Jeffrey R. Key

Cloud Detection with MODIS. Part I: Improvements in the MODIS Cloud Mask for Collection 5

Significant improvements have been made to the Moderate Resolution Imaging Spectroradiometer (MODIS) cloud mask (MOD35 and MYD35) for Collection 5 reprocessing and forward stream data production. Most of the modifications are realized for nighttime scenes where polar and oceanic regions will see marked improvement. For polar night scenes, two new spectral tests using the 7.2-m water vapor absorption band have been added as well as updates to the 3.9–12- and 11–12-m cloud tests. More non-MODIS ancillary input data have been added. Land and sea surface temperature maps provide crucial information for mid- and low-level cloud detection and lessen dependence on ocean brightness temperature variability tests. Sun-glint areas are also improved by use of sea surface temperatures to aid in resolving observations with conflicting cloud versus clear-sky signals, where visible and near-infrared (NIR) reflectances are high, but infrared brightness temperatures are relatively warm. Day and night Arctic cloud frequency results are compared to those created by the Advanced Very High Resolution Radiometer (AVHRR) Polar Pathfinder-Extended (APP-X) algorithm. Day versus night sea surface temperatures derived from MODIS radiances and using only the MODIS cloud mask for cloud screening are contrasted. Frequencies of cloud from sun-glint regions are shown as a function of sun-glint angle to gain a sense of cloud mask quality in those regions. Continuing validation activities are described in Part II of this paper.

Arctic Surface, Cloud, and Radiation Properties Based on the AVHRR Polar Pathfinder Dataset. Part II: Recent Trends

Abstract Over the past 20 yr, some Arctic surface and cloud properties have changed significantly. Results of an analysis of satellite data show that the Arctic has warmed and become cloudier in spring and summer but has cooled and become less cloudy in winter. The annual rate of surface temperature change is 0.057°C for the Arctic region north of 60°N. The surface broadband albedo has decreased significantly in autumn, especially over the Arctic Ocean, indicating a later freeze-up and snowfall. The surface albedo has decreased at an annual rate of −0.15% (absolute). Cloud fraction has decreased at an annual rate of −0.6% (absolute) in winter and increased at annual rates of 0.32% and 0.16% in spring and summer, respectively. On an annual time scale, there is no trend in cloud fraction. During spring and summer, changes in sea ice albedo that result from surface warming tend to modulate the radiative effect of increasing cloud cover. On an annual time scale, the all-wave cloud forcing at the surface has d...

Arctic Surface, Cloud, and Radiation Properties Based on the AVHRR Polar Pathfinder Dataset. Part I: Spatial and Temporal Characteristics

Abstract With broad spectral coverage and high spatial and temporal resolutions, satellite sensors can provide the data needed for the analysis of spatial and temporal variations of climate parameters in data-sparse regions such as the Arctic and Antarctic. The newly available Advanced Very High Resolution Radiometer (AVHRR) Polar Pathfinder (APP) dataset was used to retrieve cloud fraction, cloud optical depth, cloud particle phase and size, cloud-top pressure and temperature, surface skin temperature, surface broadband albedo, radiative fluxes, and cloud forcing over the Arctic Ocean and surrounding landmasses for the 18-yr period from 1982 to 1999. In the Arctic, Greenland is the coldest region with the highest surface albedo, while northeastern Russia has the highest surface temperature in summer. Arctic annual mean cloud coverage is about 70%, with the largest cloudiness occurring in September and the lowest cloudiness occurring in April. On annual average, Arctic cloud visible optical depth is about...

Arctic system on trajectory to new, seasonally ice‐free state

The Arctic system is moving toward a new state that falls outside the envelope of glacialinterglacial fl uctuations that prevailed during recent Earth history. This future Arctic is likely to have dramatically less permanent ice than exists at present. At the present rate of change, a summer ice-free Arctic Ocean within a century is a real possibility, a state not witnessed for at least a million years. The change appears to be driven largely by feedback-enhanced global climate warming, and there seem to be few, if any, processes or feedbacks within the Arctic system that are capable of altering the trajectory toward this “super interglacial” state.

High-latitude surface temperature estimates from thermal satellite data

Abstract The surface temperature of the polar regions controls sea ice growth, snow melt, and surface-atmosphere energy exchange. However, our limited knowledge of polar surfaces and atmospheres has hampered the development of methods to estimate surface temperature with satellite data. In this article, clear-sky surface-temperature retrieval algorithms for rise with the Advanced Very High Resolution Radiometer (AVHRR) and the Along Track Scanning Radiometer (ATSR) for the Arctic and the Antarctic, over ocean and land, are presented. The methods are similar to those used in estimating sea and land surface temperatures but are developed with. data specific to the polar regions. An extensive validation analysis using an annual cycle of .surface meamarements gives accuracies in the range of 0.3–2.1 K, the larger errors being attributable to the spatially variable .surface of the validation area. For homogeneous surfaces the expected accuracy is sufficient for many climate process studies.

Recent Innovations in Deriving Tropospheric Winds from Meteorological Satellites

The evolving constellation of environmental/meteorological satellites and their associated sensor technology is rapidly advancing. This is providing opportunities for creatively improving satellite-derived products used in weather analysis and forecasting. For example, the retrieval methods for deriving atmospheric motion vectors (AMVs) from satellites have been expanding and evolving since the early 1970s. Contemporary AMV processing methods are continuously being updated and advanced through the exploitation of new sensor technologies and innovative new approaches. It is incumbent upon the research community working in AMV extraction techniques to ensure that the quality of the current operational products meets or exceeds the needs of the user community. In particular, the advances in data assimilation and numerical weather prediction in recent years have placed an increasing demand on data quality. To keep pace with these demands, innovative research toward improving methods of deriving winds from sat...

Arctic ice surface temperature retrieval from AVHRR thermal channels

The relationship between AVHRR thermal radiances and the surface (skin) temperature of Arctic snow-covered sea ice is examined through forward calculations of the radiative transfer equation, providing an ice/snow surface temperature retrieval algorithm for the central Arctic Basin. Temperature and humidity profiles with cloud observations collected on an ice island during 1986–1987 are used. Coefficients that correct for atmospheric attenuation are given for three Arctic clear sky “seasons”, as defined through statistical analysis of the daily profiles, for the NOAA 7, 9, and 11 satellites. Modeled directional snow emissivities, different in the two split-window (11 and 12 μm) channels, are used. While the sensor scan angle is included explicitly in the correction equation, its effect in the dry Arctic atmosphere is small, generally less than 0.1 K. Using the split-window channels and scan angle, the rms error in the estimated ice surface temperature (IST) is less than 0.1 K in all seasons. Inclusion of channel 3 (3.7 μm) during the winter decreases the rms error by less than 0.003 K. The seasonal dependence of the coefficients is important, with errors in the range of 0.1 – 0.6 K when coefficients from one season are used with data from another. Similarly, mixing coefficients and data from different satellites results in average errors from 0.1 to 1.0 K. Overall, employing the IST coefficients results in increased accuracy of up to 0.6 K over SST coefficients developed for the North Atlantic and the Greenland Sea areas.

An Arctic Springtime Mixed-Phase Cloudy Boundary Layer Observed during SHEBA

Abstract The microphysical characteristics, radiative impact, and life cycle of a long-lived, surface-based mixed-layer, mixed-phase cloud with an average temperature of approximately −20°C are presented and discussed. The cloud was observed during the Surface Heat Budget of the Arctic experiment (SHEBA) from 1 to 10 May 1998. Vertically resolved properties of the liquid and ice phases are retrieved using surface-based remote sensors, utilize the adiabatic assumption for the liquid component, and are aided by and validated with aircraft measurements from 4 and 7 May. The cloud radar ice microphysical retrievals, originally developed for all-ice clouds, compare well with aircraft measurements despite the presence of much greater liquid water contents than ice water contents. The retrieved time-mean liquid cloud optical depth of 10.1 ± 7.8 far surpasses the mean ice cloud optical depth of 0.2, so that the liquid phase is primarily responsible for the cloud’s radiative (flux) impact. The ice phase, in turn, ...

Parameterization of shortwave ice cloud optical properties for various particle habits

[1] The relative importance of ice clouds in the climate system is highly uncertain. Measurements of their microphysical properties are sparse, especially given their complex structure and large variability in particle size, shape, and density. To better understand the role of ice clouds in the climate system, parameterizations of their radiative properties are needed. The shortwave bulk optical properties of seven ice particle shapes, or ‘‘habits,’’ are parameterized as a function of the effective ‘‘radius’’ and ice water content by integrating the scattering properties over 30 in situ size distributions. The particle habits are solid and hollow hexagonal columns, hexagonal plates, two- and three-dimensional bullet rosettes, aggregates of columns, and dendrites. Parameterizations of the volume extinction coefficient, single-scattering albedo, and the asymmetry parameter are presented for 6, 24, and 56 band shortwave schemes from 0.2 to 5.0 mm. Applications to downwelling flux and upwelling radiance calculations indicate that differences in fluxes for various habits can be more than 15%, and differences in retrievals of cloud optical depth from satellite visible reflectances can be more than 50%. INDEX TERMS: 3359 Meteorology and Atmospheric Dynamics: Radiative processes; 0360 Atmospheric Composition and Structure: Transmission and scattering of radiation; 3360 Meteorology and Atmospheric Dynamics: Remote sensing;

Clues to variability in Arctic minimum sea ice extent

Received 11 August 2005; revised 20 September 2005; accepted 26 September 2005; published 2 November 2005. [1] Perennial sea ice is a primary indicator of Arctic climate change. Since 1980 it has decreased in extent by about 15%. Analysis of new satellite-derived fields of winds, radiative forcing, and advected heat reveals distinct regional differences in the relative roles of these parameters in explaining variability in the northernmost ice edge position. In all six peripheral seas studied, downwelling longwave flux anomalies explain the most variability – approximately 40% – while northward wind anomalies are important in areas north of Siberia, particularly earlier in the melt season. Anomalies in insolation are negatively correlated with perennial ice retreat in all regions, suggesting that the effect of solar flux anomalies is overwhelmed by the longwave influence on ice edge position. Citation: Francis, J. A., E. Hunter, J. R. Key, and X. Wang (2005), Clues to variability in Arctic minimum sea ice extent, Geophys. Res. Lett., 32, L21501, doi:10.1029/ 2005GL024376.