Rebecca J. Ross

Climatology and Trends of U.S. Surface Humidity and Temperature

Abstract Climatological annual and seasonal dewpoint, specific humidity, and relative humidity maps for the United States are presented using hourly data from 188 first-order weather stations for the period 1961–90. Separate climatologies were calculated for daytime (three observations per day between 0800 and 1600 LST), nighttime (three observations per day between 2000 and 0400 LST), and the full day (eight observations per day, every 3 h). With extended datasets for the period 1961–95, trends in these same variables and temperature are calculated for each of 170 stations and for eight regions of the country. The data show increases in specific humidity of several percent per decade, and increases in dewpoint of several tenths of a degree per decade, over most of the country in winter, spring, and summer. Nighttime humidity trends are larger than daytime trends. The specific humidity increases are consistent with upward temperature trends. The upward temperature and humidity trends are also consistent w...

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...

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.

Effects on Climate Records of Changes in National Weather Service Humidity Processing Procedures

Abstract The U.S. National Weather Service has recently corrected an error in radiosonde humidity data reduction algorithms, eliminated a sonde thats processing contained another error, and recently made a further change in the humidity data reduction algorithm. They also introduced new procedures for reporting humidity from radiosondes. These changes will affect the climate records of U.S. upper-air humidity. Because the changes affect observations differently at different temperatures and relative humidities, their impact on monthly mean humidity quantities depends on the frequency with which certain conditions occur. The authors calculated the effects on monthly mean humidity quantities of five specific changes at a representative set of U.S. radiosonde stations. Some of these effects are quite substantial and should be considered in evaluating climate variability.

Estimated Impacts on Climate Records of Adaptive Strategies for Scheduling Radiosondes

Abstract Possible changes to the North American Atmospheric Observing System that are under discussion include the scheduling of radiosonde launches according to synoptic conditions. The authors estimate biases in long-term monthly mean tropospheric temperature and humidity resulting from reducing the number of radiosonde launches at a site depending on synoptic conditions. The sign and magnitudes of the biases vary considerably with geographic location and sometimes with season. Such biases would contaminate the upper-air climatology and complicate the calculation of climate trends. Approaches for minimizing these biases are explored.