Abstract

A series of observing system simulation experiments (OSSES) was conducted to assess the potential impact of the Laser Atmospheric Wind Sounder (LAWS) instrument on a 5-day forecast using the Florida State University (FSU) primitive equation multilevel spectral global circulation model. This proposed Earth Observing System satelliteborne instrument is a CO₂ Doppler lidar wind sounding system. The instrument's requirement for usable measurements is that clouds or high concentrations of tropospheric aerosols must exist within the sample volume.

Two different orbits, a 55° inclined and a 98° sun synchronous, were examined by adding simulated LAWS wind profiles into a global four-dimensional data assimilation system and comparing the analyses and forecasts to a control experiment. Also, two different sets of simulations were examined for the 55° inclined orbit. The first set followed the assumption of other previous Doppler lidar wind sounding OSSES; a global concentration of aerosols exists such that observations will be usable at each pulse location. The second set of simulations incorporated the effects of subvisual cirrus, aerosols, molecular attenuation, and sampling-scale turbulence. All simulations of LAWS wind observations are degraded when the lidar pulse encounters the earth's topography and when the downward-integrated cloud amount reaches a critical threshold.

The four-dimensional data assimilation system consists of a multivariate optimum interpolation analysis and a nonlinear normal-mode initialization using the aforementioned FSU global circulation model. In this set of assimilations only upper-air data was used with the exclusion of temperature sounder data, which may reduce the overall skill of the forecasts in the largely data-void Southern Hemisphere, which has been seen in temperature sounder OSSES.

The inclusion of LAWS wind observations exhibits an overall improvement of the forecast skill for this study. The greatest increase in skill is in the Southern Hemisphere, as can be seen in both the motion and mass fields. The 98° sun-synchronous orbit resolved the polar meteorological features much better than the 55° inclined orbit. Otherwise, the two different orbits were very similar, with the 55° inclined showing a better analysis of the tropics.

A comparison of the two different sets of simulations for the 55° inclined orbit is also revealing. The incorporation of physical effects into the LAWS simulations shows a reduced forecast skill as compared with the other set of simulated LAWS observations. However, a global increase in forecast skill can still be seen over that of the control experiment. This is an attempt at creating more “realistic” simulations, and it is difficult to draw robust conclusions without completing a large number of forecasts, examining many different meteorological phenomena.

In the future, the atmospheric physical effects will be much better incorporated into the LAWS simulated observations. The analysis of Global Backscatter Experiment data will aid in a better definition of global concentrations of aerosols and will attempt to relate specific concentrations of aerosols to chemical composition and associated large-scale meteorological features.

Abstract

This paper examines some observational aspects of the evolution of energy exchanges and differential heating during the GARP Monsoon Experiment MONEX. The main findings are that:

1. 1) A large increase in the kinetic energy of the total flow field and the nondivergent component of the flows occurs over the Arabian Sea about a week prior to the onset of monsoon rains over central India.

2. 2) The field of differential heating moves during the onset period to a favorable position for the generation of eddy available potential energy and its release to eddy kinetic energy.

3. 3) The release of the eddy available potential energy goes to enhance the kinetic energy of the divergent circulations.

4. 4) The kinetic energy of the divergent circulation does not increase much with time. This energy is shown to he transferred rapidly to the nondivergent motion via a number of interaction functions. The orientation of the divergent flows is shown to be of prime importance in these transfers during the onset, active, break and revival periods of the monsoons.

Based on the above observational findings a series of numerical prediction experiments ire conducted to examine the sensitivity of the monsoon onset to initially imposed fields of differential heating. The results of some 96 h integrations seem to confirm the large sensitivity in the evolution to the field of heating. The results of time integrations also show that the kinetic energy of the monsoon circulations increases via the rapid increase of interactions among the irrotational and nondivergent modes.

Abstract

A study of the potential vorticity budget for the low-level flows over the Arabian Sea and Indian Ocean is presented here. This study covers a 17-day period between 11 and 27 June 1979 during the GARP Monsoon Experiment (MONEX). Data sets for this study include the special observing systems for the Global Experiment, i.e., cloud winds from the geostationary satellites, surface and upper air data from the First GARP Global Experiment, FGGE and MONEX, data from low-level constant-level balloons, upper air data from dropwindsonde aircraft, and the World Weather Watch. The selection of the 17-day period is important logically for both phenomena and from the availability of the maximum data sets, over oceans during the global experiment. As many as 50 soundings per day were available from this observing system. The framework for the calculations are three equations that describe the changes in potential vorticity, absolute vorticity and the dry static stability. These equations include as relevant forcing the effects of shortwave and longwave radiative processes, shallow and deep cumulus convection, surface friction, and the air-sea transfers of momentum, heat and moisture.

The period of this study coincides with that of the onset and establishment of deep moist westerlies of the monsoons. The Somali jet established itself around 10°N during this period and the onset of monsoon rain had begun. A large increase of positive potential vorticity on the cyclonic shear side of the low-level jet is attributed to an increase of both the absolute vorticity and of the dry static stability. The mechanisms responsible for this increase are explored via detailed budget calculations. The salient mechanisms arise from the covariance of potential vorticity and differential radiative heating along the vertical, and vertical convergence of potential vorticity by the cumulus mass flux, which contributes to a generation of potential vorticity. On the other hand, the role of horizontal advection and friction seems to be opposite.

The meridional flow is countergradient, such that horizontal advection brings low values of dry static stability, absolute vorticity and potential vorticity into the region of their large positive values north of the equator. The wind-stress curl in this region is positive; it contributes to a diminution of the absolute as well as potential vorticity.

Abstract

The analyzed wind field at 850 mb during the summer monsoon experiment (MONEX) is subjected to a time series analysis to confirm the existence of a peak in the time range of 30–50 days. Having established that, this study presents a mapping of the motion field for this time scale. In this short paper we illustrate the steady meridional propagation of a train of troughs and ridges that seem to form near the equator and dissipate near the Himalayas. The meridional scale of this mode is around 300 km, and its meridional speed of propagation is ∼0.75° latitude per day. The amplitude of the wind for this mode is around 3–6 m s⁻¹. The salient contributions here are the mapping and the demonstration of a very regular behavior of this mode; its existence is here noted from a period well before the onset of monsoon, i.e., from early May to late July. Three major storms during MONEX were noted to form within the trough line of this system, and the period of major cessation of rains over the Indian subcontinent was noted to occur around the period of arrival near 20°N of a ridge line of this mode. A more detailed analysis of this strongly divergent mode will be published at a later date.

Abstract

In this paper we investigate a baroclinic boundary layer of cross-equatorial flow over the Arabian Sea with a fine-mesh (200 m) vertical resolution numerical model. The dynamical model utilizes a prescribed vertically varying pressure field from observations. The vertical variation of eddy diffusivity is parameterized. Long-term steady state on a meridional-vertical plane is obtained via integrations of the numerical model starting from an initial Ekman balance. The principal results of the study include a realistic simulation of the vertical structure of the cross-equatorial flow. The balance of forces and boundary-layer transitions between strong cross-equatorial flow, the low-level jet and an ITCZ over the northern Arabian Sea are analyzed in some detail. We note that an advective boundary layer is dominant across the equator, and the core of the low-level jet is located toward the poleward edge of the advective boundary layer. North of this region a gradual increase of upward motion is noted and this region is identified as the Intertropical Convergence Zone (ITCZ). The ITCZ occurs in a transition zone between an advective and a quasi-Ekman boundary layer.

Abstract

Mid-tropospheric cyclones are an important part of the tropical general circulation of the summer season. These are synoptic-scale disturbances which appear in the daily and monthly mean circulation maps with greatest intensity at levels near 500 mb. The structure and energetics of this type over southeast Asia are discussed in this paper. Interesting features include a warm anomaly above the cyclone and a cold anomaly below.

A five-level non-geostrophic balanced model is used in this study to obtain the distribution of vertical motion. The model includes a parameterization of cumulus-scale convection. In the middle levels, rising motions are found west of the cyclone and sinking motions to the east. This is primarily due to the thermal structure of the atmosphere and associated advection of colder air from the oceanic regions and warmer air from land areas. A marked diurnal change in the vertical velocity is noted in the computations; magnitudes are large at 0700 local time compared to 1900. This diurnal change is primarily due to changes in the wind direction and speed. The important result of this study is that both the cumulus- and synoptic-scale motions exhibit the following dual roles in the maintenance of this midlevel system:

1) Both scales contribute to a net warming of the air above the cyclone, diabatic warming by cumulus-scale motions and adiabatic warming by the descent of synoptic-scale motions.

2) The two scales oppose each other in the transformation eddy available potential energy into eddy kinetic energy. Cumulus-scale motions contribute to a net generation, while synoptic-scale motions transform kinetic energy into potential energy. This latter result is consistent with 1) because the areas of descent are somewhat closer to the warm temperature anomaly than are the areas of ascent.

Abstract

In this note, the local solution of Gill for the symmetric and the antisymmetric heat sources and sinks is extended to the entire global tropical belt by the authors. The symmetric and antisymmetric heating forced solutions of the motion field derived from the linearized shallow-water equations describe many aspects of the winter and the summer monsoon circulations. The extension, provided here, describes the response of tropical circulation to the global tropical heat sources and sinks retrieved from a satellite-based field of the outgoing longwave radiation through regression algorithms. Such a prescribed field is expanded into a low-order set of trigonometric functions in the zonal direction and parabolic cylinder functions along the meridional coordinate. Upon sequential substitution of these modes into the linearized shallow-water equations on a beta plane, it is possible to solve for the heating forced solutions in a closed analytical form for the entire Tropics. The solutions exhibit most of the climatological features of the wintertime and the summertime circulations of the lower troposphere, such as the trades of the two hemispheres and the Asian monsoon with its elements; for example, monsoon trough, cross-equatorial flows, southwest monsoon current, Mascarene high, and the subtropical anti-cyclones. Furthermore, the solutions also describe features such as the heat lows over Arabia, the Sahara, and Mexico. Overall, these solutions confirm the importance of diabatic heat sources and sinks in the shaping of the tropical climate.

These notes provide a short summary of the field phase of the GARP Winter Monsoon Experiment. The field phase commenced on 1 December 1978, with the field operations coordinated from Kuala Lumpur. The participants included scientists and technical personnel from Malaysia, Indonesia, Thailand, the Philippines, Hong Kong, Japan, Saudi Arabia, Singapore, Australia, People's Republic of China (PRC), the U.S.S.R., and the U.S.A. The observing systems, type of experiments, mission objectives, components of overall data sets, and a preliminary evaluation are presented in this short survey.

The life cycle of Supertyphoon Hope (1979) from a tropical depression stage to intensification and its eventual weakening after landfall, some 6 days later, is followed in a real-data numerical prediction experiment. The predictions are carried out with a very high-resolution global spectral model, (Spectral resolution, triangular 170 waves). The initial data for this study are obtained from the delayed reanalysis of the Global Experiment (1979). Those were the analysis of the European Centre for Medium Range Weather Forecasts (ECMWF). Initialization is based on nonlinear normal mode with physics. The model has 12 vertical layers with staggered variables. A detailed physical-dynamical model is used for these studies. This typhoon prediction is a follow-up of a recent study (Krishnamurti et al. 1989). Here we show the prediction of the life cycle at a much higher resolution. The present study illustrates a remarkable prediction of the track, structure, and intensity of the supertyphoon at the higher resolution and raises the possibility for major improvement of tropical storm prediction with real data.

Abstract

The diurnal mode of the Asian summer monsoon during active and break periods is studied using four versions of the Florida State University (FSU) global spectral model (GSM). These versions differ in the formulation of cloud parameterization schemes in the model. Observational-based estimates show that there exists a divergent circulation at 200 hPa over the Asian monsoon region in the diurnal time scale that peaks at 1200 local solar time (LST) during break monsoon and at 1800 LST during active monsoon. A circulation in the opposite direction is seen near the surface. This circulation loop is completed by vertical ascending/descending motion over the monsoon domain and its surroundings. This study shows that global models have large phase and amplitude errors for the 200-hPa velocity potential and vertical pressure velocity over the monsoon region and its surroundings. Construction of a multimodel superensemble could reduce these errors substantially out to five days in advance. This was on account of assigning differential weights to the member models based on their past performance. This study also uses a unified cloud parameterization scheme that inherits the idea of a multimodel superensemble for combining member model forecasts. The advantage of this model is that it is an integrated part of the GSM and thus can improve the forecasts of other parameters as well through improved cloud cover. It was seen that this scheme had a larger impact on forecasting the diurnal cycle of cloud cover and precipitation of the Asian summer monsoon compared to circulation. The authors show that the diurnal circulation contributes to about 10% of the rate of change of total kinetic energy of the monsoon. Therefore, forecasting this pronounced diurnal mode has important implications for the energetics of the Asian summer monsoon.