From Control of Supersonic Impinging Jets Using Microjets by Huadon Lou:

Loss of lift is another major problem when STOVL aircraft hover close to the ground. This lift loss, also referred to as “suck down force”, is due to the entrainment flow associated with the lifting jet that induces low surface pressure on the airframe, resulting in a force opposite to lift. It also is expected that the entrainment by the radial wall jet will further reduce the surface pressure on the airframe when STOVL aircraft hover close to the ground. Krothapalli et al. (1999) found the lift loss in hover increases in magnitude as the aircraft approaches the ground, and the lift loss can be as high as 60% of the main jet thrust.