Impacts of upper tropospheric clouds on GPS radio refractivity

Global positioning system radio occultations (GPS RO) are active limb soundings in which a receiver aboard a low Earth orbiting satellite tracks the coherent signal broadcast by the GPS satellites as they occult behind the Earth´s atmosphere. The GPS signal delay introduced by the atmosphere along the entire signal path can be inverted to yield high vertical resolution radio-refractivity profiles as a function of height from the lower-troposphere to the stratosphere. Using various retrieval techniques, the neutral atmosphere refractivity can yield vertical profiles of temperature and water vapor. Recently, GPS RO has been used to identify thin moist atmospheric layers and to globally map boundary layer and tropopause heights as well as tropopause sharpness with high vertical sensitivity. The expected thermodynamic cloud boundary signatures and how they would appear in GPS refractivity are discussed; numerical algorithms for detecting cloud boundaries are proposed. The algorithms are validated using nearly coincident radio occultations from the CHAMP satellite and lidar and radiosonde data over a four-night period near Hawaii. The validation shows that the algorithms correctly determine cloud boundaries whenever lidar detected clouds, even when clouds are thin or layered