Carole J. Hahn

The Effect of Moonlight on Observation of Cloud Cover at Night, and Application to Cloud Climatology

Abstract Visual observations of cloud cover are hindered at night due to inadequate illumination of the clouds. This usually leads to an underestimation of the average cloud cover at night, especially for the amounts of middle and high clouds, in climatologies based on surface observations. The diurnal cycles of cloud amounts, if based on all the surface observations, are therefore in error, but they can be obtained more accurately if the nighttime observations are screened to select those made under sufficient moonlight. Ten years of nighttime weather observations from the Northern Hemisphere in December were classified according to the illuminance of moonlight or twilight on the cloud tops, and a threshold level of illuminance was determined, above which the clouds are apparently detected adequately. This threshold corresponds to light from a full moon at an elevation angle of 6°, light from a partial moon at higher elevation, or twilight from the sun less than 9° below the horizon. It permits the use o...

A Survey of Changes in Cloud Cover and Cloud Types over Land from Surface Observations, 1971-96

From a dataset of weather observations from land stations worldwide, about 5400 stations were selected as having long periods of record with cloud-type information; they cover all continents and many islands. About 185 million synoptic reports were analyzed for total cloud cover and the amounts of nine different cloud types, for the 26-yr period 1971–96. Monthly and seasonal averages were formed for day and night separately. Time series of total-cloud-cover anomalies for individual continents show a large decrease for South America, small decreases for Eurasia and Africa, and no trend for North America. The largest interannual variations (2.7%) are found for Australia, which is strongly influenced by ENSO. The zonal average trends of total cloud cover are positive in the Arctic winter and spring, 60°–80°N, but negative in all seasons at most other latitudes. The global average trend of total cloud cover over land is small, 0.7% decade 1 , offsetting the small positive trend that had been found for the ocean, and resulting in no significant trend for the land–ocean average. Significant regional trends are found for many cloud types. The night trends agree with day trends for total cloud cover and for all cloud types except cumulus. Cirrus trends are generally negative over all continents. A previously reported decline in total cloud cover over China and its neighbors appears to be largely attributable to high and middle clouds. Global trends of the cloud types exhibit trade-offs, with convective cloud types increasing at the expense of stratiform clouds, in both the low and middle levels. Interannual variations over Europe, particularly of nimbostratus, are well correlated with the North Atlantic Oscillation; significant correlations are also found across northern Asia. Interannual variations in many parts of the Tropics are well correlated with an ENSO index. Little correlation was found with an index of smoke aerosol, in seven regions of seasonal biomass burning. In the middle latitudes of both hemispheres, seasonal anomalies of cloud cover are positively correlated with surface temperature in winter and negatively correlated in summer, as expected if the direction of causality is from clouds to temperature.

Simultaneous Occurrence of Different Cloud Types

Abstract Cloud observations from land stations and from ships in the ocean are used to investigate the frequency of observation and the co-occurrence of different cloud types, and the geographical and seasonal variations of these co-occurrences. Ground-based observations are used because they provide a more definitive identification of clouds by type than do satellite observations. The clouds are grouped into six types (cirrus + cirrostratus + cirrocumulus, altostratus + altocumulus, stratus + stratocumulus, nimbostratus, cumulus, and cumulonimbus). The results are expressed as frequency of occurrence of different cloud types and as contingency probabilities; that is, given that one cloud type is present, the probability that another particular type is also present is computed. Several sources of bias are identified, and their effects on the results are estimated. It is found that, on the average at all latitudes and in all seasons, clear skies occur more frequently, by a factor of about 4, over land than...

ISCCP Cloud Properties Associated with Standard Cloud Types Identified in Individual Surface Observations

Individual surface weather observations from land stations and ships are compared with individual cloud retrievals of the International Satellite Cloud Climatology Project (ISCCP), stage C1, for an 8-yr period (1983‐ 91) to relate cloud optical thicknesses and cloud-top pressures obtained from satellite data to the standard cloud types reported in visual observations from the surface. Each surface report is matched to the corresponding ISCCP-C1 report for the time of observation for the 280 km 3 280 km grid box containing that observation. Classes of the surface reports are identified in which a particular cloud type was reported present, either alone or in combination with other clouds. For each class, cloud amounts from both surface and C1 data, base heights from surface data, and the frequency distributions of cloud-top pressure ( pc) and optical thickness (t ) from C1 data are averaged over 158 latitude zones, for land and ocean separately, for 3-month seasons. The frequency distribution of pc and t is plotted for each of the surface-defined cloud types occurring both alone and with other clouds. The average cloud-top pressures within a grid box do not always correspond well with values expected for a reported cloud type, particularly for the higher clouds Ci, Ac, and Cb. In many cases this is because the satellites also detect clouds within the grid box that are outside the field of view of the surface observer. The highest average cloud tops are found for the most extensive cloud type, Ns, averaging 7 km globally and reaching 9 km in the ITCZ. Nimbostratus also has the greatest average retrieved optical thickness, t 20. Cumulonimbus clouds may actually attain far greater heights and depths, but they do not fill the grid box. The t ‐p c distributions show features that distinguish the high, middle, and low clouds reported by the surface observers. However, the distribution patterns for the individual low cloud types (Cu, Sc, St) occurring alone overlap to such an extent that it is not possible to distinguish these cloud types from each other on the basis of t ‐p c values alone. Other cloud types whose t ‐p c distributions are indistinguishable are Cb, Ns, and thick As. However, the t ‐p c distribution patterns for the different low cloud types are nevertheless distinguishable when all occurrences of a low cloud type are included, indicating that the different low types differ in their probabilities of co-occurrence with middle and high clouds.

Variations in Cloud Cover and Cloud Types over the Ocean from Surface Observations, 1954-2008

AbstractSynoptic weather observations from ships throughout the World Ocean have been analyzed to produce a climatology of total cloud cover and the amounts of nine cloud types. About 54 million observations contributed to the climatology, which now covers 55 years from 1954 to 2008. In this work, interannual variations of seasonal cloud amounts are analyzed in 10° grid boxes. Long-term variations O(5–10 yr), coherent across multiple latitude bands, remain present in the updated cloud data. A comparison to coincident data on islands indicates that the coherent variations are probably spurious. An exact cause for this behavior remains elusive. The globally coherent variations are removed from the gridbox time series using a Butterworth filter before further analysis.Before removing the spurious variation, the global average time series of total cloud cover over the ocean shows low-amplitude, long-term variations O(2%) over the 55-yr span. High-frequency, year-to-year variation is seen O(1%–2%).Among the cl...

A study of the diurnal behavior of boundary-layer winds at the Boulder Atmospheric Observatory

Diurnal wind variations are examined at the Boulder Atmospheric Observatory which is located 25 km east of the foothills of the Rocky Mountains. Data were obtained from a 300-m tower which was instrumented at eight levels and operated almost continuously for three weeks during September 1978. Observations on clear days, for which the diurnal heating and cooling of the local terrain slopes can be expected to affect the winds, show that daytime winds tend to be easterly (upslope) throughout the 300-m depth. At night, a temperature inversion typically develops to about 100 m. Below this level, the nocturnal flow tends to be downslope; above the inversion, a distinctly different regime of flow develops. A diurnal wind oscillation, characterized by strong southerly flow beginning near sunset and ending near midnight, occurred in the upper layer on 25% of the days during the study period. Rapid clockwise rotation of the wind vector occurred during the period of increased wind speed. This oscillation occurred only on days when the synoptic-scale geostrophic wind was southerly. It is suggested that this non-steady state behavior is an inertial oscillation affected by the diurnally varying temperature gradients and local topography.

Thirty-year trend of observed greenhouse clouds over the tropical oceans

Abstract Analysis of the 30-year trend (1952–1981) of the amounts of different cloud types, as observed from ships in the tropical oceans, has shown that, over latitudes 20°N–20°S during the time period studied, cirrus (Ci) and cumulonimbus (Cb) types have increased while cumulus (Cu) and stratus (St) types decreased or remained nearly constant. The greenhouse implications of these cloud trends are discussed.

The global distribution of observed cloudiness — a contribution to the ISCCP

Abstract Satellite-inferred overall global cloud patterns generally corroborate those derived from ground-based observations. Both show significant differences of cloudiness between the two hemispheres and over extended land as compared with ocean areas. However, the averaged latitudinal values of surface-based observed cloud amounts are about 10 percent higher than those derived from Nimbus-7 observations. The largest difference (10–20 percent) is in the subtropics of each hemisphere and at subpolar and polar latitudes during the summer. The difference in reported average global total cloud amounts is about 10 percent.

Observed Variations of Total Cloudiness and Cloud Types: Implications for the Atmospheric Radiation Budget

The results of a study of ground-based cloud observations over the period 1952–1981 for the oceans and 1971–1981 for the land are summarized here. It was found to be in agreement with Berlyand et al. (1980) that the global average total cloudiness (over land and oceans) was slightly more than 60 percent, a value that is significantly larger than that derived from earlier estimates based on considerably less observational data. Ocean areas have about 10 percent more clouds than land areas and, on average, there is about 5 percent more clouds in the Southern than in the Northern Hemisphere. The latitudinal distributions of the average total cloud amount and amounts for six different basic cloud groups are presented. The observations indicate a small increase in global total cloudiness over the 11-year period (1971–1981), as well as a marginally statistically significant increase in cirrus and cumulonimbus clouds, but a decrease in cumulus during that period.