Robert L. Grossman

Interaction of low-level flow with the Western Ghat Mountains and offshore convection in the summer monsoon

Abstract Seven-year averaged values of percent frequency of occurrence of highly reflective cloud for the months June, July, and August indicate that offshore convection is a major component of the cloudiness of the southwest monsoon. Principal areas of convection occur off of the western coats of India, Burma, Thailand, and the Philippines. This study concentrates on the area upstream of the Western Ghats Mountains of India. Analysis of a special boundary layer mission flown during the WMO/ICSU Summer Monsoon Experiment leads us to believe that partial deceleration of the monsoon flow by upstream blocking effects of the mountains initiates and maintains a vertical and horizontal motion field that could support the observed convection. Data obtained on this mission allow a large-scale momentum budget computation for the subcloud layer, which shows pressure deceleration to be significant. The budget, dominated by advection, predicts an increase of average wind speed which is observed. The pressure decelera...

Land–Atmosphere Interaction Research, Early Results, and Opportunities in the Walnut River Watershed in Southeast Kansas: CASES and ABLE

Abstract This paper describes the development of the Cooperative Atmosphere Surface Exchange Study (CASES), its synergism with the development of the Atmosphere Boundary Layer Experiments (ABLE) and related efforts, CASES field programs, some early results, and future plans and opportunities. CASES is a grassroots multidisciplinary effort to study the interaction of the lower atmosphere with the land surface, the subsurface, and vegetation over timescales ranging from nearly instantaneous to years. CASES scientists developed a consensus that observations should be taken in a watershed between 50 and 100 km across; practical considerations led to an approach combining long—term data collection with episodic intensive field campaigns addressing specific objectives that should always include improvement of the design of the long—term instrumentation. In 1997, long—term measurements were initiated in the Walnut River Watershed east of Wichita, Kansas. Argonne National Laboratory started setting up the ABLE ar...

Influence of Land Cover and Soil Moisture on the Horizontal Distribution of Sensible and Latent Heat Fluxes in Southeast Kansas during IHOP_2002 and CASES-97

Abstract Analyses of daytime fair-weather aircraft and surface-flux tower data from the May–June 2002 International H2O Project (IHOP_2002) and the April–May 1997 Cooperative Atmosphere Surface Exchange Study (CASES-97) are used to document the role of vegetation, soil moisture, and terrain in determining the horizontal variability of latent heat LE and sensible heat H along a 46-km flight track in southeast Kansas. Combining the two field experiments clearly reveals the strong influence of vegetation cover, with H maxima over sparse/dormant vegetation, and H minima over green vegetation; and, to a lesser extent, LE maxima over green vegetation, and LE minima over sparse/dormant vegetation. If the small number of cases is producing the correct trend, other effects of vegetation and the impact of soil moisture emerge through examining the slope ΔxyLE/ΔxyH for the best-fit straight line for plots of time-averaged LE as a function of time-averaged H over the area. Based on the surface energy balance, H + LE ...

Extended-Range Probabilistic Forecasts of Ganges and Brahmaputra Floods in Bangladesh

The authors have developed a new extended-range flood forecasting system for large river basins that uses satellite data and statistically rendered probabilistic weather and climate predictions to initialize basin-scale hydrological models. The forecasting system overcomes the absence of upstreamflow data, a problem that is prevalent in the developing world. Forecasts of the Ganges and Brahmaputra discharge into Bangladesh were made in real time on 1–10-day time horizons for the period 2003–08. Serious flooding of the Brahmaputra occurred in 2004, 2007, and 2008. Detailed forecasts of the flood onset and withdrawal were made 10 days in advance for each of the flooding events with correlations at 10 days ≥0.8 and Brier scores <0.05. Extensions to 15 days show useable skill. Based on the 1–10-day forecasts of the 2007 and 2008 floods, emergency managers in Bangladesh were able to act preemptively, arrange the evacuation of populations in peril along the Brahmaputra, and minimize financial loss. The particul...

Scale Dependence of Air-Sea Fluxes over the Western Equatorial Pacific

Abstract The goal of this study is to examine the horizontal scale dependence of vertical eddy flux in the tropical marine surface boundary layer and how this scale dependence of flux relates to the bulk aerodynamic relationship and the parameterization of subgrid-scale flux. The fluxes of heat, moisture, and momentum are computed from data collected from 27 NCAR Electra flight legs in TOGA COARE (The Tropical Ocean Global Atmosphere Coupled Ocean-Atmosphere Response Experiment) with flight elevations lower than 40 m and flight runs longer than 60 km. The dependence of the fluxes on two length scales are studied: the cutoff length scale, defining the averaging length over which mean components are obtained in order to partition field variables into mean and perturbation components; and the flux averaging length scale, defining the length over which products of perturbations are averaged in order to estimate vertical fluxes. Based on the characteristics of the scale dependence of fluxes, the total flux of ...

Air–Sea Interaction during an Extreme Cold Air Outbreak from the Eastern Coast of the United States

Abstract The area east of the coast of North Carolina chosen for enhanced observation during the Genesis of Atlantic Lows Experiment (GALE) has one of the highest average wintertime energy transfers from ocean to atmosphere on earth. A substantial part of this transfer occurs in the aftermath of winter storms as cold, dry air flows off of the continent over the warm Gulf Stream. We report on an aircraft investigation of boundary layer mean and turbulent structure and evaluate the Lagrangian budgets of temperature and moisture in the subcloud layer following a streamline during an extreme cold air outbreak. The maximum sea-air temperature difference was 23 K. Two aircraft were used: the NCAR Electra, which measured turbulent fluxes and investigated subcloud layer conditions, while the NASA Electra, using a radar, measured the height of cloud tops. A stratocumulus overcast was found from about 60 km offshore to the Gulf Stream core with cloud top rising downstream. East of the Gulf Stream cumulus congestus ...

Choosing the Averaging Interval for Comparison of Observed and Modeled Fluxes along Aircraft Transects over a Heterogeneous Surface

Abstract Data from the April–May 1997 Cooperative Atmosphere Surface Exchange Study (CASES-97) are used to illustrate a holistic way to select an averaging interval for comparing horizontal variations in sensible heat (H) and latent heat (LE) fluxes from low-level aircraft flights to those from land surface models (LSMs). The ideal filter can be defined by considering the degree to which filtered aircraft fluxes 1) replicate the observed pattern followed by H and LE at the surface, 2) are statically robust, and 3) retain the heterogeneity to be modeled. Spatial variability and temporal variability are computed for different filtering wavelengths to assess spatial variability sacrificed by filtering and how much temporal variability can be eliminated; ideally, spatial variability should approach or exceed temporal variability. The surface pattern to be replicated is a negative slope when H is plotted against LE for a given time. This is required for surface energy balance if H or LE vary horizontally more ...

Convective boundary layer budgets of moisture and sensible heat over an unstressed prairie

Convective boundary layer budgets of sensible heat and moisture were evaluated for 2 days over the unstressed vegetation of the tallgrass Konza National Prairie during IFC 1 of the First International Satellite Land Surface Climatology Project (ISLSCP) Field Experiment (FIFE) 87. On June 1, 1987, winds were strong, while on the other day, June 4, 1987, winds were light. Data for the budgets came from morning and afternoon missions of the NCAR King Air aircraft. In addition to the budget evaluation the study had these objectives: to estimate the area-averaged surface fluxes and compare them to independent, ground-based measurements, to estimate the near surface evaporative fraction, and to compare various evaluations of the ratio of surface to inversion fluxes (the entrainment parameter). The budget analyses suggest that horizontal and vertical advection were significant terms in the budget and cannot be ignored. Given the uncertainties in both aircraft and ground-based nstruments and methods, the surface sensible heat flux comparisons were reasonable as was the comparison for evaporative fraction. However, comparison of latent heat fluxes was unsatisfactory. In contrast to other FIFE studies, consistent underestimation of surface fluxes by the aircraft was not found. The direct evaluation of the entrainment parameter, k, was problematic and did not compare well with other studies; however, using other methods of estimation, k was greater than 0.2. Bowen ratio estimates from level aircraft runs were low compared to sounding estimates. Error analyses and the budget evaluations emphasized the fact that the FIFE area was too small for accurate estimates of gradients and near-inversion fluxes. Since larger sites may make adequate deployment of surface property measurements prohibitive, a suggestion for improvement in experimental design of aircraft missions is presented. Both days and a third (June 6, 1987) studied previously, showed morning and afternoon episodes of drying throughout the mixed layer and surface layer. A mixed layer growth model is presented which predicts that the time rate of change of potential temperature in the superadiabatic layer is about 40% of the surface temperature rate. Variations in surface evaporation rate and leaf vapor pressure deficit appeared to be strongly associated with solar angle with no modulation by drying episodes, suggesting that atmospheric forcing of plant moisture cycling was minimal during this period.

Land Surface Heterogeneity in the Cooperative Atmosphere Surface Exchange Study (CASES-97). Part I: Comparing Modeled Surface Flux Maps with Surface-Flux Tower and Aircraft Measurements

Abstract Land surface heterogeneity over an area of 71 km × 74 km in the lower Walnut River watershed, Kansas, was investigated using models and measurements from the 1997 Cooperative Atmosphere Surface Exchange Study (CASES-97) field experiment. As an alternative approach for studying heterogeneity, a multiscale atmospheric and surface dataset (1, 5, and 10 km) was developed, which was used to drive three land surface models, in uncoupled 1D mode, to simulate the evolution of surface heat fluxes and soil moisture for approximately a 1-month period (16 April–22 May 1997) during which the natural grassland experienced a rapid greening. Model validation using both surface and aircraft measurements showed that these modeled flux maps have reasonable skill in capturing the observed surface heterogeneity related to land-use cover and soil moisture. The results highlight the significance of rapid greening of grassland in shaping the surface heterogeneity for the area investigated. The treatment of soil hydrauli...

NCAR/CU Surface, Soil, and Vegetation Observations during the International H2O Project 2002 Field Campaign

Abstract The May–June 2002 International H2O Project was held in the U.S. Southern Great Plains to determine ways that moisture data could be collected and utilized in numerical forecast models most effectively. We describe the surface and boundary layer components, and indicate how the data can be acquired. These data document the eddy transport of heat and water vapor from the surface to the atmosphere (in terms of sensible heat flux H and latent heat flux LE), as well as radiative, atmospheric, soil, and vegetative factors that affect it, so that the moisture and heat supply to the atmosphere can be related to surface properties both for observational studies and tests of land surface models. The surface dataset was collected at 10 surface flux towers at locations representing the major types of land cover and extending from southeast Kansas to the Oklahoma Panhandle. At each location, the components of the surface energy budget (H, LE, net radiation, and soil heat flux) are documented each half-hour, ...