Vertical Moisture Transport Research Articles

Long-range transport of carbon monoxide from tropical ground to upper troposphere: a case study for South East Asia in October 1997

High concentrations of carbon monoxide (CO) were observed in October 1997 at the upper troposphere of the western tropical Pacific. Transport from the potential sources of CO due to biomass burnings in the tropics was investigated by using a global chemical transport model (CTM) driven by assimilated meteorological data provided from European Centre for Medium-Range Weather Forecasts (ECMWF). A CTM evaluation simulation using water vapor showed that the amount of vertical transport of moisture by large-scale flow was consistent with the precipitation predicted at the convective zone. A series of CTM simulations using 10-day emission periods of an artificial material with lifetime of 60 days indicated that vertical lifting of surface air at the Indonesian archipelago occurred in the concentrated convections west of Sumatra Island. No evidence was found that CO from the Amazon region or Africa significantly contributed to high concentrations in the western tropical Pacific. Transport formed a large-scale anvil below the tropopause by rapid vertical transport and by divergence flow. The average time required for the transport from Kalimantan and Sumatra Island to the point of high CO concentration was about 15 days. High concentrations at an altitude of 10 km in the Southern Hemisphere were transported by large-scale subsidence from the upper tropospheric maximum, which was presumably produced from the sources at Kalimantan and Sumatra Island. Estimated emissions of CO in September and October from Kalimantan and Sumatra were substantially larger than the previous estimates. Omission of chemical reaction was a possible problem for the overestimate, but not significant. The possible problems in the transport were incorrect CTM transport due to insufficient horizontal (2.5×2.5°) and vertical resolution of the CTM, and to inaccuracy in the wind fields at the upper part of the troposphere and a divergent flow pattern in the upper part of the troposphere.

Airborne turbulence measurements in the lower troposphere onboard the research aircraft Dornier 128-6, D-IBUF

The research aircraft Dormer (DO) 128-6, call-sign D-IBUF, operated by the Institute of Flight Guidance and Control of the Technical University of Braunschweig and its scientific equipment is presented. The aircraft's operational capabilities and the quality of measurements of meteorological parameters and trace constituents in the lower troposphere are discussed. Besides avionic instrumentation for VFR-flights (Visual Flight Rules) and IFR-flights (Instrument Flight Rules) at low altitudes over complex terrain, there are redundant sensors for the measurement of wind, temperature and humidity. Together with INS- (Inertial Navigation System) and GPS- (Global Positioning System) navigation a sample frequency of 25 Hz (soon 100 Hz) is realised. Using a mean ground speed of 65 m s -1 , the resolution of the measurements is less than 3 m. Since 1998, the Institute of Meteorology and Climate Research at the Forschungszentrum Karlsruhe has enlarged the aircraft's research capabilities by the integration of a sensor package for the measurement of CO, NO, NO 2 . CO 2 and O 3 with frequencies between 1 Hz and 20 Hz, and detection limits of 1 ppb. The equipment does not only allow to detect mean quantities, but also, using the eddy-correlation technique, to calculate small-scale turbulent trace gas fluxes, e.g. caused by secondary circulation systems, mountain venting, convective cells and other mass transport processes between the boundary layer and the free troposphere (handover). Such data are of great importance to many aspects of air pollution dispersion and air pollution modelling. Process studies of the small-scale vertical transport of heat, moisture and trace gases and their parametrisation are some of these aspects. The function of the sensor package is shown by calibration results, intercomparison of the measurements of redundant sensors and spectral analysis of the wind components. Examples of the detection of secondary circulation systems, of the micro structure of trace gases in urban plumes and the calculation of ozone flux profiles are given.

Development of an Antarctic Regional Climate System Model. Part II: Station Validation and Surface Energy Balance

Abstract In this, the second part of the analysis of an Antarctic regional climate system model, the model results and analyses are compared to a series of observational data from automated weather stations at a number of Antarctic stations, radiosonde launches, and surface energy balance climatology. The observational analyses show significant biases in comparison with station data, which is attributable in part to the errors in and low resolution of the elevation dataset. This is a factor in model performance also. Further, a dominant factor in the generation of the model biases is the simulation of atmospheric water vapor and cloud. The known “cold” bias in the clear-sky longwave radiation scheme is amply compensated for by excessive cloudiness in many cases. The strong vertical moisture transport contributes to the excessive cloud formation. The biases explored in detail in this paper are common to regional and global simulations of the Antarctic region and highlight areas in which model development s...

Three‐dimensional model study of organized mesoscale circulations induced by vegetation

Organized mesoscale circulations induced by vegetation discontinuities and their effects on the vertical transport of heat and moisture are investigated under realistic atmospheric conditions. A regional climate model (RegCM2) is used to simulate local circulations which result from contrasts in surface thermal forcing over land. The limited area domain, with a grid resolution of 10 km, is forced at the boundaries by analyses of observations for the 30‐day period of June 1990. Surface vegetation and soil moisture are idealized to provide strong, geometric contrasts in surface heating. Control (homogeneous surface) and perturbation (surface thermal contrast) simulations are required to isolate the circulation signal. Differences in the wind, temperature, and humidity fields show the diurnal evolution of coherent circulations during the simulated month. On three days, circulations having strength similar to a land‐sea breeze occur. Vertical fluxes of heat and moisture, which result from the circulations, show a different vertical structure and diurnal development than turbulent fluxes. The vertical transport of moisture is found to be comparable to or larger than transport by turbulent processes. Vertical transport of heat by the circulations is small. An increase is seen in the monthly total large‐scale precipitation downwind of the ascending region of the circulations. Of the large‐scale forcings studied, the circulation is most closely correlated to the mean horizontal temperature gradient.

Radiative‐convective model with an explicit hydrologic cycle: 2. Sensitivity to large changes in solar forcing

The one‐dimensional radiative‐connective equilibrium model with an explicit hydrologic cycle introduced in part 1 is used to study the sensitivity of the model's atmosphere to large changes in the solar forcing, when various cumulus convection parameterizations are used. As shown by Simpson [1927], by Komabayasi [1967], and by Ingersoll [1969] when the concentration of the absorbing gas in the atmosphere is temperature dependent, equilibrium is impossible for values of the solar forcing larger than a critical value. This result is referred to as a runaway greenhouse. The cumulus convection parameterization schemes currently in use in global climate models (GCMs) employ different assumptions about moistening. This causes the critical solar forcing above which a runaway greenhouse occurs to be very sensitive to the cumulus convection scheme employed. Furthermore, using the microphysically based cumulus convection scheme proposed by Emanuel [1991], we show that the sensitivity of the equilibrium temperature to changes in the solar forcing depends crucially on the microphysics of cumulus convection. For fixed cloud conditions, the critical solar forcing for a runaway greenhouse to occur is between approximately 1.22 and 1.49 times the global mean value for the Earth, and for clear sky conditions, it is a few percent lower. The runaway greenhouse in the experiments with the mass flux schemes generally occurs more rapidly than in the experiments with the adjustment schemes. In addition, the inability of the hard convective adjustment scheme to produce an efficient vertical transport of moisture, together with the saturation requirement for convection to occur, leads to the breakdown of the radiative‐convective equilibria when other processes are not available to provide the necessary vertical transport of water vapor.

Roll vortices in the planetary boundary layer: A review

Roll vortices may be loosely defined as quasi two-dimensional organized large eddies with their horizontal axis extending through the whole planetary boundary layer (PBL). Their indirect manifestation is most obvious in so-called cloud streets as can be seen in numerous satellite pictures. Although this phenomenon has been known for more than twenty years and has been treated in a review by one of us (R.A.Brown) in 1980, there has been a recent resurgence in interest and information. The interest in ocena/land-atmosphere interactions in the context of climate modeling has led to detailed observational and modeling efforts on this problem. The presence of rolls can have a large impact on flux modelling in the PBL. Hence, we shall review recent advances in our understanding of organized large eddies in the PBL and on their role in vertical transport of momentum, heat, moisture and chemical trace substances within the lowest part of the atmosphere.

A Modeling Study of Heating and Drying Effects of Convective Clouds in an Extratropical Mesoscale Convective System

The two-dimensional version of the cumulus ensemble model developed by Soong and Ogura is applied both to a prestorm situation and to the mature stage of the extratropical mesoscale convective system (MCS) that developed on 10–11 April 1979 (AVE-SESAME-79 I) over the central United States. The objective is to investigate the statistical properties of convection, developing in response to an imposed large-scale forcing, and the thermodynamic feedback effect of clouds on the large-scale environment in midlatitudes. The result is compared to that recently obtained by Tao for a tropical rainband. The outstanding result of the model integration for 17 h of physical time is that statistical properties of clouds averaged horizontally over 128 km of the model domain undergo temporal variations for a given time-independent large-scale forcing, rather than settling down into a steady state. When applied to a prestorm situation, the model predicts heavy precipitation that continues to fall for the first 5 h, followed by a 4 h period without precipitation. A second burst of deep convection then occurs. An analysis of the result reveals that the pause of precipitation occurs when the subcloud layer is dried up primarily due to the net vertical transport of moisture associated with clouds. Convection again starts developing when the moisture in the subcloud layer is replenished by the imposed large-scale forcing. The precipitation rate averaged over the precipitation period is found to exceed the supply of moisture by the large-scale forcing. The result implies that the fraction of moisture convergence in a vertical air column that contributes to moisten the environmental atmosphere in Kuo's cumulus parameterization scheme can be negative. Further, the result indicates the following: 1) The updraft mass flux increases with height until it reaches the local maximum at 350 mb, indicating that the cloud population is dominated by deep clouds, in contrast to the bimodal or broad spectral distribution of clouds observed in the tropics. 2) The cloud heating effect does not balance the large-scale cooling effect, reflecting the fact that the storage and horizontal advection terms are not negligibly small compared to the vertical advection term in the large-scale heat budget; and 3) The net vertical fluxes of heat and moisture are not negligibly small cormpared to condensation and evaporation processes at upper levels in the heat and moisture budgets, reflecting the fact that the atmosphere considered here is more unstably stratified and updrafts are stronger than the tropical counterparts.

“Nuclear winter”: A diagnosis of atmospheric general circulation model simulations

We investigate the adiabatic and diabatic thermal balance of an atmospheric general circulation model (GCM) under two conditions: the control case, representing today's atmosphere, and a “nuclear winter” scenario in which virtually all sunlight in northern hemisphere mid‐latitudes is absorbed in the upper troposphere by prescribed dense smoke clouds hypothesized to result from the burning of many cities in a nuclear war. We also examine the changes in moisture and cloudiness simulated by the model. Our object is to examine the reliability of existing simulations of the climatic response to assumed dense, widespread, high‐altitude smoke and to identify improvements needed in model parameterizations. We find that in the smoke‐perturbed case our model simulation of land surface temperature is particularly influenced (i.e., warmed) by parameterized diffusion of heat downward from the lower troposphere. In turn the lower troposphere over land is supplied with heat transported from the relatively warm oceans. Thermal balance in the perturbed atmosphere as a whole is dominated by intense solar heating of the upper troposphere smoke layer in mid‐latitudes balanced by parameterized dry convection and large‐scale dynamical heat transport. Clouds largely disappear in the mid to upper troposphere in smoke‐affected regions as a consequence of a decrease in local relative humidity that results from temperature increases and, to a smaller extent, from a reduction of vertical moisture transport. The computation of substantial downward vertical heat diffusion into the lowest model layer is almost certainly an overestimate for the smoke‐perturbed conditions of high vertical stability. Consequently, the use of the present diffusive parameterization will, in the absence of other errors, result in an underestimate of the magnitude of land surface cooling in the “nuclear winter” scenario. Current three‐dimensional model simulations, however, contain numerous additional omissions and approximations that make it difficult to say whether “nuclear winter” simulations, to date, produce effects that are more or less severe than those that might really occur given the hypothesized smoke amounts and distribution. We believe that the most important areas for GCM enhancement to study climatic effects of nuclear war‐generated aerosols include improved surface and planetary boundary layer processes and incorporation of radiatively active aerosol tracer transport and removal.

Inclusion of Sensible Heating in Convective Parameterization Applied to Lake-Effect Snow

Abstract A method has been designed for including the surface fluxes of water vapor and sensible heat from water surfaces in a convective parameterization scheme for a primitive equation numerical model of the atmosphere. The grid size is 48 km. Application of the model to Lake Huron and vicinity in the simulation of three lake-effect storm cases resulted in predictions of vertical transport of heat and moisture and the consequent alteration of air mass properties, as well as precipitation. The maximum grid-square-average observed precipitation over 6 h was 0.75, 0.86 and 0.54 cm for the three cases. These values may be compared to maximum predicted values for those grid squares where observations were available. The predicted maxima were 0.60, 0.76 and 0.57 cm, respectively. The model was also employed to isolate the various physical influences on precipitation. While model results were not clear-cut, they indicated that for near-lake points, heat and moisture from the lake were the principal causes of p...

Subgrid-Scale Condensation in Models of Nonprecipitating Clouds

One of the shortcomings of present condensation schemes is the assumption that a computational grid volume is either entirely saturated or entirely unsaturated, which is a crude approximation in some instances even with a relatively fine mesh. The concept of a statistical distribution of the points where condensation occurs inside a given grid volume is discussed and results are presented for diagnosing both the fraction of the grid volume containing saturated air and the liquid water content when that fraction is less than unity. An abbreviated procedure is tested in a boundary layer model of 50 m mesh for a case of nonprecipitating tropical moist convection. Two main effects upon the simulated turbulent field are found: first, the cloud activity measured by the total liquid water content or by the vertical moisture transport is increased and extends farther up, at the expense of subcloud layer moisture; second, the temporal variations within the model domain of the calculated turbulent properties of the cloud layer are decreased, which allows a given time sample of the simulated atmosphere to be more representative of the ensemble mean.

On the Vertical Transport of Soil Moisture at the Grass Field

On the Vertical Transport of Soil Moisture at the Grass Field