Aiguo Dai

The Changing Character of Precipitation

Abstract From a societal, weather, and climate perspective, precipitation intensity, duration, frequency, and phase are as much of concern as total amounts, as these factors determine the disposition of precipitation once it hits the ground and how much runs off. At the extremes of precipitation incidence are the events that give rise to floods and droughts, whose changes in occurrence and severity have an enormous impact on the environment and society. Hence, advancing understanding and the ability to model and predict the character of precipitation is vital but requires new approaches to examining data and models. Various mechanisms, storms and so forth, exist to bring about precipitation. Because the rate of precipitation, conditional on when it falls, greatly exceeds the rate of replenishment of moisture by surface evaporation, most precipitation comes from moisture already in the atmosphere at the time the storm begins, and transport of moisture by the storm-scale circulation into the storm is vital....

A Global Dataset of Palmer Drought Severity Index for 1870-2002: Relationship with Soil Moisture and Effects of Surface Warming

A monthly dataset of Palmer Drought Severity Index (PDSI) from 1870 to 2002 is derived using historical precipitation and temperature data for global land areas on a 2.58 grid. Over Illinois, Mongolia, and parts of China and the former Soviet Union, where soil moisture data are available, the PDSI is significantly correlated (r 5 0.5 to 0.7) with observed soil moisture content within the top 1-m depth during warm-season months. The strongest correlation is in late summer and autumn, and the weakest correlation is in spring, when snowmelt plays an important role. Basin-averaged annual PDSI covary closely (r 5 0.6 to 0.8) with streamflow for seven of world’s largest rivers and several smaller rivers examined. The results suggest that the PDSI is a good proxy of both surface moisture conditions and streamflow. An empirical orthogonal function (EOF) analysis of the P ← ]

Precipitation Characteristics in Eighteen Coupled Climate Models

Abstract Monthly and 3-hourly precipitation data from twentieth-century climate simulations by the newest generation of 18 coupled climate system models are analyzed and compared with available observations. The characteristics examined include the mean spatial patterns, intraseasonal-to-interannual and ENSO-related variability, convective versus stratiform precipitation ratio, precipitation frequency and intensity for different precipitation categories, and diurnal cycle. Although most models reproduce the observed broad patterns of precipitation amount and year-to-year variability, models without flux corrections still show an unrealistic double-ITCZ pattern over the tropical Pacific, whereas the flux-corrected models, especially the Meteorological Research Institute (MRI) Coupled Global Climate Model (CGCM; version 2.3.2a), produce realistic rainfall patterns at low latitudes. As in previous generations of coupled models, the rainfall double ITCZs are related to westward expansion of the cold tongue of...

Estimates of Freshwater Discharge from Continents: Latitudinal and Seasonal Variations

Annual and monthly mean values of continental freshwater discharge into the oceans are estimated at 18 resolution using several methods. The most accurate estimate is based on streamflow data from the world’s largest 921 rivers, supplemented with estimates of discharge from unmonitored areas based on the ratios of runoff and drainage area between the unmonitored and monitored regions. Simulations using a river transport model (RTM) forced by a runoff field were used to derive the river mouth outflow from the farthest downstream gauge records. Separate estimates are also made using RTM simulations forced by three different runoff fields: 1) based on observed streamflow and a water balance model, and from estimates of precipitation P minus evaporation E computed as residuals from the atmospheric moisture budget using atmospheric reanalyses from 2) the National Centers for Environmental Prediction‐National Center for Atmospheric Research (NCEP‐NCAR) and 3) the European Centre for Medium-Range Weather Forecasts (ECMWF). Compared with previous estimates, improvements are made in extending observed discharge downstream to the river mouth, in accounting for the unmonitored streamflow, in discharging runoff at correct locations, and in providing an annual cycle of continental discharge. The use of river mouth outflow increases the global continental discharge by ;19% compared with unadjusted streamflow from the farthest downstream stations. The river-based estimate of global continental discharge presented here is 37 288 6 662 km3 yr21, which is ;7.6% of global P or 35% of terrestrial P. While this number is comparable to earlier estimates, its partitioning into individual oceans and its latitudinal distribution differ from earlier studies. The peak discharges into the Arctic, the Pacific, and global oceans occur in June, versus May for the Atlantic and August for the Indian Oceans. Snow accumulation and melt are shown to have large effects on the annual cycle of discharge into all ocean basins except for the Indian Ocean and the Mediterranean and Black Seas. The discharge and its latitudinal distribution implied by the observation-based runoff and the ECMWF reanalysis-based P‐E agree well with the river-based estimates, whereas the discharge implied by the NCEP‐NCAR reanalysis-based P‐E has a negative bias.

Surface Observed Global Land Precipitation Variations during 1900–88

The authors have analyzed global station data and created a gridded dataset of monthly precipitation for the period of 1900‐88. Statistical analyses suggest that discontinuities associated with instrumental errors are large for many high-latitude station records, although they are unlikely to be significant for the majority of the stations. The first leading EOF in global precipitation fields is an ENSO-related pattern, concentrating mostly in the low latitudes. The second leading EOF depicts a linear increasing trend (;2.4 mm decade21) in global precipitation fields during the period of 1900‐88. Consistent with the zonal precipitation trends identified in previous analyses, the EOF trend is seen as a long-term increase mostly in North America, mid- to high-latitude Eurasia, Argentina, and Australia. The spatial patterns of the trend EOF and the rate of increase are generally consistent with those of the precipitation changes in increasing CO 2 GCM experiments. The North Atlantic oscillation (NAO) accounts for ;10% of December‐February precipitation variance over North Atlantic surrounding regions. The mode suggests that during high-NAO-index winters, precipitation is above normal in northern (.508N) Europe, the eastern United States, northern Africa, and the Mediterranean, while below-normal precipitation occurs in southern Europe, eastern Canada, and western Greenland. Wet and dry months of one standard deviation occur at probabilities close to those of a normal distribution in midlatitudes. In the subtropics, the mean interval between two extreme events is longer. The monthly wet and dry events seldom (probability , 5%) last longer than 2 months. ENSO is the single largest cause of global extreme precipitation events. Consistent with the upward trend in global precipitation, globally, the averaged mean interval between two dry months increased by ;28% from 1900‐44 to 1945‐88. The percentage of wet areas over the United States has more than doubled (from ;12% to .24%) since the 1970s, while the percentage of dry areas has decreased by a similar amount since the 1940s. Severe droughts and floods comparable to the 1988 drought and 1993 flood in the Midwest have occurred 2‐9 times in each of several other regions of the world during this century.

Effects of Clouds, Soil Moisture, Precipitation, and Water Vapor on Diurnal Temperature Range

The diurnal range of surface air temperature (DTR) has decreased worldwide during the last 4‐5 decades and changes in cloud cover are often cited as one of the likely causes. To determine how clouds and moisture affect DTR physically on daily bases, the authors analyze the 30-min averaged data of surface meteorological variables and energy fluxes from the the First International Satellite Land Surface Climatology Project Field Experiment and the synoptic weather reports of 1980‐1991 from about 6500 stations worldwide. The statistical relationships are also examined more thoroughly in the historical monthly records of DTR, cloud cover, precipitation, and streamflow of this century. It is found that clouds, combined with secondary damping effects from soil moisture and precipitation, can reduce DTR by 25%‐50% compared with clear-sky days over most land areas; while atmospheric water vapor increases both nighttime and daytime temperatures and has small effects on DTR. Clouds, which largely determine the geographic patterns of DTR, greatly reduce DTR by sharply decreasing surface solar radiation while soil moisture decreases DTR by increasing daytime surface evaporative cooling. Clouds with low bases are most efficient in reducing the daytime maximum temperature and DTR mainly because they are very effective in reflecting the sunlight, while middle and high clouds have only moderate damping effects on DTR. The DTR reduction by clouds is largest in warm and dry seasons such as autumn over northern midlatitudes when latent heat release is limited by the soil moisture content. The net effects of clouds on the nighttime minimum temperature is small except in the winter high latitudes where the greenhouse warming effect of clouds exceeds their solar cooling effect. The historical records of DTR of the twentieth century covary inversely with cloud cover and precipitation on interannual to multidecadal timescales over the United States, Australia, midlatitude Canada, and former U.S.S.R., and up to 80% of the DTR variance can be explained by the cloud and precipitation records. Given the strong damping effect of clouds on the daytime maximum temperature and DTR, the well-established worldwide asymmetric trends of the daytime and nighttime temperatures and the DTR decreases during the last 4‐5 decades are consistent with the reported increasing trends in cloud cover and precipitation over many land areas and support the notion that the hydrologic cycle has intensified.

Changes in Continental Freshwater Discharge from 1948 to 2004

Abstract A new dataset of historical monthly streamflow at the farthest downstream stations for the world’s 925 largest ocean-reaching rivers has been created for community use. Available new gauge records are added to a network of gauges that covers ∼80 × 106 km2 or ∼80% of global ocean-draining land areas and accounts for about 73% of global total runoff. For most of the large rivers, the record for 1948–2004 is fairly complete. Data gaps in the records are filled through linear regression using streamflow simulated by a land surface model [Community Land Model, version 3 (CLM3)] forced with observed precipitation and other atmospheric forcings that are significantly (and often strongly) correlated with the observed streamflow for most rivers. Compared with previous studies, the new dataset has improved homogeneity and enables more reliable assessments of decadal and long-term changes in continental freshwater discharge into the oceans. The model-simulated runoff ratio over drainage areas with and witho...

Estimates of the Global Water Budget and Its Annual Cycle Using Observational and Model Data

Abstract A brief review is given of research in the Climate Analysis Section at NCAR on the water cycle. Results are used to provide a new estimate of the global hydrological cycle for long-term annual means that includes estimates of the main reservoirs of water as well as the flows of water among them. For precipitation P over land a comparison among three datasets enables uncertainties to be estimated. In addition, results are presented for the mean annual cycle of the atmospheric hydrological cycle based on 1979–2000 data. These include monthly estimates of P, evapotranspiration E, atmospheric moisture convergence over land, and changes in atmospheric storage, for the major continental landmasses, zonal means over land, hemispheric land means, and global land means. The evapotranspiration is computed from the Community Land Model run with realistic atmospheric forcings, including precipitation that is constrained by observations for monthly means but with high-frequency information taken from atmosphe...

The Diurnal Cycle and Its Depiction in the Community Climate System Model

Abstract To evaluate the performance of version 2 of the Community Climate System Model (CCSM2) in simulating the diurnal cycle and to diagnose the deficiencies in underlying model physics, 10 years of 3-hourly data from a CCSM2 control run are analyzed for global and large-scale features of diurnal variations in surface air temperature, surface pressure, upper-air winds, cloud amount, and precipitation. The model-simulated diurnal variations are compared with available observations, most of which were derived from 3-hourly synoptic reports and some new results are reported for surface air temperatures. The CCSM2 reproduces most of the large-scale tidal variations in surface pressure and upper-air winds, although it overestimates the diurnal pressure tide by 20%–50% over low-latitude land and underestimates it over most oceans, the Rockies, and other midlatitude land areas. The CCSM2 captures the diurnal amplitude (1°–6°C) and phase [peak at 1400–1600 local solar time (LST)] of surface air temperature ove...

Simulation of Global Land Surface Conditions from 1948 to 2004. Part I: Forcing Data and Evaluations

Abstract Because of a lack of observations, historical simulations of land surface conditions using land surface models are needed for studying variability and changes in the continental water cycle and for providing initial conditions for seasonal climate predictions. Atmospheric forcing datasets are also needed for land surface model development. The quality of atmospheric forcing data greatly affects the ability of land surface models to realistically simulate land surface conditions. Here a carefully constructed global forcing dataset for 1948–2004 with 3-hourly and T62 (∼1.875°) resolution is described, and historical simulations using the latest version of the Community Land Model version 3.0 (CLM3) are evaluated using available observations of streamflow, continental freshwater discharge, surface runoff, and soil moisture. The forcing dataset was derived by combining observation-based analyses of monthly precipitation and surface air temperature with intramonthly variations from the National Center...