Donald P. Wylie

Surface Heat Budget of the Arctic Ocean

A summary is presented of the Surface Heat Budget of the Arctic Ocean (SHEBA) project, with a focus on the field experiment that was conducted from October 1997 to October 1998. The primary objective of the field work was to collect ocean, ice, and atmospheric datasets over a full annual cycle that could be used to understand the processes controlling surface heat exchanges—in particular, the ice–albedo feedback and cloud–radiation feedback. This information is being used to improve formulations of arctic ice–ocean–atmosphere processes in climate models and thereby improve simulations of present and future arctic climate. The experiment was deployed from an ice breaker that was frozen into the ice pack and allowed to drift for the duration of the experiment. This research platform allowed the use of an extensive suite of instruments that directly measured ocean, atmosphere, and ice properties from both the ship and the ice pack in the immediate vicinity of the ship. This summary describes the project goal...

Four Years of Global Cirrus Cloud Statistics Using HIRS

Abstract : Trends in global upper tropospheric transmissive cirrus cloud cover are beginning to emerge from a four year cloud climatology using NOAA polar orbiting HIRS multispectral infrared data. Cloud occurrence, height, and effective emissivity are determined with the CO2 slicing technique on the four years of data (June 1989 - May 1993). There is a global preponderance of transmissive high clouds, 42% on the average; about three fourths of these are above 500 hPa and presumed to be cirrus. In the ITCZ a high frequency of cirrus (grater than 50%) is found at all times; a modest seasonal movement tracks the sun. Large seasonal changes in cloud cover occur over the oceans in the storm belts at mid-latitudes; the concentrations of these clouds migrate north and south with the seasons following the progressions of the subtropical highs (anticyclones). More cirrus is found in the summer than in the winter in each hemisphere. This paper reports on the investigation of seasonal changes with multispectral observations from polar orbiting HIRS (High resolution Infrared Radiation Sounder).

Trends in Global Cloud Cover in Two Decades of HIRS Observations

The frequency of cloud detection and the frequency with which these clouds are found in the upper troposphere have been extracted from NOAA High Resolution Infrared Radiometer Sounder (HIRS) polar-orbiting satellite data from 1979 to 2001. The HIRS/2 sensor was flown on nine satellites from the Television Infrared Observation Satellite-Next Generation (TIROS-N) through NOAA-14, forming a 22-yr record. Carbon dioxide slicing was used to infer cloud amount and height. Trends in cloud cover and high-cloud frequency were found to be small in these data. High clouds show a small but statistically significant increase in the Tropics and the Northern Hemisphere. The HIRS analysis contrasts with the International Satellite Cloud Climatology Project (ISCCP), which shows a decrease in both total cloud cover and high clouds during most of this period.

MODIS Global Cloud-Top Pressure and Amount Estimation: Algorithm Description and Results

Abstract The Moderate Resolution Imaging Spectroradiometer (MODIS) on the NASA Earth Observing System (EOS) Terra and Aqua platforms provides unique measurements for deriving global and regional cloud properties. MODIS has spectral coverage combined with spatial resolution in key atmospheric bands, which is not available on previous imagers and sounders. This increased spectral coverage/spatial resolution, along with improved onboard calibration, enhances the capability for global cloud property retrievals. MODIS operational cloud products are derived globally at spatial resolutions of 5 km (referred to as level-2 products) and are aggregated to a 1° equal-angle grid (referred to as level-3 product), available for daily, 8-day, and monthly time periods. The MODIS cloud algorithm produces cloud-top pressures that are found to be within 50 hPa of lidar determinations in single-layer cloud situations. In multilayer clouds, where the upper-layer cloud is semitransparent, the MODIS cloud pressure is representa...

Two Years of Cloud Cover Statistics Using VAS

Abstract Statistics of cloud characteristics over North America have been accumulated for the past 2 yr. The frequency of cloud cover with the associated heights and infrared attenuation were charted using the C02 channel radiometric data from the VISSR Atmospheric Sounder (VAS). Cloud top pressures were determined from the ratio of VAS CO2 channel radiances in a radiative transfer equation formulation. Cloud emissivities were then calculated from infrared window channel observations The VAS C02 derived cloud top height and emissivity assignments have been found to be reliable in most cloud type, including thin cirrus clouds where other techniques have been inconsistent. Observations since 1985 reveal that 20%–30% of the United States was covered with thin semitransparent clouds (radiative attenuation was less than 95%), 45% was covered with thick opaque clouds, and 25%–35% had clear sky conditions. It is likely that 5% of the opaque cloud should have been identified as semitransparent cirrus. The geograp...

Seasonal and Diurnal Changes in Cirrus Clouds as Seen in Four Years of Observations with the VAS

Abstract GOES VAS multispectral observations in the carbon dioxide absorption band at 15 μm have been used to compile cloud-cover statistics over the continental United States for the past 4 years. The CO2 technique calculates both cloud-top pressures and effective emissivities and reliably distinguishes semitransparent cirrus from opaque clouds. The frequency of semitransparent cirrus clouds exhibits small seasonal variation; they are generally present 25%–30% of the time in all seasons. Diurnal variations of semitransparent cirrus are found only in the summer months and correspond to diurnal variations in convection in the Rocky Mountains and southeastern United States, increases of 20% in cirrus are noted subsequent to the convective cloud activity. In the winter months, no diurnal change in semitransparent cirrus is detected. Attempts to correlate cirrus with some common atmospheric features reveal that a majority of cirrus occurred where dynamic parameters indicate rising vertical motion but that con...

Comparison of the Climatologies of High-Level Clouds from HIRS and ISCCP

Abstract Comparison of individually matched analyses of high-level cloudiness from the High-Resolution Infrared Sounder (HIRS) CO2-slicing analysis and the International Satellite Cloud Climatology Project (ISCCP) analysis of satellite data for 4 months shows that the former reports about 0.12 more high-level clouds than the latter. Almost all of the difference in high-level cloud amounts occurs as differences of thin cirrus, defined by infrared emissivity ϵ < 0.5 or τvis < 1.3, consistent with a previous comparison of Stratospheric Aerosol and Gas Experiment II and ISCCP. Some of this difference may be caused by the large field of view of the HIRS instrument. Over oceans the differences in cirrus cloud amounts are caused by the higher sensitivity of the HIRS analysis to optically thin clouds, aided by a small high bias of the sea surface temperatures used in the HIRS cloud detection step. Over land the higher detection sensitivity of the HIRS analysis was partially offset by the effect of large low biase...

The 27–28 October 1986 FIRE Cirrus Case Study: Meteorology and Clouds

Abstract Detailed descriptions of the rawinsonde-resolved meteorological conditions (3-hourly soundings) associated with a succession of five distinct mesoscale cirrus cloud regimes, which were observed intensely over a 36-hour period, is given. The synoptic scale systems in which these features were embedded are described and a brief overview of the experiment are given. Regional analyses of the static stability structure and vertical motion are presented and interpreted with respect to the characteristics of the corresponding cloud fields as deduced from satellite and lidar observations. The cloud fields exhibited a high degree of persistent mesoscale organization on scales of 20–500 km reflecting corresponding scales of dynamic and thermodynamic structure/variability as on the synoptic scale. Cloud generation was usually confined to layers less than 1 km deep (typically 0.5 km in depth) and cellular organization was evident in most cases irrespective of the thermal stratification. Multilayered developm...

Understanding Satellite Cirrus Cloud Climatologies with Calibrated Lidar Optical Depths

Abstract Optical depth measurements of transmissive cirrus clouds were made using coincident lidar and satellite data to improve our interpretation of satellite cloud climatologies. The University of Wisconsin High Spectral Resolution Lidar was used to measure the optical depth of clouds at a wavelength of 532 nm, while the GOES and AVHRR window channel imagers provided measurements at a wavelength of 10.8 µm. In single-layer cirrus clouds with a visible optical depth greater than 0.3, the ratio of the visible to the IR optical depth was consistent with the approximate 2:1 ratio expected in clouds comprised of large ice crystals. For clouds with visible optical depths <0.3, the visible/IR ratios were nearly always <2. It is likely that this reflects a measurement bias rather than a difference in cloud properties. Most cirrus clouds observed in this study were more than 1 km thick and were often comprised of multiple layers. Supercooled liquid water layers coexisted with the cirrus in 32% of the cases exam...

An Application of a Geostationary Satellite Rain Estimation Technique to an Extratropical Area

Abstract The use of geostationary satellite data for estimating precipitation in non-tropical areas was explored with data taken in Montreal, Canada. The previous studies using geostationary images for rain estimation have concentrated primarily on tropical clouds (Griffith et al., 1978; Stout, et al., 1979). The intent of this study was to evaluate the applicability of using these data and techniques in other geographical areas. The Montreal area provided a wide range of weather situations common to midlatitudes for which the techniques could be tested. Because of the many variables in this area (different cloud types, moisture availability, temperature vertical structure and others) the rain rates of the cloud areas varied. Large differences in rain rates between the days studies in Montreal were found. The Montreal data also had rain rates that were considerably smaller than found in the tropical studies. To explain these differences the environments of the clouds were investigated using sounding data....