William P. Elliott

On the utility of radiosonde humidity archives for climate studies

This paper considers the use of upper-air data from radiosondes in long-term climate studies. The accuracy and precision of radiosonde humidity measurements, including temperature and pressure measurements used in calculating them, and their effects on the precision of reported and derived variables are estimated. Focusing on the U.S. radiosonde system, we outline the history of changes in instruments and reporting practices and attempt to assess the implications of such changes for studies of temporal variations in lower-tropospheric water vapor. Changes in biases in the data are highlighted, as these can lead to misinterpretation of climate change. We conclude that the upper-air data record for the United States is not homogeneous, especially before 1973. Because of problems with the humidity data in cold, dry conditions, the water vapor climatology in the upper troposphere, nominally above the 500-mb level, is not well known.

Tropospheric Water Vapor Climatology and Trends over North America: 1973–93

Abstract Here 21 years of radiosonde observations from stations in the Western Hemisphere north of the equator were analyzed for trends in tropospheric water vapor. Mean fields of precipitable water and relative humidity at several levels we shown. Annual trends of surface-500 mb precipitable water were generally increasing over this region except over northeastern Canada. When trends were expressed as a percentage of the climatological mean at each station, the trends south of ∼45°N represent a linear rate of increase of 3%–7% decade−1. Trends in the upper portion of this layer, 700–500 mb, were as large or larger than those of the middle (850–700 mb) or lower layer and were consistent in sign. Annual trends in dewpoint generally agree in sign with trends in temperature. However, the dewpoint trends tended to be larger than those of temperature. This was consistent with the annual increases found in relative humidity over this period. Relative humidity increased except in Canada, Alaska, and a few statio...

Radiosonde-Based Northern Hemisphere Tropospheric Water Vapor Trends

Abstract Trends in tropospheric water vapor at Northern Hemisphere radiosonde stations are presented for two periods;1973–95 and 1958–95. Stations with incomplete or inhomogeneous temporal records were identified and excluded from the analysis. For the 1973–95 period, trends in surface–500-mb precipitable water and in specific humidity, dewpoint, and temperature at the 850-mb level are shown. At most stations in this analysis, precipitable water, specific humidity, and dewpoint temperature have increased along with temperature over the period. An exception is Europe, over which temperature increased but humidity slightly decreased. Water vapor increases are larger, more uniform, and more significant over North America than over Eurasia, and the differences in trend magnitude and sign between the two regions may be attributable to changes in the late 1970s that affected North America more than Eurasia. Seasonal and annual correlations of surface–500-mb precipitable water with temperature, dewpoint temperat...

Space and Time Scales of Global Tropospheric Moisture

Abstract Radiosonde data from a global 118-station network are used to determine the spatial and temporal scales of variability of tropospheric water vapor. Various sources of possible error and bias in the data are analyzed. Changes in instrumentation at U.S. stations are shown to have a considerable influence on the record; information on comparable changes in other countries is not readily available. Mean monthly data are shown to be acceptable at tropical nations but not at high-latitude stations, where the nonlinear dependence of saturation vapor pressure on temperature, coupled with large temperature ranges, leads to biases of up to 10% in mean monthly specific humidity. A series of three empirical orthogonal function analyses (for the tropics, North America, and the globe) of specific humidity at the surface, 850-mb, 700-mb, and 500-mb levels is presented. All three show evidence of a shift in the specific humidity field in the winter of 1976/77, with generally lower values from the beginning of th...

Relationships between tropospheric water vapor and surface temperature as observed by radiosondes

Using radiosonde data from 50 stations for 1973–1990, we quantify relationships between surface air temperature (Ts) and precipitable water vapor (W) for different time scales. Monthly mean observations are fairly well described by an equation of the form ln W = A + B Ts, but the coefficients A and B depend on the Ts range considered. At high Ts, the relationship is poor. This relationship and relationships between sea surface temperature (SST) and W based on satellite microwave observations over oceans are in remarkably good agreement over restricted SST ranges. Monthly and annual anomalies of W and Ts are well correlated only outside the tropics, but on longer time scales, there is some evidence of positive trends in both W and Ts at most of the stations studied. Thus the relationship between W and Ts depends on the time scales and geographic region considered.

Recent Changes in NWS Upper-Air Observations with Emphasis on Changes from VIZ to Vaisala Radiosondes

Abstract Recent changes in the NWS upperair observing network are listed and an analysis is presented of the change in 1995 from VIZ to Vaisala radiosonde instruments. Results are shown for 14 stations at 850, 700, 500, 100, and 50 hPa and atboth 0000 and 1200 UTC. This change in radiosonde type resulted in detectable shifts in the means of temperature, height, dewpoint, and relative humidity at the time of the change. The largest differences in temperature occur at the 100- and50-hPa levels and are likely related to radiation effects on the sensors. However, the magnitude and, in some cases, the sign of the difference was found to vary depending on pressure level, time of day, region, and season. Thus, no singlenetworkwide adjustment to eliminate artificial shifts seems possible.

Annual cycles of tropospheric water vapor

To understand better the annual cycles of atmospheric humidity, radiosonde data were used to create climatologies of temperature, dew point, relative humidity, and precipitable water in the lower troposphere for 56 locations around the world for the period 1973–1990. On the basis of the annual ranges of relative humidity at the surface and at the 850, 700, and 500 mbar levels and the ratio of the annual maximum to minimum surface to 500-mbar precipitable water, we have defined five humidity regimes: (1) middle- and high-latitude continental, (2) middle- and high-latitude oceanic, (3) midlatitude monsoon, (4) tropical oceanic, and (5) tropical monsoon. For each regime we describe the annual cycles of temperature and humidity variables and discuss phase relationships among them. Relative humidity ranges are small in the first two regimes, where precipitable water and temperature vary in phase. Relative humidity ranges in the other three regimes are moderate to large, and in the tropics the annual march of horizontal moisture advection and vertical convection, not temperature, controls seasonal humidity variations. These results suggest that the assumption of constant relative humidity made in some climate models is not always justified and that precipitable water is not a strong function of temperature in the tropics.

Variations of cloudiness, precipitable water, and relative humidity over the United States : 1973-1993

Water vapor variables were calculated from radiosonde stations in each of six U.S. regions for which cloudiness observations were available. Associations among variations of cloudiness, precipitable water and relative humidity during the 1973–93 period over the U.S. as a whole and in the six regions were determined. Trends in the quantities as well as relations among seasonal variations have been examined. There was a general increase in precipitable water in each season and in each region and a suggestion of increases in relative humidity. These were not accompanied by a general increase in cloudiness that was significant. However, there were significant correlations between seasonal anomalies of cloudiness and both relative humidity and precipitable water.

Regional trends of surface and tropospheric temperature and evening‐morning temperature difference in northern latitudes: 1973–93

Trends for the 21-year period 1973–1993 of temperatures at the surface and in the troposphere were analyzed from radiosonde observations in eight north-latitude regions. The seasonal consistency of the surface and tropospheric trends of temperature and evening-morning temperature difference was examined for these regions. In most seasons the surface temperature trends were positive in Eurasia and western N. America but were negative in central N. America and eastern Canada. The regions with sizable and statistically significant surface temperature trends usually had significant tropospheric trends up to 500 mb and sometimes to 300 mb. An exception was central Asia, where the strong warming was confined below 850 mb. The significant tropospheric trends showed a tendency to decrease in magnitude with height. Trends of the evening minus morning temperature difference were also analyzed in two sectors, Canada and Russia, where the radiosonde launches occurred at 5am and pm ±2hrs. Trends were fairly consistent in sign between the surface and lower troposphere in both sectors and up to 300 mb over Canada. Most Canadian trends were significant and represented a decreasing range between evening and morning temperatures. Significant Russian trends were primarily in summer.

Lower-Tropospheric Humidity–Temperature Relationships in Radiosonde Observations and Atmospheric General Circulation Models

Abstract Annual and seasonal correlations between temperature and both specific and relative humidity are presented based on radiosonde station data over the tropical Pacific Ocean and North America. Results are presented for the surface and the 850-, 700-, and 500-hPa levels. The correlations between anomalies of temperature and relative humidity are generally negative, and those between temperature and specific humidity are generally positive. Longitudinal differences in the pattern of correlations are found both in low latitudes and over midlatitude North America. In particular, near-zero or negative temperature–specific humidity correlations are found in the western United States at and below 700 hPa (especially in summer) and over the western tropical Pacific at 700 and 500 hPa (especially in winter). The observed correlation patterns are compared with those of 12 atmospheric general circulation model (AGCM) simulations. Simulated high-latitude correlation patterns qualitatively agree with observatio...