P. Ya. Groisman

Observed Variability and Trends in Extreme Climate Events: A Brief Review*

Variations and trends in extreme climate events have only recently received much attention. Exponentially increasing economic losses, coupled with an increase in deaths due to these events, have focused attention on the possibility that these events are increasing in frequency. One of the major problems in examining the climate record for changes in extremes is a lack of high-quality, long-term data. In some areas of the world increases in extreme events are apparent, while in others there appears to be a decline. Based on this information increased ability to monitor and detect multidecadal variations and trends is critical to begin to detect any observed changes and understand their origins.

The Accuracy of United States Precipitation Data

Abstract Precipitation measurements in the United States (as well as all other countries) are adversely affected by the gauge undercatch bias of point precipitation measurements. When these measurements are used to obtain areas averages, particularly in mountainous terrain, additional biases may be introduced because most stations are at lower elevations in exposed sites. Gauge measurements tend to be underestimates of the true precipitation, largely because of wind-induced turbulence at the gauge orifice and wetting losses on the internal walls of the gauge. These are not trivial as monthly estimates of this bias often vary from 5% to 40%. Biases are larger in winter than in summer and increase to the north in the United States due largely to the deleterious effect of the wind on snowfall. Simple spatial averaging of data from existing networks does not provide an accurate evaluation of the area-mean precipitation over mountainous terrain (e.g., over much of the western United States) since most stations...

Empirical Data on Contemporary Global Climate Changes (Temperature and Precipitation)

Abstract New data are presented on the changes of mean global surface air temperature and annual precipitation over extratropical continents of the Northern Hemisphere. Global warming occurred during the last century with a mean trend of 0.5°C/100 years. It is shown that for the same period the annual precipitation over the land in the 35°–70°N zone increased by 6%. The observed variations of precipitation coincide with the results of general circulation modeling of doubled CO2 equilibrium climate change by sign but contradict by scale.

Overcoming Biases of Precipitation Measurement: A History of the USSR Experience

Abstract Documenting the instrumentally observed precipitation climate record presents many challenges because scientists must rely on data from stations that undergo many changes in the course of their operation. Detecting changes from such networks is essential for adequate understanding of climate and global change. As an illustrative example, we review the history of the instrumentally observed precipitation in the USSR. In the USSR, similar to other countries, numerous problems must be addressed before reliable estimates of precipitation can be made. The types of problems range from inadequate and changing exposures of raingages to varying sampling periods used to measure precipitation. Using information about measurement procedures, instrument intercomparisons, and field studies, various methods have been devised to overcome biases in the measurements.

Assessing Surface–Atmosphere Interactions Using Former Soviet Union Standard Meteorological Network Data. Part II: Cloud and Snow Cover Effects

Groisman and Genikhovich developed a method to obtain direct estimates of surface turbulent heat fluxes. The authors now apply it to the territory of the former Soviet Union using the 3-/6-h data of 257 stations for the past several decades to assess the sensitivity of sensible heat flux to cloud and snow cover. This property was quantified for bare soil landscapes over the entire country. During the day, the presence of clouds is associated with low values of sensible heat flux from the surface to the atmosphere. At night (and during the day in winter in high latitudes), the sign of the effect is different, but because the direction of sensible heat flux is also different (from the atmosphere to the surface), the presence of clouds again reduces the turbulent heat exchange between the bare soil and the atmosphere. The estimates of ‘‘overall cloud effect’’ on summer sensible heat flux are compared with similar estimates from five general circulation models to assess the abilities of these GCMs to reproduce the response of this flux to cloud cover change. Snow on the ground is associated with temperature depression. When the effect of this depression is excluded, the presence of snow on the ground is generally associated with less water vapor in the lower troposphere under clear-sky conditions, while the evaporation rate and sensible heat flux are higher than average.