, middle‐tropospheric RH, opaque cloud cover, and cloud top altitude increase together with SST, while optically thin cloud cover decreases. Over SSTs >302 K, middle‐tropospheric RH, opaque cloud cover, and opaque cloud top altitude decrease with SST, while the cover of optically thin clouds increases. Interestingly, the altitude of high clouds (not the cloud top) increases monotonically with SST from 299 to 305 K on a range of space and time scales, and the altitude of optically thin clouds remains higher than that of opaque clouds. The observed relationships on different time and space scales are compared to simulations of a global atmospheric model. Despite systematic biases, the model reproduces the sensitivity of the middle‐tropospheric RH and cloud altitude to SST rather well but fails to reproduce the variations of the balance between high opaque cloud cover and high optically thin cloud cover.

Improved Detection of Interannual Cloud Variability over the Southern Hemisphere Using Legacy Satellites

AbstractShifts in deep tropical convection and midlatitude jet streams both manifest themselves in high cloud anomalies. Such anomalies may play a significant role in local to global climate processes. This work investigates how high cloud properties covary with two primary interannual modes of variability in the Southern Hemisphere (SH): El Niño–Southern Oscillation (ENSO) and the southern annular mode (SAM). In contrast to several recent studies that utilize the latest remote sensing datasets (e.g., CloudSat), we employ a novel combination of imager and sounder data from legacy satellite instruments. Using these legacy data, we confirm the poleward shift of high cloud fields in the SH midlatitudes with SAM seen in other recent studies and characterize the opposing impacts of SAM and ENSO on the South Pacific convergence zone and Southern Hemisphere storm tracks. Furthermore, we demonstrate that the standard deviation of brightness temperature data from the window channel acts as a surrogate for high cloud fraction in the tropics and midlatitudes. Our results reconcile apparent differences in recent studies and suggest that brightness temperature standard deviations are climate relevant, in addition to being largely insensitive to instrument calibration.

Responses of Tropical Deep Convection to the QBO: Cloud-Resolving Simulations

Abstract Observational studies suggest that the stratospheric quasi-biennial oscillation (QBO) can modulate tropical deep convection. The authors use a cloud-resolving model with a limited domain, representing a convective column in the tropics, to study the mechanisms of this modulation. The large-scale circulation is parameterized using the weak temperature gradient (WTG) approximation, under which the parameterized large-scale vertical motion acts to relax the horizontal-mean temperature toward a specified reference profile. Temperature variations typically seen in easterly and westerly phases are imposed in the upper troposphere and lower stratosphere of this reference profile. The responses of convection are studied over different sea surface temperatures, holding the reference temperature profile fixed. This can be thought of as studying the response of convection to the QBO over different “relative SSTs” and also corresponds to different equilibrium precipitation rates in the control simulation. The equilibrium precipitation rate shows slight increases in response to a QBO easterly phase temperature perturbation over small SST anomalies and strong decreases over large SST anomalies, and vice versa for the QBO westerly phase perturbation. A column moist static energy budget analysis reveals that the QBO modulates the convective precipitation through two pathways: it changes the high-cloud properties and thus the column radiative cooling, and it alters the shape of the large-scale vertical motion and thus the efficiency of energy transport by the large-scale flow. The nonmonotonicity of the precipitation response with respect to relative SST results from the competition of these two effects.

Climatology of High Cloud Dynamics Using Profiling ARM Doppler Radar Observations

AbstractIce cloud properties are influenced by cloud-scale vertical air motion. Dynamical properties of ice clouds can be determined via Doppler measurements from ground-based, profiling cloud radars. Here, the decomposition of the Doppler velocities into reflectivity-weighted particle velocity Vt and vertical air motion w is described. The methodology is applied to high clouds observations from 35-GHz profiling millimeter wavelength radars at the Atmospheric Radiation Measurement Program (ARM) Southern Great Plains (SGP) climate research facility in Oklahoma (January 1997–December 2010) and the ARM Tropical Western Pacific (TWP) site in Manus (July 1999–December 2010). The Doppler velocity measurements are used to detect gravity waves (GW), whose correlation with high cloud macrophysical properties is investigated. Cloud turbulence is studied in the absence and presence of GW. High clouds are less turbulent when GW are observed. Probability density functions of Vt, w, and high cloud macrophysical properties for the two cloud subsets (with and without GW) are presented. Air-density-corrected Vt for high clouds for which GW (no GW) were detected amounted to hourly means and standard deviations of 0.89 ± 0.52 m s−1 (0.8 ± 0.48 m s−1) and 1.03 ± 0.41 m s−1 (0.86 ± 0.49 m s−1) at SGP and Manus, respectively. The error of w at one standard deviation was estimated as 0.15 m s−1. Hourly means of w averaged around 0 m s−1 with standard deviations of ±0.27 (SGP) and ±0.29 m s−1 (Manus) for high clouds without GW and ±0.22 m s−1 (both sites) for high clouds with GW. The midlatitude site showed stronger seasonality in detected high cloud properties.

A method for comparing properties of cirrus clouds in global climate models with those retrieved from IR sounder satellite observations

A methodology to compare cloud properties simulated by global climate models with those retrieved from observations by satellite-based infrared (IR) sounders has been developed. The relatively high spectral resolution in the CO2 absorption band of these instruments leads to especially reliable cirrus properties, day and night. Additionally, bulk microphysical properties can be retrieved for semi-transparent cirrus, based on the observed spectral emissivity differences between 8 and 11 µm. The particular intention of this study is to compare macro-and microphysical properties of high cloudiness as represented by the model simulations and the satellite data. For this purpose, a method has been developed to process the model output to be comparable to the satellite measurements, as in other observational simulator packages (for example the ISCCP-simulator). This simulator method takes into account i) the differences in horizontal resolution of the model and the observations, ii) the specific observation time windows, iii) the determination of the pressure of a cloud system, identified with the pressure at the middle of the uppermost cloud, and iv) the selection of high clouds with specific cloud optical thickness ranges for the microphysical property retrieval using IR sounder data. Applying this method to simulations by the global climate model ECHAM and TOVS satellite observations has important effects. The frequency of high clouds selected from the model output by using the method is significantly smaller than the total frequency of high cloudiness in the model. Largest differences occur around the equator where the zonal mean frequency of high cloudiness is reduced by about 30 % (relative change). The selection method is essential for the comparison of modelled and observed microphysical properties of high clouds. The selection of high clouds from the ECHAM simulation according to the optical thickness range of the TOVS data results in a reduction of the mean water path of high clouds by factors of more than 3 compared to the case where also high clouds of other optical thicknesses are considered. Furthermore, the selection by optical thickness causes a significant increase in the mean effective cloud particle diameter. These changes significantly reduce the differences between the simulation and the observations. The method can also be applied for comparisons with other IR sounder climatologies such as from AIRS and IASI. © Gebruder Borntraeger, Stuttgart 2010.

High Cloud Properties from Three Years of MODIS Terra and Aqua Collection-4 Data over the Tropics

Abstract This study surveys the optical and microphysical properties of high (ice) clouds over the Tropics (30°S–30°N) over a 3-yr period from September 2002 through August 2005. The analyses are based on the gridded level-3 cloud products derived from the measurements acquired by the Moderate Resolution Imaging Spectroradiometer (MODIS) instruments aboard both the NASA Earth Observing System Terra and Aqua platforms. The present analysis is based on the MODIS collection-4 data products. The cloud products provide daily, weekly, and monthly mean cloud fraction, cloud optical thickness, cloud effective radius, cloud-top temperature, cloud-top pressure, and cloud effective emissivity, which is defined as the product of cloud emittance and cloud fraction. This study is focused on high-level ice clouds. The MODIS-derived high clouds are classified as cirriform and deep convective clouds using the International Satellite Cloud Climatology Project (ISCCP) classification scheme. Cirriform clouds make up more than 80% of the total high clouds, whereas deep convective clouds account for less than 20% of the total high clouds. High clouds are prevalent over the intertropical convergence zone (ITCZ), the South Pacific convergence zone (SPCZ), tropical Africa, the Indian Ocean, tropical America, and South America. Moreover, land–ocean, morning–afternoon, and summer–winter variations of high cloud properties are also observed.

Radiometric validation of the Atmospheric Infrared Sounder over the Antarctic Plateau

Validation of infrared satellite instruments, after they have been launched into orbit, has traditionally relied on views of the relatively warm sea surface. The Antarctic Plateau provides a complementary validation target from space because it is also homogeneous over large areas, yet it is relatively cold. During summer, cloud‐free conditions occur often and the atmospheric humidity is very low with values of total column water vapor around 1 mm. Radiance validation experiments were conducted at Dome C, Antarctica (75°S, 123°E; 3280 m), in January 2003 and again in December 2003/January 2004 for the Atmospheric Infrared Sounder (AIRS). The Polar Atmospheric Emitted Radiance Interferometer (PAERI) measured upwelling and downwelling infrared radiance at the surface during overpasses of NASA's Earth Observing System Aqua satellite. Radiosondes were launched coincidentally with many of these overpasses. These data are used to calculate the outgoing spectral radiance at the top of the atmosphere (TOA), which is then compared with spectral radiance measurements from AIRS in selected frequency bands. At the 95% confidence level, the brightness temperature differences are −0.12 ± 0.24 K (M‐07), −0.15 ± 0.23 K (M‐08), and −0.13 ± 0.24 K (M‐09). These results validate the AIRS radiances at low temperatures (230 to 250 K) relative to the TOA calculations between 788 and 973 cm−1. The cold bias may be partially explained by subvisible or undetected clouds. Ground‐based validation experiments over the Antarctic Plateau can yield valuable information about the low‐radiance performance of infrared satellite instruments. However, one must use ground‐based instruments with a large enough field of view to average over the significant radiance variations that exist across individual snow dunes. Validation of AIRS at low radiance is important because many of the geophysical parameters that are of scientific interest are actually retrieved from low radiance values, including upper tropospheric humidity and properties of high clouds.

Simulated sensitivity of the earth's radiation budget to 'changes in cloud properties

Simulations are made of the long-term mean clear-sky and total-scene components of the earth's radiation budget (ERB), using detailed solar and thermal infrared radiative transfer schemes. These employ as input a wide range of multi-year observations of atmospheric, surface and cloud parameters. For clear skies, differences between the simulations and results from the Earth Radiation Budget Experiment (ERBE) are generally within the expected accuracy of the observations and calculations, with the most notable disagreement at high latitudes. For cloudy skies, the agreement is less good, but still generally within the systematic error bounds. the absorbed shortwave radiation (ASR) is simulated least well over oceanic regions, with the pattern of disagreement resembling the pattern of response when low cloud cover or liquid water path is perturbed; this suggests that the representation of low cloud cover or liquid water path dominates the ASR discrepancy between the simulation and ERBE. the simulated outgoing longwave radiation (OLR), which is sensitive to the high-cloud emissivity, differs most from the ERBE OLR in deep convective regions; whilst assuming the high cloud to be black lessens agreement between the simulation and observations in some areas, agreement is substantially improved in others. This indicates that better knowledge of high-cloud emissivity remains an important issue. the spatial and temporal variations of the sensitivity of the ERB to perturbations to low, mid and high cloud properties are investigated. Principal results include (a) the dominant importance of cloud amount, as compared to cloud temperature, in determining the sensitivity of the ERB to high-cloud properties, and (b) the greater sensitivity of the net ERB to low-cloud properties than to those of mid and high cloud. A major uncertainty in the calculations is the degree of cloud overlap; the inclusion of overlapping clouds is found to be capable of reversing the net high-cloud sensitivities in certain locations. Global mean sensitivity to low cloud perturbations are compared with previous results obtained using a GCM. the simulated sensitivity of the ERB to low cloud cover is substantially larger than in the previous results, although the effects of changes in liquid water path are similar.

Simulated sensitivity of the earth's radiation budget to 'changes in cloud properties

AbstractSimulations are made of the long‐term mean clear‐sky and total‐scene components of the earth's radiation budget (ERB), using detailed solar and thermal infrared radiative transfer schemes. These employ as input a wide range of multi‐year observations of atmospheric, surface and cloud parameters. For clear skies, differences between the simulations and results from the Earth Radiation Budget Experiment (ERBE) are generally within the expected accuracy of the observations and calculations, with the most notable disagreement at high latitudes. For cloudy skies, the agreement is less good, but still generally within the systematic error bounds. the absorbed shortwave radiation (ASR) is simulated least well over oceanic regions, with the pattern of disagreement resembling the pattern of response when low cloud cover or liquid water path is perturbed; this suggests that the representation of low cloud cover or liquid water path dominates the ASR discrepancy between the simulation and ERBE. the simulated outgoing longwave radiation (OLR), which is sensitive to the high‐cloud emissivity, differs most from the ERBE OLR in deep convective regions; whilst assuming the high cloud to be black lessens agreement between the simulation and observations in some areas, agreement is substantially improved in others. This indicates that better knowledge of high‐cloud emissivity remains an important issue. the spatial and temporal variations of the sensitivity of the ERB to perturbations to low, mid and high cloud properties are investigated. Principal results include (a) the dominant importance of cloud amount, as compared to cloud temperature, in determining the sensitivity of the ERB to high‐cloud properties, and (b) the greater sensitivity of the net ERB to low‐cloud properties than to those of mid and high cloud. A major uncertainty in the calculations is the degree of cloud overlap; the inclusion of overlapping clouds is found to be capable of reversing the net high‐cloud sensitivities in certain locations. Global mean sensitivity to low cloud perturbations are compared with previous results obtained using a GCM. the simulated sensitivity of the ERB to low cloud cover is substantially larger than in the previous results, although the effects of changes in liquid water path are similar.

A comparative study of the observations of high clouds and simulations by an atmospheric general circulation model

The importance of clouds in the upper troposphere (cirrus) for the sensitivity of the Earth's climate e.g., requires that these clouds be modeled accurately in general circulation model (GCM) studies of the atmosphere. Bearing in mind the lack of unambiguous quantitative information on the geographical distribution and properties of high clouds, the simulated distribution of upper tropospheric clouds in a spectral GCM is compared with several satellite-derived data-sets that pertain to high clouds only, for both winter and summer seasons. In the model, clouds are assumed to occupy an entire gridbox whenever the relative humidity exceeds 99%: otherwise the grid box is assumed to be free of cloud. Despite the simplicity of the cloud prediction scheme, the geographical distribution of the maxima in the model's upper tropospheric cloud cover coincides approximately with the regions of the observed maxima in the high cloud amount and their frequency of occurrence (e.g., intertropical convergence zone and the monsoon areas). These areas exhibit a minimum in the outgoing longwave radiation (OLR; Nimbus-7) and are also coincident with regions of heavy precipitation. The model, with its relatively simple cloud formation scheme, appears to capture the principal large-scale features of the tropical convective processes that are evident in the satellite and precipitation datasets, wherein the intense, upward motion is accompanied by condensation and the spreading of thick upper tropospheric layers of high relative humidity and cloudiness in the vicinity of the tropical rainbelt regions.