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H. Mark Helfand

Abstract

Analysis of the simulation of seasonal change by the GLAS model of the general circulation reveals deficiencies in the simulation of tropospheric temperature and of convective cloud cover. These interrelated deficiencies are due to a spurious doubling from January to July in the convective cloud cover of the Northern Hemisphere. The spurious doubling, in turn, is due to the oversensitivity of cumulus convection, in the GLAS model, to the specific humidity of the lower atmosphere. The oversensitivity is enhanced by a feedback mechanism which perpetuates the existence of deep, penetrative convective clouds at certain preferred locations.

The cumulus parameterization scheme has been modified to more realistically relate the onset of cumulus convection to the relative humidity of the lower atmosphere. The modified parameterization has improved the simulation of tropospheric temperature, planetary albedo and convective cloud cover as well as their seasonal variations. Comparison of this experiment with its control has shown a high degree of interrelation among these fields in the GLAS model and has demonstrated the sensitivity of the atmospheric heat budget to the design of the cumulus parameterization scheme. Also, the modification to the cumulus scheme has demonstrated a plausible mechanism to explain the correlation between convective cloud cover and relative humidity in the real atmosphere.

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H. Mark Helfand

Abstract

A simple scheme to parameterize the vertical mixing of horizontal momentum by cumulus convection has been added to the GLAS model of the general circulation. This cumulus friction term had little effect on the simulation of the general circulation, however, until deep, “hot tower” clouds were added to the model's cumulus cloud spectrum. Therefore, two January simulations have been run with the new, extended cumulus spectrum: an experiment with cumulus friction and a control without cumulus friction. Cumulus friction has strengthened the winter Hadley circulation in the experiment by 14% and has smoothed the mean meridional wind field. The zonally averaged, mean westerly shear of the tropics has decreased, and the tropical “bulge” of the jet stream has diminished. Contrary to the results of previous simulations by the GLAS model, which have included a significant amount of vertical momentum mixing, eddy kinetic energy has actually increased slightly in the present experiment.

The results of this experiment support the idea that the intensity of the mean meridional circulation is regulated by the atmosphere's angular momentum budget. Changes in the zonally-averaged, mean Coriolis force correlate well with the new cumulus friction term. The intensification of the winter Hadley circulation is a direct response of the mean meridional flow field to the downward cumulus flux of relative angular momentum in the winter hemisphere.

The figure obtained for the strengthening of the Hadley cell by cumulus convection is in line with Schneider's (1977) results. The stronger Hadley circulation and most of the structural changes of the mean meridional wind field render the GLAS simulation closer to Oort and Rasmusson's (1970, 1971) observations. A more realistic simulation of the mean meridional wind field, however, will require a more realistic mean zonal wind field as well as an improved cumulus parameterization scheme which realistically predicts very deep clouds and which makes full use of the available vertical resolution of the GLAS model.

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H. Mark Helfand and Eugenia Kalnay

Abstract

A simple mechanism is proposed to help explain the observed presence in the atmosphere of open or closed cellular convection. If convection is produced by cooling concentrated near the top of the cloud layer, as in radiative cooling of stratus clouds, it develops strong descending currents which are compensated by weak ascent over most of the horizontal area, and closed cells result. Conversely, heating concentrated near the bottom of a layer, as when an air mass is heated by warm water, results in strong ascending currents compensated by weak descent over most of the area, or open cells. This mechanism, unlike that of Hubert (1966), does not invoke vertical variations of the eddy diffusion coefficients, and is similar to the one suggested by Stommel (1962) to explain the smallness of the oceans’ sinking regions.

This mechanism is studied numerically by means of a two-dimensional, nonlinear Boussinesq model. For this purpose we define open (closed) convection as a convective circulation pattern in which the majority of the fluid has a descending (ascending) motion. An internal heat source-sink destabilizes a layer of fluid adding no net heating. A steady state is attained. The resulting circulation is closed, as expected, when the cooling is concentrated near the upper surface, and the heating is spread throughout the lower region. The mean lapse rate is unstable in the upper half of the fluid and stable in its lower half. Conversely, the circulation is open when heating is concentrated near the bottom. In this case, the lower half of the fluid has an unstable mean lapse rate, and the upper half of the fluid is stable.

The numerical results indicate that the width of the plume produced by the cooling in the upper part of the layer or by the heating in the lower part of the layer is largely independent of the degree of vertical asymmetry of the internal heating profile. On the other hand, the compensating motion occupies a region which becomes broader as the heating profile becomes more asymmetric. In other words, if cooling is very concentrated near the top of the layer with heating spread throughout the rest of the region or if heating is very concentrated near the bottom with cooling spread throughout, the generated closed or open cells have an aspect ratio much larger than 1. These results may help explain the large aspect ratios observed in atmospheric convection.

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H. Mark Helfand and Siegfried D. Schubert

Abstract

The Great Plains region of the United States is characterized by some of the most frequent and regular occurrences of a nocturnal low-level jet (LLJ). While the LLJ is generally confined to the lowest Kilometer of the atmosphere, it may cover a substantial region of the Great Plains, and typically reaches maximum amplitudes of more than 20 m s−1.

A two-month, springtime simulation with the Goddard Earth Observing System (GEOS-1) atmospheric general circulation model (AGCM) has produced a Great Plains LLJ with a vertical and temporal structure, directionality, and climatological distribution that compare favorably with observations. The diurnal cycle of the low-level flow is dramatic and coherent over a subcontinental area that includes much of the western United States and northern Mexico. This cycle can be interpreted as the nightly intrusion of the anticyclonic, subtropical gyre (associated with the Bermuda high) into the North American continent as surface friction decreases. The AGCM also simulates a pair of northerly LLJ maxima off the California coast, which seem to correspond to observations of a so-called “Baja Jet.” Other apparently related diurnal variations extending well into the upper troposphere are documented and compared with observations.

The time-averaged climatological picture of the low-level flow is dominated over land by the nocturnal phase of the diurnal cycle, in which surface friction is minimal and wind speeds are strongest. This pattern, with its zones of strong convergence, is characteristic of an unsteady, strongly forced flow. Over the open ocean, the mean low-level flow is more reminiscent of a smooth, climatological pattern.

Analysis of the simulated moisture budget for the continental United States reveals a horizontally confined region of strong southerly moisture transport with a strong diurnal cycle in the region of the Great Plains LLJ, as has been found in observations of water vapor transport. The LLJ plays a key role in that budget by transporting almost one-third of all the moisture that enters the continental United States with most of the influx from the LLJ (slightly less than two-thirds of it) entering during the 12 nighttime hours. However, it is the mean flow pattern and not covariances associated with the diurnal cycle that contribute most significantly to the total time-mean moisture transport. Covariances on the synoptic and longer timescales contribute only about one-fifth of the total time-mean transport of moisture in the jet region, and covariances on the diurnal timescale are negative and negligible despite the strong diurnal signal in the wind.

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H. Mark Helfand and Juan Carlos Labraga

Abstract

No abstract available.

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Siegfried D. Schubert, H. Mark Helfand, Chung-Yu Wu, and Wei Min

Abstract

Subseasonal variations in warm-season (May–August) precipitation over the central and eastern United States are shown to be strongly linked to variations in the moisture entering the continent from the Gulf of Mexico within a longitudinally confined “channel” (referred to here as the Texas corridor or TC). These variations reflect the development of low-level southerly wind maxima (or jets) on a number of different timescales in association with distinct subcontinental and larger-scale phenomena. On the diurnal timescale, the TC moisture flux variations are tied to the development of the Great Plains low-level jet. The composite nighttime anomalies are characterized by a strong southerly moisture flux covering northeast Mexico and the southern Great Plains, and enhanced boundary layer convergence and precipitation over much of the upper Great Plains. The strongest jets tend to be associated with an anomalous surface low over the Great Plains, reflecting a predilection for periods when midlatitude weather systems are positioned to produce enhanced southerly flow over this region. On subsynoptic (2–4 days) timescales the TC moisture flux variations are associated with the development and evolution of a warm-season lee cyclone. These systems, which are most prevalent during the early part of the warm season (May and June), form over the central Great Plains in association with an upper-level shortwave and enhanced upper-tropospheric cross-mountain westerly flow. A low-level southerly wind maximum or jet develops underneath and perpendicular to the advancing edge of enhanced midtropospheric westerlies. The clash of anomalous southerly moisture flux and a deep intrusion of anomalous northerly low-level winds results in enhanced precipitation eventually stretching from Texas to the Great Lakes. On synoptic (4–8 days) timescales the TC moisture flux variations are associated with the propagation and intensification of a warm-season midlatitude cyclone. This system, which also occurs preferentially during May and June, develops offshore and intensifies as it crosses the Rocky Mountains and taps moisture from the Gulf of Mexico. Low-level southerly wind anomalies develop parallel to the mid- and upper-level winds on the leading edge of the trough. Widespread precipitation anomalies move with the propagating system with reduced rainfall occurring over the anomalous surface high, and enhanced rainfall occurring over the anomalous surface low. On still longer timescales (8–16 days) the variations in the TC moisture transport are tied to slow eastward-moving systems. The evolution and structure of the mid- and low-level winds are similar to those of the synoptic-scale system with, however, a somewhat larger zonal scale and spatially more diffuse southerly moisture flux and precipitation anomalies.

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