Low-level Stratiform Clouds Research Articles

Summertime Evolution of Decadal Sea Surface Temperature Anomalies in the Midlatitude North Pacific

Abstract To clarify the summertime evolution of decadal sea surface temperature (SST) anomalies and related physical processes in the midlatitudes of the North Pacific, numerical solutions of a three-dimensional bulk mixed layer model are analyzed, focusing on the contribution of the net shortwave radiative forcing at the sea surface. A quantitative heat budget analysis for the ocean mixed layer relating to late-1980s decadal SST change reveals that the decadal SST anomalies decay from late spring to early summer over the entire midlatitudes of the North Pacific. This quasi-seasonal decay of the decadal SST anomalies is controlled by an anomalous local thermal damping (i.e., anomalous surface heat fluxes). From midsummer to early autumn the anomalous net shortwave radiation flux associated with a meridional shift of the storm track acts to induce strong seasonal damping of the decadal SST anomaly in the northern Kuroshio–Oyashio Extension region. In contrast, in the north of the subtropical frontal region, the net shortwave radiation flux anomaly, which results from changes in low-level stratiform cloud cover, plays a major role in seasonally enhancing the decadal SST anomaly. Consequently, the SST anomalies formed by these radiative forcings cause significant variations in the local thermal damping rate at the sea surface over the period from late summer to early autumn.

Daytime Cycle of Low-Level Clouds and the Tropical Convective Boundary Layer in Southwestern Amazonia

Abstract During the wet season in the southwestern Amazon region, daytime water transport out of the atmospheric mixed layer into the deeper atmosphere is shown to depend upon cloud amounts and types and synoptic-scale velocity fields. Interactions among clouds, convective conditions, and subcloud-layer properties were estimated for two dominant flow regimes observed during the 1999 Tropical Rainfall Measuring Mission component of the Brazilian Large-Scale Biosphere–Atmosphere (TRMM-LBA) field campaign. During daytime the cloud and subcloud layers were coupled by radiative, convective, and precipitation processes. The properties of cloud and subcloud layers varied according to the different convective influences of easterly versus westerly lower-tropospheric flows. The most pronounced flow-regime effects on composite cloud cycles occurred under persistent lower-tropospheric flows, which produced strong convective cloud growth with a near absence of low-level stratiform clouds, minimal cumulative attenuation of incoming solar irradiance (∼25%), rapid daytime mixed-layer growth (>100 m h−1), and boundary layer drying (0.22 g kg−1 h−1), high convective velocities (>1.5 m s−1), high surface buoyancy flux (>200 W m−2), and high latent heat flux (600 W m−2) into cloud layer. In contrast, persistent westerly flows were less convective, showing a strong morning presence of low-level stratiform genera (>0.9 cloud amount), greater cumulative attenuation of incoming solar irradiance (∼47%), slower mixed-layer growth (<50 m h−1) with a slight tendency for mixed-layer moistening, and a delayed peak in the low-level cumuliform cloud cycle (2000 versus 1700 UTC). The results reported in this article indicate that numerical models need to account for cloud amounts and types when estimating water vapor transport to the cloud layer.

PAFOG—a new efficient forecast model of radiation fog and low-level stratiform clouds

The new one-dimensional forecast model PAFOG for radiation fogs and low-level stratiform clouds will be presented. The aim of the model is to improve the local visibility forecast on airports and other traffic locations where fog and low-level stratus frequently occur. PAFOG has been developed on the basis of the microphysical fog model MIFOG of Bott et al. [J. Atmos. Sci. 47 (1990) 2153]. To obtain a numerically efficient model, the detailed spectral cloud microphysics of MIFOG has been replaced by the parameterization scheme of Chaumerliac et al. [J. Geophys. Res. 92 (1987) 3114]. Furthermore, according to Siebert et al. [Beitr. Phys. Atmos. 65 (1992a) 93], a model for low vegetation is included in PAFOG so that now fog evolution as influenced by different types of vegetation can also be accounted for. The performance of PAFOG has been tested by comparing the model results with routine observations of the German Weather Service. Nine different weather periods comprising a total of 45 days have been investigated. In 41 cases, PAFOG yields agreement with the observations in terms of occurrence or nonoccurrence of fog or stratiform clouds. During radiation fogs, the calculated and observed visibilities are quite similar. However, in the model simulations the formation of dense fogs tends to be somewhat delayed. From the case studies with stratiform clouds, it is seen that cloud evolution in time and space strongly depends on the value of the large-scale subsidence. Since this quantity is not available from measurements, it must be provided by means of a numerical weather forecast model.

Tropical Atlantic air‐sea interaction and its influence on the NAO

An atmospheric general circulation model (AGCM) is forced with a prescribed SST dipole anomaly in the tropical Atlantic to investigate the cause of cross‐equatorial SST gradient (CESG) variability and its teleconnection to the extratropics. The model response bears a striking resemblance to observations in both the tropics and extratropics. The tropical response is robust and can act to reinforce the prescribed SST anomalies through wind‐induced evaporation. A new feedback mechanism involving low‐level stratiform clouds in the subtropics is also identified in the model and observations. The tropical SST dipole forces a barotropic teleconnection into the extratropics that projects onto the North Atlantic Oscillation (NAO). It further induces the extratropical portion of the North Atlantic SST tripole when the AGCM is coupled with an ocean mixed layer model. CESG variability thus appears to be the centerpiece of a pan‐Atlantic climate pattern observed to extend from the South Atlantic to Greenland.

The GOES I–M Imagers: New Tools for Studying Microphysical Properties of Boundary Layer Stratiform Clouds

This study reviews the capability of the advanced imagers on Geostationary Operational Environmental Satellites (GOES) I-M to provide quantitative information about bulk microphysical properties of low-level stratiform clouds, namely, cloud liquid water path (LWP) and droplet effective radius (r e ). Previous studies show that accurate estimates of cloud LWP from GOES imagers are possible, as evaluated from both ground-based and spaceborne passive microwave measurements, provided care is taken in vicarious calibration of the visible channel. GOES estimates of r e have yet to be validated. However, the r e versus LWP relationship derived from GOES and Special Sensor Microwave/Imager data shows good agreement with theory. The unique high-temporal sampling of the imager allows for detailed study of daytime characteristics of cloud microphysical properties and, possibly, indirect aerosol effect. Microphysical information for drizzling marine stratocumuli was also obtained, which was confirmed by direct comparison to ship-based C-band radar during the 1997 Tropical Eastern Pacific Process Study. From the promising results obtained thus far, GOES I-M imager data should be of great value in future field experiments involving low-level stratiform clouds.

A Microphysical Retrieval Scheme for Continental Low-Level Stratiform Clouds: Impacts of the Subadiabatic Character on Microphysical Properties and Radiation Budgets

Abstract Using measurements from the Department of Energy’s Atmospheric Radiation Measurement Program, a modified ground-based remote sensing technique is developed and evaluated to study the impacts of the subadiabatic character of continental low-level stratiform clouds on microphysical properties and radiation budgets. Airborne measurements and millimeter-wavelength cloud radar data are used to validate retrieved microphysical properties of three stratus cloud systems occurring in the April 1994 and 1997 intensive observation periods at the Southern Great Plains site. The addition of the observed cloud-top height into the Han and Westwater retrieval scheme eliminates the need to invoke the adiabatic assumption. Thus, the retrieved liquid water content (LWC) profile is represented as the product of an adiabatic LWC profile and a weighting function. Based on in situ measurements, two types of weighting functions are considered in this study: one is associated with a subadiabatic condition involving cloud...

Net Shortwave Fluxes over the Ocean

Abstract Net surface shortwave fluxes (Qsw) computed from National Aeronautics and Space Administration/Langley satellite data are compared with Qsw from reanalyses of the European Centre for Medium-Range Weather Forecasts (ERA) and the National Centers for Environmental Prediction (NCEP). The mean and variability of Qsw is examined for the period 1983–91, with a focus on the tropical and summer hemisphere oceans during June, July, August (JJA) and December, January, February (DJF). Both reanalyses exhibit a positive bias, indicating too much sunlight is absorbed at the surface, in regions where low-level stratiform clouds are most common, but a negative bias in regions where cumuliform clouds are the dominant cloud type. The ERA has a greater intermonthly variability during JJA than the satellite data over most of the Pacific, especially north of 40°N and in the central and eastern equatorial Pacific. The NCEP variability in JJA is also larger than the satellite estimates over the North Pacific and the e...

Holographic in-situ measurements of the spatial droplet distribution in stratiform clouds

Ground based in-situ measurements on the small-scale structure of low-level stratiform clouds have been performed utilizing the HOlographic Droplet and Aerosol Recording system (HODAR) of the University of Mainz, Germany. 6 holograms recorded during stratus cloud events on the Kleiner Feldberg Taunus Mountain Observatory (Frankfurt, Germany) were reconstructed in the laboratory and analysed by means of an automated data extraction and image processing system. In post-processing, each originally recorded droplet population was subjected to 2 statistical methods: (1) the sub-cell scanning analyses with statistical “Fishing” tests and (2) measurements of inter-droplet distance frequency distributions. Based on these analyses, significant deviations between the measured spatial droplet distributions and theoretical random distributions were found.

Cloud factor and seasonality of the indirect effect of anthropogenic sulfate aerosols

A method for evaluation of the indirect shortwave effect of anthropogenic sulfate aerosols based on the empirically derived cloud climatology is described and applied. The method is simple and computationally efficient which allows many sensitivity tests to be conducted. The tests presented focus mainly on the effects of cloud amounts and types as well as on seasonality of the indirect forcing. We found the indirect shortwave forcing estimated from cloud climatology (−1.1 W m−2, with a hemispheric difference of 0.4 W m−2) is close to that obtained from general circulation model simulations in previous studies. The hemispheric forcing has a strong seasonal cycle with the northern hemisphere (NH) forcing exceeding that of the southern hemisphere (SH) during the NH spring and summer, and the SH forcing prevailing during the SH spring and summer. We also estimate that single‐layer St/Sc contribute most to the aerosol indirect forcing (60%) with an additional 25% from St/Sc overlapped with Ci/Cs/Cc and 15% from St/Sc overlapped with As/Ac. The upper and middle level clouds overlapping the low level stratiform cloud layers reduce the indirect forcing by more than a factor of 2 (from −2.5 to −1.1 W m−2). The estimate of the indirect forcing of −1.1 W m−2 is based on climatological mean values of transmittance for Ac/As and Ci/Cs/Cc. The transmittance of these clouds, however, may vary in a wide range resulting in a large regional and seasonal variability of the aerosol indirect forcing.

Radiatively Driven Interactions between Stratocumulus and Synoptic Waves

Quasi-geostrophic disturbances on a midlatitude beta-plane channel forced by radiative heating perturbations due to synoptic-scale variations of low-level stratiform cloud are considered. The longitudinal phase of the cloud is linked to that of the low-level streamfunction. Cloudiness is wavelike with cooling above cloudy areas that decreases exponentially with height. No perturbation cooling occurs above cloud-free areas. Two background flows are considered: one is constant and the other has a jet centered near the tropopause. Forced linear steady waves are found. Though infinitesimal amplitude disturbances are considered, the problem is nonlinear because of the coupling between cloud and winds. The resulting structures are sensitive to the phase shift between cloud and streamfunction with strongest coupling for cloud to the west of surface troughs. The waves have vertical scales on the order of the troposphere depth. The stationary structures capture the summertime pattern of stratocumulus off California and its linkage to the mid-Pacific ridge. When zonal-mean cloud cooling is allowed, the mean westerlies are strengthened in the northern half of the channel near the lower boundary. Synoptic-scale amplitudes respond to the mean cloud cooling by increasing (decreasing) just above the cloud in the northern (southern) part of the channel.

Simulation of cloud radiative forcing with the ECMWF model

Radiative fluxes and corresponding cloud forcings derived from Earth Radiation Budget Experiment (ERBE) measurements are compared with similar quantities produced by the T63 version of the European Centre for Medium Range Weather Forecasts (ECMWF) model for January 1986. Monthly mean values and standard deviations of the radiation fields on a global scale are considered as well as regional diagnostics over the areas showing discrepancies between observations and model simulations. Clouds in the storm tracks of the Northern Hemisphere are correctly represented. Shortwave and longwave impacts of clouds in tropical convective areas are overestimated whereas low-level stratiform clouds on the eastern parts of subtropical oceans are missing. Cloud liquid water content diagnosed from the saturation mixing ratio is shown to give too small optical thickness at cold temperatures. This is particularly seen in the underestimation of the shortwave radiative impact of the clouds in the higher latitudes of the Southern Hemisphere.

Upper Cloud Movements in the Equatorial Regions of the Atlantic

RECENT communications to your paper have given the motion of the upper clouds from the eastward in the equatorial regions of the Atlantic. My observations (the result of having passed through these regions sixteen times in sailing-ships) give the motion of the upper clouds from the westward: and the motion of the intermediate cloud layers, consisting of the high low-level stratiform clouds (cirro-cumulus and such like), from a point somewhat to the north of east on the north side of the equator. Intermediate clouds are rare in the equatorial regions south of the Line. The high low-level clouds are constantly being confounded with the true high clouds.