Engineering topics

Hartmann, Dennis L.

Publications and source records attributed to Hartmann, Dennis L..

10 records

NASA NTRS1993

Large-scale effects on the regulation of tropical sea surface temperature

The dominant terms in the surface energy budget of the tropical oceans are absorption of solar radiation and evaporative cooling. If it is assumed that relative humidity in the boundary layer remains constant, evaporative cooling will increase rapidly with sea surface temperature (SST) because of the strong temperature dependence of saturation water vapor pressure. The resulting stabilization of SST provided by evaporative cooling is sufficient to overcome positive feedback contributed by the decrease of surface net longwave cooling with increasing SST. Evaporative cooling is sensitive to small changes in boundary-layer relative humidity. Large and negative shortwave cloud forcing in the regions of highest SST are supported by the moisture convergence associated with largescale circulations. In the descending portions of these circulations the shortwave cloud forcing is suppressed. When the effect of these circulations is taken into account by spatial averaging, the area-averaged cloud forcing shows no sensitivity to area-averaged SST changes associated with the 1987 warming event in the tropical Pacific. While the shortwave cloud forcing is large and important in the convective regions, the importance of its role in regulating the average temperature of the tropics and in modulating temperature gradients within the tropics is less clear. A heuristic model of SST is used to illustrate the possible role of large-scale atmospheric circulations on SST in the tropics and the coupling between SST gradients and mean tropical SST. The intensity of large-scale circulations responds sensitivity to SST gradients and affects the mean tropical SST by supplying dry air to the planetary boundary layer. Large SST gradients generate vigorous circulations that increase evaporation and reduce the mean SST.

Hartmann, Dennis L.↗

NASA NTRS1993

The seasonal cycle of low stratiform clouds

The seasonal cycle of low stratiform clouds is studied using data from surface-based cloud climatologies. The impact of low clouds on the radiation budget is illustrated by comparison of data from the Earth Radiation Budget Experiment with the cloud climatologies. Ten regions of active stratocumulus convection are identified. These regions fall into four categories: subtropical marine, midlatitude marine, Arctic stratus, and Chinese stratus. With the exception of the Chinese region, all the regions with high amounts of stratus clouds are over the oceans. In all regions except the Arctic, the season of maximum stratus corresponds to the season of greatest lower-troposphere static stability. Interannual variations in stratus cloud amount also are related to changes in static stability. A linear analysis indicates that a 6 percent increase in stratus fractional area coverage is associated with each 1 C increase in static stability. Over midlatitude oceans, sky-obscuring fog is a large component of the summertime stratus amount. The amount of fog appears to be related to warm advection across sharp gradients of SST.

Klein, Stephen A.↗

NASA NTRS1993

Spurious changes in the ISCCP dataset

The International Satellite Cloud Climatology Project (ISCCP) data set for July 1983-December 1990 exhibits a long term decrease in mean cloud optical depth and mean cloudtop temperature which is large compared to the mean for this period. It is here suggested that this decrease is an artifact of incomplete normalization of the visible channel on successive polar orbiters employed as the ISCCP's calibration standard; more accurate calibration techniques are required for the establishment of long-term climate trends.

Klein, Stephen A.↗

NASA NTRS1992

The effect of cloud type on Earth's energy balance - Global analysis

The role of fractional area coverage by cloud types in the energy balance of the earth is investigated through joint use of International Satellite Cloud Climatology Project (ISCCP) C1 cloud data and Earth Radiation Budget Experiment (ERBE) broadband energy flux data for the one-year period March 1985 through February 1986. Multiple linear regression is used to relate the radiation budget data to the cloud data. Comparing cloud forcing estimates obtained from the ISCCP-ERBE regression with those derived from the ERBE scene identification shows generally good agreement except over snow, in tropical convective regions, and in regions that are either nearly cloudless or always overcast. It is suggested that a substantial fraction of the disagreement in longwave cloud forcing in tropical convective regions is associated with the fact that the ERBE scene identification does not take into account variations in upper-tropospheric water vapor. On a global average basis, low clouds make the largest contribution to the net energy balance of the Earth, because they cover such a large area and because their albedo effect dominates their effect on emitted thermal radiation. High, optically thick clouds can also very effectively reduce the energy balance, however, because their very high albedos overcome their low emission temperatures.

Hartmann, Dennis L.↗

NASA NTRS1992

The effect of cloud type on earth's energy balance - Results for selected regions

International Satellite Cloud Climatology Project (ISCCP) C1 cloud information is compared with planetary albedo, outgoing longwave radiation (OLR), and net radiation measured at the top of the atmosphere by the Earth Radiation Budget Experiment (ERBE). Principal component analysis indicates that the day-to-day variations of the abundances of the 35 cloud types of the C1 data are correlated with each other, so that for many purposes the data set can be well represented by about five cloud types. Using stepwise multiple regression, the ISCCP C1 data can be used to predict the day-to-day variations of the energy balance measured by ERBE for 2.5-deg regions. Total fractional area coverage of cloudiness is a relatively poor predictor of radiation budget quantities. If the total fractional area coverage by clouds is divided into contributions from several distinct cloud types, the fractional coverages by these several cloud types will together form a much better prediction of radiation budget quantities than the single variable of total fractional-area cloud coverage. The regression equations can be used to estimate the net effect of clouds on the radiation balance and the contributions from particular types of clouds to albedo, OLR, and net radiation.

Ockert-Bell, Maureen E.↗

NASA NTRS1992

Theoretical support for the Airborne Antarctic Ozone Experiment

This investigation was to provide theoretical support during and after the deployment of NASA research aircraft to Punta Arenas, Chile during August and September of 1987 to conduct the Airborne Antarctic Ozone Experiment. The experiment was very successful in demonstrating the role of anthropogenic chlorine in producing the ozone hole over Antarctica during September and October of 1987. The PI worked primarily on using tracer data from the ER-2 aircraft to show that transport could not have caused the ozone hole in 1987, and that transport of chemical species into the polar vortex was very weak during the period of the experiment. The presence of gravity waves was also very apparent in the ER-2 data, and papers were published on this analysis and on the use of meteorological analyses to position the aircraft within the vortex.

Hartmann, Dennis L.↗

NASA NTRS1991

Diurnal variations of outgoing longwave radiation and albedo from ERBE scanner data

The scanning instruments of the Earth Radiation Budget Experiment provide measurements of instantaneous broadband albedo and outgoing longwave radiation (OLR) with a spatial resolution of about 50 km. Data from the Earth Radiation Budget Satellite (ERBS), which is in an orbit that precesses through local time at the rate of one hour every three days, can be used to describe the mean, hourly diurnal variations in the distribution of OLR and albedo on this scale. Much of this variation is caused by cloud type and amount changes. Two-dimensional histograms show the coevolution of OLR and albedo with the diurnal cycle, and the distribution of albedo-OLR pairings associated with the cloud distribution in a particular region and season. The albedo-OLR pairing characterizes a cloud type and determines its net effect on the energy balance at the top of the atmosphere. Diurnal variations in cloud type and amount in many regions are sufficient to cause substantial errors in radiation budget quantities and cloud properties estimated from observations taken from a single sun-synchronous orbit. Errors in estimated net radiation can be as large as 50 W/sq m for oceanic stratus regions and for land regions during summer.

Hartmann, Dennis L.↗

NASA NTRS1991

On the net radiative effectiveness of clouds

The net effect of clouds on the top-of-the-atmosphere energy fluxes was investigated by measuring the relative magnitudes of the effects of clouds on outgoing longwave radiation and on reflected solar radiation. The ratio between these two effects, expressed in terms of the cloud factor angle which measures the departure of the net radiative effect of clouds from a neutral effect, was estimated in two independent ways, using data from the Earth Radiation Budget Experiment. Both estimates of the cloud radiative effectiveness indicate that the global average net radiative effect of today's clouds is cooling, with the decrease in the absorbed solar radiation being greater than the decrease in the outgoing longwave radiation.

Hartmann, Dennis L.↗

NASA NTRS1991

The dynamics of the stratospheric polar vortex and its relation to springtime ozone depletions

Recent aircraft observations have determined the structure of polar vortices during winter and their relationship to polar ozone depletions, based on high dynamical isolation and the extremely low temperatures required for stratospheric cloud formation. The aircraft data reveal large gradients of potential vorticity and concentrations of conservative trace species at the transition from high-latitude to polar air, implying that the inward mixing of heat and constituents is strongly inhibited, and that the perturbed polar stratospheric chemistry associated with the ozone hole is isolated from the rest of the stratosphere until the vortex breaks up in late spring. It is therefore the overall polar vortex which limits the annual polar ozone depletions' maximum area-coverage.

Schoeberl, Mark R.↗

NASA NTRS1989

Reconstruction of the constituent distribution and trends in the Antarctic polar vortex from ER-2 flight observations

The measurements of ozone, ClO, and N2O concentrations in the south polar region taken aboard the ER-2 aircraft during the Airborne Antarctic Ozone Experiment are analyzed using conservative coordinate transformations to potential temperature-N2O and potential temperature-potential vorticity space. The latter transformation is equivalent to interpreting trace species observations within the modified Lagrangian mean (MLM) coordinate system. The results show that the MLM transformed ozone concentration decreases at about 0.06 ppmv per day between 20- and 16-km altitude inside the polar vortex during the mid-August to mid-September period. These ozone changes are collocated with the region of high ClO concentration. Outside the chemically perturbed region, at the highest aircraft altitudes, ozone concentration systematically increases, suggesting a diabatic cooling of the order 0.3-0.6 K/day.

Schoeberl, Mark R.↗