Search Results

You are looking at 1 - 5 of 5 items for :

  • Author or Editor: Y. C. Sud x
  • Monthly Weather Review x
  • Refine by Access: All Content x
Clear All Modify Search
Y. C. Sud
and
A. Molod

Abstract

The GLA (Goddard Laboratory for Atmospheres) GCM (general circulation model) was employed to investigate the influence of surface albedo and evapotranspiration anomalies that could result from the hypothetical semiarid vegetation over North Africa (including the Sahara desert) on its July circulation and rainfall. In the first experiment a soil moisture anomaly was prescribed over North Africa, whereas in the second experiment a soil moisture plus surface albedo anomaly was prescribed over North Africa. These two experiments used the first version of the GCM with the old parameterization of evaporation from failing rain drops and were compared with a control run that was made with climatologically normal boundary conditions. The third experiment had the soil moisture and surface albedo anomalies of the second experiment and was run with the second version of the model that included a recently modified parameterization of evaporation of falling rain. It was compared to its control that had climatologically normal boundary conditions.

The results of the first experiment show that the increased soil moisture and its dependent evapotranspiration produces a cooler and moister PBL over North Africa that is able to support enhanced moist convection and rainfall in Sahel and southern Sahara. The results of the second experiment show that the lower surface albedo yields even higher moist static energy in the PBL and further enhances the local moist convection and rainfall. The third experiment, with the modified rain-evaporation parameterization, produces hydrological cycle and accompanying rainfall anomalies that were quite similar to those of the second experiment specifically over the anomaly region; however, some differences between the second and third experiments were evident in distant regions. These differences suggest the importance of a different and/or a better parameterization of falling rain.

Full access
Y. C. Sud
and
G. K. Walker

Abstract

A rain evaporation and downdraft parameterization is designed to complement the cumulus convection scheme of the Goddard Laboratory for Atmospheres General Circulation Model (GLA GCM). The scheme invokes (i) a diagnostic determination of the commencement level of rain-evaporation-induced downdrafts, (ii) a method for calculating downdraft mass fluxes emanating from different levels of the atmosphere, and (iii) an explicitly prescribed overall fraction of rain evaporation within the downdraft.

The parameterization was tested with the GATE [GARP (Global Atmospheric Research Program) Atlantic Tropical Experiment] phase III data in a fully prognostic mode and with the entire atmospheric and surface forcings prescribed with data. It was found that the near-surface downdraft cooling largely mitigates the observed surface sensible heating. In the absence of this cooling, the boundary layer must get rid of the surface heat flux by spurious turbulent transport, which becomes significant in simulations that ignore both the rain evaporation and downdrafts. The time mean as well as root-mean-square errors in the vertical temperature profiles are somewhat larger for simulations without downdrafts and are much larger for simulations without both downdrafts and rain evaporation. The downdrafts are found to produce a useful correction in the simulated near-surface temperature and humidity in GCM simulations, and in that way, the parameterization improves the simulation of tropospheric temperature and humidity. In a one-year comparison of GLA GCM simulations with and without downdrafts, the former produced better precipitation climatology and surface temperatures.

Full access
N. Wolfson
,
R. Atlas
, and
Y. C. Sud

Abstract

A series of numerical forecast experiments has been conducted with the Goddard Laboratory for Atmospheres (GLA) fourth-order general circulation model in order to study the role of (i) sea-surface temperature (SST) anomalies in the North Pacific, (ii) soil moisture anomalies over the continental United States, as derived from observations of precipitation and surface temperature, and (iii) solar radiational forcing in the maintenance and breakdown of the severe heat wave over the United States in the summer of 1980.

Results from these experiments show opposite effects of the SST and soil moisture anomalies on the model's medium-range (ten-day) simulations of the maintenance of the heat wave. The lower than normal soil moisture over the United States during the summer of 1980 resulted in reduced surface evaporation, higher ground temperature, increased sensible heat flux from ground to air, higher surface temperature, lower sea-level pressure and higher 500 mb height over much of the Great Plains in the model forecasts. In contrast, the SST anomalies in the North Pacific during the same period resulted in an enhanced northerly flow of cooler and dryer air, increased evaporation, decreased ground and air temperature and reduced 500-mb heights over the Great Plains. These results suggest that once established, soil moisture deficits contribute to maintaining warm, dry conditions. Although long-term effects of North Pacific SST anomalies may be to create or enhance the heat wave, ten-day simulations showed that such anomalies contributed to lower temperatures over shorter time scales.

A limited number of experiments, with modified solar radiational forcing, showed a dramatic weakening of the heat wave pattern in the model forecasts and indicated that the interaction of changing solar declination with the prevailing synoptic situation was probably responsible for the breakdown of the heat wave in September 1980.

Full access
Y. C. Sud
,
Winston C. Chao
, and
G. K. Walker

Abstract

A coarse (4° &times 5° × 9-sigma level) version of the Goddard Laboratory for Atmospheres (GLA) General Circulation Model (GCM) was used to investigate the influence of a cumulus convection scheme on the simulated atmospheric circulation and hydrologic cycle. Two sets of integrations, each containing an ensemble of three summer (June, July, and August) simulations, were produced. The first set, containing control cases, included a state-of-the-art cumulus parameterization scheme in the GCM; whereas the second set, containing experiment cases, used the same GCM but without the cumulus parameterization. All simulations started from initial conditions that were taken from analysis of observations for three consecutive initial times that wore only 12 h apart beginning with 0000 UTC 19 May 1988. The climatological boundary conditions—sea surface temperature, snow, ice, and vegetation cover-were kept exactly the same for all the integrations. The ensemble sets of control and experiment simulations are control and differentially analyzed to determine the influence of a cumulus convection scheme on the simulated circulation and hydrologic cycle.

The results show that cumulus parameterization has a very significant influence on the simulated circulation and precipitation. The influence is conspicuous in tropical regions, interior of continents in the Northern Hemisphere, and some oceanic regions. The upper-level condensation heating over the intertropical convergence zone (ITCZ) is much smaller for the experiment simulations as compared to the control simulations; correspondingly, the Hadley and Walker cells for the control simulations are also weaker and are accompanied by a weaker Ferrel cell in the Southern Hemisphere. The rainfall under the rising branch of the southern Ferrel cell (at about 50°S) does not increase very much because boundary-layer convergence poleward reduces the local evaporation. Overall, the difference fields show that experiment simulations (without cumulus convection) produce a cooler and less energetic atmosphere. The vertical profile of the zonally averaged diabatic heating also shows large differences in the tropics that are physically consistent with accompanying differences in circulation. Despite producing a warmer and wetter planetary boundary layer (PBL) in the tropics (20°S–20°N), the control simulations also produce a warmer but drier 400-mb level. The moisture transport convergence fields show that while only the stationary circulation is affected significantly in the PBI, both the stationary and eddy moisture transports are altered significantly in the atmosphere above the PBL. These differences no only reaffirm the important role of cumulus convection in maintaining the global circulation, but also show the way in which the presence or absence of a cumulus parameterization scheme can affect the circulation and rainfall climatology of a GCM.

Full access
Shian-Jiann Lin
,
Winston C. Chao
,
Y. C. Sud
, and
G. K. Walker

Abstract

A generalized form of the second-order van Leer transport scheme is derived. Several constraints to the implied subgrid linear distribution are discussed. A very simple positive-definite scheme can be derived directly from the generalized form. A monotonic version of the scheme is applied to the Goddard Laboratory for Atmospheres (GLA) general circulation model (GCM) for the moisture transport calculations, replacing the original fourth-order center-differencing scheme. Comparisons with the original scheme are made in idealized tests as well as in a summer climate simulation using the full GLA GCM. A distinct advantage of the monotonic transport scheme is its ability to transport sharp gradients without producing spurious oscillations and unphysical negative mixing ratio. Within the context of low-resolution climate simulations, the aforementioned characteristics are demonstrated to be very beneficial in regions where cumulus convection is active. The model-produced precipitation pattern using the new transport scheme is more coherently organized both in time and in space, and correlates better with observations. The side effect of the filling algorithm used in conjunction with the original scheme is also discussed, in the context of idealized tests.

The major weakness of the proposed transport scheme with a local monotonic constraint is its substantial implicit diffusion at low resolution. Alternative constraints are discussed to counter this problem.

Full access