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NASA NTRS1994

Deep cumulonimbus cloud systems in the tropics with and without lightning

The thunderstorm frequency over the oceans during the Global Atmospheric Research Program Atlantic Tropical Experiment is quantified by examination of over 20,000 surface hourly observations from research ships. The overall thunderstorm frequency is one thunderstorm day per ship per month. There were many examples of intense mesoscale systems, such as squall lines, passing over the ships, extending to 13-17 km in altitude, but that nevertheless produce few reports of lightning. This reinforces the idea, based on data from other tropical ocean regions and from global satellite data, that in spite of the ubiquitous 'hot towers' over tropical oceans, marine cumulonimbus produce little lightning. Climatological data from the monsoon regions of the Tropics are analyzed to reveal that during periods of onshore flow and heavy rainfall the oceanic regime of high rainfall but little lightning moves onshore. A rain-thunderstorm ratio is defined and used to characterize convective rainfall regimes as continental (relatively little) or maritime (relatively great) rainfall compared to the number of thunderstorm days. In regions such as West Africa and south Asia, the seasonal rainfall peak is actually accompanied by a thunderstorm minimum. It is further suggested that the data support the idea, not original here, that vertical velocities in oceanic cumulonimbus clouds tend to be low compared with continental clouds. It is hypothesized that most oceanic storms have updrafts weaker than a posssible threshold value, below which the supercooled liquid water, large ice particles, and ice-ice collisions are not present in the mixed-phase region in sufficient concentrations for electrification leading to lightning.

Zipser, Edward J.↗
NASA NTRS1994

A comparison of eastern North American seismic strain-rates to glacial rebound strain-rates

Glacial rebound strain-rates computed using a simple Laurentide glacial loading model are of the order of 10(exp -9) per year within the region of glaciation and extending several hundred kilometers beyond. The horizontal strain-rates receive approximately equal contributions from horizontal and vertical velocities, a consequence of the spherical geometry adopted for the Earth model. In the eastern United States and southeastern Canada the computed strain-rates are 1-3 orders of magnitude greater than an estimate of the average seismic strain-rate (Anderson, 1986) and approximately 1 order of magnitude greater than predicted erosional strain-rates. The predicted glacial rebound strain-rates are not, in general, oriented in such a way as to augment the observed state of deviatoric stress, possibly explaining why the seismic strain-rates are much smaller than the glacial rebound strain-rates. An exception to this may be seismically active regions in the St. Lawrence valley.

James, Thomas S.↗
NASA NTRS1995

Variability of moisture in the upper troposphere as inferred from TOVS satellite observations and the ECMWF model analyses in 1989

Large-scale variability of moisture in the upper troposphere is examined using TIROS (Television Infrared Observation Satellite) Operational Vertical Sounder (TOVS) satellite observations and European Centre for Medium-Range Weather Forecasts (ECMWF) model analyses from 1989 in the latitude band from 40 deg N to 40 S. To compare these dissimilar datasets, upwelling radiances were computed for the 6-7 micrometer water vapor band from the ECMWF temperature and moisture analyses, and these computed radiances were compared to the corresponding TOVS satellite observations. The ECMWF-based radiances reproduce the general locations and seasonal cycle of the TOVS-observed moisture features, particularly after an improved convective parameterization scheme was adopted by ECMWF in May 1989. However, the ECMWF analysis scheme still results in much milder lateral moisture gradients and seasonal contrasts than indicated by the TOVS observations. Seasonally, the upper troposphere in each hemisphere dries in winter and moistens in summer, but there are regions in each hemisphere that run counter to this seasonal trend, apparently depending on continental- and monsoon-scale dynamics. Dynamically, the TOVS-observed regions of significant subtropical dryness are correlated with persistent subsidence indicated by ECMWF 300-mb vertical velocity analyses. The TOVS radiance observations indicate large variations in space and time of the upper-tropospheric moisture field, which are not fully captured by the ECMWF analyses.

Salathe, Eric P., Jr.↗
NASA NTRS1996

Parallelization and Visual Analysis of Multidimensional Fields: Application to Ozone Production, Destruction, and Transport in Three Dimensions

The three-dimensional, spectral transport model used in the current project was first successfully integrated over climatological time scales by Dr. Guang Ping Lou for the simulation of atmospheric N2O using the United Kingdom Meteorological Office (UKMO) 4-dimensional, assimilated wind and temperature data set. A non-parallel, FORTRAN version of this integration using a fairly simple N2O chemistry package containing only photo-chemical reactions was used to verify our initial parallel model results. The integrations reproduced the gross features of the observed stratospheric climatological N2O distributions but also simulated the structure of the stratospheric Antarctic vortex and its evolution. Subsequently, Dr. Thomas Kindler, who produced much of the parallel version of our model, enlarged the N2O model chemistry package to include N2O reactions involving O(D-1) and also introduced assimilated wind data from NASA as well as UKMO. Initially, transport calculations without chemistry were run using Carbon-14 as a non-reactive tracer gas with the result that large differences in the transport properties of the two assimilated wind data sets were apparent from the resultant Carbon-14 distributions. Subsequent calculations for N2O, including its chemistry, with the two input winds data sets with verification from UARS satellite observations have refined the transport differences between the two such that the model's steering capabilities could be used to infer the correct climatological vertical velocity fields required to support the N2O observations. During this process, it was also discovered that both the NASA and the UKMO data contained spurious values in some of the higher frequency wave components, leading to incorrect local transport calculations and ultimately affecting the large scale properties of the model's N2O distributions, particularly at tropical latitudes. Subsequent model runs with wind data that had been filtered to remove some of the high frequency components produced much more realistic N2O distributions. During the past few months, the UKMO wind data base for a complete two-year period was processed into spectral form for model use. This new version of the input transport data base now includes complete temperature fields as well as the necessary wind data. This was done to facilitate advanced chemical calculations in the parallel model which often depend upon temperature. Additional UKMO data is being added as it becomes available.

Schwan, Karsten↗
NASA NTRS1994

Vorticity and Vertical Motions Diagnosed from Satellite Deep-Layer Temperatures

Spatial fields of satellite-measured deep-layer temperatures are examined in the context of quasigeostrophic theory. It is found that midtropospheric geostrophic vorticity and quasigeostrophic vertical motions can be diagnosed from microwave temperature measurements of only two deep layers. The lower- ( 1000-400 hPa) and upper- (400-50 hPa) layer temperatures are estimated from limb-corrected TIROS-N Microwave Sounding Units (MSU) channel 2 and 3 data, spatial fields of which can be used to estimate the midtropospheric thermal wind and geostrophic vorticity fields. Together with Trenberth's simplification of the quasigeostrophic omega equation, these two quantities can be then used to estimate the geostrophic vorticity advection by the thermal wind, which is related to the quasigeostrophic vertical velocity in the midtroposphere. Critical to the technique is the observation that geostrophic vorticity fields calculated from the channel 3 temperature features are very similar to those calculated from traditional, 'bottom-up' integrated height fields from radiosonde data. This suggests a lack of cyclone-scale height features near the top of the channel 3 weighting function, making the channel 3 cyclone-scale 'thickness' features approximately the same as height features near the bottom of the weighting function. Thus, the MSU data provide observational validation of the LID (level of insignificant dynamics) assumption of Hirshberg and Fritsch.

Spencer, Roy W.↗
NASA NTRS1996

Turbulent Compressible Convection with Rotation: Flow Structure and Evolution - Part 1

The effects of Coriolis forces on compressible convection are studied using three-dimensional numerical simulations carried out within a local modified f-plane model. The physics is simplified by considering a perfect gas occupying a rectilinear domain placed tangentially to a rotating sphere at various latitudes, through which a destabilizing heat flux is driven. The resulting convection is considered for a range of Rayleigh, Taylor, and Prandtl (and thus Rossby) numbers, evaluating conditions where the influence of rotation is both weak and strong. Given the computational demands of these high-resolution simulations, the parameter space is explored sparsely to ascertain the differences between laminar and turbulent rotating convection. The first paper in this series examines the effects of rotation on the flow structure within the convection, its evolution, and some consequences for mixing. Subsequent papers consider the large-scale mean shear flows that are generated by the convection, and the effects of rotation on the convective energetics and transport properties. It is found here that the structure of rotating turbulent convection is similar to earlier nonrotating studies, with a laminar, cellular surface network disguising a fully turbulent interior punctuated by vertically coherent structures. However, the temporal signature of the surface flows is modified by inertial motions to yield new cellular evolution patterns and an overall increase in the mobility of the network. The turbulent convection contains vortex tubes of many scales, including large-scale coherent structures spanning the full vertical extent of the domain involving multiple density scale heights. Remarkably, such structures align with the rotation vector via the influence of Coriolis forces on turbulent motions, in contrast with the zonal tilting of streamlines found in laminar flows. Such novel turbulent mechanisms alter the correlations which drive mean shearing flows and affect the convective transport properties. In contrast to this large-scale anisotropy, small-scale vortex tubes at greater depths are randomly orientated by the rotational mixing of momentum, leading to an increased degree of isotropy on the medium to small scales of motion there. Rotation also influences the thermodynamic mixing properties of the convection. In particular, interaction of the larger coherent vortices causes a loss of correlation between the vertical velocity and the temperature leaving a mean stratification which is not isentropic.

Brummell, Nicholas H.↗
NASA NTRS1997

Anthropogenic Sulfate, Clouds, and Climate Forcing

This research work is a joint effort between research groups at the Battelle Pacific Northwest Laboratory, Virginia Tech University, Georgia Institute of Technology, Brookhaven National Laboratory, and Texas A&M University. It has been jointly sponsored by the National Aeronautics and Space Administration, the U.S. Department of Energy, and the U.S. Environmental Protection Agency. In this research, a detailed tropospheric aerosol-chemistry model that predicts oxidant concentrations as well as concentrations of sulfur dioxide and sulfate aerosols has been coupled to a general circulation model that distinguishes between cloud water mass and cloud droplet number. The coupled model system has been first validated and then used to estimate the radiative impact of anthropogenic sulfur emissions. Both the direct radiative impact of the aerosols and their indirect impact through their influence on cloud droplet number are represented by distinguishing between sulfuric acid vapor and fresh and aged sulfate aerosols, and by parameterizing cloud droplet nucleation in terms of vertical velocity and the number concentration of aged sulfur aerosols. Natural sulfate aerosols, dust, and carbonaceous and nitrate aerosols and their influence on the radiative impact of anthropogenic sulfate aerosols, through competition as cloud condensation nuclei, will also be simulated. Parallel simulations with and without anthropogenic sulfur emissions are performed for a global domain. The objectives of the research are: To couple a state-of-the-art tropospheric aerosol-chemistry model with a global climate model. To use field and satellite measurements to evaluate the treatment of tropospheric chemistry and aerosol physics in the coupled model. To use the coupled model to simulate the radiative (and ultimately climatic) impacts of anthropogenic sulfur emissions.

Ghan, Steven J.↗
NASA NTRS1997

Wave Driven Exothermic Heating in the Mesopause Region

A full-wave propagation model was developed that describes the propagation of gravity waves from the Earth's surface to the upper boundary, which can be placed anywhere between 150 and 500 km altitude. The model includes a realistic background atmosphere, and includes the effects of mean horizontal winds and their vertical shears, mean vertical temperature gradients, the eddy and molecular diffusion of heat and momentum, and the effects of ion-drag. This model solves five coupled second-order differential equations (continuity, momentum, and energy) in the vertical coordinate to derive the perturbation variables u', v', w' (horizontal and vertical velocity components), T' (temperature) and p' (pressure). The upper boundary can be automatically selected based on tests using the radiation condition at the upper boundary, wherein the height is increased until the wave is experiencing severe dissipation at the upper boundary, ensuring that substantial absorption occurs for any waves reflected from the upper boundary. The determination of wave amplitude is a key requirement of wave energetics. Therefore, the fullwave model has been applied to airglow observations in order to determine wave amplitudes as a function of altitude. This was accomplished by using the full-wave model output to drive a chemistry perturbation module that describes minor species perturbations and the resulting airglow perturbations. The full-wave output was multiplied by an altitude-independent factor such that the modeled and observed relative airglow intensity perturbations were equal. The effects of mean winds were included in these studies, and found to be the most important model input affecting the calculations (being more important than the choice of eddy diffusion profiles and chemical kinetic coefficients). In one study (Hickey et al., 1997a) these winds could not be well estimated from the measurements, whereas in the second study (Hickey et al.,1997b) the mean were well defined with a sodium wind-temperature lidar.

Hickey, Michael P.↗
NASA NTRS1961

Landing Characteristics of a Lenticular-Shaped Reentry Vehicle

An experimental investigation was made of the landing characteristics of a 1/9-scale dynamic model of a lenticular-shaped reentry vehicle having extendible tail panels for control after reentry and for landing control (flare-out). The landing tests were made by catapulting a free model onto a hard-surface runway and onto water. A "belly-landing" technique in which the vehicle was caused to skid and rock on its curved undersurface (heat shield), converting sinking speed into angular energy, was investigated on a hard-surface runway. Landings were made in calm water and in waves both with and without auxiliary landing devices. Landing motions and acceleration data were obtained over a range of landing attitudes and initial sinking speeds during hard-surface landings and for several wave conditions during water landings. A few vertical landings (parachute letdown) were made in calm water. The hard-surface landing characteristics were good. Maximum landing accelerations on a hard surface were 5g and 18 radians per sq second over a range of landing conditions. Horizontal landings on water resulted in large violent rebounds and some diving in waves. Extreme attitude changes during rebound at initial impact made the attitude of subsequent impact random. Maximum accelerations for water landings were approximately 21g and 145 radians per sq second in waves 7 feet high. Various auxiliary water-landing devices produced no practical improvement in behavior. Reduction of horizontal speed and positive control of impact attitude did improve performance in calm water. During vertical landings in calm water maximum accelerations of 15g and 110 radians per sq second were measured for a contact attitude of -45 deg and a vertical velocity of 70 feet per second.

Blanchard, Ulysse J.↗
NASA NTRS1959

Approach and Landing Investigation at Lift-Drag Ratios of 2 to 4 Utilizing a Straight-Wing Fighter Airplane

A series of landings was performed with a straight-wing airplane to evaluate the effect of low lift-drag ratios on approach and landing characteristics. Landings with a peak lift-drag ratio as low as 3 were performed by altering the airplane configuration (extending speed brakes, flaps, and gear and reducing throttle setting). As lift-drag ratio was reduced, it was necessary either to make the landing pattern tighter or to increase initial altitude, or both. At the lowest lift-drag ratio the pilots believed a 270 deg overhead pattern was advisable because of the greater ease afforded in visually positioning the airplane. The values of the pertinent flare parameters increased with the reduction of lift-drag ratio. These parameters included time required for final flare; speed change during final flare; and altitude, glide slope, indicated airspeed, and vertical velocity at initiation of final flare. The pilots believed that the tolerable limit was reached with this airplane in the present configuration, and that if, because of a further reduction in lift-drag ratio, more severe approaches than those experienced in this program were attempted, additional aids would be required to determine the flare-initiation point.

Matranga, Gene J.↗
NASA NTRS1998

A Simulation of Biological Prosesses in the Equatorial Pacific Warm Pool at 165 deg E

A nine-year simulation (1984-1992) of biological processes in the equatorial Pacific Warm Pool is presented. A modified version of the 4-component (phytoplankton, zooplankton, nitrate and ammonium) ecosystem model by McClain et al. (1996) is used. Modifications include use of a spectral model for computation of PAR and inclusion of fecal pellet remineralization and ammonium nitrification. The physical parameters (horizontal and vertical velocities and temperature) required by the ecosystem model were derived from an improved version of the Gent and Cane (1990) ocean general circulation model (Murtugudde and Busalacchi, 1997). Surface downwelling spectral irradiance was estimated using the clear-sky models of Frouin et al. (1989) and Gregg and Carder (1990) and cloud cover information from the International Satellite Cloud Climatology Project (ISCCP). The simulations indicate considerable variability on interannual time scales in all four ecosystem components. In particular, surface chlorophyll concentrations varied by an order of magnitude with maximum values exceeding 0.30 mg/cu m in 1988, 1989, and 1990, and pronounced minimums during 1987 and 1992. The deep chlorophyll maximum ranged between 75 and 125 meters with values occasionally exceeding 0.40 mg/cu m. With the exception of the last half of 1988, surface nitrate was always near depletion. Ammonium exhibited a subsurface maximum just below the DCM with concentrations as high as 0.5 mg-atN/cu m . Total integrated annual primary production varied between 40 and 250 gC/sq m/yr with an annual average of 140 gC/sq m/yr. Finally, the model is used to estimate the mean irradiance at the base of the mixed layer, i.e., the penetration irradiance, which was 18 Watts/sq m over the nine year period. The average mixed layer depth was 42 m.

McClain, Charles R.↗
NASA NTRS1996

Chlorine Deactivation in the Lower Stratospheric Polar Regions during late Winter: Results from UARS

Recovery from enhanced chlorine conditions in the lower stratospheric polar regions of both hemispheres is investigated using data from the Upper Atmosphere Research Satellite (UARS). Microwave Limb Sounder (MLS) measurements of ClO within the polar vortices are used to infer ClO(sub x) (ClO + 2Cl202) abundances that are then correlated with simultaneous Cryogenic Limb Array Etalon Spectrometer (CLAES) measurements of ClON02 and Halogen Occultation Experiment (HALOE) measurements of HCl obtained starting within 5 days of the end of the MLS and CLAES high-latitude observing periods in each hemisphere. Time series of vortex-averaged mixing ratios are calculated on two potential temperature surfaces (585 K and 465 K) in the lower stratosphere for approximately month-long intervals during late winter: August 17 - September 17, 1992, in the southern hemisphere and February 12 - March 16, 1993, in the northern hemisphere. The observed mixing ratios are adjusted for the effects of vertical transport using diabatic vertical velocities estimated from CLAES tracer data. In the northern hemisphere, the decrease in ClO, is balanced on both surfaces by an increase in ClON02- In the southern hemisphere, continuing polar stratospheric cloud activity prevents ClO from undergoing sustained decline until about September 3. In contrast to the northern hemisphere, there is no significant chemical change in vortex-averaged ClON02 at 465 K, and there is an apparent decrease in ClON02 at 585 K, even after the enhanced ClO abundances have started to recede. Results from the SLIMCAT chemical transport model initialized with UARS data and run with OH + ClO yields HCl + 02 as an 8% channel suggest that the primary recovery product in the south during this time period is not ClON02, but HCl. HALOE HCl mixing ratios are extrapolated back to the time of the MLS and CLAES data. At 585 K, the chlorine budget can be made to balance by extrapolating HCl back to a value of 0.6 parts per billion by volume (ppbv) at the beginning of the study period; at 465 K, the contribution from extrapolated HCl is not sufficient to offset the loss in ClOx, and there is a slight imbalance between the decrease in reactive chlorine and the change in chlorine reservoirs. The difficulty in closing the chlorine budget in the southern hemisphere may arise from complications caused by ongoing activation, incomplete photochemical assumptions, and/or inadequate data quality.

Santee, M. L.↗
NASA NTRS1999

Effects of Precipitation on Ocean Mixed-Layer Temperature and Salinity as Simulated in a 2-D Coupled Ocean-Cloud Resolving Atmosphere Model

A two-dimensional coupled ocean-cloud resolving atmosphere model is used to investigate possible roles of convective scale ocean disturbances induced by atmospheric precipitation on ocean mixed-layer heat and salt budgets. The model couples a cloud resolving model with an embedded mixed layer-ocean circulation model. Five experiment are performed under imposed large-scale atmospheric forcing in terms of vertical velocity derived from the TOGA COARE observations during a selected seven-day period. The dominant variability of mixed-layer temperature and salinity are simulated by the coupled model with imposed large-scale forcing. The mixed-layer temperatures in the coupled experiments with 1-D and 2-D ocean models show similar variations when salinity effects are not included. When salinity effects are included, however, differences in the domain-mean mixed-layer salinity and temperature between coupled experiments with 1-D and 2-D ocean models could be as large as 0.3 PSU and 0.4 C respectively. Without fresh water effects, the nocturnal heat loss over ocean surface causes deep mixed layers and weak cooling rates so that the nocturnal mixed-layer temperatures tend to be horizontally-uniform. The fresh water flux, however, causes shallow mixed layers over convective areas while the nocturnal heat loss causes deep mixed layer over convection-free areas so that the mixed-layer temperatures have large horizontal fluctuations. Furthermore, fresh water flux exhibits larger spatial fluctuations than surface heat flux because heavy rainfall occurs over convective areas embedded in broad non-convective or clear areas, whereas diurnal signals over whole model areas yield high spatial correlation of surface heat flux. As a result, mixed-layer salinities contribute more to the density differences than do mixed-layer temperatures.

Li, Xiaofan↗
NASA NTRS2000

Fallspeeds and Vertical Air Motions in Stratiform Rain Derived from ER-2 Doppler Radar Observations

The Tropical Rain Measuring Mission (TRMM) conducted several intensive field validation campaigns for improved understanding of Tropical precipitation systems. Two of the campaigns (TEFLUN in Florida and Texas, and LBA in Brazil) utilized: the NASA ER-2 high-altitude (20 km) remote sensing aircraft instrumented with the ER-2 Doppler Radar (EDOP), the University of North Dakota Citation microphysics aircraft, and the NCAR S-POL polarization radar. This paper focuses on EDOP-derived fallspeeds and vertical velocities in the rain regions of two stratiform cases (5 September 1998 along the east coast of Florida, and 17 February 1999 in Amazonia in Brazil). These cases were sampled in situ microphysically by the Citation and reported elsewhere in this meeting; the main emphasis of this paper will be on the airborne radar measurements and inferences from them.

Heymsfield, G. M.↗
NASA NTRS2000

BOREAS AFM-06 Mean Temperature Profile Data

The Boreal Ecosystem-Atmosphere Study (BOREAS) Airborne Fluxes and Meteorology (AFM)-6 team from the National Oceanic and Atmospheric Adminsitration/Environment Technology Laboratory (NOAA/ETL) operated a 915-MHz wind/Radio Acoustic Sounding System (RASS) profiler system in the Southern Study Area (SSA) near the Old Jack Pine (OJP) tower from 21 May 1994 to 20 Sep 1994. The data set provides temperature profiles at 15 heights, containing the variables of virtual temperature, vertical velocity, the speed of sound, and w-bar. The data are stored in tabular ASCII files. The mean temperature profile data are available from the Earth Observing System Data and Information System (EOSDIS) Oak Ridge National Laboratory (ORNL) Distributed Active Archive Center (DAAC). The data files are available on a CD-ROM (see document number 20010000884).

Wilczak, James↗
NASA NTRS2000

Vertical Structure of Stratiform Rain During TEFLUN-B and TRMM-LBA as Derived From an Airborne Radar Perspective

The NASA aircraft instrumented with airborne radars overflew a number of stratiform rain systems during the Texas and Florida Underflights Experiment-B TEFLUN-B) and TRMM-LBA (LBA). TEFLUN-B was conducted near Melbourne Florida during late Summer 1998 for validation of the TRMM satellite. LBA was conducted in Rondonia, Brazil in the Amazon region. Key in these campaigns were the ER-2 and DC-8 aircraft, as well as extensive ground-validation networks including the NCAR S-band (S-POL) polarization radar and University of North Dakota Citation II in situ microphysics measurements. This paper compares the vertical structure of precipitation and vertical motions for four cases: 5 September 1998 and 13 August 1998 during TEFLUN-B and 17 February 1999 and 21 February 1999 during LBA. Both these stratiform systems were within close range of the S-POL radar, and were sampled with downlooking radars and radiometers on the ER-2 (and DC-8 during LBA). Emphasis in the paper is on statistics of the vertical hydrometeor profiles, and their relation to the vertical velocity structure deduced from the ER-2 Doppler Radar (EDOP) measurements. These profiles are used to identify whether growth regions exist as would be expected with mesoscale updrafts often associated with stratiform regions. Finally, rain rates in the stratiform rain derived from EDOP and S-POL are compared.

Heymsfield, Gerald M.↗
NASA NTRS2001

Characterizing the Influence of the General Circulation on Marine Boundary Layer Clouds

The seasonal and intraseasonal variability of boundary layer cloud in the subtropical eastern oceans are studied using combined data from the International Satellite Cloud Climatology Project (ISCCP) and the European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis. Spectral analysis reveals that most of the time variability of cloud properties occurs on seasonal to annual time scales. The variance decreases one to two orders of magnitude for each decade of time scale decrease, indicating that daily to monthly time scales have smaller, but non-negligible variability. The length of these dominant time scales suggests that the majority of the variability is influenced by the general circulation and its interaction with boundary layer turbulence, rather than a product of boundary layer turbulence alone. Previous datasets have lacked the necessary resolution in either time or in space to properly characterize variability on synoptic scales; this is remedied by using global satellite-retrieved cloud properties. We characterize the intraseasonal subtropical cloud variability in both hemispheres and in different seasons. In addition to cloud fraction, we examine variability of cloud optical thickness - cloud top pressure frequency distributions. Despite the large concentration of research on the variability of Northern Hemisphere (NH) regions during summer, it is noted that the largest amplitude intraseasonal variability in the NH regions occurs during local winter. The effect of intraseasonal variability on the calculation and interpretation of seasonal results is investigated. Decreases in seasonally averaged cloud cover, optical thickness and cloud top pressure from the May-through-September season to the November-through-March season are most apparent in the NH regions. Further analysis indicates that these changes are due to an increase in frequency, but a decrease in the persistence of synoptic events. In addition, changes in cloud top pressure and optical thickness characteristics from the summer to winter seasons indicate that the NH subtropics undergo a change in dynamic regime with season. This change appears in the cloud fields as a shift from the more commonly seen lower-altitude, thicker optical thickness clouds to higher-altitude, thinner clouds. The latter cloud-type is associated with the lower sea level pressure, upward vertical velocity phase of the synoptic wave. Intraseasonal changes in cloud properties in the Southern Hemisphere and NH summer are much smaller in amplitude. Although they also appear to be linked to changes in the large-scale dynamics, similarly to NH winter variations, the relationships are more ambiguous due to the small amplitudes and longer time scales. We attempt to interpret some of these relationships using the results of the Betts and Ridgway (1989) box model. However, these results cannot consistently explain the patterns when results from all regions are considered, implying that this model may not adequately explain all the processes involved in the variability.

Rozendaal, Margaret A.↗
NASA NTRS2001

Precipitation Efficiency in the Tropical Deep Convective Regime

Precipitation efficiency in the tropical deep convective regime is analyzed based on a 2-D cloud resolving simulation. The cloud resolving model is forced by the large-scale vertical velocity and zonal wind and large-scale horizontal advections derived from TOGA COARE for a 20-day period. Precipitation efficiency may be defined as a ratio of surface rain rate to sum of surface evaporation and moisture convergence (LSPE) or a ratio of surface rain rate to sum of condensation and deposition rates of supersaturated vapor (CMPE). Moisture budget shows that the atmosphere is moistened (dryed) when the LSPE is less (more) than 100 %. The LSPE could be larger than 100 % for strong convection. This indicates that the drying processes should be included in cumulus parameterization to avoid moisture bias. Statistical analysis shows that the sum of the condensation and deposition rates is bout 80 % of the sum of the surface evaporation rate and moisture convergence, which ads to proportional relation between the two efficiencies when both efficiencies are less han 100 %. The CMPE increases with increasing mass-weighted mean temperature and creasing surface rain rate. This suggests that precipitation is more efficient for warm environment and strong convection. Approximate balance of rates among the condensation, deposition, rain, and the raindrop evaporation is used to derive an analytical solution of the CMPE.

Li, Xiaofan↗