Philip Ardanuy

On the Onset Vortex of the Summer Monsoon

Abstract In this paper we examine the evolution of the low-level flow over the Arabian Sea during the onset of the summer monsoon. A detailed examination of the onset vortex that forms over the Arabian Sea just prior to the commencement of heavy rains over central India is carried out. The unique aspect of this study is the use of data sets from the Global Atmospheric Research Program (GARP) Monsoon Experiment (MONEX) from a variety of observing platforms. These include winds from geostationary satellites, constant level balloons, dropwindsonde aircraft and an enhanced World Weather Watch network. The data sets were analyzed for a 46-day period from 16 May through 30 June 1979. A number of calculations were performed with this analysis. Of major interest is a finding that the kinetic energy of the zonal flow over the central Arabian Sea increases by an order of magnitude one week prior to the commencement of monsoon rain over central India. This study provides a MONEX time-averaged analysis for the low-le...

Shortwave, longwave, and net cloud-radiative forcing as determined from Nimbus 7 observations

We studied the effect of clouds on the Earths radiation budget at the top of the atmosphere. Both the northern hemisphere summer and winter are considered in terms of cloud-radiative forcing, which describes the total effect of clouds on the Earths radiation budget components. The cloud forcing is obtained by using statistically generated models correlating simultaneous observations of cloud properties and radiation budget parameters obtained from observations from Nimbus 7 satellite observations. The models estimate the broadband albedo and longwave flux throughout the range of variation, including periods of extensive to near-total cloud cover where more simplistic representations fail. As such, the models are used to estimate period-averaged clear-sky radiation budget components. Knowledge of the cloudy-sky and clear-sky radiation budget components permits the computation of cloud-radiative forcing. We show that the net effect of clouds is to cool the Earth. On the average, for an Earth observed to be 52.1% cloud covered, the net cloud forcing is −26.8 Wm−2. This net cooling is obtained from a longwave warming of 24.1 Wm−2 and a shortwave cooling of −50.9 Wm−2. Over much of the Earth there is a general compensation between the warming and cooling effects of clouds. However, over the middle- and high-latitude oceans the albedo effect of clouds is dominant. This is particularly true in each summer hemisphere, where the effects of persistent cloudiness, low oceanic albedos, and high solar insolation combine to drive the net cloud forcing strongly negative. The net effect of cloud forcing cools the Earth, primarily due to the effect of clouds in the summer hemisphere oceans.

A Three-Dimensional Planetary Boundary Layer Model for the Somali Jet

Abstract This paper is an extension of an earlier study on the planetary boundary layer dynamics of the low level monsoonal flow over the Arabian Sea (Krishnamurti and Wong, 1979), where the long term steady state motion field for a boundary layer was determined using a zonally symmetric model with a prescribed pressure field. In that study we examined the balance of forces in the surface layer and the planetary boundary layer for regions across the equator, across and along the low-level Somali jet, and across an intertropical convergence zone. The important role of advective accelerations in the near-equatorial balance of forces was demonstrated. The important study is based on a three-dimensional model that removes the restriction of zonal symmetry. This mesoscale fine mesh model, with a horizontal resolution of ∼55 km and a vertical resolution of 200 m, is integrated to examine the evolution of three-dimensional planetary boundary layer flows for prescribed three-dimensional pressure patterns. The obs...

In-Flight Calibration of the Nimbus-7 Earth Radiation Budget (ERB) Sensors. Part II: Short-Term Perturbations

Abstract This paper describes the production calibration adjustment algorithms used to remove thermal perturbation and stray light noise signals from the Nimbus-7 earth radiation budget (ERB) measurements. Sunlight, both direct and scattered from the sensor baffles, contaminated the ERB measurements at satellite sunrise and sunset. The problem covered subsatellite solar zenith angles from 90° to 120° and reduced the usefulness of the longwave spectral radiation measurements. Scattered light corrections are made from 90° to 99° while orbit-by-orbit interpolation is used frown 99° to 121°. Tests indicate that in the mean the midpoint interpolation error is less than 1 W m−2 with a standard deviation of about 5 W m−2. Thermal perturbations on the total channel 12 (0.2–50 μm) appeared to be always less than 0.3%. However, the Suprasil-W domes on the otherwise similar shortwave channels 13 and 14 in some way helped produce thermal perturbations of up to 6% or more in channel 13 (0.2–3.8 μm and up to 3% or more...

Evolution of the Southern Oscillation as observed by the Nimbus-7 ERB experiment

Abstract The Nimbus-7 satellite has been in a 955-km, sun-synchronous orbit since October 1978. The Earth Radiation Budget (ERB) experiment has taken approximately 8 years of high-quality data during this time, of which 7 complete years have been archived at the National Space Science Data Center. A final reprocessing of the wide- field-of-view channel dataset is underway. Error analyses indicate a long-term stability of 1% better over the length of the data record. As part of the validation of the ERB measurements, the archived 7-year Nimbus-7 ERB dataset is examined for the presence and accuracy of interannual variations including the Southern Oscillation signal. Zonal averages of broadband outgoing longwave radiation indicate a terrestrial response of more than 2 years to the oceanic and atmospheric manifestations of the 1982–83 El Nin/Southern Oscillation (ENSO) event, especially in the tropics. This signal is present in monthly and seasonal averages and is shown here to derive primarily from atmosphe...

In-Flight Calibration of the Nimbus-7 Earth Radiation Budget (ERB) Sensors. Part III: Long-Term Changes

Abstract Sensitivity changes in the four wide-field-of-view (WFOV) Nimbus-7 earth radiation budget (ERB) sensors were monitored over a 9-yr period (November 1978–October 1987) by use of a number of reference sources. The sun was the primary reference and was used to check the shortwave (SW; about 0.2–4 μm) sensitivities on the twin total channels 11 and 12. The longwave (LW; greater than 4 μm) sensitivity in channel 12 was checked by a time series analysis of the nighttime mean global terrestrial signal, but the method could not be usefully applied to channel 11 because it was shuttered too much of the time. The accuracy of this type of analysis was verified by comparing a similar shortwave time series analysis with the solar calibration results. It was also checked by comparing channel 12 nighttime measurements with those from the companion scanner. The scanner had a built-in blackbody for calibration, but the scanner failed after 20 months. As a result of this comparison, a bias adjustment of 12.6 W m−2...

Evaluating the design of an earth radiation budget instrument with system simulations. Part 2: Minimization of instantaneous sampling errors for CERES-I

Abstract Much of the new record of broadband earth radiation budget satellite measurements to be obtained during the late 1990s and early twenty-first century will come from the dual-radiometer Clouds and Earths Radiant Energy System Instrument (CERES-1) flown aboard sun-synchronous polar orbiters. Simulation studies conducted in this work for an early afternoon satellite orbit indicate that spatial rms sampling errors of instantaneous CERES-I shortwave flux estimates will range from about 8.5 to 14.0 W m−2 on a 2.5° latitude and longitude grid resolution. Root-mean-square errors in longwave flux estimates are only about 20% as large and range from 1.5 to 3.5 W m−2. These results are based on an optimal cross-track scanner design that includes 50% footprint overlap to eliminate gaps in the top-of-the-atmosphere coverage, and a “smallest” footprint size to increase the ratio in the number of observations lying within to the number of observations lying on grid area boundaries. Total instantaneous measurem...

The constrained inversion of Nimbus-7 wide field-of-view radiometer measurements for the Earth Radiation Budget

Abstract The results of a constrained, wide field-of-view (WFOV) radiometer measurement deconvolution are presented and compared against higher resolution results obtained from the Earth Radiation Budget (ERB) Experiment on the Nimbus-7 satellite and from the Earth Radiation Budget Experiment (ERBE). The method is applicable to both longwave and shortwave observations and is specifically designed to treat the problem of anisotropic reflection and emission at the top of the atmosphere (TOA), and low signal-to-noise ratios that arise regionally within the observation field. The latter occur, for example, near the earths terminator where measured WFOV shortwave signals contain increasing percentages of instrument and modeling errors. Ridge regression and meridional smoothing are used to quell the resulting “local” instability and permit the recovery of a global solution. An optimized retrieval is obtained by tuning the constraints until the recovered solution matches, as well as possible, a known higher res...

In-Flight Calibration of the Nimbus-7 Earth Radiation Budget (ERB) Sensors. Part I: A Thermal Model for the Shortwave Channels

Abstract Much of the early record of spectrally broadband earth radiation budget (ERB) measurements was taken by the ERB instrument launched on the Nimbus-7 spacecraft in October 1978. The wide-field-of-view (WFOV) sensors measured the emitted and reflected radiation from November 1978 through January 1993, and the first nine years have been processed into a stable, long-term dataset. However, heating and cooling of the ERB experiment introduced thermal perturbations in the original measurements that were only significant in the shortwave (SW) channels. These sensors were covered by spherical filter domes to absorb incident longwave (LW) radiation. In this paper, a thermal regression model—the thermal calibration adjustment table (CAT)—is developed to track and remove these thermal signals from the SW data. The model relies on instrument temperatures within and near the surface of the ERB instrument, and the observed nonzero nighttime sensor readings represent the thermal signals. Confidence that the mode...