General Circulation Model Precipitation Research Articles

Simulation of streamflow in a macroscale watershed using general circulation model data

General circulation models (GCMs) currently perform vertical water and energy balances at 20‐ or 30‐min time intervals for grid points 2°–4° apart but generally contain no information on the land‐phase transfer of water between grid points or within watersheds. As a result, they operate on an incomplete hydrological cycle. This study combines a hydrological model with a GCM for a macroscale watershed. A water balance was carried out at 12‐hour time intervals for a 10‐year period using the Canadian Climate Centre GCM II data set for grid points within and surrounding the 1.6×106 km2 Mackenzie River Basin in northwestern Canada. The water surpluses from each relevant grid point were accumulated to provide a simulated hydrograph at the outlet of the Mackenzie River. A hydrological model was calibrated and verified using 5 years of recorded climatological and hydrometric data as well as land cover data from classified National Oceanic and Atmospheric Administration advanced very high resolution radiometer images. The climatological outputs from the GCM (precipitation, temperature, and evaporation) were then used as inputs to the hydrological model, generating a second hydrograph for the Mackenzie River. The results show that using the hydrological model with the GCM data produces a better representation of the recorded flow regime. The study provides a means of verifying the performance of the GCM and is a first step in developing a continental‐scale hydrological model which will, ultimately, form a part of a full model of the global hydrological cycle.

Transport and residence times of tropospheric aerosols inferred from a global three‐dimensional simulation of210Pb

A global three‐dimensional model is used to investigate the transport and tropospheric residence time of210Pb, an aerosol tracer produced in the atmosphere by radioactive decay of222Rn emitted from soils. The model uses meteorological input with 4°×5° horizontal resolution and 4‐hour temporal resolution from the Goddard Institute for Space Studies general circulation model (GCM). It computes aerosol scavenging by convective precipitation as part of the wet convective mass transport operator in order to capture the coupling between vertical transport and rainout. Scavenging in convective precipitation accounts for 74% of the global210Pb sink in the model; scavenging in large‐scale precipitation accounts for 12%, and scavenging in dry deposition accounts for 14%. The model captures 63% of the variance of yearly mean210Pb concentrations measured at 85 sites around the world with negligible mean bias, lending support to the computation of aerosol scavenging. There are, however, a number of regional and seasonal discrepancies that reflect in part anomalies in GCM precipitation. Computed residence times with respect to deposition for210Pb aerosol in the tropospheric column are about 5 days at southern midlatitudes and 10–15 days in the tropics; values at northern midlatitudes vary from about 5 days in winter to 10 days in summer. The residence time of210Pb produced in the lowest 0.5 km of atmosphere is on average four times shorter than that of210Pb produced in the upper atmosphere. Both model and observations indicate a weaker decrease of210Pb concentrations between the continental mixed layer and the free troposphere than is observed for total aerosol concentrations; an explanation is that222Rn is transported to high altitudes in wet convective updrafts, while aerosols and soluble precursors of aerosols are scavenged by precipitation in the updrafts. Thus210Pb is not simply a tracer of aerosols produced in the continental boundary layer, but also of aerosols derived from insoluble precursors emitted from the surface of continents. One may draw an analogy between210Pb and nitrate, whose precursor NOxis sparingly soluble, and explain in this manner the strong correlation observed between nitrate and210Pb concentrations over the oceans.

Global aspects of the Los Alamos general circulation model hydrologic cycle

The global hydrologic cycle in the Los Alamos general circulation model (GCM) is compared to available global observations. Global observations of the water vapor, water‐vapor flux and water‐vapor flux divergence are derived from the National Meteorological Center's final analysis for the period 1986–1989. The new precipitation data set of Legates and Willmott (1990) is used for the global precipitation observations. Global evaporation is derived as a residual of the precipitation and water‐vapor flux divergence. There are a number of similarities as well as discrepancies between the GCM and observations. The large‐scale nondivergent and divergent GCM circulations are remarkably similar to the observed circulations; the large‐scale GCM precipitation and evaporation patterns are also qualitatively similar to observations. Discrepancies are mainly quantitative and small‐scale in nature: the GCM atmosphere is relatively dry which results in a slightly greater evaporation and precipitation rate than is observed; the GCM South Pacific convergence zone is displaced too far to the northwest.

Assessment of NCAR general circulation model precipitation in comparison with observations

In this study precipitation fields from six different present-day climate experiments are compared to observed data. The six NCAR general circulation model experiments are: (a) prescribed sea-surface temperature (SST) variable hydrology CCM1, (b) prescribed SST fixed hydrology CCM1, (c) mixed layer CCM1, (d) prescribed SST w/ BATS CCM0B (e) mixed layer CCM0A, and (f) mean annual CCM0A. In general, predicted precipitation compares more favorably with observations for the case of prescribed SSTs, than for the overly zonal energy balance ocean experiments. However, despite shortcomings, the energy balance ocean experiments have utility for climate change experiments due to their lack of bias associated with specifying ocean temperature distributions. The most favorable comparisons are in the subtropics and mid-latitudes, followed by the tropics and poles. The model is best at reproducing large scale features such as the sub-tropical deserts and mid-latitude frontal precipitation. It is poorest at stimulating features that are dependent upon sub-resolution processes such as orographic uplift and convection. The model reproduces the major observed trends between the solstice seasons, such as lower land/ocean precipitation fractions in DJF, although many regional inaccuracies exist. Continental precipitation is generally excessive in the model, although the global combined continental and oceanic (CAO) averages are quite close to the observed value. Observed precipitation typically accounts for between 0.3 and 0.4 of the global spatial variance in the model data.

Assessment of NCAR general circulation model precipitation in comparison with observations

In this study precipitation fields from six different present-day climate experiments are compared to observed data. The six NCAR general circulation model experiments are: (a) prescribed sea-surface temperature (SST) variable hydrology CCM1, (b) prescribed SST fixed hydrology CCM1, (c) mixed layer CCM1, (d) prescribed SST w/ BATS CCM0B (e) mixed layer CCM0A, and (f) mean annual CCM0A. In general, predicted precipitation compares more favorably with observations for the case of prescribed SSTs, than for the overly zonal energy balance ocean experiments. However, despite shortcomings, the energy balance ocean experiments have utility for climate change experiments due to their lack of bias associated with specifying ocean temperature distributions. The most favorable comparisons are in the subtropics and mid-latitudes, followed by the tropics and poles. The model is best at reproducing large scale features such as the sub-tropical deserts and mid-latitude frontal precipitation. It is poorest at stimulating features that are dependent upon sub-resolution processes such as orographic uplift and convection. The model reproduces the major observed trends between the solstice seasons, such as lower land/ocean precipitation fractions in DJF, although many regional inaccuracies exist. Continental precipitation is generally excessive in the model, although the global combined continental and oceanic (CAO) averages are quite close to the observed value. Observed precipitation typically accounts for between 0.3 and 0.4 of the global spatial variance in the model data.