The mutual interaction of cross-equatorial twin cyclones, its effect, and the effect of the planetary vorticity gradient on their motion are investigated using barotropic nondivergent and shallow water models. In comparing the behavior of a single cyclone to that of twin cyclones, it is found that each cyclone tends to accelerate the other and to turn its track westward. The causes of these changes are illuminated by investigating the motion of a single northern cyclone in a mean flow that resembles the one induced by a southern cyclone. The meridionally varying environmental vorticity gradient induced by the southern cyclone plays a key role in the explanation of the northern cyclone motion. This gradient tends to cancel the planetary vorticity gradient around the northern cyclone, resulting in a lower effective &Beta that produces a weaker wave number 1 (WN1) asymmetry and consequently a slower motion. As the cyclone moves northward, the effective &Beta increases, and so does the WN1 asymmetric circulation, which in turn accelerated the cyclone. Even in the linear case, the mean varying vorticity gradient causes the WN1 circulation to be no longer east-west, but to turn counterclockwise. This effect is due to the development of a strong anticyclonic circulation that is at a maximum in the northeast region as a consequence of vorticity conservation. When the nonlinear effects are included, the &Beta gyres are turned further counterclockwise by the symmetric circulation, which is stronger than in the single cyclone case. Mutual interaction of tropical twin cyclones may increase the duration of the resulting Westerly Wind Bursts by reducing the poleward drift. The role of divergence effects on the twin cyclones' evolution is examined using a shallow water simulation with the same cyclone wind profile as in the nondivergent case. It is found that the divergence tends to slow down the cyclone motion. We suggest that this effect is due to the potential vorticity conservation (compared to absolute vorticity conservation for nondivergent flows) that tends to reduce the produced WN1 asymmetry.

The second ARM (Atmospheric Radiation Measurement) Enhanced Shortwave Experiment (ARESE II) used a single aircraft flying above the north central Oklahoma Southern Great Plains ARM central facility to measure the atmospheric absorption in the column of air between the surface and the Twin Otter altitude ceiling around 7 km on both clear and overcast days. For this experiment, three types of broadband radiometers were used to measure upwelling and downwelling shortwave flux on the aircraft at 7 km. This provided redundancy that was lacking in the first ARESE. Further, all instruments used on the ground and on the aircraft were calibrated on the ground against the same radiation standard. Preflight and postflight comparisons of the broadband instruments used on the aircraft during the flight series with the standard, and comparisons of the ground instruments with the standard during the flights, suggest agreement much better than the target uncertainty of 20 W/m2 at the 95% confidence level. Comparisons of the standard and the aircraft instruments for low-altitude passes on clear days indicate more uncertainty as expected for a nonstationary platform. The estimated uncertainty in the measured column absorption based on the difference in measured net irradiances at the surface and near 7 km at the 95% confidence level is 20 W/m2.

We describe a radiation experiment performed at the Kaashidhoo Climate Observatory (KCO), Republic of Maldives during the Indian Ocean Experiment (INDOEX) field campaign in February through March 1999. Both the total solar broadband (0.3 to 3.81 microns) and visible spectral narrowband (7 spectral channels in the region from 0.4 to 0.7 microns) quantities were measured for the total (hemispherical), direct solar, and diffuse (including forward scattering) components of the radiation field. The aerosol optical depth at 500 nm, obtained from one of the narrowband spectral channels, ranged from approximately 0.2 to 0.7 during the period encompassing the intensive field phase (IFP) of INDOEX. The diurnally averaged atmospheric forcing efficiencies determined for the total solar broadband and visible spectrum are -72.2 +/- 5.5 W/m2 and -38.5 +/- 4.0 W/m2, respectively. Model simulations driven by in-situ measurements and realistic aerosol optical properties infer a single scattering albedo of 0.874 +/- 0.028 at 500 nm.

Data obtained by the ScaRaB instrument on the Meteor 3 satellite have been analyzed and compared to satellite (GOES-8), aircraft (RAMS), and surface (BSRN, SIROS, and RAMS) measurements of irradiance obtained during the Atmospheric Radiation Measurements Enhanced Shortwave Experiment (ARESE). It is found that the ScaRaB data covering the period from March 1994 to February 1995 (the instrument's operational lifetime) indicate excess absorption of solar radiation by the cloudy atmosphere in agreement with previous aircraft, surface, and GOES-8 results. The full ScaRaB data set combined with BSRN and SIROS surface observations gives an average all-sky absorptance of 0.28. The GOES-8 data set combined with RAMS surface observations gives an average all-sky absorptance of 0.26. The aircraft data set (RAMS) gives a mean all-sky absorptance of 0.24 (for the column between 0.5 and 13 km).