Coastal Clouds Research Articles

Improving the combined use of reflectance and thermal channels for ocean and coastal cloud detection for the Sea and Land Surface Temperature Radiometer (SLSTR)

Reflectance imagery is used to aid daytime cloud detection in thermal remote sensing. This paper presents new approaches to utilization of reflectance for sea surface temperature (SST) remote sensing for the Sea and Land Surface Temperature Radiometer (SLSTR). SLSTR is an along-track scanning sensor with a complex instrumental viewing geometry, and no co-registration of the fields-of-view of reflectance and thermal pixels. Reflectance channels have twice the spatial resolution of thermal channels, and observations are placed on compatible “image grids” for the convenience of users of Level-1 data. We highlight limitations of simple methods, based on these image grids, of using reflectance imagery to inform cloud detection at the thermal resolution. We present improvements from averaging the N-nearest reflectance observations directly to the infrared instrument geometry, where N = 10 is chosen in this study when using the A and B stripes together. We show that the standard deviation of the N-nearest reflectance observations is another calculable quantity of use to improve the discrimination of clouds in the infrared image, beneficially reducing the weight placed on coarser-scale thermal spatial variability. The developments are illustrated by case studies in coastal zones over optically bright waters and around strong ocean fronts, and the benefit for SST products is quantified by the impacts on coverage and validation statistics. In a case study over optically bright waters, the clear-sky fraction increases from 46.2% to 93.1%. Coastal zone validation shows a 23.8-31.6% reduction in the false alarm rate and a corresponding 27.1-33.3% increase in the cloud detection true skill score. Globally the robust standard deviation for clear-sky matches between SLSTR-A and drifting buoys reduces from 0.3 to 0.29 with a 6% reduction in data due to improved screening of scattered cloud.

Downwind control of oceanic air by land: the land wake and its sensitivity to CO2

Oceans are well-known to be directly altered by global climate forcings such as greenhouse gas changes, but how oceans are indirectly influenced by land and its response to such forcings remains less explored. Here, we assess the present-day and projected future state of a little-explored feature of the climate system—a ‘land wake’ in relative humidity downwind of the east coast of North America, consisting of low-humidity continental air extending roughly 1000 km over the Atlantic ocean. The wake exists throughout the year, but is supported by high continental temperatures in summer and low continental moisture in winter. The wake is well represented in an ensemble of global climate models (GCMs), qualitatively matching reanalysis data. Under increasing atmospheric CO2, the land wake intensifies in GCM simulations through two pathways: the radiative effects of CO2 on surface temperatures, and the biogeochemical effect of CO2 on terrestrial vegetation. Vegetation responses to increased CO2 alter the summer wake from Florida to Newfoundland, and both the radiative and biogeochemical effects of CO2 drive reductions in coastal cloud cover. These changes illustrate the potential of rapidly changing terrestrial climate to influence coastal regions and the ocean environment downwind of continents through both light conditions and the energy balance of the surface ocean.

A high-resolution record of coastal clouds and fog and their role in plant distributions over San Clemente Island, California

San Clemente Island (SCI), located in the Southern California Bight, is owned and operated by the U.S. Navy and is home to endemic species, including federally threatened or endangered plants and birds. The SCI ecosystem is influenced by the presence of warm season low-level clouds that shade, cool and, especially when in the form of fog, moisten the environment. We created a new cloud and fog satellite-derived albedo product for SCI at a higher resolution than previous datasets. The record spans 23 summers (1996–2018, May - Sep). The spatial resolution is ∼1 km and the temporal resolution is half hourly (nominally 0600 to 1800 PST). Using Geostationary Operational Environmental Satellite (GOES)-WEST visible band measurements, it was discovered that small (typically on the order of less than 5 km) geographical misalignment of the satellite images was common. The biological ramifications of such a shift could be significant. Thus, to provide a useful 1 km product for a narrow island such as SCI it was necessary to correct misalignments. Misalignment of albedo was easily apparent in clear sky images at the interface of land and water. This concept was used in the automated correction. The northwest coast of SCI is the cloudiest/foggiest area. June, on average, is the cloudiest month on SCI. The intra-day variability reaches ∼20% cloud albedo while interannual and monthly variability are ∼10%. To demonstrate the records’ utility in understanding ecological phenomena and patterns, we used the dataset to model plant distributions. We found monthly mean cloud albedo values, a proxy for cloud frequency and persistence, were among the most important environmental variables in understanding plant distributions. The vegetation models indicate locations with appropriate conditions, including clouds and fog during critical periods of the year, for particular vegetation types and thus, can inform restoration and management activities.

Characteristics of CCN activation and cloud microphysics over the east coast of India during the Northeast Monsoon onset

Airborne observations conducted during the Northeast Monsoon onset as part of the Cloud Aerosol Interaction and Precipitation Enhancement Experiment (CAIPEEX 2011) are used to link the activation properties of aerosols with the observed cloud microphysics. This study illustrates the significant spatial variability in the aerosol spectrum, cloud condensation nuclei (CCN) activation characteristics, and the cloud droplet spectral properties over a coastal and an inland location. High concentrations of Aitken mode and black carbon aerosols were observed in the free troposphere and are attributed to the convectively transported smoke and aerosols from the coastal boundary layer. The assumption of an internally mixed organic and inorganic aerosol composition provided a better CCN closure over assumptions of either purely organic or purely inorganic aerosols. The coastal clouds were equally polluted as the inland clouds with high cloud droplet number concentrations near the cloud base. The vertical distribution of cloud droplet spectral characteristics was similar in both coastal and inland clouds. An increase in droplet number concentration up to 2 km above the cloud base indicated a prominent influence of submicron particles on the cloud microphysical parameters. Evidence for an enhanced concentration of supercooled drops above the freezing level up to temperatures below −12 °C is documented. The secondary ice production was evident through observations of graupel and snow particles. Heavy loading of aerosols near the cloud base leads to enhanced mixed-phase processes in these clouds.

A Satellite Survey of Cloud Cover and Water Vapor in the Southwestern USA and Northern Mexico

Cloud cover and water vapor conditions in the southwestern USA and northern Mexico were surveyed as a preparatory work for the Thirty Meter Telescope (TMT) in situ site testing program. Although the telescope site is already selected, the TMT site testing team decided to make public these results for its usefulness for the community. Using 58 months of meteorological satellite observations between 1993 July and 1999 September, different atmospheric parameters were quantified from data of the 10.7 μm and of 6.7 μm windows. In particular, cloud cover and water vapor conditions were identified in preferred areas. As a result of the aerial analysis, 15 sites of existing and potential telescope were selected, compared, and ranked in terms of their observing quality. The clearest sites are located along the spine of the Baja peninsula and into southern California on mountain peaks above the temperature inversion layer. A steep gradient of cloudiness was observed along the coast where coastal cloud and fog are trapped below the inversion layer. Moving from west to east over the continent, a significant increase in cloudiness was observed. The analysis shows that San Pedro Mártir, San Gorgonio Mountain and San Jacinto Peak have the largest fraction of clear sky conditions (∼74%). The site with the optimal combination of clear skies and low precipitable water vapor is Boundary Peak, Nevada. An approach based in satellite data provided a reliable method for sites comparison.

Coastal Clouds at the Eastern Margin of the Southeast Pacific: Climatology and Trends

Abstract The climatology and recent trends of low-level coastal clouds at three sites along the northern Chilean coast (18.3°–23.4°S) are documented based upon up to 45 years of hourly observations of cloud type, coverage, and heights. Consistent with the subtropical location, cloud types are dominated by stratocumuli having greatest coverage (>7 oktas) and smaller heights (600–750 m) during the nighttime of austral winter and spring. Meridionally, nighttime cloud fraction and cloud-base heights increase from south to north. Long-term trends in mean cloud cover are observed at all sites albeit with a seasonal modulation, with increasing (decreasing) coverage in the spring (fall). Consistent trend patterns are also observed in independent sunshine hour measurements at the same sites. Cloud heights show negative trends of about 100 m decade−1 (1995–2010), although the onset time of this tendency differs between sites. The positive cloud fraction trends during the cloudy season reported here disagree with previous studies, with discrepancies attributed to differences in datasets used or to methodological differences in data analysis. The cloud-base height tendency, together with a less rapid lowering of the subsidence inversion base height, suggests a deepening of the coastal cloud layer. While consistent with the tendency toward greater low-level cloud cover and the known cooling of the marine boundary layer in this region, these tendencies are at odds with a drying trend of the near-surface air documented here as well. Assessing whether this intriguing result is caused by physical factors or by limitations of the data demands more detailed observations, some of which are currently under way.

North American west coast summer low cloudiness: Broadscale variability associated with sea surface temperature

Six decades of observations at 20 coastal airports, from Alaska to southern California, reveal coherent interannual to interdecadal variation of coastal low cloudiness (CLC) from summer to summer over this broad region. The leading mode of CLC variability represents coherent variation, accounting for nearly 40% of the total CLC variance spanning 1950–2012. This leading mode and the majority of individual airports exhibit decreased low cloudiness from the earlier to the later part of the record. Exploring climatic controls on CLC, we identify North Pacific Sea Surface Temperature anomalies, largely in the form of the Pacific Decadal Oscillation (PDO) as well correlated with, and evidently helping to organize, the coherent patterns of summer coastal cloud variability. Links from the PDO to summer CLC appear a few months in advance of the summer. These associations hold up consistently in interannual and interdecadal frequencies.

Geostrophic Wind Dependent Probabilistic Irradiance Forecasts for Coastal California

Coastal California has enormous potential for rooftop solar photovoltaic (PV) energy production. To reduce grid-integration costs, accurate and certain solar energy forecasts are required. However, frequent marine layer fog and stratus conditions limit the accuracy of numerical weather prediction models, especially during the summer. Thus, irradiance uncertainty is large and probabilistic forecast intervals are generally wide. To produce narrow and meaningful forecast intervals, the correlation of uncertainty to local meteorological conditions describing synoptic-scale atmospheric flow was considered. Specifically, the direction and magnitude of geostrophic flow were used as an indicator of coastal cloud cover probability to produce regime-dependent forecast intervals. The method was tested in summer 2011 for coastal California. The forecast interval was smaller than that provided by previous methods for all except clear conditions and contained 80% of irradiance measurements.

The influence of summertime fog and overcast clouds on the growth of a coastal Californian pine: a tree-ring study

The coast of California is home to numerous rare, endemic conifers and other plants that are limited in distribution by drought sensitivity and the summer-dry climate that prevails across most of the state. Ecologists have long assumed that some coastal plant populations survived the early Pleistocene transition to a warmer and drier environment because they benefit from frequent fog and stratus clouds that provide water and shade during the rainless summer. One such population is that of Torrey pine (Pinus torreyana ssp. Insularis) on Santa Rosa Island in Channel Islands National Park. Here we report that the tree-ring width record from this population indicates strong growth sensitivities to summer fog drip and cloud shading. We quantified the effects of summer cloud cover by comparing ring-width indices to coastal airport cloud-frequency records (1944-2004). For the first time observed, summertime cloud frequency correlated positively with ring-width indices, regardless of whether the effect of rainfall was first removed from the ring-width record. The effect of ground-level fog was strongest in July early mornings (03:00 PST, R(2) = 0.262, P < 0.0002). The effect of clouds high enough to provide shade but not fog water was also strongest in July, but climbed steadily throughout the day before becoming strongest in late afternoon (16:00-18:00 PST, R(2) = 0.148, P < 0.004). Correlations were substantially stronger in years with higher soil moisture, suggesting that growth response to summer clouds is strongly affected by pre-summer rainfall. A change in the height and/or timing of coastal cloud formation with climate change would likely affect this and other populations of California's coastal vegetation.

Mesoscale forcing of a boundary layer jet along the California coast

Mesoscale numerical model simulations were examined to investigate the thermal and topographic forcing of a boundary layer jet along the northern California coast from 0000 UTC May 2 to 1200 UTC May 3, 1990. Routine surface and upper air observations, satellite, and higher‐resolution coastal observations (R/V Point Sur and Doppler wind profiler data) documented the diurnal evolution of a coastal marine atmospheric boundary layer (MABL), coastal cloud structure, and mesoscale features such as the coastal jet and land‐sea breeze circulations that were reasonably well forecast by the numerical model. Sensitivity model simulations to coastal baroclinicity showed that the diurnal development and intensification of the coastal MABL jet located north of the San Francisco Bay area was a result of thermal forcing due to differential land‐sea heating. Simulations in which the coastal baroclinicity was held fixed in time at a minimal strength showed no diurnal development of the MABL jet. The strength of the jet was approximated well by the thermal wind relationship, with the jet core located in the sloping marine inversion layer within one Rossby radius of deformation from the coastline. The steepness of the marine inversion at the coastline was enhanced by regions of strong MABL divergence just offshore and strong convergence onshore resulting from the sea breeze circulation. Modifications to coastal terrain in two separate sensitivity model simulations illustrated that the primary effect of coastal topography was to act as a barrier to the onshore intrusion of higher momentum offshore air. This blocking was due to a topographically forced vertical circulation with a strong descending branch located at the coast. This downslope flow and adiabatic warming steepened the MABL inversion at the coast and enhanced the coastal baroclinicity and the coastal jet. Simulations without coastal terrain showed a jet structure that was spatially diffuse across the coastline and displaced more shoreward. Simulations in which coastal valleys were removed and replaced by a larger coastal mountain range resulted in little modification to the coastal jet intensity or offshore displacement.

Real-Time Forecasting of the Western Australian Summertime Trough: Evaluation of a New Regional Model

The real-time prediction of the location, strength, and structure of the summertime heat trough is a major forecasting problem over Western Australia. Maximum temperatures, wind strength and direction along the west coast, low-level coastal cloud, and thunderstorm activity are vulnerable to forecast errors in the heat trough. This study has three main parts. First, prediction errors of the operational Australian region numerical weather prediction (NWP) model were quantified over the period December 1991 to February 1992. Second, a newly developed regional NWP model, which will be the next operational regional model, was compared with the current operational model. The new model has more efficient numerics than the present operational model, allowing higher-resolution forecasts and a more sophisticated representation of physical processes. The third part was a set of sensitivity experiments to assess the relative importance of the differences. The dominant errors in the current operational model are a large westward bias in the trough location, a wide spread of errors in the intensity of the low in the northern section of the heat trough, a sizable range of coastal pressure gradient errors, and a northward bias in the latitude of the subtropical ridge axis between longitudes 110° and 120°E. It was demonstrated that these errors are reduced significantly in the new model, especially the subtropical ridge error, which has been virtually eliminated. The sensitivity studies revealed the importance of each of the differences between the models, and that the relative impact varies from case to case.

Direct damage to vegetation caused by acid rain and polluted cloud: definition of critical levels for forest trees.

The concept of critical levels was developed in order to define short-term and long-term average concentrations of gaseous pollutants above which plants may be damaged. Although the usual way in which pollutants in precipitation (wet deposition) influence vegetation is by affecting soil processes, plant foliage exposed to fog and cloud, which often contain much greater concentrations of pollutant ions than rain, may be damaged directly. The idea of a critical level has been extended to define concentrations of pollutants in wet deposition above which direct damage to plants is likely. Concentrations of acidity and sulphate measured in mountain and coastal cloud are summarised. Vegetation at risk of injury is identified as montane forest growing close to the cloud base, where ion concentrations are highest. The direct effects of acidic precipitation on trees are reviewed, based on experimental exposure of plants to simulated acidic rain, fog or mist. Although most experiments have reported results in terms of pH (H(+) concentration), the accompanying anion is important, with sulphate being more damaging than nitrate. Both conifers and broadleaved tree seedlings showing subtle changes in the structural characteristics of leaf surfaces after exposure to mist or rain at or about pH 3.5, or sulphate concentration of 150 micromol litre(-1). Visible lesions on leaf surfaces occur at around pH 3 (500 micromol litre(-1) sulphate), broadleaved species tending to be more sensitive than conifers. Effects on photosynthesis and water relations, and interactions with other stresses (e.g. frost), have usually been observed only for treatments which have also caused visible injury to the leaf surface. Few experiments on the direct effects of polluted cloud have been conducted under field conditions with mature trees, which unlike seedlings in controlled conditions, may suffer a growth reduction in the absence of visible injury. Although leaching of cations (Ca(2+), Mg(2+), K(+)) is stimulated by acidic precipitation, amounts leached are small compared with root uptake, unless soils have been impoverished. This aspect of the potential effects of acidic precipitation is best considered in terms of the long-term critical-load of pollutants to the soil. Given the practical difficulties in monitoring cloud water composition, a method for defining critical levels is proposed, which uses climatological average data to identify the duration and frequency of hill cloud, and combines this information with measured or modelled concentrations of particulate sulphate in the atmosphere, to derive cloud water concentrations as a function of cloud liquid water content. For forests within 100 m of the cloud base the critical levels of particulate sulphate, corresponding to solution concentrations in the range 150-500 micromol litre(-1), are in the range 1-3.3 microg S m(-3). These concentrations are observed over much of central Europe, suggesting that many montane forests are at risk of direct effects of fossil-fuel-derived pollutants in cloud.

Numerical simulation of coastal clouds when solar radiation is blocked by smoke

Large fires ignited in a major nuclear exchange would inject a considerable amount of smoke into the atmosphere. This smoke could block the incident solar radiation from reaching the ground. Without the input of solar radiation on the ground, the temperature over the land surface would drop more rapidly than the temperature over the ocean, leading to formation of a thermal gradient at the continental coastlines. This temperature gradient has been hypothesized to induce a flow field that would initiate precipitation which could rapidly scavenge much of the smoke. This hypothesis is being investigated using an enhanced version of the Colorado State University Mesoscale Model to simulate air flow, clouds and precipitation along continental coastlines. The model has been modified to include the formation of fog and clouds, removal of moisture by precipitation, and the transport and diffusion of smoke. Also, the long wave radiation parameterization has been changed to take into account the effects of clouds. For both east and west coasts, with a moderate westerly synoptic wind and with solar radiation blocked by smoke, continental cooling leads to the formation of fog or low clouds over land. When fog forms, the layer of most rapid cooling moves to the top of the cloud while the potential temperature inside the cloud becomes nearly constant. As higher layers become saturated, the cloud expands vertically, and the liquid water content increases until precipitation falls from the cloud. Over the ocean, the atmosphere cools radiatively while the sea surface remains at constant temperature thus reducing the thermal stability and enhancing the vertical mixing of moisture which is injected into the atmosphere from the sea. Eventually a layer of precipitating cloud forms over the ocean that is very similar to the clouds over land. Once these similar cloud layers have formed, the cooling rates over land and sea become nearly identical, and there is no mechanism to produce the horizontal temperature gradients needed for vertical growth of clouds and precipitation into the upper levels of the atmosphere where they can scavenge the injected smoke.

Light‐absorbing material extracted from cloud droplets and its effect on cloud albedo

Using the counterflow virtual impactor, a new instrument for sampling cloud droplets, measurable levels of light‐absorbing material were found to exist inside droplets in stratocumulus clouds off the coast of southern California. Eighty percent of the samples of droplet residue material had light absorption coefficients ranging from 6 to 20 × 10−8 m−1 (where m−1 means per meter of distance in the cloud, not in the cloud droplet nor in the dried bulk aerosol material). Calculated soot concentrations were between 23 and 79 ng soot g−1 of cloud water. These values are in general agreement with aerosol, rainwater, and snow measurements from other experiments and suggest that the coastal clouds were influenced by continental air. Aircraft measurements determined that the sampled clouds had average liquid water contents of 0.24–0.31 g m−3, effective droplet radii of 5.0–7.8 μm, and optical thicknesses of 14–28. Radiative transfer calculations indicate that even at the most sensitive wavelength, the maximum amount of light‐absorbing material from cloud air and inside cloud droplets in this experiment would not significantly alter the albedos of the clouds that were measured. The same amount could possibly affect the albedo of much thicker clouds or of snowpacks (which have relatively large particles and optical thicknesses).

Evidence of the biogenic nuclei involvement in Antarctic coastal clouds

Evidence of the biogenic nuclei involvement in Antarctic coastal clouds

Discussion of Papers Presented at the Symposlum: Evidence of Biogenic Nuclei Involvement in Antarctic Coastal Clouds

Discussion of Papers Presented at the Symposlum: Evidence of Biogenic Nuclei Involvement in Antarctic Coastal Clouds

石狩湾上の"Coastal Clouds"について

In winter, early in the morning, there was sometimes observed a low cloud layer near the coast of Ishikari Bay, Hokkaido. Still and motion pictures of this type of cloud layer were taken from the top of Mt. Teine. The results indicate that these clouds seem to have been caused by the mixing of relatively warm air over the bay and cold air flowing out from the Ishikari Plain. These "coastal clouds" occurred only on a clear morning which follows a clear night, and they were found only over the coast of the bay. The observed clouds were small in scale, and it did not produce precipitation. It is expected, however, that such clouds may occur on a larger scale in other places, and that it may produce precipitation even if there is no front nor mountain.

A Note on the South American Coastal Cloud

A Note on the South American Coastal Cloud